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REHEARSAL PROCESSES IN WORKING MEMORY AND
SYNCHRONIZATION OF BRAIN AREAS
Franziska Kopp*
#
, Erich Schröger* and Sigrid Lipka
#
*University of Leipzig, Institute of General Psychology
#
University of Leipzig, Institute of Linguistics
E-mail: fkopp@rz.uni-leipzig.de
Abstract
In this study we investigated brain activity characteristics of rehearsal processes in working
memory using analysis of EEG coherence as a measure of synchronization of brain areas. In
a delayed serial recall paradigm we enabled subjects to carry out rehearsal in one condition
and disrupted it by irrelevant speech in another condition. Results show that rehearsal-
specific changes of coherence duration are located at central electrode sites (C3, Cz, C4) in
the Gamma frequency range (35-47 Hz). These changes in synchronization follow our
behavioural data that show the classical irrelevant speech effect. Additionally, we found
significant coherence changes between frontal and parietal electrodes in the Theta band (4-
7.5 Hz) suggesting that rehearsal components can be dissociated from processes reflecting
mental effort for retention.
Describing psychological processes by the investigation of quantitative external measures has
a long tradition and goes back to the beginnings of psychophysics in the 19
th
century. During
the last few years, however, one can notice an endeavour to find internal characteristics of
psychological processes by analyzing brain activity as part of neuropsychological research.
This trend results from the great development of imaging techniques on the one hand and on
the other hand from the difficulty of describing more complex cognitive processes by
traditional behavioural methods such as reaction time or error rate measures. Thus, analyzing
brain activity online provides a new and exciting method to investigate temporal and spatial
characteristics of so-called higher cognitive processes like memory.
In this study we examined a special part of working memory, namely the subvocal rehearsal
process. It comprises active maintenance of memory traces and renewal of decaying
representations in a short-term store. As one part of the phonological loop in the influential
working memory model of Baddeley (1999) rehearsal was proposed on the basis of
experiments that examined the disruption of the phonological loop by articulatory suppression
or irrelevant speech. In the 1980s and 1990s numerous findings were reported showing which
materials disrupt the phonological loop (and hence rehearsal processes) under which
conditions. One outcome which is relevant for our experiment showed that presentation of
irrelevant speech considerably interrupted phonological rehearsal whereas presentation of
unstructured noise did not (Salamé & Baddeley, 1987).
Neuropsychological studies using fMRI or PET gave evidence for the activation of frontal
and parietal brain areas in working memory tasks (e.g. Smith & Jonides, 1997). Other
activation studies proved the participation of several brain areas in rehearsal: especially
premotor and prefrontal regions (consistent with the hypothesis that during rehearsal, motor
programs are executed like in a recall situation but without overt articulation) (e.g., Fujimaki
et al., 1999). However, these structures have to cooperate functionally in order to carry out
psychological processes. One special mechanism of this interaction has recently been
discussed above all, that is the coupling by synchronization of electrical activity. In our
experiment we investigated the synchronization of rehearsal-related brain areas using EEG
coherence as an indicator. Coherence gives evidence of the degree of interrelatedness with
respect to frequency (this can vary between 0 and 1).
As for our experiment, we hypothesized that during rehearsal the duration of high coherence
should be long in the prefrontal and premotor regions, and between those two regions.
Additionally, disruption of rehearsal is predicted to lead to smaller coherence durations in
these areas. For the behavioural data we predicted the classical irrelevant speech effect.
Method
In our experiment 12 subjects (10 female, 2 male, age 19-29 years, students and
professionals) were tested in 3 memory and 3 control conditions. As the paradigm for our
memory task we used delayed serial recall (Fig. 1.a). Lists of 5 items were presented
sequentially on a PC screen (bisyllabic concrete German nouns similar to the stimuli used by
Weiss et al., 2000; they were balanced in frequency and semantic relatedness) with a
presentation rate of 1 sec. per item and an ISI of 250 msec. After each list subjects had to
retain the items in working memory for an interval of 10 sec. Three question-marks at the end
of the retention interval prompted subjects to recall items aloud.
Fig. 1. Schematic representation of a trial for the memory tasks (1.a.) and for the non-
memory control tasks (1.b.)
The three memory conditions differed during the 10 sec. retention interval: In one condition
there was silence in the retention interval intended to enable subjects to subvocally rehearse
the items (condition quiet). In another condition irrelevant speech (10 sec. digitalized radio
recordings without background noise or music) was presented via headphones during the
retention interval to prevent subjects from rehearsing items (condition speech). In the last
condition (condition noise) subjects heard white noise during the retention phase in order to
expose them to auditory stimulation without disrupting rehearsal (Salamé & Baddeley 1987).
Additionally, we tested subjects in 3 control conditions (Fig. 1.b) without memory demands.
A fixation cross was presented on an empty screen for 10 sec. During this time, in some trials,
a small point appeared for 100 msec. in a region within a few millimetres around the fixation
Retention
interval:
- quiet
- noise
- speech
ISI
250 msec.
1 sec.
Auster
1 sec.
Pokal
1 sec.
Hügel
1 sec.
Feuer
1 sec.
+
10 sec.
???
1.a
1.b
Control
interval:
- quiet
- noise
- speech
10 sec.
+
???
cross. Upon the presentation of the three question-marks at the end of the 10 sec., subjects had
to say if the point had been there or not. This task was used to control attention in the interval.
Only "no"-trials were analysed. The 3 control conditions differed like the memory conditions
in quiet, noise and speech.
During the experiment we recorded EEGs using the 10/20 system with 19 scalp electrodes
and the nose as reference point. The Neuroscan system made recordings at a rate of 250 Hz.
We used Ag-AgCl electrodes and the modular Easy-cap system.
Results and Discussion
Behavioural data
Fig. 2 shows the percentages of correctly recalled word lists in the memory condition
averaged over all subjects. As predicted, noise and quiet did not differ, whereas speech
differed significantly both from quiet and from noise. These results show the classical
irrelevant speech effect.
As for the non-memory control tasks, correct answers reached a level of over 99 % in each
condition and conditions did not differ, in line with our predictions.
Fig. 2. Behavioural results of the memory tasks which show the mean percentages of
correctly recalled lists in all three memory conditions.
EEG data
For the calculation of coherence only artefact-free trials were analyzed with the SpecTrial and
SpecPara program based on ARMA models (Schack et al., 1999). This method of analysis
allows us to get coherence values with high time and frequency solution. To test our
hypotheses, we examined electrode pairs within the frontal region (F7, F3, Fz, F4, F8), within
the central region (C3, Cz, C4), between frontal and central electrode sites (combinations of
F7, F3, Fz, F4, F8 with C3, Cz, C4) and between frontal and parietal regions (combinations
of F3, Fz, F4 with P3, Pz, P4) (Kopp & Sommerfeld, 2000). Based on previous findings
reported in the literature we decided to analyze three frequency bands: Beta1 (13-20 Hz),
Theta (4-7.5 Hz) and Gamma (35-47 Hz) (Sommerfeld et al., 1999, Tallon-Baudry et al.,
1999, Sarnthein et al., 1998). Coherence was computed for a 2 sec. interval within the 10 sec.
retention interval (the period between 2 and 4 sec. after the onset of the 10 sec. interval). We
computed coherence duration as the sum of all periods of high coherence, i.e., coherence
30
35
40
45
50
55
60
65
quiet noise speech
condition
mean % of correctly
recalled lists
levels above the threshold of 0.7 within the 2 sec. interval (the threshold resulted from an
inspection of coherence histograms). With coherence duration as the dependent variable,
statistical tests were carried out separately for each subject because levels of coherence vary
considerably between subjects and averaged data do not fulfil requirements of ANOVA.
Comparison memory vs. control
Single comparisons (t-tests) were carried out for the conditions memory quiet vs. control
quiet, memory noise vs. control noise and memory speech vs. control speech to extract the
contribution of working memory demand. Fig. 3 shows electrode pairs tested as significant in
at least 10 cases of all statistical comparisons. As one can see, several neuronal networks
contribute to processing of the memory tasks, showing activity in all three frequency bands.
Obviously the Theta band seems to contribute to working memory more than the others and
shows differences especially at fronto-parietal electrode pairs. Common to all frequency
bands are cooperations in the central area. Our implicit assumption that language-specific
effects would occur at left lateral sites was not confirmed.
Fig. 3. Electrode combinations for the frequency bands Beta1, Theta and Gamma showing
differences in coherence duration between memory and control tasks. Solid lines mean
significant increases for memory compared to control conditions, the dashed line represents a
decrease for memory compared to control.
Comparisons within the memory tasks
We found that subjects showed very different memory profiles and consequently very
different profiles of brain activity: Some of them could recall all 5 items of a list without
effort, others had real problems. For some subjects speech had a large disrupting effect on
rehearsal, whereas for others speech caused nearly no decrease in performance. Thus it made
sense to divide subjects into groups in our analysis of the memory conditions.
Two groups were formed based on the strength of the irrelevant speech effect. The first group
comprised all subjects with a strong irrelevant speech effect (subjects 3, 5, 8, 9, 10, 11, 12)
and the second group consisted of those who were hardly disrupted by irrelevant speech
(subjects 1, 2, 4, 6, 7). Statistical analyses of these groups revealed significant differences in
the Gamma frequency band at central electrode sites. Fig. 4.a illustrates these results with one
representative of each subject group at C3-Cz. As one can see the Gamma coherence duration
Beta1
(13-20 Hz)
Theta
(4-7.5 Hz)
Gamma
(35-47 Hz)
decreases significantly in the speech condition for subject 5 who had a strong irrelevant
speech effect. Gamma coherence duration does not decrease in subject 7. The results for these
two subjects were representative for their respective groups. Additionally, we found
significant correlations between recall performance and coherence durations (Gamma band at
central electrode sites) only for the subjects with the strong irrelevant speech effect (between
0.26 and 0.43). This rehearsal effect at central electrode positions is consistent with the
suggestion that rehearsal is supported by motor programs, as is recall, but without overt
articulation.
Fig. 4.a. Gamma coherence durations at the central electrode pair C3-Cz for subject 7 (no
irrelevant speech effect) and subject 5 (strong irrelevant speech effect) indicating rehearsal.
Fig. 4.b. Theta coherence durations at the fronto-parietal electrode pair Fz-P3 in subject 4
(high working memory capacity) and subject 11 (low working memory capacity) indicating
mental effort for retention.
As mentioned above subjects also varied concerning their working memory capacity. Based
on a measure of working memory capacity, digit span, we divided subjects again into 2
groups: one with subjects of high capacity (subjects 4, 5, 8, 10, 12) and the other with subjects
of low capacity (subjects 3, 9, 1, 2, 6, 7, 11). Statistical comparisons of these groups revealed
significant differences in fronto-parietal electrode combinations in the Theta frequency band
as shown in Fig. 4.b (for two typical members of each group) for the electrode pair Fz-P3.
Subjects with low capacity, such as subject 11, have significantly longer coherence durations
overall compared to subjects with high capacity (see subject 4). Regarding differences
between memory conditions, results show that high-capacity subjects show an increase of
Theta coherence durations in noise and in speech compared to quiet, that means an increase
with task difficulty. Subjects with low capacity do not show any differences between memory
conditions. Fronto-parietal coupling of brain activation especially in the Theta frequency
range has been described before as an indicator for mental effort for retention (Sommerfeld et
al., 1999). This could be reflected in our data as well. Subjects with high working memory
capacity, i.e., with more resources, use less effort to retain 5 items in working memory and
they increase their effort with increasing memory demands. However, for low capacity
C3-Cz
400
500
600
700
800
900
1000
1100
1200
quiet noise speech
condition
Gamma coherence duration
su b j 7
su b j 5
Fz-P3
400
500
600
700
800
900
1000
1100
1200
quiet noise speech
condition
Theta coherence duration
su b j 4
su b j 11
4.a
4.b
subjects task requirements are high from the outset and remain high for all memory
conditions.
In sum, the EEG coherence analysis proved to be a very useful tool to show which brain areas
cooperate during rehearsal processes in a working memory task. Furthermore, group
differences in recall performance corresponded to differences in the EEG coherence
measures. Our study also demonstrated that coherence analysis is a useful method to
dissociate different components of working memory, i.e., rehearsal processes and mental
effort for retaining items.
Acknowledgement
We would like to thank Esther Herrmann and Frank-Michael Schleif for their assistance in
data collection and analysis and Dr. Erdmute Sommerfeld for her advice in all matters of
coherence. This work was supported by Deutsche Forschungsgemeinschaft.
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