The role of isochrony in speech perception in noise

Abstract

The role of isochrony in speech--the hypothetical division of speech units into equal duration intervals--has been the subject of a long-standing debate. Current approaches in neurosciences have brought new perspectives in that debate through the theoretical framework of predictive coding and cortical oscillations. Here we assess the comparative roles of naturalness and isochrony in the intelligibility of speech in noise for French and English, two languages representative of two well-established contrastive rhythm classes. We show that both top-down predictions associated with the natural timing of speech and to a lesser extent bottom-up predictions associated with isochrony at a syllabic timescale improve intelligibility. We found a similar pattern of results for both languages, suggesting that temporal characterisation of speech from different rhythm classes could be unified around a single core speech unit, with neurophysiologically defined duration and linguistically anchored temporal location. Taken together, our results suggest that isochrony does not seem to be a main dimension of speech processing, but may be a consequence of neurobiological processing constraints, manifesting in behavioural performance and ultimately explaining why isochronous stimuli occupy a particular status in speech and human perception in general.

Full text

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The role of isochrony in speech
perception in noise
Vincent Aubanel* & Jean‑Luc Schwartz
The role of isochrony in speech—the hypothetical division of speech units into equal duration
intervals—has been the subject of a long‑standing debate. Current approaches in neurosciences have
brought new perspectives in that debate through the theoretical framework of predictive coding
and cortical oscillations. Here we assess the comparative roles of naturalness and isochrony in the
intelligibility of speech in noise for French and English, two languages representative of two well‑
established contrastive rhythm classes. We show that both top‑down predictions associated with the
natural timing of speech and to a lesser extent bottom‑up predictions associated with isochrony at
a syllabic timescale improve intelligibility. We found a similar pattern of results for both languages,
suggesting that temporal characterisation of speech from dierent rhythm classes could be unied
around a single core speech unit, with neurophysiologically dened duration and linguistically
anchored temporal location. Taken together, our results suggest that isochrony does not seem to
be a main dimension of speech processing, but may be a consequence of neurobiological processing
constraints, manifesting in behavioural performance and ultimately explaining why isochronous
stimuli occupy a particular status in speech and human perception in general.
A fundamental property of mammalian brain activity is its oscillatory nature, resulting in the alternation between
excitable and inhibited states of neuronal assemblies1. e crucial characteristic of heightened excitability is that
it provides, for sensory areas, increased sensitivity and shorter reaction times, ultimately leading to optimised
behaviour. is idea formed the basis of the Active Sensing theoretical framework2,3 and has found widespread
experimental support.
Oscillatory activity in relation to speech, a complex sensory signal, has initially been described as speech
entrainment, or tracking, a view which proposes that cortical activity can be matched more or less directly to
some characteristics of the speech signal such as the amplitude envelope4,5. e need to identify particular events
to be the support of speech tracking has in turn prompted the question of which units would oscillatory activity
entrain to. e syllable has usually been taken as the right candidate6,7, given the close match, under clear speech
conditions, between the timing of syllable boundaries and that of the amplitude envelope’s larger variations. ese
conditions are however far from being representative of how speech is usually experienced: connected speech is
notoriously characterised by the lack of acoustically salient syllable boundaries8.
In parallel, early works on speech rhythm, also inspired by evident similarities with the timing of music9, have
led scholars to focus on periodic aspects of speech. e isochrony hypothesis extended impressionistic descrip-
tions of speech sounding either morse-like or machine gun-like10,11, and led to the rhythmic class hypothesis12,
stating that languages fall into distinct rhythmic categories depending of which unit is used to form the isoch-
ronous stream. Two main classes emerged, stress-timed languages (e.g., English) based on isochronous feet and
syllable-timed languages (e.g., French) which assume equal-duration syllables. Still, the isochrony hypothesis and
the related rhythmic class hypothesis, in spite (or by virtue) of their simple formulation and intuitive account,
have been the source of a continuous debate (see review in13).
e way current theories are formulated as reviewed above, isochrony in speech would present some advan-
tages: speech units delivered at an ideal isochronous pace would be maximally predictable and lead to maximum
entrainment, through alleviating the need for potentially costly phase-reset mechanisms14. However, naturally
produced speech is rarely isochronous, if at all, and this departure from a hypothetical isochronous form, that
is, variation of sub-rhythmic unit durations is in fact used to encode essential information at all linguistic (pre-
lexical, lexical, prosodic, pragmatic, discursive) and para-linguistic levels. Two apparently contradictory hypoth-
eses are therefore at play here, the rst one positing a benecial role for isochrony in speech processing, and the
second one seeing natural speech timing as a gold standard, with any departure from it impairing its recognition.
In this study we attempt to disentangle the role of the two temporal dimensions of isochrony and natural-
ity in speech perception. We report on two experiments conducted separately on spoken sentences in French
and English, each representative of the two rhythmic classes. For this aim, we exploited the Harvard corpus for
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English15 and its recently developed French counterpart, the Fharvard corpus16. Both corpora contain sentences
composed of 5–7 keywords, recorded by one taker for each language. Sentences were annotated at two hierarchi-
cal rhythmic levels: the accent group level and the syllable level, respectively forming the basis of the two main
language rhythmic classes mentioned above. We retimed naturally produced sentences to an isochronous form
(or a matched anisochronous form, see hereaer) by locally compressing or elongating speech portions cor-
responding to rhythmic units at the two corresponding levels (accent group and syllable). Retiming was opera-
tionalised around P-centres, that is, the time at which listeners report the occurrence of the unit17,18, and which
provide crucial pivotal events at the meeting point between bottom-up acoustic saliency cues and top-down
information about the onset of linguistic units. Unmodied time onsets of either accent (acc) or syllable (syl)
rhythmic units served as a reference for the natural rhythm (NAT) condition, from which isochronous (ISO) and
anisochronous (ANI) conditions were dened. Altogether, this provided 5 temporal versions of each sentence in
each corpus: the unmodied natural version (NAT), the isochronous stimuli at the accent (ISO.acc) and syllable
(ISO.syl) levels, and the anisochronous stimuli at the accent (ANI.acc) and syllable (ANI.syl) levels. e ANI
conditions were controls for the ISO conditions through the application of identical net temporal distortions
from the NAT sentences though in a non-isochronous way (see “Methods”).
We then evaluated the consequences of these modications of naturalness towards isochrony in the ability of
listeners to process and understand the corresponding speech items. Sentence stimuli were mixed with stationary
speech-shaped noise to shi comprehension to below-ceiling levels. en, for both languages separately, the set
of the ve types of sentences in noise was presented to native listeners, and the proportion of recognised key-
words was taken as the index of the intelligibility of the corresponding sentence in the corresponding condition.
We show that naturalness is the main ingredient of intelligibility, while isochrony at the syllable level—but not
at the accent group level, whatever the rhythmic class of the considered language—plays an additional though
quantitatively smaller benecial role. is provides for the rst time an integrated coherent framework combin-
ing predictive cues related to bottom-up isochrony and top-down naturalness, describing speech intelligibility
properties independently on the language rhythmic class.
Results
Natural timing leads to greater intelligibility than either isochronously or anisochronously
retimed speech in both languages. We rst report the eect of temporal distortion on intelligibility,
separately by retiming condition, for the two languages. Figure1 shows intelligibility results as the proportion of
keywords correctly recognised by French and English listeners (top panel) and the temporal distortion applied to
sentences in each condition (bottom panel, see “Methods”, Eq.(1) for computation details). Net temporal distor-
tion from natural speech at the condition level appears to be reected in listeners’ performance, with increased
temporal distortion associated with decreased intelligibility in both languages.
Extending the analysis done for the English data and reported in19, we tted a generalised linear mixed-eect
model to the French data. Table1 gathers the results of simultaneous generalised hypotheses on the condition
eects formulated separately for each language.
As is veried in Fig.1 and in the rst 4 rows of Table1, intelligibility of unmodied naturally timed sentences
for French was signicantly higher than sentences in any temporally-modied conditions. is result replicates
what was obtained for English, and conrms that any temporal distortion leads to degraded intelligibility. In
contrast to English however, where accent-isochronously retimed sentences were signicantly more intelligible
than accent-anisochronously retimed ones, no such eect is observed for French (Table1 row 5). Similarly, the
tendency for an isochronous versus anisochronous intelligiblity dierence at the syllable level observed in English
is absent in French (Table1 row 6). Indeed, an overall benet of isochronous over anisochronous transformation
is observed for English but not in French, when combining the two rhythmic levels (Table1 row 7).
As shown by the last row of Table1, syllable level distortion led to greater intelligibility decrease, than accent-
level distortion, in both French and English. is relates to the greater amount of distortion applied to syllable-
level over the accent-level modications applied to sentences, see Fig.1, bottom panel.
In sum, while temporal distortion appears to be the main predictor of intelligibility for both languages, the
independent role of isochrony seems to dier between both languages. We present in the next section evidence
for a common pattern underlying these surface dierences.
Syllable‑level isochrony plays a secondary role in both languages, even in naturally timed sen‑
tences. We dened several rhythm metrics to quantify, for each sentence in any temporal condition, the
departure of either accent group or syllable timing from two canonical rhythm types: natural or isochronous
rhythm. For the two hierarchical rhythmic levels considered here, this amounts to 4 metrics altogether: depar-
ture from naturally timed accent groups or syllables (respectively dnat.acc and dnat.syl), and departure from
isochronous accent group or syllables (respectively diso.acc and diso.syl, see “Methods” and Table5).
Figure2 shows intelligibility scores as a function of the temporal distortion applied to the sentences along
the 4 metrics, for all 5 experimental conditions, for English and French.
We analysed the joint role of isochrony and naturality in the dierent temporal conditions using logistic
regression modelling (see “Methods”). We conducted three separate analyses by grouping natural, isochronous
and anisochronous sentences respectively. is was done to avoid including subsets of data where a given metric
would yield a zero-value by design (see Fig.2). e three analyses are presented in the next subsections, each
corresponding to a highlighted region in Fig.2.
Departure from isochrony in naturally timed sentences (Fig. 2 region A). Naturally-timed sen-
tences have a null departure from naturality by design, but their departure from an isochronous form at both
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●
●
●
●
●
●
●
●
●
●
French English
0.40
0.45
0.50
0.55
0.60
prop. word correct
Intelligibility
●
●
●
●
●
●
●
●
●
●
NAT
ISO.acc
ANI.acc
ISO.syl
ANI.syl
NAT
ISO.acc
ANI.acc
ISO.syl
ANI.syl
0.0
0.1
0.2
0.3
0.4
0.5
δ
●
●
●
●
●
NAT
ISO.acc
ANI.acc
ISO.syl
ANI.syl
Temporal distortion
Figure1. Top panel: proportion of words correctly recognised in each experimental condition for French and
English. Error bars show 95% condence intervals over 26 and 27 subjects in the two languages respectively.
Bottom panel: average sentence temporal distortion (
δ
function computed on speech units matching the
temporal condition, see Eq.(1)). By construction, temporal distortion is null for natural sentences (NAT
condition) and identical for isochronously and anisochronously retimed sentences at a given rythmic level, that
is, in ISO.acc and ANI.acc conditions on one hand, and in ISO.syl and ANI.syl conditions on the other hand.
Error bars show 95% condence intervals over 180 sentences for French and English. Data for English was
previously reported in19.
Table 1. Simultaneous generalised hypotheses tests for the eect of condition on intelligibility, formulated
on two independent models for French and English respectively. From le to right: comparison tested and,
for each language, comparison estimate and associated z and p values, with classical visual signicativity
indication. Data for English has been previously reported in19.
Row Comparison
French English
Est. z p Est. z p
1 ISO.acc, NAT
−0.509
−10.94
<0.001
***
−0.545
−12.16
<0.001
***
2 ANI.acc, NAT
−0.420
−9.09
<0.001
***
−0.722
−16.06
<0.001
***
3 ISO.syl, NAT
−0.862
−18.45
<0.001
***
−1.017
−22.40
<0.001
***
4 ANI.syl, NAT
−0.820
−17.59
<0.001
***
−1.127
−24.68
<0.001
***
5 ISO.acc, ANI.acc
−0.088
−1.93
0.273 0.177 4.00
<0.001
***
6 ISO.syl, ANI.syl
−0.042
−0.91
0.878 0.110 2.43
0.092.
7 ISO, ANI
−0.130
−2.01
0.233 0.287 4.54
<0.001
***
8 syl, acc
−0.753
−11.59
<0.001
***
−0.878
−13.80
<0.001
***
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the accent and the syllable level can be evaluated by the diso.acc and diso.syl metrics respectively. We therefore
included in the analysis the diso.acc and diso.syl metrics, and discarded the dnat.acc and dnat.syl metrics (see
Fig.2 regionA). Starting from the initial logistic regression model predicting intellibility with the full interac-
tion of language (French and English), diso.acc and diso.syl, as xed eects, we found that the simplest equivalent
model was a model with only diso.acc and diso.syl factors without interaction (see Table2 and “Methods”).
e resulting model shows that for natural sentences, intelligibility is positively correlated with departure from
accent isochrony (i.e., increased accent group irregularity is associated with better intelligibility) and negatively
correlated with departure from syllable isochrony (i.e., the more isochronous naturally timed syllables are, the
better the sentence is recognised). Importantly, this result does not depend on the language considered, with
both French and English showing the same pattern of results. Fixed eect sizes are markedly small, as the major-
ity of the variance is explained by random eects, as expected with the material used here. But xed eects are
nevertheless real and quantitatively important, as seen on Fig.2.
Departure from natural timing in isochronously retimed sentences (Fig. 2 region B). Next we
assessed to what extent intelligibility in isochronous conditions can be predicted from the departure from natu-
ral rhythm, at the accent and syllable levels (see Fig.2 regionB). From an initial fully interactional model with
language, dnat.acc and dnat.syl predictors, the simplest equivalent model consisted of only the dnat.syl factor
(see Table3).
is indicates that in conditions where sentences are isochronously transformed, intelligibility is signicantly
negatively correlated with departure from natural syllabic rhythmicity. Crucially, departure from accent group
natural rhythm does not play a role, and the results are identical for both languages considered.
Figure2. Intelligibility as a function of temporal distortion, as measured by the four metrics (rows) dened
in Table5. Data are grouped according to experimental condition (colors), the type of modication of the
experimental condition (column groupings) and language (columns). ree subsets of data are highlighted
for subsequent analysis (see text): (A) departure from isochrony of naturally timed sentences; (B) departure
from natural rhythm of isochronously retimed sentences; (C) departure from natural rhythm and isochrony
of anisochronously retimed sentences. Regression lines show linear modelling of the data points, disregarding
subject and sentence random variation.
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Departure from isochrony and natural timing in anisochronously retimed sentences (Fig. 2
region C). In this last step we evaluated whether intelligibility of anisochronously retimed sentences could be
predicted by a combination of the four rhythmic distortion metrics (diso.acc, diso.syl, dnat.acc and dnat.syl, see
Fig.2 regionC). Indeed, anisochronously retimed speech departs from both natural and isochronous canonical
forms of timing, and in particular all four metrics have non all-zero values for these sentences. From an initial
fully interactional model crossing language with the four rhythm metrics, we found that the simplest equivalent
model consisted of the additive model of factors dnat.syl and diso.syl (see Table4).
ese results rene and extend what was obtained in the previous two analyses. First, the rhythmic unit of
accent group (whether dened at the stressed syllable level in English or at the accentual phrase level in French)
does not provide any explanatory power in predicting intelligibility in anisochronously timed speech. Second, the
role of natural syllable timing is conrmed and is the strongest predictor of intelligibility in that model as shown
by its z-value (Table4B) and its eect size (Table4C). ird, a role for the departure from isochronously-timed
syllables is detected. is means that in these conditions where the timing of speech is most unpredictable, there
is a tendency for isochronous syllables to be associated with increased intelligibility. We note however that there is
a necessary correlation between dnat and diso metrics for anisochronous sentences, exemplied by the fact that a
close-to-isochronous natural sentence has to be distorted by a low diso value to be rendered isochronous, and that
its anisochronous counterpart, being distorted by the same quantity by design, will be close to both the natural
and the isochronous version. A quantitative analysis conrmed this (Pearson’s product-moment correlation
Table 2. (A) Initial (m1) and equivalent simpler (m2) model for the role of departure from isochrony in
naturally timed sentences. e formulae of the xed eects are given for the two models, and the result of
a likelihood-ratio test between the two models is given on the right of the vertical separator. (B) m2 model
coecients, with associated p values. (C) Fixed-eect sizes with lower and upper condence levels.
Model summary Likelihood-ratio test (m1, m2)
Fixed eects AIC
χ2
Df
p
(> χ
2)
(A) Model selection
m1 language
∗
diso.acc
∗
diso.syl 6728.4 3.36 5 0.65
m2 diso.acc+ diso.syl 6721.7
Estimate SE zvalue Pr(>|z|)
(B) Equivalent model (m2) coecients
(Intercept) 0.8633 0.3230 2.673 0.00752**
diso.acc 1.0651 0.4633 2.299 0.02150*
diso.syl
−1.5151
0.5757
−2.632
0.00850**
Eect
R2
Lower CL Upper CL
(C) Fixed-eects size
m2 0.028 0.014 0.048
diso.syl 0.018 0.006 0.034
diso.acc 0.013 0.004 0.028
Table 3. (A) Initial (m3) and equivalent simpler (m4) models for the role of departure from natural rhythm in
isochronously retimed sentences.e formulae of the xed eects are given for the two models, and the result
of a likelihood-ratio test between the two models is given on the right of the vertical separator. (B) m4 model
coecients, with associated p values. (C) Fixed-eect sizes with lower and upper condence levels.
Model summary Likelihood-ratio test (m3, m4)
Fixed eects AIC
χ2
Df
p
(> χ
2)
(A) Model selection
m3 language
∗
dnat.acc
∗
dnat.syl 13,507 6.12 6 0.41
m4 dnat.syl 13,502
Estimate SE zvalue Pr(>|z|)
(B) Equivalent model (m4) coecients
(Intercept) 0.7382 0.1424 5.185
2.16e
−
07
***
dnat.syl
−2.6231
0.2749
−9.541
<2e−16
***
Eect
R2
Lower CL Upper CL
(C) Fixed-eects size
m4 0.045 0.033 0.058
dnat.syl 0.045 0.033 0.058
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between diso.syl and dnat.syl: French:
r
=
0.72, p<0.01
; English:
r
=
0.56, p<0.01
). e specic contribution
of syllabic isochrony to intelligibility therefore appears to be small at best for anisochronous sentences. Finally,
as for above analyses, this pattern of result applies indistinctively to both French and English.
Discussion
In the current study, we set to characterise the possible role of isochrony in speech perception in noise. Isochrony
is contrasted to naturalness, where the former refers to an ideally perfectly regular timing of speech units, while
the latter to the timing of speech units as they occur in naturally produced speech. We included a third set of
anisochronous conditions, in which the timing of speech events bear the same degree of temporal distortion from
naturally timed speech as isochronous speech, while being irregular. We tested temporally modied sentences
at the accent and the syllable levels, two hierarchically nested linguistic levels in English and French. ese two
languages are traditionally described as being representative of two distinct rhythmic classes, based on a hypo-
thetical underlying isochrony of accent versussyllable units respectively in natural speech production12,13,20.
A rst important result of this study is the replication from English to French that isochronous forms of
speech are always less intelligible than naturally timed speech. In fact, in a paradigm as the current one, where
the internal rhythmic structure of speech is changed but the sentence duration remains the same, any temporal
distortion to the naturally produced timings of speech units appears to be detrimental, with the amount of
temporal distortion being a strong predictor of intelligibility decrease. is result goes against the hypothesis
that isochronous speech, by virtue of a supposedly ideally regular timing of its units, would be easier to track,
by alleviating the need for constant phase-resetting. For traditional linguistic accounts too, these results further
debunk the isochrony hypothesis, assuming that produced speech would be based on an underlying ideally
isochronous form12, see review in13,20. In fact, our results suggest that the natural timing statistics are actively
used by listeners in decoding words, even though they are encountered in a single instance by listeners.
At the condition level, while an advantage of isochronous over anisochronous forms of speech was found for
English19, no such trend was observed for French. While possible prosodic or idiosyncratic eects might account
for this dierence (see Supplementary Materials), this led us to examine, separately by condition, the relationship
between intelligibility and the four timing metrics, namely departure from the natural timing of speech units and
departure from a hypothetical underlying isochronous form of those speech units. is analysis unveiled a rather
consistent portrait across the two talkers of the two languages, according to the type of sentence retiming. For
naturally-timed sentences, intelligibility correlates with the degree of departure from an ideal isochronous form
of the sentence at the syllable level. at is, naturally isochronous sentences at the syllable level are signicantly
better recognised than naturally anisochronous sentences. For isochronously retimed sentences, intelligibility
was strongly correlated with departure from naturality (see also Fig.1, bottom). Importantly, this result is valid
only for the syllable level—departure from natural timing of accent groups did not explain intelligibility vari-
ation. For anisochronous sentences, the two syllabic rhythm metrics, that is, departure from naturally timed
syllabes and departure from syllabic isochrony, were found to be correlated with intelligibility, though the latter
to a much lesser extent. is shows that both temporal dimensions of syllabic rhythm are actively relied upon
in speech comprehension. In addition, the simultaneous variation of the two temporal dimensions for this type
of stimuli enabled to provide an indication of the relative size of these eects, since the role of departure from
naturally timed syllables was about ten times stronger than the role of departure from isochronously timed syl-
lables (Table4C)—a ratio however possibly underestimated owing to the necessary correlation that exists between
Table 4. (A) Initial (m5) and equivalent simpler (m6) model for the role of departure from isochrony and
natural rhythm in anisochronously retimed sentences. e formulae of the xed eects are given for the two
models, and the result of a likelihood-ratio test between the two models is given on the right of the vertical
separator. (B) m6 model coecients, with associated p values. (C) Fixed-eect sizes with lower and upper
condence levels.
Model summary Likelihood-ratio test (m5, m6)
Fixed eects AIC
χ2
Df
p
(> χ
2)
(A) Model selection
m5 language
∗
dnat.acc
∗
dnat.syl
∗
diso.acc
∗
diso.syl 13,610 39.35 29 0.095
m6 dnat.syl+ diso.syl 13,591
Estimate SE z value Pr(>|z|)
(B) Equivalent model (m6) coecients
(Intercept) 0.9614 0.1669 5.760
8.38e−09
***
dnat.syl
−2.9418
0.3852
−7.637
2.22e−14
***
diso.syl
−0.5311
0.2742
−1.937
0.0527.
Eect
R2
Lower CL Upper CL
(C) Fixed-eects size
m6 0.059 0.046 0.073
dnat.syl 0.030 0.021 0.041
diso.syl 0.003 0.000 0.007
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the two metrics, by design for these sentences. Again, the regularity of accent group timing did not provide any
signicant increase in intelligibility in any condition—contrary to natural sentences, as we will discuss later.
Crucially, all three analyses did not nd language as a contributing factor in explaining intelligibility, with the
same pattern of result applying to speech produced by both English and French talkers. Of course, as we tested
only one talker in each language, there is the possibility that an observed dierence (or lack thereof) between
languages might be imputable to confounded talker characteristics (see Supplementary Material for an analysis
of talker idiosyncrasies on a range of acoustic parameters). With that caveat in mind, given that those languages
were selected as being representative of dierent rhythmic classes, the undiscriminative nature of the results
may have two implications. First, it suggests that isochrony eects could apply across the board to any language,
independently of their rhythmic patterning at a global level—though, of course, this will have to be tested on
other languages and multiple speakers in future studies. Second, our results suggest that the syllable is a core
unit in temporal speech processing. While the primary role for the syllable in speech processing has long been
proposed7,21,22, (but see8) the fact that isochrony eects are stronger for syllable than accent groups even for a
stress-timed language (which would supposedly favour accent-group isochrony), indicates that the temporal scale
associated with the syllable is key—rather than its linguistic functional value. Instead, our results further support
the usefulness of the notion of a neurolinguistic unit dened at the crossroads of linguistic and neurophysiologi-
cal constraints, such as the so-called “theta-syllable”23 which would apply universally to languages independently
of their linguistically-dened rhythm class. We suggest that this unit combines high-level perceptually-grounded
temporal anchoring, with the P-centre being a good candidate, with physiologically-based temporal constraints
determined by neural processing in the theta range of 4–8Hz (or up to 10Hz, see24,25).
Taken together, the data presented here are compatible with a model of cortical organisation of speech pro-
cessing with co-occurring and inter-dependent bottom-up and top-down activity25,26. While early studies have
focussed on exogenous eect of rhythm on cortical activity, the role of endogenous activity is being progressively
acknowledged and understood27. In this study, we use intelligibity as a proxy for successful cortical process-
ing, following the long established link between intelligibility and entrainment28–30. Our data allow, across two
languages with markedly dierent rhythmic structures, to suggest unifying principles: the dependence of intel-
ligibility of naturally-timed sentences to their underlying isochrony suggests a bottom-up response component
that benets from regularity at the syllable level. In isochronous sentences, the strong response variation with
departure from the sentences’ natural rhythm indicates that learned patterns are a strong determinant of speech
processing. Unifying both distortion types, anisochronous sentences display a graded dependence to both of
temporal distortions, and additionally provide a quantication of the relative magnitude of the eects, since in
this type of sentences, top-down processing has a stronger role than bottom-up information. ese hypotheses
need to be backed-up with neurophysiological data, and a rst work in that framework is reported in31.
A point of interest of our results is the positive correlation in natural sentences between intelligibility and
departure from accent-level isochrony in both English and French. Local intelligibility variations associated
with regular versus irregular accent-level prominences could explain this eect: irregular accent-level units in a
sentence might trigger fewer but louder prominences in a sentence compared to isochronously distributed accent
units, which in turn may prove more intelligibile when mixed with a xed signal-to noise ratio over the entire
sentence, as opposed to a more uniform distribution of energetic masking over the sentences. e psychoacoustic
consequences of this hypothesis are dicult to control and outside the scope of the current study but could open
productive avenues in speech intelligibility enhancing modications32.
e sentence-long speech material employed here can be considered a good match to ecologically valid speech
perception conditions (over more classical controlled tasks involving lexical decision or segment-level categorical
perception33). In everyday communicative settings however, listeners do make use of a much larger temporal
context for successfully decoding speech, eciently incorporating semantic, prosodic or discursive cues from a
temporal span in the range of minutes rather than seconds. It is however dicult to predict the potential eect
that isochronously retimed speech might produce in a broader temporal context, as it may become an explicit
feature of the stimulus to be perceived. Indeed, none of the participants in the current study reported the isoch-
ronous quality of modied speech, but with sustained stimulation beyond the duration of a sentence, listeners
may be able to project a temporal canvas onto upcoming speech. One could hypothesise both a facilitatory
eect due to the explicit nature of the isochronous timing, as found in rhythmic priming studies34–36, but also a
detrimental eect due to the sustained departure from optimally-timed delivery of information found in natural
speech, as defended here. e relevance of examining long-term oscillatory entrainment in the delta and theta
range also needs to be posed, although phase-locking eects beyond the duration of a phrase seem unlikely37,38.
In conclusion, in light of the data presented here, we propose an account of the role of isochrony in speech
perception which unies previous hypotheses on the role of isochrony in speech production and perception39,40
but, importantly, quanties its contribution, with respect to the “elephant-in-the-room” factor, that is, natural
timing of speech. First, to avoid any ambiguity, we propose that isochrony does not have a primary role in
speech perception or production. Indeed, through the eect of the temporal manipulations of speech reported
here, it is clear that isochronous forms of speech do not provide a benet in speech recognition in noise. In fact,
the natural timing statistics of speech appear to contain an essential ingredient used by listeners in the form of
learned information, to which listeners attend to by recruiting top-down cortical activity, and allows them to
process sentences even though they are presented for the rst time and degraded by noise. We however show
that isochrony does play a role, albeit a secondary one, which is compatible with a by-product output of neuronal
models of oscillatory activity41–43. Importantly, the theta-syllable provides the pivot for isochrony in speech and
oscillations in the brain in this context. Entrainment to oscillatory activity has received a lot of attention in the
recent years and received broad experimental support, in particular in specic experimental settings where the
material is explicitely presented in an isochronous form (in visual, auditory, musical contexts38,44–47). e fact
isochrony eects are observed in a context where isochrony is not an explicit characteristic of the experimental
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material suggests that these eects could reect physiological mechanisms which, if sustained, can lead to the
kind of behavioural outcomes reported in controlled setups. In sum, we suggest that isochrony is not a require-
ment for successful speech processing but rather a kind of underlying timing prior actively suppressed by the
necessities of exible and ecient linguistic information encoding. Because of plausible neuroanatomical archi-
tecture of cortical processing however, isochrony seems to represents a canonical form of timing that can be
seen as an attractor, and thereby enjoys a special status in perception and production generally, such as poetry,
music and dance.
Methods
Speech material and annotation. e speech material consisted of sentences, taken from the Harvard
corpus for English15 and the Fharvard corpus for French16. Both corpora are highly comparable in their struc-
ture: they both contain 700 sentences or more, each sentence is composed of 5–7 keywords (exactly 5 keywords
for French), with a mild overall semantic predictability, and sentences are phonetically balanced into lists of 10
sentences. For the current study, we used a subset of 180 sentences for each corpus, recorded by a female talker
for English48 and by a male talker for French49. Each sentence subset was randomly sampled from all sentences
of each corpus.
Sentences were annotated at two hierarchical rhythmic levels: the accent group and the syllable level. e
syllable level was taken as the lowest hierarchical rhythmical unit in both languages, and the accent group was
dened as the next hierarchical step up the rhythmic hierarchy, i.e., the stressed-syllable in English and the
accentual phrase in French50,51. While annotation of stressed syllables in English was relatively straightforward,
accentual phrase boundaries prove more dicult to determine unambiguously, as they combine several factors,
from the syntactic structure of the sentence to the particular intonational pattern used by the talker producing
the sentence. We relied on an independent annotation of accent group boundaries done by three native French
speakers. From an initial set of 280 sentences, 181 sentences received a full inter-annotator agreement, and the
rst 180 sentences of that set were selected for the current study.
e P-centre associated to a rhythmic unit corresponds tothe onset of the vowel associated with that unit
in the case of simple consonant-vowel syllables, and is usually advanced in the case of complex syllable onsets,
or when the vowel is preceded by a semi-vowel. In both corpora, P-centre events were rst automatically posi-
tioned following an automatic forced alignement procedure52, then manually checked and corrected if necessary,
typically when a schwa was inserted or deleted in a given syllable. Given the hierarchical relationship between
accent groups and syllables, the accent group P-centre aligns with the corresponding syllable P-centre. See
Supplementary Materials for auditory examples of annotated sentences at the accent group and syllable levels.
Stimuli. Sentences were temporally modied by locally accelerating or slowing down adjacent successive
speech segments. Unmodied time onsets of either accent (acc) or syllable (syl) rhythmic units served as a refer-
ence for the natural rhythm (NAT) condition, from which isochronous (ISO) and anisochronous (ANI) condi-
tions were dened, as detailed below.
We rst dened t the reference time series identifying the time onsets of the N corresponding rhythmic units of
a sentence anked with the sentence endpoints, i.e.,
t=t0,t1,...,tN,tN+1
, with
t0
and
tN+1
respectively the start
and end of the sentence. We noted d the associated inter-rhythmic unit durations, i.e.,
di
=
ti+1
−
ti,i
=
0, ...,N
.
We then dened the target time series
t′
and the associated durations
d′
, the resulting values aer temporal
transformation. e initial and nal portions of the sentence were le unchanged, i.e.,
t′
0
=t
0
,t
′
1
=t
1
,t
′
N
=t
N
and
t′
N+1
=t
N
+
1
, hence
d′
0
=d
0
, and
d′
N
=d
N
. For the ISO condition, the target durations were set to the average
duration of the corresponding intervals in the reference time series NAT, i.e.,
d
′
i
=
t
N
−t
1
N−1
,
i=1, ...,N−1
. For
the ANI condition, we imposed that sentences were temporally transformed by the same quantity as isochronous
sentences, but resulted in an unpredictable rhythm. We achieved this by using the following simple heuristic, con-
sisting of applying an isochronous transformation to the time-reversed rhythmic units events. First, the reference
time series was replaced by a pseudo reference time series made of pseudo events such that successive pseudo ref-
erence durations were the time reversal of the original reference durations, i.e.,
rev(di)
=
dN−i,i
=
1, ...,N
−
1
;
then, the target time series ANI were computed from this reversed sequence by equalising the temporal distance
of the pseudo events as in the ISO condition.
Temporal transformation was then operated by linearly compressing or expanding successive speech segments
identied by the reference time series by applying the duration ratio of target to reference speech segments, i.e.,
applying a time-scale step function
τ
i=
d′
i
di
,i=1, ...,
N
to the speech signal of the sentence. is was achieved
using WSOLA53, a high-quality pitch-preserving temporal transformation algorithm.
Altogether, we obtained 5 temporal versions of each sentence in the corpus: the unmodied natural version
(NAT), the isochronous stimuli at the accent (ISO.acc) and syllable (ISO.syl) levels, and the anisochronous stim-
uli at the accent (ANI.acc) and syllable (ANI.syl) levels. Figure3 shows the result of the temporal transformation
of an example sentence for the rst 3 conditions. See Supplementary Materials for examples of retimed stimuli.
Final experimental stimuli were constructed by mixing the temporally transformed sentence with speech-
shaped noise at a signal-to-noise ratio of
−3
dB. is value was determined in previous studies16 to elicit a
keyword recognition score of around 60% in the unmodied speech condition, in turn providing maximum
sensitivity for the dierence between unmodied and modied speech conditions. Speech-shaped noise was
constructed, separately for each talker, by ltering white noise with a 200-pole LPC lter computed on the con-
catenation of all sentences of the corpus recorded by that talker.
Temporal distortion metrics. We quantied the net amount of temporal distortion
δ
applied to a given
sentence by computing the root mean square of the log-transformed time-scale step function
τ
associated to that
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sentence. Log-transformation was done so that compression and elongation by inverse values (e.g.,
×1
2
and
×2
respectively) would contribute equally to the overall distortion measure (i.e.,
log
(1
2
)
2
=log(2)
2
). Binary loga-
rithm was used here. Using our notation referring to discrete events, the temporal distortion from the reference
time series t with N events to the target time series
t′
can therefore be written:
where
i=1, ...,N
is the event index,
τ
the time-scale factors linking t and
t′
and d the duration between succes-
sive reference events, as dened in the preceding section. Note that the
di
term in the numerator emerges from
the grouping of samples between successive reference events, since for all these samples
τi
values are constant
by design.
By design, individual sentences undergo an identical amount of temporal distortion in isochronous and
anisochronous transformations. By extending the application of the
δ
function to time instants other than the
ones used for stimulus construction, we introduce additional metrics to evaluate the departure of the rhythm
of any sentence – whether temporally modied or not – to two canonical rhythm types associated with the
sentence: its unmodied natural rhythm and its isochronous counterpart. For the two hierarchical rhythmic
levels considered here, this amounts to 4 new metrics: departure from naturally timed accent groups or syllables
(respectively dnat.acc and dnat.syl), and departure from isochronous accent group or syllables (respectively diso.
acc and diso.syl, see Table5).
(1)
δ
=

N
i=1(log τi)2di

N
i=1
d
i
Figure3. Annotation of an example sentence (translation: e red neon lamp makes his/her hair iridescent), in
its original unmodied natural timing (A) and transformed isochronous forms at the accent (B) and syllable
(C) levels. For each panel, from top to bottom: spectrogram with target boundaries used for the transformation
overlaid in dashed lines, accent group onsets (red), syllable onsets (orange), phonemes and words.
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Participants and procedure. Data for English are based on 26 participants (21 females) with mean age
of 20.9 (SD = 6.3), all speaking Australian English as a native language, with no known audition troubles and
are described in detail in19. We include the data for English previously reported in19 for the purpose of compar-
ing with the French data. New analyses are conducted on this dataset (see “Results”). All experimental proto-
cols were approved by the Human Research Ethics Committee of Western Sydney University under the refer-
ence H9495. For French, 27 participants (15 females), with mean age of 26.7 years (SD = 8.8) were recruited
from the student and sta population of the University of Grenoble-Alpes (UGA) and were given a 15euro gi
card as compensation for their participation. We checked that all participants met the selection criteria which
included speaking French as a native language, and having no known audition troubles. No data was removed
from the initial set. All experimental protocols were approved by UGA’s ethical committee (CERGA, agreement
IRB00010290-2017-12-12-33). For both English and French studies, all methods were carried out in accordance
with relevant guidelines and regulations, and informed consent was obtained from all participants.
For both language groups, participants were given written instructions and the experimenter gave comple-
mentary information when necessary. Participants then sat in front of a computer screen, where short on-screen
instructions were given before each block of the experiment. Participants were presented with speech and noise
mixtures played binaurally through Beyer Dynamic DT 770 Pro 80 ohm closed headphones at a comfortable level
set at the beginning of the experiment and maintained constant throughout the experiment. Participants had
to type what they heard on a keyboard and press “Enter” to trigger the presentation of the next stimuli. Stimuli
were grouped by condition forming 5 blocks of 36 sentences each. An additional 5 stimuli from each condition
were presented as practice in the beginning of the experiment and were not used for further analysis. e order
of conditions was counterbalanced across participants, and the order of sentences was pseudo-randomised for
each participant. e order of the practice sentences was xed across all participants, and the order of conditions
for practice sentences was also xed, to: NAT, ISO.acc, ANI.acc, ISO.syl, ANI.syl.
Scoring. Sentences were automatically scored with custom-made scripts that matched keywords with listen-
ers typed responses. A dictionary of variants was used in both languages to correct for homophones, full letter
spelling of digits and common spelling mistakes (e.g., *‘ciggar’ corrected to ‘cigar’ in English, and ‘*cigne’ cor-
rected to ‘cygne’ (‘swan’) in French. Each sentence received a nal score as the proportion of keywords correctly
recognised.
e accuracy of the automatic scoring was evaluated on a 530-sentence subset of the listeners responses
(around 5.5% of all 9450 responses), randomly and uniformly sampled across subjects, conditions and languages.
e subset was manually scored, and we found that 98% of the sentences were correctly scored by the automatic
procedure, with the 10 incorrectly scored sentences each having no more than one word typed with a spelling
mistake which was absent in the dictionary (and therefore added to it for future studies).
Data modelling. e eect of experimental condition on intelligibility was analysed for French following
previously reported analysis of the English data19. A generalised linear mixed-eect model (function glmer
from the R package lme454) was tted to intelligibility scores, including a random term intercept by subject.
Normal distribution of residuals was visually veried. Generalised simultaneous hypothesis were formulated
and tested with the function glht from the R package multcomp55, which corrects for multiple comparisons.
Analysis of the contribution of the temporal distortion metrics to intelligibility was performed on com-
bined French and English data. ree models were tted for each of the three subsets of the data (see Fig.2 and
“Results”). Fixed eects were language (French and English) and the non all-zero valued metrics for the given
data subset. Random eect structure included a term by sentence and by participant. For each model, we report
an analysis of variance between the initial full model and the minimal equivalent model. e latter is obtained
from the initial model by incrementally removing xed-eect terms until no term can be removed without
Table 5. Analysis metrics to evaluate departure from naturally-timed and isochronous forms at the accent
and syllable level (rows) in each of the 5 experimental conditions (columns). Each argument of the
δ
function
(Eq.1) is a time series of either accent (acc) or syllable (syl) onsets, as they occur in a given experimental
condition. For example,
tsylISO.acc
represents the syllable onsets of a sentence as they occur in the transformed
ISO.acc experimental condition. Note that some of these distortions are equal to 0 by construction: they
are dnat.acc and and dnat.syl for NAT sentences, and diso.acc and diso.syl for ISO.acc and ISO.syl sentences
respectively.
NAT ISO.acc ISO.syl
dnat.acc
δ(taccNAT ,taccNAT )
δ(taccNAT ,taccISO.acc )
δ(t
acc
NAT ,t
acc
ISO.syl )
dnat.syl
δ(tsylNAT ,tsylNAT )
δ(t
syl
NAT ,t
syl
ISO.acc )
δ(t
syl
NAT ,t
syl
ISO.syl )
diso.acc
δ(taccISO.acc ,taccNAT )
δ(taccISO.acc ,taccISO.acc )
δ(t
accISO.acc
,t
acc
ISO.syl )
diso.syl
δ(t
syl
ISO.syl ,t
syl
NAT )
δ(t
syl
ISO.syl ,t
syl
ISO.acc )
δ(t
syl
ISO.syl ,t
syl
ISO.syl )
ANI.acc ANI.syl
dnat.acc
δ(taccNAT ,taccANI.acc )
δ(t
acc
NAT ,t
acc
ANI.syl )
dnat.syl
δ(tsylNAT ,tsylANI.acc )
δ(t
syl
NAT ,t
syl
ANI.syl )
diso.acc
δ(taccISO.acc ,taccANI.acc )
δ(taccISO.acc ,taccANI.syl )
diso.syl
δ(t
syl
ISO.syl ,t
syl
ANI.acc )
δ(t
syl
ISO.syl ,t
syl
ANI.syl )
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signicantly changing the explained variance. Visual distribution of residuals was checked. Fixed-eects size are
computed with the function r2beta from the R packager2glmm56).
Data availability
Example stimuli illustrating P-centre annotation and temporal modiction as well as computer code for the tem-
poral distortion metrics are included in Supplementary Materials. Experimental stimuli and listeners’ responses
data are available for download at: https ://doi.org/10.5281/zenod o.39664 75.
Received: 29 April 2020; Accepted: 28 October 2020
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Acknowledgements
is work was supported by the European Research Council under the European Community’s Seventh Frame-
work Program (FP7/2007-2013 Grant Agreement No. 339152, “Speech Unit(e)s”). We thank Christine Nies for
her help in collecting data and Silvain Gerber for assistance with statistical analysis.
Author contributions
V.A. and J.-L.S. conceived and designed the study, supervised data collection, analysed the data and wrote the
paper.
Competing interests
e authors declare no competing interests.
Additional information
Supplementary information is available for this paper at https ://doi.org/10.1038/s4159 8-020-76594 -1.
Correspondence and requests for materials should be addressed to V.A.
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