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Cognition, 25 (1987) 71-102
Functional parallelism in spoken word-recognition
WILLIAM D. MARSLEN-WILSON*
Max-Planck-lnstitut fijr Psycholinguistik,
Nijmegen, and MRC Applied Psychology
Unit, Cambridge
Abstract
The process of spoken word-recognition breaks down into three basic func-
tions, of access, selection and integration. Access concerns the mapping of the
speech input onto the representations of lexical form, selection concerns the
discrimination of the best-fitting match to this input, and integration covers the
mapping of syntactic and semantic information at the lexical level onto higher
levels of processing. This paper describes two versions of a “cohort”-based
model of these processes, showing how if evolves from a partially interactive
model, where access is strictly autonomous but selection is subject to top-down
control, to a fully bottom-up model, where context plays no role in the process-
es of form-based access and selection. Context operates instead at the interface
between higher-level representations and information generated on-line about
the syntactic and semantic properties of members of the cohort. The new model
retains intact the fundamental characteristics of a cohort-based word-recogni-
tion process. It embodies the concepts of multiple access and multiple assess-
ment, allowing a maximally efficient recognition process, based on the princi-
ple of the contingency of perceptual choice.
1. Introduction
To understand spoken language is to relate sound to meaning. At the core
of this process is the recognition of spoken words, since it is the knowledge
representations in the mental lexicon that provide the actual bridge between
sounds and meanings, linking the phonological properties of specific word-
*I thank Uli Frauenfelder and Lorraine Tyler for their forbearance as editors, and for their comments on
the manuscript. I also thank Tom Bever and two anonymous reviewers for their stimulating criticism of
previous drafts. The first version of this paper was written with the support of the Department of Experimental
Psychology, University of Cambridge, which I gratefully acknowledge. Reprint requests should be sent to
William Marslen-Wilson, MPI fiir Psycholinguistik, Wundtlaan 1, 6525 XD Nijmegen, The Netherlands
OOlO-0277/87/$10.10 0 1987, Elsevier Science Publishers B.V.
72 W. D. Marslen- Wilson
forms to their syntactic and semantic attributes. This duality of lexical repre-
sentation enables the word-recognition process to mediate between two radi-
cally distinct computational domains-the acoustic-phonetic analysis of the
incoming speech signal, and the syntactic and semantic interpretation of the
message being communicated. In this paper, I am concerned with the conse-
quences of this duality of representation and of function for the organisation
of the word-recognition process as an information-processing system.
The overall process of spoken word-recognition breaks down into three
fundamental functions. These I will refer to as the access, the selection, and
the integration functions. The first of these, the access function, concerns the
relationship of the recognition process to the sensory input. The system must
provide the basis for a mapping of the speech signal onto the representations
of word-forms in the mental lexicon. Assuming some sort of acoustic-phone-
tic analysis of the speech input, it is a representation of the input in these
terms that is projected onto the mental lexicon.
The integration function, conversely, concerns the relationship of the rec-
ognition process to the higher-level representation of the utterance. In order
to complete the recognition process, the system must provide the basis for
the integration, into this higher level of representation, of the syntactic and
semantic information associated with the word that is being recognised.
Finally, and mediating between access and integration, there is the selec-
tion function. In addition to accessing word-forms from the sensory input,
the system must also discriminate between them, selecting the word-form
that best matches the available input.
These three functional requirements have to be realised in some way in
any model of spoken word-recognition. They need to be translated into claims
about the kinds of processes that subserve these functions, and about the
processing relations between them during the recognition of a word. I will
begin the discussion here by considering the way that the access and selection
functions are realised, and their relationship to the integration function. How
far do access, selection, and integration correspond to separate processing
stages in the recognition of a spoken word, and to what extent do they
operate in computational isolation from one another?
I will develop the argument here in its approximate historical sequence.
In Section 2 I will argue that, while the accessing of the mental lexicon is a
strictly autonomous, bottom-up process, there seems to be a close computa-
tional dependency between the process of selecting the word-form that best
matches the sensory input and the process of integrating the syntactic and
semantic properties of word-forms with their utterance context. The charac-
teristics of the real-time transfer function of the system suggest that the selec-
tion phase of the recognition process cannot depend on bottom-up informa-
Functional parallelism in spoken word-recognition 73
tion alone, and that contextual constraints also affect its outcome. This, as I
will show in Section 3, led to the first version of the cohort model: a parallel,
interactive model of spoken word-recognition. In Section 4 I will examine the
properties and predictions of this early model. In Section 5 I will show how
this model now needs to be modified. In particular, I will argue that it needs
to incorporate the concept of activation, and I will re-examine the role of
top-down interaction in the on-line recognition process, suggesting a model
where different information sources are integrated together to give the per-
ceptual output of the system, but where they do not, in the conventional
sense, interact. In particular, I argue for the autonomy of form-based selec-
tion, as well as for the autonomy of form-based access.
2. The earliness of spoken word-recognition
The crucial constraint on the functional properties of access and selection is
the earliness of correct selection. This I define as the reliable identification
of spoken words, in utterance contexts, before sufficient acoustic-phonetic
information has become available to allow correct identification on that basis
alone. If this can be demonstrated, then it places strong restrictions not only
on how the selection process is organised, but also on the ways in which
representations are initially accessed from the bottom-up.
To prove early selection, two things must be established. The first is how
long it takes to recognise a given word. This reflects the timing with which
the selection function is completed. The second is whether the acoustic-pho-
netic information available at this estimated selection-point is or is not suffi-
cient, by itself, to support correct identification.
The major techniques for establishing the timing of on-line word-recogni-
tion-thereby answering the first of these two questions-involve fast reac-
tion-time tasks. Typical examples are the shadowing and the identical
monitoring tasks, where the listener responds directly to the words he hears-
either by repeating them aloud, or by making a detection response to a
word-target. The mean reaction-times in such tasks, measured from word-
onset, can be used as a direct estimate of selection-time, subject to a correc-
tion factor to allow for the time it takes to execute the response.’ Typical
values obtained in these tasks (for one- and two-syllable content words heard
‘The use of a correction factor compensates for the fact that a monitoring reaction-time of, for example,
250 ms, does not mean that the word was not identified until 250 ms of it had been heard. There is undoubtedly
Verne lag between the internal decision process and the external evidence that this decision has been made.
The correction factor reflects this.
74 W. D. Marslen- Wilson
in normal utterance contexts) are of the order of 250-275 ms, which, with a
correction factor of 50-75 ms, gives a mean selection-time of around 200 ms
(e.g., Marslen-Wilson, 1973, 1985; Marslen-Wilson & Tyler, 1975, 1980).
Similar values can be obtained, more indirectly, from reaction-time tasks
where the listeners are asked to respond, not to the word itself, but to some
property of the word whose accessibility for response depends on first iden-
tifying the word in question. Examples of this are the rhyme-monitoring
results reported by Marslen-Wilson & Tyler (1975, 1980) and others (e.g.,
Seidenberg & Tanenhaus, 1979), and at least some research involving the
phoneme-monitoring task (e.g., Marslen-Wilson, 1984; Morton & Long,
1976). By subtracting an additional constant from the response-times in these
tasks, to take into account the extra phonological matching processes they
involve, one again arrives at selection-times for words in context of the order
of 200 ms from word-onset.
But these estimates are only half of the equation. It is also necessary to
establish whether or not the acoustic-phonetic information available at these
selection-points is sufficient for correct selection. For the research described
above, this could only be done indirectly, by estimating the average number
of phonemes that could be identified within 200 ms of word-onset, and then
using that estimate to determine how many words would normally still be
consistent with the input. If, as the available measurements suggest, 200 ms
would only be enough to specify an initial two phonemes, then there would
on average be more than 40 words still compatible with the available input
(this estimate is based on the analysis of a 20,000-word phonetic dictionary
of American English (Marslen-Wilson, 1984)). The limitation of this indirect
inference to early selection is that it cannot take into account possible coar-
ticulatory and prosodic effects. This could lead to an underestimate of the
amount of sensory information actually available to the listener after 200 ms.
The second main technique allows a more direct measure of the sufficiency
of the acoustic-phonetic input available at the estimated selection-point. This
is the gating task, as developed by Grosjean (1980), and exploited by Tyler
and others (e.g., Salasoo & Pisoni, 1985; Tyler & Wessels, 1983). Listeners
are presented with successively longer fragments of a word, at increments
ranging (in different experiments) from 20 to 50 ms, and at each increment
they are asked to say what they think the word is, or is going to become.
This tells us exactly how much acoustic-phonetic input the listener needs to
hear to be able to reliably identify a word under various conditions. In the
original study by Grosjean (1980), we find that subjects needed to hear an
average of 199 ms of a word when it occurred in sentential context, as op-
posed to 333 ms for the same acoustic token presented in isolation.
Because of the unusual way the auditory input is presented in the gating
Functional parallelism in spoken word-recognition 75
task, there has been some criticism of its validity as a reflection of normal
word-recognition processes. Since the listener hears the same fragments re-
peated many times in sequence, this might encourage abnormal response
strategies. This objection is met by Cotton and Grosjean (1984) and Salasoo
and Pisoni (1985), whose subjects heard only one fragment for any given
word, and where the pattern of responses matched very closely the results
for the same words when presented as complete sequences to each subject.
It is also possible that responses are distorted by the effectively unlimited
time-in comparison to normal listening-that listeners have available to
think about what the word could be at each presentation. This objection is
met by Tyler and Wessels (1985), in an experiment where subjects also heard
only one fragment from each word, and where they responded by naming the
word as quickly as possible. Mean naming latencies were 478 ms from frag-
ment offset, and the response patterns again closely corresponded to those
obtained without time-pressure.
In a recent study (Brown, Marslen-Wilson, & Tyler, unpublished) we have
combined reaction-time measures for words heard normally with gating tests
for the same words. This provides the most direct evidence presently available
for early selettion. In the first half of the experiment, subjects monitored
pairs of sentences for word targets, with a mean reaction-time for words in
normal contests of 241 ms. This gives an estimated selection-time of 200 ms
or less. In the second part of the experiment, the target-words were edited
out of the stimulus tapes and presented, as isolated words, to a different set
of subjects in a standard gating task. The mean identification-time estimated
here was 301 ms, indicating that the words were being responded to in the
monitoring task some 100 ms before sufficient acoustic-phonetic information
could have accumulated to allow recognition on that basis alone.2
Given, then, that we have accurate and reliable estimates of the two vari-
ables in our equation, simple arithmetic tells us that content words, heard in
utterance contexts, can usually be selected-and, indeed, recognised-earlier
than would be possible if just the acoustic-phonetic input was being taken
into account. Naturally, as Grosjean and Gee (1987, this issue) point out,
some words-especially function words and short, infrequent content
words-will often not be recognised early. In fact, under certain conditions
of temporary ambiguity, as Grosjean (1985) has documented, “late” selection
will occur, where the word is not only not recognised early, but may not even
be identified until the word following it has been heard. These observations
nonetheless do not change the significance of the fact that a large proportion
‘There is still the problem here of factoring out the purely acoustic-phonetic effects of removing words
from their contexts. We are investigating this in current research.
76 W. D. Murslen- Wilson
of words are selected early. A theory of lexical access has to be able to
explain this, just as it has to deal with late selection as well. Late selection,
however, places far weaker constraints on the properties of the recognition
process than does early se1ection.j
A different type of objection is methodological in character. It is argued
that none of the tasks used to establish early selection are measuring “real”
word-recognition. Instead, by forcing subjects to respond unnaturally early,
they elicit some form of sophisticated guessing behaviour. Forster (1981), for
example, argues that when a subject responds before the end of the word,
as in the shadowing task, he must in some way be guessing what the word
will be, on the basis of fragmentary bottom-up cues plus knowledge of con-
text.
Such objections, however, have little force. First, because the claim that
subjects are responding “unnaturally early” does not have any independent
empirical basis. There is no counter evidence, from “more natural” tasks,
showing that under these conditions different estimates of recognition-time
are obtained-nor is the notion “more natural task” itself easy to defend
except in terms of subjective preference. Secondly, to distinguish under these
conditions between “perception of the target word and guessing” (Forster,
1981, p. 490; emphases in original) is to assume, as a theoretical a priori, a
particular answer to the fundamental questions at issue.
Forster apparently wants to rule out, as an instance of normal perception,
cases where the listener responds before all of the sensory information poten-
tially relevant to that response has become available. But this presupposes a
theory of perception where there is a very straightforward dependency be-
tween the sensory input and the corresponding percept. The claims that I am
trying to develop here allow for the possibility of a less direct causal relation-
ship between the sensory input and the percept (see Marcel, 1983, for a
discussion of some related issues). These claims may or may not prove to be
correct. But one cannot settle the issue in advance by excluding evidence on
the grounds that it conflicts with the theoretical assumptions whose validity
one is trying to establish. If one is advancing the view that normal perception
is just the outcome of the integration of partial bottom-up cues with contex-
tual constraints, then it is not an argument against this view simply to assert
that perception under these conditions is not perception.
‘It should also bc clear. contrary to Grosjean and others, that the phenomenon of “late selection”, does
not constitute a problem for theories, like the cohort model, which emphasise the real-time nature of the
word-recognition proccsa. Actwation-based versmns of the cohort model, as discussed in Section 5, and as
modcllcd. for example. in the McClelland and Elman (1986) TRACE model, functmn equally well indepen-
dent of whether the critical sensory information arrives before or after the word boundary (as classically
defined).
Functional parallelism in spoken word-recognition 77
3. Implications of early selection
Early selection means that the acoustic-phonetic and the contextual con-
straints on the identity of a word can be integrated together at a point in time
when each source of constraint is inadequate, by itself, to uniquely specify
the correct candidate. The sensory input can do no more than specify a class
of potential candidates, consisting of those entries in the mental lexicon that
are compatible with the available input. Similarly, the current utterance and
discourse context provides a set of acceptability criteria that also can do no
more than delimit a class of potentially appropriate candidates. It is only by
intersecting these two sets of constraints that the identity of the correct can-
didate can be derived at the observed selection-point. It is this that forces a
parallel model of access and selection, and that poses intractable difficulties
for any model which depends on an autonomous bottom-up selection process
to reliably identify the single correct candidate for submission to subsequent
processing stages (e.g., Forster, 1976, 1979, 1981).
To see this, consider the major functional requirements that early selection
places upon the spoken word-recognition system. These are the requirements
of multiple access, of multiple assessment, and of real-time efficiency. They
reflect the properties the recognition system needs to have if it is to integrate
sensory and contextual constraints to yield mean selection-times of the order
of 200 ms.
Multiple access is the accessing of multiple candidates in the original map-
ping of the acoustic-phonetic input onto lexical representations. The sensory
input defines a class of potential word-candidates, and, in principle, all of
these need to be made available, via a multiple access process, to the selection
phase of spoken word-recognition. The second requirement is the require-
ment for multiple assessment. If contextual constraints are to affect the selec-
tion phase at a point in time when many candidates are compatible with the
sensory input, then the system must provide a mechanism whereby each of
these candidates can be assessed for their syntactic and semantic appropriate-
ness relative to the current context.
The final, and critical, requirement is for real-time efficiency. The system
must be organised to allow these access and assessment activities to take
place in real time, such that the correct candidate can be identified-and
begin to be integrated into an utterance-level representation-within about
200 ms of word-onset.
These requirements, taken together, cannot be met by a serial process
moving through the decision space one item at a time (cf. Fahlman, 1979).
They point, instead, to some form of parallel or distributed recognition model
(e.g., Hinton & Anderson, 1981). But they do not, however, uniquely deter-
78 W. D. Marslen- Wilson
mine the form of such a model. In particular, they do not unambiguously
dictate the manner in which the word-recognition process is divided up into
distinct processing stages. But they do place strong constraints on the func-
tional properties of the recognition model. The strategy that I have followed,
therefore, is to propose a model which rather literally embodies these con-
straints, and then to use this model as a heuristic starting-point for a detailed
investigation of the properties of on-line speech processing. Accordingly, I
will begin here by describing the first version of this model and the predictions
it makes. In a later section, I will discuss the ways the model now needs to
be expanded and modified.
The model in question, labelled an “active direct access model” in Marslen-
Wilson and Welsh (1978), but now usually referred to as the “cohort model”,
evolved out of an analysis of Morton’s logogen model (as stated in Morton,
1969) and of the Forster “bin” model (Forster, 1976). As originally stated, it
meets the requirements of multiple access and multiple assessment by assum-
ing a distributed, parallel processing system. In this system, each individual
entry in the mental lexicon is assumed to correspond to a separate computa-
tionally active recognition unit. This unit represents a functional coordination
of the acoustic-phonetic and of the syntactic and semantic specifications as-
sociated with a given lexical entry.
Given such an array of recognition elements, this leads to the characteristic
“cohort” view of the recognition process, with its specific claims about the
way this process develops over time. A lexical unit is assumed to become
active when the sensory input matches the acoustic-phonetic pattern specified
for that unit. The model prohibits top-down activation of these units in nor-
mal word-recognition, so that only the sensory input can activate a unit.
There is no contextually driven pre-selection of candidates, so that words
cannot become active as potential percepts without some bottom-up (sensory)
input to the structures representing these words.
Early in the word, when only the first 100-150 ms have been heard, then
the recognition devices corresponding to all of the words in the listener’s
mental lexicon that begin with this initial sequence will become active-
thereby meeting the requirement for multiple access.4 This subset of active
elements, constituting the word-initial cohort, monitors both the continuing
sensory input, and the compatibility of the words that the elements represent
‘The notion of “activity” will bc examined more closely in Section 5. What it means here is that each
lexical recognition unit, as a computationally independent pattern-matching device, can respond to the presence
of a match with the signal. Ail words that could match the input are matched by it, and this changes the state
of the relevant pattern matching devices, thereby differentiating them from the other devices in the system,
which do not match the current input.
Functional parallelism in spoken word-recognition 79
with the available structural and interpretative context-which meets the re-
quirement for multiple assessment. A mismatch with either source of con-
straint causes the elements to drop out of the pool of potential candidates.
This means that there will be a sequential reduction over time in the initial
set of candidates, until only one candidate is left. At this point, the correct
word-candidate can be recognised, and the correct word-sense, with its struc-
tural consequences, is incorporated into the message-level representation of
the utterance. This is a system that allows for optimal real-time efficiency,
since each word will be recognised as soon as the accumulating acoustic-
phonetic information permits, given the available contextual constraints.5
In terms of the issues raised earlier in this paper, the model treats the
initial access phase as a functionally separable aspect of the recognition pro-
cess. It does not do this by postulating an independent processing component
which performs the access function-in the style, for example, of the
peripheral access files proposed by Forster and others (e.g., Forster, 1976;
Norris, 1981). It assumes, instead, that the processing mechanisms underlying
word-recognition can only be engaged by a bottom-up input. It is the speech
signal, and only the speech signal, that can activate perceptual structures in
the recognition lexicon. ’ This has the effect of making access functionally
autonomous, without having to make claims about additional levels and pro-
cesses.
Once the word-initial cohort has been accessed, and the model has entered
into the selection phase, then top-down factors begin to affect its behaviour.
It is this that allows the model to account for early selection. When a word
is heard as part of a normal utterance, then both sensory and contextual
constraints contribute jointly to a process of mapping word senses onto
‘The sequential cohort recognition process is sometimes treated as if it were equivalent to following a path
down a “pronunciation tree”. This is a branching structure, starting from a single phoneme (e.g., /t/), and
branching at each subsequent phonetic choice point. By following the path to its terminal node one arrives at
the correct word--trespass, tress, rrend, or whatever. This captures in a limited sense the sequential decision
process represented in the cohort model. Where it fails, however, is to capture the treatment of context in
the cohort model, In a pronunciation tree, it is only when one reaches the terminal node that one can know
what word one is hearing. It is only at this point, therefore, that the syntactic and semantic information
associated with this word can be accessed. and made available for interaction with context. But the cohort
model-and the evidence on which it is based-require context to be able to operate much earlier in the word,
to help select the correct word even before the sensory input could have uniquely identified it. The pronunci-
ation tree is neither an adequate model of human word-recognition nor an accurate depiction of the cohort
model.
“It is not an argument against this claim to point out that one can often predict what someone is going to
say before they say it. There is no doubt that this is true. But to be able to predict what someone will say is
(i) not the same as having the percept that they have actually said it, nor (ii) is it evidence that this knowledge
can penetrate, top-down, into the mental lexicon. and change the state of the basic recognition devices-and
it is this that’s at issue here.
80 W. D. Marslen- Wilson
higher-level representations. The way this is realised in the model is by allow-
ing the semantic and syntactic appropriateness of word-candidates to directly
affect their status in the current cohort, which causes the selection process to
converge on a single candidate earlier than it would if only acoustic-phonetic
constraints were being taken into consideration.
Even in this rough and ready form-that is, as stated in Marslen-Wilson
and Welsh (1978) and Marslen-Wilson and Tyler (1980)-the model serves
its heuristic purpose. It makes a number of strong predictions, which not only
differentiate it from other models, but also, more importantly, raise novel
and testable questions about the temporal microstructure of spoken word-rec-
ognition. In the next section of this paper I will summarise the research by
myself and others into three of these major predictions: The model’s claims
about the concept of “recognition-point”, about optimal real-time analysis,
and about the early activation of multiple semantic codes.
4. Some predictions of the cohort model
4.1. The concept of recognition-point
The unique feature of the cohort model is its ability to make predictions
about the precise timing of the selection and integration process for any
individual word in the language. Other models have had essentially nothing
to say about the recognition process at this level of specificity. The cohort
model, in contrast, provides a theoretical basis for predicting the recognition-
point for any given word. This is the point at which, starting from word-onset,
a word can be discriminated from the other members of its word-initial
cohort, taking into account both contextual and sensory constraints. For
many words--especially monosyllables-this point may only be reached when
all of the word has being heard. But for longer words-and for words of any
length heard in constraining contexts-the recognition-point can occur well
before the end of the word.7
‘In a recent paper, Lute (1986) argues against the notion of recognition-point on the grounds that most
common words are monosyllables and that most monosyllables (as he establishes by searching a lexical data-
base) do not become unique until the end of the word or after. There are a number of problems WI& his
argument.
The first is that he does not take into account the role of prosodic structure and of various types of
anticipatory coarticulation in the recognition process. These will not only position the recognition-point earlier
than a purely phonemic analysis would indicate, but will also reduce the potential problem created by short
words that are also the first syllables of longer words. The second is that the claims of the cohort model derive,
in the first instance, from observations of word-recognition in context, where even monosyllables are normally
recognised before all of them have been heard (see Section 2 above). Thirdly, the important claim of the cohort
Functional parallelism in spoken word-recognition 81
Take, for example, the word “trespass”. If this word is heard in isolation,
then its recognition-point-the point at which it can be securely identified-is
at the /p/, since it is here that it separates from words like “tress” and “tres-
tle”. The recognition-point for the same word in context might be at the first
/s/, however, if these competitors were syntactically or semantically excluded.
Similar predictions can be derived for any word in any context, given a specifi-
cation of the word-initial cohort for that word, and of the constraints deriv-
able from the context in which it is uttered.
The crucial hypothesis underlying the notion of recognition-point is a claim
about the contingency of the recognition process. The identification of a word
does not depend simply on the information that a given word is present. It
also depends on the information that other words are not present. The word
“trespass”, heard in isolation, is only identifiable at the /p/ if the decision
process can take into account, in real-time, the status of other potential
word-candidates. The calculation of recognition-points directly reflects this.
If these predicted recognition-points are experimentally validated, then this
rules out all models of spoken word-recognition that do not allow for these
dependencies.
4.1.1. Evidence for recognition-points
Paralleling the various types of evidence for early selection summarised in
Section 2, the evidence for the psychological validity of recognition-points
derives from a mixture of reaction-time and gating tasks. In a first experiment
(Marslen-Wilson, 1978; Marslen-Wilson, 1984) response-latencies in a
phoneme-monitoring task were found to be closely correlated with re-
cognition-points, both as calculated a priori on the basis of cohort analysis,
and as operationally defined in a separate gating task.
In phoneme-monitoring, the subject is asked to monitor spoken materials
for a phoneme target defined in advance. There are two major strategies
listeners can use to do this (cf. Cutler & Norris, 1979). I exploited here the
lexical strategy, where the listener determines that a given phoneme is present
by reference to his stored phonological knowledge of the word involved.
When this strategy is used, response-latency is related to the timing of word
identification, since the phonological representation of the word in memory
cannot be consulted until it is known which word is being heard. If cohort
theory correctly specifies the timing of word-identification, then there should
model is, in any case, not whether the recognition-point falls early or late relative to the word-boundary, but
rather that the word is uniquely discriminated as soon as the available constraints (sensory. contextual) make
it possible for the system to do so. Wherever the recognition-point falls, that is where the listener should
identify the word in question. And for content words heard in utterance context, this will be, more often than
not, before all of the word has been heard.
82 W. D. Marslen- Wilson
be a close dependency between the monitoring response and the distance
between the phoneme-target and the recognition-point for that word. In par-
ticular, response-latency should decrease the later the target occurs relative
to the recognition-point, since there will be less of a delay before the subject
can identify the word and access its phonological representation.
I evaluated this question for a set of 60 three-syllable words, which con-
tained phoneme targets varying in position from the end of the first syllable
until the end of the third syllable. I had already confirmed that a lexical
strategy was being used for these stimuli, since overall response-latencies
dropped sharply over serial-positions, compared to a control set of nonsense
words where there was no change in latency as a function of position (for
further details, see Marslen-Wilson, 1984). The cohort structure of the mate-
rials was analysed to determine the recognition-point for each word, and the
distances measured between the recognition-points and the monitoring
targets. These recognition-points could occur as much as two or three
hundred ms before or after the target-phoneme.
A linear regression analysis showed that there was a close relationship
between these distances and the monitoring response (r = +.89).’ The vari-
ations in distance accounted for over 80% of the variance in the mean laten-
ties for the 60 individual words containing targets. This strong correlation
with phoneme-monitoring latency shows that recognition-points derived from
cohort analysis have a real status in the immediate, on-line processing of the
word. The subjects in this experiment were using a lexical strategy, so that
their response-latencies reflected the timing of word-recognition processes,
and the cohort model correctly specified the timing of these processes for the
words involved.
These results were checked in a follow-up study, which used the gating
task to operationally define the recognition-points for the same set of mate-
rials. Gating offers a variety of methods for calculating recognition-points,
depending on whether or not confidence ratings are taken into account. The
most satisfactory results are obtained when confidence ratings are included,
since this reduces the distorting effects of various response biases. Gating
recognition-points were therefore defined as the point in a word at which
85% of the subjects had correctly identified the word, and where these sub-
jects were at least 85% confident.’ These operationally derived recognition-
‘The correlation is positive because the earlier the recognition-point occurs, relative to the position of the
target phoneme (which is also the point from which response-time is measured), the longer the subjects have
to wait until they can identify the word, access its phonological representation. and then make their response.
“The exact percentage chosen as criteria1 is not critical. Setting the level at 80 or 90%, for example, gives
equivalent results.
Functional parallelism in spoken word-recognition 83
points correlated very highly both with the previous set of recognition-points
(calculated on an a priori basis) and with the phoneme-monitoring response
latencies (r = +.92).
The comparison between gating recognition-points and a priori recogni-
tion-points is further evidence that the cohort model does provide a basis for
correctly determining when a word can be recognised. The point at which a
word becomes uniquely identifiable, as established through an analysis of
that word’s initial cohort, corresponds very well to the point at which listeners
will confidently identify a word in the gating task. This has been confirmed
for a new set of materials, and, in particular, extended to words heard in
utterance contexts, in a recent study by Tyler and Wessels (1983). The gating
recognition-points calculated in this study are indeed the points at which a
single candidate is left, and this point is not only quite independent of the
total length of the word, but also varies in the manner predicted by the theory
as a function of the availability of contextual constraints.
4.1.2. Implications of on-line recognition-points
The evidence for the psychological reality of the recognition-points
specified by cohort analysis poses severe problems for certain classes of word-
recognition model. The recognition-points were calculated on the basis not
only of the positive information accumulating over time that a given word
was present, but also, and equally important, the information that certain
other words were not present. There is nothing, for example, about trespass
by itself that predicts a recognition-point at the /p/-or indeed, anywhere else
in the word. It is only in terms of the relationship of trespass to its initial
cohort that the recognition-point can be computed. This contingency of the
recognition response on the state of the ensemble of alternatives is in conflict
with the basic decision mechanisms employed both by logogen-based theories
and by serial search theories.
The results exclude, first, those recognition-models that depend on a self-
terminating serial search, in the manner of Forster’s models of access and
selection (Forster, 1976, 1979, 1981). In this type of model, word-forms are
stored in peripheral access files. These access files are organised into “bins”,
with the words within any one bin arranged in sequential order according to
frequency. Once a bin has been accessed, there follows a serial search through
the contents of the bin, terminating as soon as a word-form is encountered
which matches the search parameters. The search must be self-terminating,
since it is this that gives the model its ability to deal with frequency effects-
frequent words are recognised more quickly because they are encountered
earlier in the search process. Such a procedure could only take into account
the status of competing word-candidates if they were higher in frequency
84 W. D. Marslen- Wilson
than the actual best match. This would not predict the correct recognition-
points.
It is in general a problem for sequential search models if the outcome of
the recognition process needs to reflect the state of the entire ensemble of
possibilities, since this makes the process extremely sensitive to the size of
this ensemble. In fact, evidence I will cite later shows that the timing of
word-recognition processes is not affected by the number of alternatives that
need to be considered. Parallel access and selection processes are far better
suited to the task of providing information about the status of several word-
candidates simultaneously. But this by no means guarantees the suitability of
all parallel models.
One type of parallel model that is excluded by the present results (as well
as by the data reported in the next section) are the logogen-based models.
These models depend on the accumulation of positive evidence within a single
recognition device as the basis for recognition. Each device has a decision
threshold, and the word that is recognised is the one whose corresponding
recognition device (or logogen) crosses the threshold first, without reference
to the state of any other recognition devices. The model has no mechanism for
allowing the behaviour of one unit to take into account the behaviour of
other units in the ensemble. This means that it has no basis for computing
the recognition-point for a given word as a function of the timing with which
that word emerges from the ensemble of its competitors, and, therefore,
cannot explain the effectiveness of cohort-based recognition-points in ac-
counting for response variation in the phoneme-monitoring task.
4.2. Optimal real-time analysis
The evidence for the psychological reality of recognition-points is also evi-
dence for a more general claim about the properties of the word-recognition
system. In a distributed model of access and selection, information coming
into the system is made simultaneously available to all of the processing
entities to which it might be relevant. This makes the system capable, in
principle, of extracting the maximum information-value from the speech sig-
nal, in real-time as it is heard.
The information-value of the signal is defined with respect to the informa-
tion that it provides, over time, for the discrimination of the correct word-can-
didate from among the total set of possible words that might be uttered. To
use this information in an optimally efficient manner requires an access and
selection process that can continuously assess the sensory input against all
possible word-candidates. It is only by considering all possible lexical in-
terpretations of the accumulating sensory input that the system can be sure,
Functional parallelism in spoken word-recognition 85
on the one hand, of not selecting an incorrect candidate, and, on the other,
of being able to select the single correct candidate as soon as it becomes
uniquely discriminable-that is, at the point where all other candidates be-
come excluded by the sensory input. A series of experiments, using an audit-
ory lexical decision task, show that listeners do have access, in real time, to
information about the sensory input that could only have derived from an
analysis process with these properties (Marslen-Wilson, 1980, 1984).
These experiments focused on the discrimination of nonwords, rather than
on the timing of real-word recognition, because this made it possible to ask
a wider range of questions about the processes of access and selection. The
nonword stimuli-which the subjects heard mixed in with an equal number
of real words-were constructed by analysing the cohort structure of sets of
English words. The sequence “trenker”, for example, becomes a nonword at
the /k/, since there are no words in English beginning with /tren/ which have
/k/ as a continuation. The use of this type of material allowed us to ask the
following questions.
First, can listeners detect that a sound sequence is a nonword at precisely
the point where the sequence diverges from the existing possibilities in En-
glish-that is, from the offset of the last phoneme in the nonword sequence
that could be part of the beginning of a real word in English? If the selection
process does continuously assess the incoming speech against possible word-
candidates, then decision-time should be constant relative to critical phoneme
offset. It should be independent both of the position of the critical phoneme
in the sequence, and of the length of the sequence as a whole.
The results were unambiguous. Decision-time, measured from the offset
of the last real word phoneme, was remarkably constant, at around 450 ms.“’
It was unaffected either by variations in the position of the nonword point
(from the second to the fifth phoneme in the sequence), or by variations in
the length of the nonword sequences (from one to three syllables). It appears
that not only is there a continuous lexical assessment of the speech input, but
also that this input itself is not organised into processing units any larger than
a phoneme.
This latter point was investigated in a subsequent experiment (Marslen-
Wilson, 1984), which looked specifically at the role of a larger unit-the
syllable-in access and selection. If the speech input is fed to the mental
“‘We can also look at the results in terms of the relationship between overall reaction-time (measured from
sequence onset) and the delay from sequence onset until the offset of the critical phoneme. In an optimal
system, the slope of this relationship should approach 1.0, since reaction-time from sequence onset should
increase as a linear function of the delay until the sequence becomes a nonword. The outcome is very close
to this, with an observed slope of +.90, and with a correlation coefficient of + .97.
86 W. D. Marslen- Wilson
lexicon in syllable-sized chunks, then nonword decision-time, which depends
on access to the lexicon, should increase the further the critical phoneme is
from the end of the syllable. To test this, I used nonword sequences where
the critical phoneme was either at the beginning, in the middle, or at the end
of a syllable. This variation in position had no effect on decision-time, which
remained constant at around 4.50 ms. The absence of any delay for syllable-in-
ternal targets shows that subjects do not need to wait until the end of a
syllable to make contact with the lexicon. This is consistent with recent evi-
dence (Cutler, Mehler, Norris, & Segui, 1983) that the syllable does not
function as a processing unit in English.
The absence of length effects in these experiments appears to be fatal for
standard logogen models. A weak point in this type of model, as I have noted
elsewhere (Marslen-Wilson & Welsh, 1978), is its treatment of nonwords.
A logogen-based recognition system cannot directly identify a nonword, since
recognition depends on the triggering of a logogen, and there can be no
logogen for a nonword. The system can only determine that a nonword has
occurred if no logogen fires in response to some sensory input. But to know
that no logogen will fire, it must wait until all of the relevant input has been
heard. In the present experiment, therefore, nonword decision-times should
have been closely related to item length. In fact, there was no relationship
at all between these two variables.
The predicted effect of length derives directly from the fundamental deci-
sion mechanism around which logogen-based recognition models are con-
structed. The failure of this prediction means that we must reject such
mechanisms as the building blocks for models of spoken word-recognition.
The second main question I was able to ask, using nonword stimuli, ad-
dressed more directly the claim for a parallel access and selection process. A
major diagnostic of a parallel, as opposed to serial system, is its relative
insensitivity to set size effects. For a distributed system like the cohort model,
it need make no difference to the timing of the word-recognition decision
whether two candidates have to be considered or two hundred. In either case,
the timing of the selection process reflects the point at which a unique solution
emerges. .This is purely a matter of cohort structure, and has nothing to do
with the number of alternatives per se. For a serial process, however, which
moves through the alternatives in the decision space one item at a time, an
increase in the number of alternatives must mean an increase in decision-
time.
I investigated this in two experiments, in which I varied the size of the
“terminal cohort” of sets of nonword sequences. This refers to the number
of real words that are compatible with the nonword sequences at the point
where they start to become nonwords-that is, at the offset of the last real-
Functional parallelism in spoken word-recognition 87
word phoneme. To make the nonword decision, all of these words presuma-
bly need to be analysed, to determine whether the subsequent speech input
is a possible continuation of any of them. In the first experiment, the size of
these terminal sets varied from one to 30. In the second, replicating the first,
the range was from one to over 70. In neither case did I find an effect of
set-size. Decision-time was constant, as predicted by the model, from the
offset of the last real-word phoneme in the sequence, irrespective of whether
only one real word remained consistent with the input up to this decision
point, or of whether more than 70 still remained. This is evidence against any
sequential search model of spoken word-recognition, whether self-termi-
nating or not.
4.3. The early activation of multiple semantic codes
The two preceding sections focused on the way the cohort model leads one
to think about the relationship between the sensory input and the mechanisms
of access and selection. Here I consider the role of contextual constraints in
the operation of these mechanisms.
The cohort model places severe restrictions on the ways in which contex-
tual variables can affect the access and selection process. In particular, it
prohibits the top-down pre-selection of potential word-candidates. It is the
sensory input that activates the initial set of candidates, which can then be
assessed against context. There is no top-down flow of activation (or inhibi-
tion) from higher centers, but, rather, the bottom-up activation of the syntac-
tic and semantic information associated with each of the word-forms that has
been accessed.
This has two major consequences. It means, first, that contextual con-
straints cannot prevent the initial accessing (i.e., the entry into the word-ini-
tial cohort) of words that do not fit the context. There is already indirect
evidence for this from earlier work on lexical ambiguity (e.g., Seidenberg,
Tanenhaus, Leiman, & Bienkowski, 1982; Swinney, 1979). More recently,
research by Tyler (1984) and Tyler and Wessels (1983) shows that subjects
in the gating task produce a substantial proportion of contextually inappro-
priate responses at the earlier gates-that is, when they have heard between
50 and 200 ms of the word. These are responses that are compatible with the
available sensory input, but which do not fit the semantic and syntactic con-
text in which these fragments occur. The existence of these responses at the
early gates is evidence for the priority given by the system to the bottom-up
input, and for the inability of context to suppress the initial activation of
inappropriate candidates.
The second major consequence is that early in the recognition process
88 W. D. Marslen-Wilson
there will be the activation of multiple semantic and syntactic codes.” If
contextual constraints are to affect the selection process, they can only do so,
within this framework, if they have access to the syntactic and semantic prop-
erties of the potential word-candidates. This information must be made avail-
able not only about the word that is actually being heard, but also about the
other words that are compatible with the sensory input-that is, the other
members of the current cohort.
We have evaluated these two claims by using cross-modal priming tasks to
tap the activation of different semantic codes early in the recognition process.
In these experiments (Marslen-Wilson, Brown, & Zwitserlood, in preparation;
Zwitserlood, 1985), the subjects heard spoken words, and made lexical decision
judgements to visual probes that were presented concurrently with these words.
Previous research by Swinney and his colleagues (e.g., Onifer & Swinney,
1981; Swinney, 1979) had shown that lexical decisions to visually presented
stimuli are facilitated when these words are associatively related to spoken
words that are being presented at the same time.
The spoken words in our experiments were drawn from pairs of words
such as CAPTAIN and CAPTIVE, which only diverge from each other rela-
tively late in the word-in this case at the onset of the vowel following the
/t/-burst. The visual probes, to which the subjects made their lexical decisions,
were semantically associated with one or the other member of the pair of
spoken words-in this case, for example, the probes might be the words
SHIP and GUARD, where SHIP is frequently produced as an associate to
CAPTAIN but never to CAPTIVE, and vice versa for GUARD. The critical
variable, however, was the timing with which the visual probes were present-
ed, relative to the separation-point in the spoken words. We contrasted two
probe positions in particular: an Early position, where the probe appeared
just before the separation-point, and a Late position, where it occurred at
the end of the word, well after the separation-point.
The cohort model claims that both CAPTAIN and CAPTIVE will be ac-
cessed early in the selection process, and that this will make available the
semantic codes linked to both of them. If this is correct, then there should
be facilitation of the lexical decision for both visual probes when they occur
“It is important not to equate the kind of activation being postulated here with the activation effects
detected by Swinney (1979) and Seidenberg et al. (1982) in experiments using homophones. In these experi-
ments, subjects hear a complete word-form-like “bug” or “rose”-that has two or more different meanings.
Under these conditions. there is a strong activation of both meanings, which appears to persist for as much
as a second after word offset. This is not the same as the phenomena predicted here, where the transient
match, early in the word, of the incoming signal with a number of different word-forms leads to the transient
activation of the semantic and syntactic codes associated with these forms. These effects are only the faint
precursors of the activation effects to bc cxpectcd when there is a full match of the input to a given word-form,
1s in the homophone experiments.
Functional parallelism in spoken word-recognition 89
in the Early position. Decision-time for SHIP and GUARD should, there-
fore, be affected equally when these probes are presented on or before the /t/
in either CAPTIVE or CAPTAIN. In contrast, when the probes are present-
ed in the Late position, then only the probe related to the actual word should
be facilitated. If the word is CAPTAIN, for example, there should be facili-
tation of SHIP at the end of the word but not of GUARD.
This pattern should hold both for isolated words and for the same words
in context. If the initial access, first of word-forms, and then of the syn-
tactic and semantic information associated with these word-forms, is trig-
gered from the bottom-up, and if contextual effects can only operate on this
information after it has been accessed in this way, then the presence or
absence of contextual constraints should not affect the pattern of activation
of semantic codes at the early positions.
In a series of experiments this was exactly what we found. For words in
isolation we see facilitation of both probes for the Early locations, but only
facilitation of one probe at the Late positions (Marslen-Wilson et al., in
preparation; Zwitserlood, 1985). The same pattern holds for words in context
(Zwitserlood, 1985). The differential facilitation of probes associated with
contextually appropriate as opposed to contextually less appropriate words
only begins to appear after about 200 ms. At earlier probe positions, there
is evidence for the activation of semantic codes linked to contextually inap-
propriate words, just as we find for words in isolation.
These results support the fundamental claim of the cohort model that the
recognition process is based not only on multiple bottom-up access, but also
on multiple contextual assessment (as discussed in Section 3). They also dem-
onstrate that the involvement of contextual variables early in the selection
process takes place under highly constrained conditions. No contextual pre-
selection is permitted, and context cannot prevent the accessing and activa-
tion of contextually inappropriate word-candidates.
These conclusions distinguish the first version of the cohort model both
from standard autonomy models and from standard interactive models. The
cohort model differs from autonomous models, because it allows contextual
variables to affect the selection process. But it shares with autonomy theories
the assumption that initial access is autonomous, in the sense that top-down
inputs cannot activate perceptual structures in the recognition lexicon.
This partial “autonomy” distinguishes the cohort model from theories
which do permit top-down influences on initial access. One example is the
logogen model, where logogens can be activated by inputs from the cognitive
system as well as by bottom-up inputs. Another, more topical example, is the
interactive activation model put forward by Rumelhart and McClelland
90 W. D. Marslen- Wilson
(1981), and recently applied to spoken word-recognition in the form of the
TRACE model (Elman & McClelland, 1984). This is an approach that can
accommodate many of the phenomena driving the cohort model-and, in-
deed, this was what it was initially designed to do.
It is not clear, however, whether TRACE (or its predecessor COHORT),
with its mixture of excitatory connections between levels and inhibitory con-
nections within levels, can accommodate the pattern of semantic activation
described here for members of the same cohort heard in context and isolation.
It should, first, predict differential patterns early in recognition for the con-
textually appropriate word, because of feed-forward from excitatory top-
down connections. Secondly, because of the inhibitory connections between
units within a level, there should be very little early activation of competing
words like CAPTAIN and CAPTIVE. They should mutually inhibit each
other until after their separation-point. Neither prediction is consistent with
our results.
Finally, it is worth remembering that any evidence which reinforces the
claims for multiple contextual assessment also serves to underline the funda-
mental inability of sequential search models to explain the observed prop-
erties of the on-line transfer-function of the recognition system-namely, the
convergence of two sets of criteria, sensory and contextual, onto a unique
solution within 200 ms of word-onset.
5. Information and decision in the cohort model: Some revisions and
extensions
The results summarised in the preceding sections illustrate the value of the
cohort approach as a basis for research into spoken word-recognition, and
they support the accuracy of the claims it embodies about the functional
characteristics of the recognition process. Nonetheless, it is also clear that the
internal structure of the model, as originally stated, is over-simplified and
inadequate on several counts.
In this final section of the paper I want to discuss some problems with the
handling of information and decision in the cohort theory. These problems
concern the nature of the information coming into the system, the way that
information is represented within the system, and the way in which decisions
are taken to exclude or include candidates for selection and recognition.
I will argue, in particular, that the cohort model has to move away from
its binary concept of information and decision, where candidates are either
in the cohort or out of it, towards a more fluid form of organisation, incorpo-
rating the concept of activation. The rationale for this derives, first of all,
Functional parallelism in spoken word-recognition 91
from some recent evidence for the role of word-frequency in the early stages
of access and selection.
5.1. Activation and word-frequency
As originally stated, the cohort model made no mention at all of word-fre-
quency. The main reason for this was the absence of compelling evidence
that word-frequency was an effective variable in the kinds of on-line analysis
processes with which the model is concerned. The older research in this area
(e.g., Broadbent, 1967; Howes, 1957; Morton, 1969; Pollack, Rubinstein, &
Decker, 1960) showed that word-frequency affects the intelligibility of spoken
words heard in noise. But it was never clear whether these were immediate
perceptual effects or due to post-perceptual response biases.
More recent research, using reaction-time techniques, was flawed by its
failure to take into account the distribution of information over time in
spoken words. Unless the high and low frequency words in an experiment
are matched for recognition-point, and unless reaction-time is measured with
respect to this point, then any measures of response-time to the two different
classes of stimuli are difficult to interpret. This is the problem, for example,
with the auditory lexical decision data reported by McCusker, Holley-Wilcox,
and Hillinger (1979) and by Blosfeld and Bradley (1981). Both studies show
faster response times to high frequency as opposed to low frequency monosyl-
lables. But in each case reaction-time was measured from word-onset, with
no correction for possible variations in recognition-point,
Two new studies provide better evidence for the role of word-frequency.
In a preliminary study I looked at lexical decision latencies for matched pairs
such as STREET and STREAK, where the recognition-point for each word
is in the word-final stop-consonant. I2 This means that reaction-time can be
measured from comparable points in each member of the pair-in this case,
from the release of the final stop. For a set of 3.5 matched pairs, with mean
frequencies, respectively, of 130 per million and 3 per million, there was a
considerable advantage for the high-frequency words (387 vs. 474 ms).
Evidence of a different sort shows that these frequency effects can be
detected early in the selection process. This evidence comes from the research
on the early activation of semantic codes (see Section 4.3), where we found
that the frequency of the spoken words being heard indirectly affected the
amount of priming of the concurrent visual probe.
The effective variable was the difference in frequency between the word
‘*This was research carried out under my supervision by R. Sanders and E. Eden in 1983, as part of an
undergraduate research project in the Cambridge Department of Experimental Psychology.
92 W. D. Mmslen- Wilson
being heard and its closest competitor-in this experiment usually the other
member of the stimulus pair. For the Early probes, presented before the
spoken words had separated from each other, we regularly found more faci-
litation for the probe related to the more frequent member of the pair, with
the size of this effect varying according to the size of the frequency difference
between the two words.
The word CAPTAIN, for example, is more frequent than its close com-
petitor CAPTIVE. For visual probes presented in the Early position, just
before the /t/, there would be more facilitation of SHIP (the probe related
to CAPTAIN) than of GUARD (the probe related to CAPTIVE), irrespec-
tive of whether the word actually being heard was CAPTAIN or CAPTIVE
(Marslen-Wilson et al., in preparation). But for Late probes, presented at
the end of the spoken word, these effects of relative frequency had disap-
peared, so that only the probe associated with the actual word being heard
would be facilitated. Comparable effects were found by Zwitserlood (1$X35),
in a study where the relative frequency of the members of such pairs was
systematically varied.
These appear to be genuine perceptual effects, reflecting competition be-
tween different candidates early in the selection process. Alternative explana-
tions, in terms of post-perceptual response-bias, can be excluded. If there are
any bias effects in the data, they will reflect the properties of the visual
probes rather than the spoken words, since it was the visual probes the sub-
jects were actually responding to. They were not being asked to make any
judgements about the identity or lexical status of the spoken words, nor, in
general, did they seem to be aware that there was a relationship between
these words and the visual probes. Furthermore, since the effects hold only
for the Early probes, they reflect the state of the system during the selection
phase, and not after it is completed.
Finally, and most significantly for the activation argument, these effects
are transient. The effects of relative difference in frequency have dissipated
by the time the Late probes are presented (between 200 to 300 ms later).
What we appear to be picking up earlier in the word is a temporary advantage
accorded to frequent word-forms, where the size of this advantage reflects
the degree of differential activation of word-forms and their closest com-
petitors.
Related transient effects can be seen in some other studies. For example,
Blosfeld and Bradley (1981) only found significant frequency effects for
monosyllables. In disyllabic words, lexical decision time did not vary accord-
ing to frequency. This is because lexical decision is a task where the listener
needs to wait until the end of the word before making a positive response,
to make sure that he is not hearing a nonword. If the end of the word comes
Functional parallelism in spoken word-recognition 93
significantly later than the recognition-point, as will usually be the case for
disyllables, then the effects of word-frequency at the recognition-point will
have dissipated when the time comes for the subject to respond.‘” Finally, in
the gating task the effects of frequency appear systematically only at the
earliest gates (Tyler, 1984).
On the basis of this, I conclude the following. We can still assume that all
word-forms which match a given input will be accessed by that input, and
will remain active candidates as long as there is a satisfactory match with the
sensory input. However, the response of higher-frequency word-forms ap-
pears to be enhanced in some way, such that the level of activation of these
elements can rise more rapidly, per unit information, than the activation of
less frequent elements (cf. Grosjean & Itzler, 1984).
This means that, early in the word, high-frequency words will be stronger
candidates than lower-frequency words, just because their relative level of
activation will be higher. This transient advantage is what the priming data
reflect. And since the selection process is dependent on the emergence of
one candidate from among a range of competitors, this should lead to faster
recognition-times for high-frequency words than for low-frequency words,
especially for low-frequency words with high-frequency competitors. This is
because the activation of high-frequency competitors will take longer to drop
below the level of the low-frequency candidate, once the critical sensory
information has become available which excludes this high-frequency com-
petitor.
To adopt this kind of account means that the behaviour of the cohort
system can no longer be characterised in terms of the simple presence or
absence of positive or negative information. Elements are not simply switched
on or off as the sensory and contextual information accumulates, until a
single candidate is left standing. Instead, the outcome and the timing of the
recognition process will reflect the differential levels of activation of success-
ful and unsuccessful candidates, and the rate at which their respective activa-
tion levels are rising and falling.
Some recent attempts to model a cohort-like analysis process have, in fact,
represented the behaviour of the system in these or very similar terms (e.g.,
Elman & McClelland, 1984; Marcus, 1981, 1984; McClelland & Elman, 1986;
Nusbaum & Slowiaczek, 1983). The results of these simulations show that an
activation-based system is capable of exhibiting the main characteristics of a
cohort selection process, with the correct candidate emerging from among its
“1 should note, however, that recent research by Fraurnfcldcr (personal communication) has failed to find
this fall-off of frequency effects for disyllables.
94 W. D. Marslen- Wilson
competitors as the discriminating acoustic-phonetic information starts to ac-
cumulate.
But apart from being strongly suggested by the word-frequency data, the
activation concept has advantages in other respects. In particular, it enables
us to deal in a more satisfactory manner with a second set of issues raised by
the cohort model’s treatment of information and decision. These concern the
nature of the sensory and contextual input to the decision process, and the
way that the matching of these inputs to lexical representations affects this
process.
5.2. Matching processes in access and selection
In the initial formulation of the cohort model it was assumed that the match-
ing process was conducted on an all-or-none basis. The sensory and the con-
textual input either did or did not match the specifications for a given candi-
date. If it did not, then the candidate would be dropped from the cohort.
The trouble with this account is that it makes the successful outcome of
the recognition process dependent on an unrealistically perfect match be-
tween the specifications of the correct candidate and the properties of the
sensory input and the context. I will begin with the problems raised by varia-
bility in the bottom-up input.
5.2.1. Matching the sensory input
The cohort model emphasises the role of sensory information in determin-
ing the scope and characteristics of the access and selection process. It is this
that determines the membership of the word-initial cohort, and that has the
priority in determining which candidates remain in the cohort and which are
dropped. The available evidence suggests that this is the correct view to take
(see Section 4.3).
To take this view, however, is to run the risk of making the recognition
process too sensitive to noise and variation in the sensory input. If sensory
information is the primary determinant of cohort membership, and if the
matching process operates on an all-or-none basis, then even a small amount
of variability in the sensory signal could lead to problems in recognition, with
the correct word-candidate either never making it into the word-initial cohort,
or being dropped from it for spurious reasons.
In fact, the human spoken word-recognition system seems to be remark-
ably indifferent to noise in the signal, so long as the disrupted input occurs
in an utterance context. Even when deviations are deliberately introduced
into words-as in the mispronunciation detection task (Cole, 1973; Marslen-
Wilson & Welsh, 1978)-listeners often fail to notice them. Over 70% of
Functional parallelism in spoken word-recognition 95
small changes (i.e., changes by a single distinctive feature) are not detected
when they occur in words in utterance contexts, even though the same
changes are readily detectable in isolated syllables (Cole, 1973).
To accommodate this type of result, the model must find some way of
permitting deviant words to enter the cohort. The model can only allow
context to compensate for deficiencies in the bottom-up specification of the
correct candidate if this candidate nonetheless manages to find its way into
the cohort.
There are two aspects to the solution of this problem. The first follows
from the activation-based selection process sketched out in the previous sec-
tion. This is not a decision process that requires all-or-none matching, since
to discriminate the correct candidate it is not necessary to systematically
reduce the cohort to a single member. Selection does not depend on simple
presence or absence in the cohort, but on relative goodness of fit to the
sensory input. This makes it in principle possible for candidates that do not
fully match the sensory input to participate nonetheless in the recognition
process.
The second aspect of the solution involves the model’s assumptions about
the nature of the input. The system will respond quite differently to deviant
or noisy input, depending on the description under which this input is fed
into the decision process. The more highly categorised the output of acoustic-
phonetic analysis, the greater the problems that will be caused by variability
and error (cf., Klatt, 1980). In fact, if the cohort model is going to be able
to allow contextual constraint to compensate for bottom-up variability, then
the input to the lexicon cannot be anything as abstract as a string of
phonemes. Instead, a representation is required which preserves more infor-
mation about the acoustic-phonetic properties of the input-for example, a
representation,in terms of a feature matrix.
To see this, consider the consequences of minor disruptions of the signal
when we adopt different assumptions about the input. Suppose that the dis-
turbance is such that a word-initial voiced stop-for example, /be/-is mis-
identified as a voiceless stop (/pe/). If the input to the word-recognition system
takes the form of a string of phonemic labels, then this error will have drastic
consequences for the membership of the cohort. A match will be established
for all words beginning with /pe/, and these will be strongly activated. But
the word intended by the speaker, beginning with a lb/, will receive no acti-
vation at all.
In contrast, if the input is specified in terms of a set of feature values, then
such an error will have much less drastic consequences. A minimal pair like
/b/ and /p/ only differ in their specifications along one feature parameter-in
this case voicing. Even if a wrong assignment is made on this parameter, the
96 W. D. Marslen- Wilson
input will still match the specifications for /b/ words along all of the other
parameters. This means much less differentiation in the degree of match and
mismatch between the /be/ and the /pe/ sets, so that the word-form intended
by the speaker has a much better chance of receiving sufficient activation to
be treated as a candidate for selection and recognition. In other words, the
system will become more tolerant of minor deviations in the sensory input.
To assume a less highly categorised input to the lexicon does not sacrifice
the ability of the system to discriminate among different alternatives. There
is no inherent advantage to making phonemic distinctions at a pre-lexical
decision stage, and the choice between two phonemes can be made just as
well at the lexical level, as part of the choice between two words. In each
case, the decision takes into account the same bottom-up information. The
advantage of making the decision at the lexical level is that it enables the
system to delay committing itself to final decisions about the properties of
the sensory input until the other information relevant to this decision-includ-
ing the lexical status of different alternatives and their contextual roles-can
be taken into account (Klatt, 1980).
5.2.2. Matching the context
The evidence that selection is intimately bound up with integration lies at
the heart of the argument for a distributed model of spoken word-recognition.
But despite this, the way that the first version of the cohort model handles
the relationship between selection and contextual constraints is seriously
flawed.
Early statements of the model (e.g., Marslen-Wilson & Welsh, 1978) assert
that candidates drop out of the pool of word-candidates when they do not fit
the specifications of context, in the same way as when they do not fit the
accumulating sensory input. This runs into similar problems to the all-or-none
assumptions about sensory matching that I have just discussed. For the sen-
sory input, the problem was to explain how mispronounced, or otherwise
deviant words could nonetheless still be correctly identified. For context, the
problem is to explain how contextually anomalous words can be identified
(e.g., Norris, 1981).
Commonsense experience, as well as experimental evidence, tells us that
contextually inappropriate words can, in fact, be readily perceived and iden-
tified, so long as they are unambiguously specified in the signal. In a recent
experiment, for example, we compared monitoring latencies to the same
target under conditions where it was either normal with respect to its context,
or was anomalous in varying degrees of severity (Brown et al., unpublished).
Consistent with earlier results, there was a clear effect of anomaly. Response
latency to the word GUITAR increased by 27 ms over normal when it occur-
Functional parallelism in spoken word-recognition 97
red in an implausible context (“John buried the guitar”), and by a further 22
ms when it occurred in a semantically anomalous context (“John drank the
guitar”). But equally clearly, these anomalies are not causing a major break-
down of the recognition process. In the semantically anomalous condition,
for example, response-latencies remain well below 300 ms, and the error rate
is essentially zero. Even for grossly anomalous targets (“John slept the
guitar”), where verb sub-categorisation constraints are also violated, re-
sponse-time is still a relatively rapid 320 ms, and the error-rate remains low.
The relative speed and accuracy of correct selection for contextually inap-
propriate candidates is a reflection of the principle of bottom-up priority
(Marslen-Wilson & Tyler, 1980, 1983). The system is organised so that it
cannot override unambiguous bottom-up information. This means that there
is a considerable asymmetry in the degree to which context can override
bottom-up mismatch as opposed to the ability of bottom-up information to
override contextual mismatch. If the sensory input clearly differentiates one
candidate from all others, then that is the candidate that will emerge from
the perceptual process, irrespective of the degree of contextual anomaly. If
contextual variables clearly indicate a given candidate, it will nonetheless not
emerge as the choice of the system unless it also fits the bottom-up input
(within the limits of variation indicated earlier).
The clear implication of this is that context does not function to exclude
candidates from the cohort. There is no all-or-none matching with context,
and no all-or-none inclusion or exclusion of candidates on this basis. This
parallels the points made earlier (Section 4.3), prohibiting top-down influ-
ences upon initial access. It looks as if contextual factors can neither deter-
mine which candidates can enter the cohort, nor which candidates must leave
it. If we accept this conclusion, then there are two lines we can follow. One
is to maintain an interactive model, but to restrict the kinds of top-down
effects that are permitted. Since inhibitory effects are now excluded, context
will only have facilitatory effects, perhaps by increasing the level of activation
of candidates that fit the current context. Alternatively, we can turn towards
a different type of model, where no top-down interactions of any sort are
permitted. Different types of information are integrated together on-line to
produce the perceptual output of the system, but they do not interact in the
conventional sense. I will explore here the possibilities for this second kind
of account.
The effects of context, within the general framework I have adopted in
this paper, reflect the processing relationship between selection and integra-
tion. This is the relationship between, on the one hand, the set of potential
word-candidates, triggered from the bottom-up, and, on the other, the
98 W. D. Marslen- Wilson
higher-level representation of the current utterance and discourse. This con-
textual representation provides a structured interpretative framework against
which the senses associated with different word-forms can be assessed. In a
non-interactive model, this framework does not, itself, operate directly on
the activation levels of different candidates. These activation levels are a
measure of the relative goodness of fit of the candidates to the bottom-up
input, and context does not tamper with this measure.
We can capture, instead, the phenomena of early selection, and of contex-
tual compensation for bottom-up deficiency, by exploiting the capacity of a
parallel system for multiple access and multiple assessment. These will lead
to a form of on-line competition between the most salient candidates (those
most strongly activated by the sensory input) to occupy the available sites in
the higher-level representation. Once the appropriate senses associated with
a given word-form have been bound to these locations in the representation,
then we can say that recognition has taken place.t4
The speed with which this is accomplished will be the joint function of two
variables: the extent to which the bottom-up fit for a given candidate differen-
tiates it from its competitors, and the extent to which the contextual match
similarly differentiates it. The facilitatory and compensatory effects of context
reflect the tendency of the system to commit itself to a particular structural
interpretation even though the sensory input may not have fully differentiated
the word-form associated with this interpretation. The reason for this lack of
full bottom-up differentiation may be either temporal-not all of the sensory
input relevant to the decision has been heard yet, or it may be substantive-
the sensory input is simply inadequate by itself to indicate a unique candidate.
On this account, both access and certain aspects of selection are autonom-
ous processes, in the sense that they are driven strictly from the bottom-up.
Whether the speech signal is heard in context or in isolation, the basic pat-
tern-matching routines of the system will operate in the same way, providing
information about the goodness of fit of the sensory signal to the array of
lexical representations of word-forms.
This means that when the signal is heard in isolation, we will get something
approximating the commonsense concept of word-recognition-that is, a pro-
cess of form-based selection culminating in the explicit decision that a given
word-form is present. But when the signal is heard in context-and note that
normal context is fluent conversational speech-there need be no explicit
form-based recognition decision. Selection-viewed as the decision that one
particular word-form rather than another has been heard-becomes a by-
“It is at this point (Marslen-Wilson & Welsh, 1978) that the output of the system becomes perceptually
available.
Functional parallelism in spoken word-recognition 99
product of the primary process of mapping word-senses into higher-level
representations. The bottom-up access and selection processes provide the
essential basis for rapid on-line comprehension processes, but they provide
no more than a partial input to an integrative system that is only peripherally
concerned with identifying word-forms, and whose primary function is to
uncover the meanings that the speaker is trying to communicate.
5.2.3. The new cohort
In the preceding section of this paper 1 have suggested a number of mod-
ifications in the way that the cohort concept should be realised as a processing
model. These include the use of the activation concept, the introduction of
frequency effects into the early stages of the recognition process, the specifi-
cation of the bottom-up input in terms of some form of sub-phonemic rep-
resentation, and the exclusion of top-down contextual influences on the state
of the actual lexical recognition units. What do these changes mean for the
central concepts of the approach, with its emphasis on the contingency of
perceptual choice, and on the processing concept of the word-initial cohort?
By moving away from the concept of all-or-none matching against sensory
and contextual criteria, and by adopting an activation metaphor to represent
the goodness of fit of a given candidate to the bottom-up input, the model
abandons the convenient fiction that the cohort is a discrete, well-demarcated
entity in the mental life of the listener. The selection process does not depend
on the membership of the cohort reducing to a single candidate. It depends
instead on the process of mutual differentiation of levels of activation of
different candidates. The operation of the system still reflects the state of the
entire ensemble of possibilities, but the state of this ensemble is no longer
represented simply in terms of the all-or-none presence or absence of differ-
ent candidates.
Functionally, however, the cohort still exists. The effective core of salient
candidates will be much the same as it would have been under an all-or-none
regime. Although very many candidates will be momentarily activated as
aspects of their phonological representations transiently match the accumulat-
ing input, the preceding and subsequent input will not match, and they will
fall back into semi-quiescence. It takes some amount of time and input for
candidates to start to participate fully in the selection and integration process.
The effect of this is that the set of candidates which must be discriminated
among will look very similar to the membership of the word-initial cohort as
defined on an all-or-none basis. But by not defining it on this all-or-none
basis, the system becomes far better equipped to deal with the intrinsic and
constant variability of the speech signal.
Overall, none of the modifications I have suggested change the fundamen-
100 W. D. Marslen- Wilson
tal functional characteristics of the cohort-based word-recognition process. It
still embodies the concepts of multiple access and multiple assessment, allow-
ing a maximally efficient recognition process, based on the principle of the
contingency of real-time perceptual choice.
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La reconnaissance de mots (dans la chaine parlee) englobc trois fonctions fondamentales: I’acces, la selection
et I’intcgration. L’accPs se refere a I’appareillement de I’onde sonore avec les representations de formes
lcxicales; la sdection. designe la discrimination du meilleur “pareil” (match) lexical avec Ic stimulus. et I’inr&
grution recouvre l’appareillement de I’information syntaxique et semantique avec les niveaux de traitement
superieures.
Cet article decrit comment deux versions d’un mod& (de type “cohorte”) rendent compte de ces proces-
sus. en tracant son evolution a partir d’une premiere version comportant un principe d’interaction partielle
oti I’accts est strictement autonome mais oti la selection est soumise a des controles “de haut en has” vers une
deuxieme version (a fonctionnement entiercment “de bas en ham”‘) oti le contexte n’intervient plus dam les
processus d’acces et de selection.
Par consequent, le contexte n’intervient qu’a I’interface entre les representations superieures et I’informa-
tion generee en temps reel sur les proprietds syntaxiques et semantiques des membres du cohorte. Ce nouveau
modelc garde intactes les caractdristiques essentielles d’un proccssus de reconnaissance de type cohorte. II
integre les notions d’acces ct d’evaluation multiples permettant ainsi un processus de reconnaissance optimal
fond6 sur Ic principe de contingence de choix perceptif.
