International Journal of Psychophysiology 38 Ž2000. 211᎐223
Oscillatory gamma activity in humans: a possible role
for object representation
Olivier BertrandU , Catherine Tallon-Baudry
Mental Processes and Brain Acti¨ ation Laboratory, INSERM U280, 151 cours Albert Thomas, 69003 Lyon, France
Received 4 October 1999; accepted 6 June 2000
Abstract
The coherent representation of an object has been suggested to be established by the synchronization in the
gamma range Ž20᎐100 Hz. of a distributed neural network. So-called ‘40-Hz’ activity in humans could reflect such a
mechanism. We have presented here experimental evidence supporting this hypothesis, both in the visual and
auditory modalities. However, different types of gamma activity should be distinguished, mainly the evoked 40-Hz
response and the induced gamma activities. Only induced gamma activities seem to be related to coherent object
representations. In addition, their topography depends on sensory modality and task, which is in line with the idea
that they reflect the oscillatory synchronization of task-dependent networks. They can also be functionally and
topographically distinguished from the classical evoked potentials and from the alpha rhythm. It was also proposed
that the functional role of gamma oscillations is not restricted to object representation established through
bottom-up mechanisms of feature binding, but also extends to the cases of internally driven representations and to
the maintenance of information in memory. 䊚 2000 Elsevier Science B.V. All rights reserved.
Keywords: Synchrony; Gamma oscillation; 40-Hz; Object representation; Feature binding; EEG; Evoked potentials
1. Introduction
Synchronized oscillatory activity in the gamma
range Ž30᎐100 Hz. could play a major role in
U
Corresponding author. Tel.: q33-472-68-19-22; fax: q33472-68-19-02.
E-mail address: [email protected] ŽO. Bertrand..
linking the different areas involved in the same
object representation into a unified, coherent percept. Indeed, the distributed nature of the network coding for a given object in a particular task
is now well documented, in particular by many
neuroimaging studies Žfor review see Courtney et
al., 1997.. Following the theoretical proposal from
Milner Ž1974., von der Malsburg and Schneider
Ž1986., the neural structures participating in this
0167-8760r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 8 7 6 0 Ž 0 0 . 0 0 1 6 6 - 5
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O. Bertrand, C. Tallon-Baudry r International Journal of Psychophysiology 38 (2000) 211᎐223
network might be dynamically linked by oscillatory synchronous firing. Over the last decade, this
hypothesis has been supported by a growing body
of experimental findings in anesthetized and
awake animals ŽGray et al., 1989; Kreiter and
Singer, 1996; for review see Singer and Gray,
1995..
The inspection of the human EEGrMEG literature since the 1950s reveals numerous reports of
gamma oscillatory activities in various sensory
modalities and tasks ŽGalambos et al., 1981; Pantev et al., 1991; Pfurtscheller et al., 1994, among
many others: Sem-Jacobsen et al., 1956; Sheer,
1989; for review see Tallon-Baudry and Bertrand,
1999.. At first glance, these observations may
appear rather disparate. Two major factors contribute to this impression: Ž1. different electrophysiological phenomena have been very often
gathered under the same term ‘40-Hz activity’ in
an undifferentiated manner; and Ž2. a common
framework for interpretation is somehow lacking.
The three different types of 40-Hz activities
observed in humans Žnamely, evoked, induced
and steady-state responses. ŽGalambos, 1992. will
be presented. Experimental evidence showing that
induced gamma activity can be related to the
generation of unified object representation will be
given. The functional specificity of induced gamma
oscillations, as compared to the evoked gamma
response or classical evoked potential, will be
assessed.
2. The three different types of ‘40-Hz’ activities
2.1. Steady-state response
Steady-state evoked potentials are obtained in
response to periodically modulated stimuli Žauditory, visual or somatosensory.. They have a sinusoidal-like waveform at the driving stimulus frequency, and show amplitude maxima in different
frequency bands, particularly at approximately 40
Hz. They could reflect the superposition of transient early evoked components ŽGalambos et al.,
1981; Morgan et al., 1996..
2.2. E¨ oked gamma
A transient oscillatory evoked response has
been observed in the first 100 ms following auditory, visual or somatosensory stimuli. It has the
general property of evoked responses to be quite
strictly phase-locked to stimulus onset. It could
thus be easily detected in the narrow-band filtered average evoked potential. It has first been
described in the auditory modality ŽPantev et al.,
1991; Ribary et al., 1991., and later in the visual
modality ŽSannita et al., 1995; Tallon et al., 1995.
at approximately 40 Hz.
This evoked gamma response, mainly studied in
the auditory modality, has never been systematically compared to the classical evoked components occurring in the same latency range. An
alternative interpretation is that this 40-Hz evoked
response could be part of a large-band complex
response. Indeed, narrow-band filtering could artificially create an oscillatory phenomenon. In
addition, no specific functional role of this gamma
response has been demonstrated so far in the
framework of the object representation hypothesis.
2.3. Induced gamma
The third type of gamma response that has
been observed in the human EEG, namely induced gamma activity, could be related to the
object representation hypothesis. It appears as
bursts of oscillatory activity with a latency jitter
from trial to trial ŽFig. 1a, dashed box.. Such an
induced gamma response cancels out by classical
stimulus-locked potential averaging ŽFig. 1b.. It
thus requires specific detection methods based on
spectral analysis, yielding to positive power values
that could sum up in the average across trials
ŽFig. 1c..
Since both the latency and frequency of oscillatory bursts are a priori not known, a timefrequency representation of the signal is more
appropriate ŽMakeig, 1993; Tallon-Baudry and
Bertrand, 1999.. The approach we propose is
based on a wavelet transform which provides a
better compromise between time and frequency
resolution than the more classical moving short-
O. Bertrand, C. Tallon-Baudry r International Journal of Psychophysiology 38 (2000) 211᎐223
213
Fig. 1. Ža.. Successive simulated EEG trials with an early gamma response phaselocked to stimulus onset Žfull line boxes. and a late
gamma burst jittering in latency Ždashed line boxes.. Žb.. Averaging in the time domain across trials leads to the conventional
evoked potential. Žc.. Time-frequency power representation Žwavelet transform. of the evoked gamma response. The x-axis is time,
and the y-axis is frequency. The gray scale codes the variations of power Žpositive or negative. with respect to a prestimulus
baseline. The non-phase-locked activity cancels out. Žd.. Time-frequency power computed for each singe trial. Že.. Average of
time-frequency powers across trials. The induced gamma response is clearly visible.
term Fourier transform ŽSinkkonen et al., 1995..
To detect stimulus-induced activities, a waveletbased analysis Žcomplex Gaussian wavelet, fully
detailed in Tallon-Baudry and Bertrand Ž1999. is
applied on each single trial ŽFig. 1d., and timefrequency powers are then averaged across trials
ŽFig. 1e.. It is thus possible to identify nonphase-locked activity as long as its signal-to-noise
ratio is high enough and its latency jitter does not
exceed the wavelet duration. What is usually considered are the variations of gamma power with
respect to a particular baseline level that is usually taken in the pre-stimulus period.
Evidence will be given that such induced activities could reflect the activation of an object representation, both in the visual and auditory modality.
3. Functional importance of visually induced
gamma activity
If synchronized oscillatory activity indeed plays
a role in linking together the different neural
regions involved in the same object processing, it
should be enhanced whenever a coherent percept
is being built. So far, we have tested three different experimental paradigms involving the activation of an object representation.
In the first case ŽTallon-Baudry et al., 1996.,
subjects were presented with coherent triangles
or an incoherent control ŽFig. 2a.. Both coherent
stimuli Žillusory and real triangles. elicit a significantly increased induced gamma response compared to the no-triangle stimulus, at approximately 280 ms ŽFig. 2b,c.. In this experiment, all
stimuli share the same type of physical properties;
however, only the perception of the illusory and
real triangles involves some binding between the
different parts of the image to build a coherent
representation. The increased induced gamma response could thus correspond to the synchronized
oscillatory cell assembly reflecting the activation
of this representation. These results are in keeping with those of Lutzenberger et al. Ž1995.,
214
O. Bertrand, C. Tallon-Baudry r International Journal of Psychophysiology 38 (2000) 211᎐223
Muller
et al. Ž1996, 1997.; see also this issue
¨
ŽMuller
et al., 2000. who observed an increase in
¨
gamma power on response to coherently moving
bars or with ambiguous rotating figures.
On the other hand, the evoked potentials do
not vary according to stimulus coherency: two
successive effects are observed, the illusory triangle eliciting a more prominent temporo-occipital
Fig. 2.
negativity at approximately 250 ms and the real
triangle a parietal positivity at 320 ms ŽFig. 2d.. In
addition, in a similar experiment, Herrmann and
Mecklinger Ž2000. observed an increase of the
early evoked gamma response to the target stimulus.
However, the general hypothesis on the functional role of oscillatory synchronization does not
O. Bertrand, C. Tallon-Baudry r International Journal of Psychophysiology 38 (2000) 211᎐223
215
Fig. 3. Ža.. Example of the stimuli used. A naive subject perceives such a stimulus as meaningless blobs. However, subjects can be
trained to detect the Dalmatian dog that is hidden in the picture. The outlines of the Dalmatian dog have been extracted Žrightmost
column. to provide the reader with the object searched for. Žb.. Time-frequency plots Žgrand average across 13 subjects. at electrode
O1, in response to the Dalmatian dog picture in naive Žleft. or trained Žright. subjects. The induced gamma activity at 280 ms after
stimulus onset is massively increased in trained subjects. This could indicate that it reflects the activation of the internal
representation of the Dalmatian dog that is needed to perform the task. An evoked gamma response peaking at 100 ms at Cz was
also present in these data, but not visible at electrode O1. This evoked gamma response did not vary with stimulus conditions. Žc..
Topographical map of the induced gamma response Žback view of the head..
only predict an increase in gamma power when an
object representation is being built from the stimulus presented, as in the experiences above, but
also whenever an internal representation is being
activated through top-down processes or retrieved
from memory.
Fig. 2 Ža.. Stimuli. Subjects were presented with two coherent stimuli Žillusory and real triangles. and a not coherent stimulus
Ž‘no-triangle’ stimulus.. They had to count the occurrences of a curved illusory triangle. Correct performance of this task indicates
that subjects perceive correctly illusory contours throughout the whole experiment. Žb.. Time᎐frequency plots Žgrand average across
eight subjects, 150 trials per subject and stimulus condition. at electrode Cz, in response to the illusory triangle Žleft. and to the
no-triangle stimulus Žright.. At this electrode, both evoked and induced gamma responses could be observed. An evoked gamma
response could be seen at 100 ms and 40 Hz, in response to all stimuli. It did not vary with stimulus coherency, and peaked at Cz.
Later, an induced gamma response appears at approximately 280 ms, extending from 30 to 60 Hz. This induced gamma activity was
much more pronounced in response to coherent stimuli than in response to the no-triangle stimulus, and peaked at more occipital
electrodes. Žc.. Topographical maps of the mean 250᎐350-ms, 30᎐70-Hz energy in response to the three stimulus types Žback view of
the head.. Both the illusory and real triangles elicit a prominent occipital induced gamma activity Žarrows.. Differences between the
two triangles are not significant. This induced gamma activity could thus reflect the spatial binding between the different parts of
the picture into a coherent triangle representation. Žd.. Topographical maps Žback views. of the classical evoked potentials, at the
two latencies where significant differences can be observed. At 250 ms, the illusory triangle elicits a more pronounced occipital and
bilateral negativity Žarrows. than the two other stimuli. At 320 ms, the parietal positivity is extended over the right hemisphere in
response to the real triangle. The effects observed in the evoked potentials are thus different from those in the gamma range.
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O. Bertrand, C. Tallon-Baudry r International Journal of Psychophysiology 38 (2000) 211᎐223
In a second experiment ŽTallon-Baudry et al.,
1997., we tested whether the activation of an
internal object representation also elicits a strong
gamma activity. We used a modified version of
the well-known Dalmatian dog picture ŽFig. 3..
Naive subjects perceive this kind of pictures as
meaningless black blobs on a gray background.
However, once trained, subjects can detect the
Fig. 4.
Dalmatian dog that is hidden in the picture, provided that they know what they are looking for. In
other words, they need to activate an internal
representation of the dog to be able to dissociate
it from the background. To ensure the perception
of the Dalmatian dog, the subjects had to discriminate between two dogs Žhead rightward or
leftward.. When subjects actively search for the
O. Bertrand, C. Tallon-Baudry r International Journal of Psychophysiology 38 (2000) 211᎐223
dog, the induced gamma response is massively
enhanced, suggesting that the activation of a
mental representation through top-down mechanisms also involves the oscillatory synchronization
of a distributed cell assembly. It should be noted
that, in this experiment, the increase of gamma
activity occurs earlier than any change in the
evoked potentials.
A third prediction was that gamma activity
should be present during the rehearsal of an
object representation in short-term memory ŽTallon-Baudry et al., 1998, 1999.. To test this prediction, subjects had to perform a delayed-matchingto-sample task, with rounded shapes as stimuli
ŽFig. 4a.. An induced gamma appears in response
to the first stimulus, at occipital electrodes. During the delay, a sustained gamma activity is
observed. It disappears in a control condition in
which sustained attention, but no stimulus memorization is required ŽFig. 4b..
The topography of the sustained gamma activity appearing during rehearsal in memory ŽFig.
4c. shows both an occipito-temporal and a frontal
maximum, which is in line with the hypothesis of
the synchronization of an occipito-frontal network in visual short-term memory tasks ŽFuster,
1997.. In addition, the topographies of gamma
activity in the delayed-matching-to-sample task,
in the Dalmatian dog ŽFig. 3c. and in the triangles
ŽFig. 2c. experiments are different, suggesting that
different gamma networks can be activated depending on the task to be performed.
217
Another component of the response, in the
beta range Ž15᎐20 Hz., was also enhanced during
the delay in the memory condition compared to
the control condition. Its topographical distribution suggests the existence of a second memoryrelated occipito-frontal network, not overlapping
the one in the gamma range ŽFig. 4c,d.. No effect
was observed in the alpha Ž8᎐12 Hz. range during
the delay ŽFig. 4e.. In addition, the manipulation
of the delay duration ŽTallon-Baudry et al., 1999.
revealed that no component of the evoked potential was likely to reflect the rehearsal of the first
stimulus in short-term memory. Indeed, only the
oscillatory activities in the gamma and beta range
showed a sustained time-course during the delay
compatible with an active maintenance of information in visual short-term memory. However, we
did not investigate oscillations in the theta band
which may play a role in memory processes Žvon
Stein and Sarnthein, 2000; Klimesch et al., 1997..
4. Auditory evoked and induced gamma responses
4.1. E¨ oked 40-Hz response
A transient 40-Hz oscillatory component has
been observed in the first 100 ms following a
transient acoustic stimulus in the averaged evoked
response from either electrical or magnetic signals. It consists of a small amplitude response,
Fig. 4 Ža.. Left: protocol in the memory condition. A first stimulus was presented for 400 ms, followed by a delay of 800 ms. After
the second stimulus onset, subjects had to press a button when the two shapes presented were identical. Right: protocol in the
control condition. At the end of the trial, the fixation point could either stay bright or dim lightly. Subjects were instructed to press
a button in response to the dimming. In this control condition, it was not necessary to maintain the first stimulus in short-term
memory. Žb.. Time-frequency plots Žgrand average across 13 subjects. at electrode C3, in the memory Žleft. and control Žright.
conditions. An induced gamma response to the first stimulus onset can be seen at approximately 280 ms in both conditions, even
though it was larger in the memory condition. It was followed by an induced gamma response to stimulus offset. During the delay,
induced gamma activity was observed in the memory condition only, suggesting that it could reflect the rehearsal of the first
stimulus representation in visual short-term memory. Žc᎐f.. Topographical maps during the delay, in the memory and control
conditions. Žc.. In the memory condition, the energy in the gamma range is distributed over occipital and frontal electrodes, which is
in line with the known functional anatomy of human visual short-term memory. Žd.. In the beta range, the energy is also higher in
the memory condition, over occipital and frontal electrodes. However, the maxima of activity in the beta and gamma ranges are not
overlapping. Že.. No difference between conditions can be observed in the alpha range. Žf.. In the evoked potentials, an
occipito-parietal negativity shows up at the beginning of the delay Ž750 ms.. A more parieto-central negativity develops later Ž1050
ms.; another experiment using randomized delay duration suggests that this second negativity is a contingent-negative-variation-like
wave.
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O. Bertrand, C. Tallon-Baudry r International Journal of Psychophysiology 38 (2000) 211᎐223
phase-locked to the stimulus onset and occurring
in the same time range as the middle-latency
auditory components ŽFig. 5a.. Pantev et al. Ž1991.
revealed from magnetic recordings that this 40-Hz
response is generated at least partially in the
auditory cortex. Similarly, we have shown
ŽBertrand and Pantev, 1994. that the scalp distribution of the electrical 40-Hz response presents a topographical pattern characterized by a
polarity reversal across the Sylvian fissure of both
scalp potential and scalp current density. This
suggests neural generators of alternating orientation in a rather focal area, in or near the auditory
cortex ŽFig. 5b., as observed for middle-latency
ŽPa. or N100 components. However, from MEG
signals, these neural sources were found 5 mm
more anterior and more medial than the sources
of the slow auditory component peaking at 100
ms ŽN100. ŽPantev et al., 1993.. Furthermore, the
sources of the 40-Hz components do not follow a
tonotopic organization, as opposed to the sources
of the low-frequency evoked components in the
Fig. 5. Stimuli: 50 ms tone burst Ž1000 Hz. presented binaurally Žrandom ISI, mean s 700 ms. to subjects watching a silent video
movie. Ža.. Averaged evoked response at electrode Cz Ž1000 stimuli, grand average across 10 subjects., wide-band Ž0᎐50 Hz. and
narrow-band Ž35᎐50 Hz. digitally filtered. Oscillations appearing in the first 70 ms could be related to the successive middle-latency
components. Žb.. Scalp topography of two successive peaks Ž38 and 50 ms. of the filtered gamma response. Potential and scalp
current density Žsurface Laplacian. distributions show a polarity reversal across a line delineating the Sylvian fissure. These patterns
are very similar to those classically observed for Pa Ž30 ms. and N100 components. Žc.. Time-frequency representation at electrode
Cz of the average evoked response, with the time-course of the mean power between 35 and 50 Hz, and its topography at the
latency of the maximum Ž30 ms.. Žd.. Average across trials of the time-frequency representations with a baseline power level in the
pre-stimulus Žy200 to y50 ms.. The time-course of the mean power between 35 and 50 Hz shows a decrease followed by a
decrease. The topography at the latency of the maximum Ž210 ms. clearly differs from that of the evoked gamma power. Note the
difference in amplitude between the induced and the evoked gamma responses.
O. Bertrand, C. Tallon-Baudry r International Journal of Psychophysiology 38 (2000) 211᎐223
same latency range Žcomponents Pa approx. 30
ms and N100 approx. 90 ms. ŽBertrand and Pantev, 1994.. This may then suggest the contribution
of functionally distinct neural populations. However, due to the narrow filters used Ž30᎐50 Hz.,
this 40-Hz response may also correspond to the
superimposition of successive evoked components
having sources following distinct tonotopic organization.
An alternative interpretation suggested that oscillatory 40-Hz spontaneous activity may be reset
and enhanced by sensory stimuli, thus leading to
a response phase-locked to the stimulus ŽRibary
et al., 1991.. These authors also claimed that it is
initiated in the thalamus and propagates through
cortico-thalamic loops with rostro-caudal sweeps.
This interpretation was proposed because a
fronto-occipital phase-shift of the oscillating
waves was observed over the head. However, these
data may be explained as well by several active
sources in or near the auditory cortex and slightly
delayed in time.
The amplitude characteristics with stimulus rate
of the magnetic 40-Hz response and the N100
component were found to be different, thus suggesting the contribution of neural generators having distinct habituation pattern ŽPantev et al.,
1993.. This response disappears during deep and
REM sleep ŽLlinas and Ribary, 1993., and is
enhanced by selective attention to the acoustic
input ŽTiitinen et al., 1993.. The 40-Hz response
seems also to be related to the temporal integration of two successive click stimuli ŽJoliot et al.,
1994.. Finally, in a passive odd-ball paradigm, no
difference was found in the 40-Hz response
between standard and deviant tones ŽTiitinen et
al., 1994..
However, in none of these experiments was the
40-Hz response compared to the multiple components of the wide-band response occurring in the
very same latency range, namely the middlelatency components Ž30᎐70 ms.. When comparisons were made in these studies, they concerned
only the 40-Hz maximum peak and N100 component occurring in successive time-periods. The
functional specificity of this response as a unitary
event thus remains to be established.
219
4.2. Induced gamma acti¨ ity
Induced gamma activity, not phase-locked to
the stimulus, was observed on the human scalp
during a simple acoustic detection task ŽJokeit
and Makeig, 1994.. The strength of the induced
response increased between 200 and 400 ms after
stimulus onset in subjects who reacted rapidly,
whereas it increased before stimulus onset for
subjects who reacted more slowly. Another study
based on passive and active auditory paradigms
has reported differences in the EEG gamma
power, 200᎐300 ms after subjects heard standard
tones compared to when they heard deviant tones
ŽMarshall et al., 1996..
In an experiment where subjects were passively
listening 1000 Hz tone-bursts of 50 ms duration
ŽBertrand et al., 1999., we detected in the EEG
an induced oscillatory response characterized by
a reduction Žbefore 150 ms. followed by an increase Žpeaking between 150 and 300 ms. of
gamma power with respect to the pre-stimulus
level ŽFig. 5d.. This temporal pattern of activation
is similar to that reported from epipial electrode
array placed over the auditory cortex of anaesthetized rats after acoustic click presentation
ŽFranowicz and Barth, 1995.. Furthermore, the
scalp topography of the induced gamma response,
with a maximum spreading over parietal electrodes ŽFig. 5d., clearly differs from that of the
evoked 40-Hz response as well as later auditory
evoked components ŽN100 and P200., suggesting
different neural sources.
A frequency discrimination paradigm was used
to test for a possible role of induced gamma
activity in acoustic object representation
ŽBertrand et al., 1998; Tallon-Baudry and
Bertrand, 1999.. Frequent standard tones Ž1000
Hz. and rare deviant tones, targets Ž1040 Hz. and
distractors Ž1080 Hz., were presented to the subjects who had to detect targets by a button press.
The tone frequencies were chosen such that the
task could not be performed automatically Ž85%
of correct responses.. The inter-stimulus interval
was constant Ž1.4 s., and subjects were aware that
at least three standard tones were delivered
between two deviants. As in the passive listening
situation, although with five times higher ampli-
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O. Bertrand, C. Tallon-Baudry r International Journal of Psychophysiology 38 (2000) 211᎐223
tude, the induced gamma response to standard
tones was characterized by a power decrease Žapprox. 100 ms. followed by an increase Žapprox.
250 ms..
The gamma response to the second standard
tone Žsecond STD. following a deviant was compared to that of the last standard tone Žlast STD.
preceding a deviant Žwhen the subjects were highly
expecting a target to occur soon.. We found ŽFig.
6a. that the induced gamma response peaks significantly later for the last STD Žapprox. 500 ms.
than for the second STD Žapprox. 250 ms. and
remains longer Žup to 1000 ms.. Furthermore,
following deviant tones Žtarget or distractor. the
gamma activity does not show any increased power
but rather a transient decrease followed by a
return to baseline.
The time averaged evoked potentials after the
second and last STD showed a very clear difference characterized by a large parieto-central positive component between 200 and 400 ms ŽFig. 6b..
These evoked responses start to differ in the
same time period as the induced gamma responses, which present different onset slopes. However, while the ERP differences are rather restricted in time Ž200᎐400 ms., the gamma response to the last STD remains sustained for a
much longer time Ž200᎐1000 ms.. These different
time-courses suggest that induced gamma oscillations and ERP components reflect distinct neural
processes.
The prolonged induced gamma activity after
several standard tones could be interpreted in
terms of increased attention oriented towards an
Fig. 6. Stimuli: 50 ms pure tone burst were delivered binaurally with a constant ISI Ž1.4 s.: frequent standard tones Ž1000 Hz. and
rare deviant tones Žtarget at 1040 Hz or distractor at 1080 Hz.. Subjects had to press a button after each target tone, and they were
aware that at least three standards were delivered between two deviants. Induced and evoked responses were computed separately
for the second standard Žsecond STD in full thin line. following a deviant and the last standard Žlast STD in full thick line.
preceding a deviant. Ža.. Time-course of the mean power between 35 and 50 Hz Žinduced gamma response. is presented for second
STD, last STD and target tones. The baseline level is the same for the three curves: pre-stimulus period Žy250 to y50 ms. of the
second STD tone. The last STD elicits a more prolonged gamma activity that may be related to increased attention or pitch
rehearsal in memory. Bottom: topography of the prolonged gamma response at 450 ms. Žb.. Evoked responses of the same tones
showing clear differences between the second and the last STD tones between 200 and 400 ms. The response to the target shows a
P300 component at approximately 400 ms. Bottom: topography of the increased positivity after the last tone Ž250 ms.. In the time
period when the induced gamma response of the second and the last STD are significantly different Ž400᎐800 ms., the
corresponding evoked responses are identical.
O. Bertrand, C. Tallon-Baudry r International Journal of Psychophysiology 38 (2000) 211᎐223
expected target. The subjects reported that, because of the difficulty of the task, they had to
actively maintain in memory each of the shortduration standard tones to detect small stimulus
frequency differences. The sustained gamma
could thus be also interpreted in terms of rehearsal of the pitch representation required to
correctly perform the task. Furthermore, after the
presentation of a deviant tone, the gamma activity
is no longer increased, suggesting that the frequency discrimination is performed by faster different processes. This result in the auditory
modality is in line with our working hypothesis
proposing that gamma oscillations should emerge
during tasks requiring activation of an object representation.
221
less clear-cut, since they both show the same type
of variation with the task, although with different
topographies.
With the only restriction concerning beta activities, the induced gamma activity was the only
component of the response to vary in accordance
with the predictions of the object representation
hypothesis. In addition, the task-dependency of
gamma activity suggests that it reflects the activation of a task-oriented network. These results
thus give strong support to the idea that the
establishment of a coherent and unified percept is
achieved through the oscillatory synchronization
in the gamma range of a distributed neural network.
Acknowledgements
5. Conclusions
This work was supported by grants from the
Rhone-Alpes
Region Ž1997᎐2000..
ˆ
Do induced gamma activities and evoked potentials reflect distinct processes? Several arguments stand in favor of a clear distinction between
these two phenomena. Indeed, the topographies
of induced gamma and evoked potentials can be
very different, as for example during the delay in
the visual short-term memory experiment ŽFig. 4.,
or fairly close, as in the acoustic frequency discrimination task ŽFig. 6.. The effects observed on
induced gamma activity can occur at shorter latencies than those observed in the evoked potentials Žas in the Dalmatian dog experiment., or at
longer latencies Žas in the acoustic frequency
discrimination task.. Therefore, the precedence
of evoked potentials effects on induced gamma
modulations cannot be considered to be a general
rule. Finally, the effect of stimulus features or
task is different on the evoked potential and on
the induced gamma activity.
Could gamma oscillations be harmonics of
lower-frequency rhythms ŽJurgens et al., 1995.?
The visual short-term memory experiment provides evidence that induced gamma and alpha
rhythms are functionally and topographically distinct. However, in the same experiment, the relationships between gamma and beta activities are
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