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The Journal of Neuroscience, June 1, 1998, 18(11):4244-4254
Induced  -Band Activity during the Delay of a Visual Short-Term
Memory Task in Humans
Catherine
Tallon-Baudry,
Olivier
Bertrand,
Franck
Peronnet, and
Jacques
Pernier
Mental Processes and Brain Activation Laboratory, Institut National
de la Santé et de la Recherche Médicale u280,
F-69003 Lyon, France
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ABSTRACT |
It has been hypothesized that visual objects could be represented
in the brain by a distributed cell assembly synchronized on an
oscillatory mode in the  -band (20-80 Hz). If this hypothesis is
correct, then oscillatory -band activity should appear in any task
requiring the activation of an object representation, and in particular
when an object representation is held active in short-term memory:
sustained -band activity is thus expected during the delay of a
delayed-matching-to-sample task. EEG was recorded while subjects
performed such a task. Induced (e.g., appearing with a jitter in
latency from one trial to the next) -band activity was observed
during the delay. In a control task, in which no memorization was
required, this activity disappeared. Furthermore, this -band
activity during the rehearsal of the first stimulus representation in
short-term memory peaked at both occipitotemporal and frontal
electrodes. This topography fits with the idea of a synchronized
cortical network centered on prefrontal and ventral visual areas.
Activities in the  band, in the 15-20 Hz band, and in the averaged
evoked potential were also analyzed. The -band activity during the
delay can be distinguished from all of these other components of the
response, on the basis of either its variations or its topography. It
thus seems to be a specific functional component of the response that
could correspond to the rehearsal of an object representation in
short-term memory.
Key words:
-band; 40 Hz; oscillations; cell assembly; visual
short-term memory; delayed-matching-to-sample task; vision; human; EEG
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INTRODUCTION |
How is a particular visual object
represented in the brain? It has been proposed that the specific
combination of attributes corresponding to one object could be coded by
an assembly of neurons distributed in different functional areas, the
distinctive sign of neurons belonging to the same assembly being the
synchronization of their discharge on an oscillatory mode (Milner,
1974 ; von der Malsburg and Schneider, 1986; Singer and Gray, 1995).
Epochs of synchronization on an oscillatory mode in the -band
(20-80 Hz) have been observed in various species and neural structures
(for review, see Singer and Gray, 1995; Engel et al., 1997). Moreover,
in support of the above hypothesis, evidence for enhanced -band
synchronization in response to a visually coherent object has been
obtained in anesthetized (Eckhorn et al., 1988; Gray et al., 1989;
Engel et al., 1991; Freiwald et al., 1995; Brosch et al., 1997) and
awake (Gray and Di Prisco, 1997) cats and awake monkeys (Kreiter and
Singer, 1992, 1996), as well as in EEGs in humans (Lutzenberger et al.,
1995; Müller et al., 1996, 1997; Tallon-Baudry et al., 1996).
Oscillatory synchronization in the -band thus could underlie the
feature binding process. Another experiment in humans suggests that an
oscillatory assembly is also activated when the internal representation
of an object is needed (Tallon-Baudry et al., 1997): when subjects were
required to find an hidden object in a picture (e.g., they had to
activate the representation of the object searched for), enhanced
-band activity was observed. Altogether, these results suggest that
the neural correlate of the activation of a visual object
representation is the oscillatory synchronization of an assembly, no
matter whether the activation of this representation is triggered by an
external input (or bottom-up process), as in feature binding, or
induced by an internal event (or top-down process), as in the visual
search task described above.
If the hypothesis of a correspondence between the activation of an
object representation and the oscillatory synchronization of a neural
assembly is correct, then induced -band activity should be elicited
in any task requiring the activation of an object representation. The
experiment presented here was designed to test whether such an
oscillatory activity could be detected when an object representation is
held active in short-term memory. Thirteen subjects performed a
delayed-matching-to-sample task (see Fig. 1A). During
the delay, subjects had to hold the representation of the first
stimulus S1 in memory, and sustained -band activity is expected. A
control task was designed (dimming condition; see Fig.
1B) in which S1 does not have to be memorized.
Finally, it should be emphasized that in all the studies quoted above,
the functionally relevant epochs of oscillatory synchronization are not
strictly triggered by stimulus onset; rather, they appear with a jitter
in latency from one trial to the next. Our analysis will thus focus on
so called "induced" or "nonphase-locked to stimulus onset"
activities.
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MATERIALS AND METHODS |
Subjects. Thirteen right-handed subjects (five
females, mean age 24 years) gave their informed and written consent to
participate in the study. All of the subjects had normal or corrected
to normal vision.
Procedure. The experiment was divided into two conditions: a
memory condition and a dimming condition (Fig.
1). Before the recordings, subjects were
instructed on which task they were to perform. The order in which the
conditions were presented was counterbalanced between subjects. The
shapes used were smooth, without any sharp angle, to avoid any explicit
spatial description of the stimulus; rather, we hoped the subjects
would hold active a "photographic" representation of S1 during the
delay. Stimuli were presented in black on a light gray background on a
video monitor (refresh rate 106 Hz). They were positioned within a disk with an inner radius of 0.88° and an outer radius of 1.23°, at a
viewing distance of 1.5 m. In the memory condition, one trial consisted of (Fig. 1A) (1) red fixation cross for 800 msec, (2) S1 presentation for 400 msec, (3) a blank screen where only
the red fixation cross remained for 800 msec, and (4) S2 presentation for 400 msec. S2 could be either different (no-go trial) or identical (go trial) to S1. In the dimming condition, one trial consisted of (1)
red fixation cross for 800 msec, (2) S1 presentation for 400 msec, (3)
red fixation cross for 800 msec, and (4) fixation cross (400 msec) of a
dimmed red (no-go trials) or the same red (go trials). In both
conditions, the intertrial interval was randomized between 2 and 3 sec.
The proportion of go trials was 20%, with no more than two consecutive
go trials. The difficulty of the task was adjusted to the subject's
performance (see below). Subjects were instructed to press a mousekey
with the right finger on go trials and to answer accurately rather than
fast. They were also instructed to blink only during intertrial
intervals.
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Figure 1.
Experimental design. In the memory condition, two
stimuli, S1 and S2, were presented for 400 msec separated by a 800 msec
delay. Subjects were to detect matching S2 (20% of the trials). In the
dimming condition, no second stimulus appeared. Instead, the fixation
cross could either dim (80% of the trials) or remain the same (20%),
in which case subjects had to respond. In this control condition, S1
does not have to be memorized and acts only as a warning stimulus.
These two conditions were presented in two successive recording
sessions. We expected to find induced -band activity reflecting S1
representation rehearsal during the delay in the memory condition but
not in the dimming condition.
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Stimuli construction. Stimuli were strictly parametrized by
12 anchor points of polar coordinates
(ri, i), with
i = 2 /12(i  1) and
ri being randomly chosen at each trial between
rmin and rmax,
corresponding to visual angles of 0.88° and 1.23°, respectively. The interpolation function between four successive points
(r1,1)..(r4,4) is given by Lagrange's polynomial:
where i and j vary between 1 and 4. Only
the inner segment, between
(r2,2) and
(r3,3), is drawn. The same
procedure is applied until the 11 segments of the curve are drawn.
Adjustment of the difficulty of the task to the subject's
performance. When S2 does not match S1, it is derived from S1 by modulating the radii of the 12 anchor points:
ri(S2) = ri(S1) × (1 ± coeff). The sign of the modulation
(increase or decrease ri) is chosen
randomly and independently for each anchor point. The value of
coeff is set at 12% at the beginning of a block of recording and constantly modified according to the subject's
performance computed on the last 20 trials. If this moving average
performance falls below 90%, coeff is increased by 1%, and
the task becomes easier. On the contrary if the performance is above
90%, coeff is decreased by 1%, and the task becomes more
difficult. The upper and lower limit values of coeff were
set at 20% and 4%, respectively.
In the dimming task, no second shape appears, but the intensity of the
red fixation cross may decrease. The difficulty of this task is also
modulated according to the subject's performance. When the moving
average performance falls below 90%, the intensity decrement of the
screen red channel is increased by one, and the task becomes easier; if
the value of the intensity decrement is decreased by one, the task
becomes more difficult.
Recordings. EEG was recorded continuously at a sampling rate
of 1000 Hz (0.1-320 Hz analog bandwidth) from 17 Ag-AgCl electrodes referenced to the nose. Electrode impedances were kept below 5 k .
Their locations, according to the international 10-20 system, are Iz,
P7, O1, O2, P8, POz, P3, Pz, P4, C3, Cz, C4, F3, Fz, and F4. Two
electrodes (OM1, OM2) were placed halfway between Iz and the mastoids.
Electrode placement on the head was computer-assisted (Echallier et
al., 1992). Horizontal eye movements were monitored (electrode Yh), and
a rejection threshold was set for each subject at one-fourth of the
potential value corresponding to a saccade of 4°. Four blocks of
~70 trials (56 no-go and 14 go trials) each were recorded in each
condition. Epochs containing artifacts (EEG > 100 µV or
EOG > threshold) were rejected off-line.
Time-frequency (TF) transformation of the data. We were
interested in the identification and characterization of oscillatory activities induced by a stimulation. Because neither latency nor frequency of these oscillatory bursts was known a priori, a method that
preserves both types of information was chosen: the time-frequency (TF) representation based on a wavelet transform of the signals (Tallon-Baudry et al., 1996, 1997). The main advantage of this approach, compared with the short-term Fourier transform approach (Makeig, 1993), is that the duration of the window of analysis depends
on the frequency band: the higher the central frequency, the shorter
the window duration and the wider the frequency band. This method thus
provides a better compromise between time and frequency resolutions
(Sinkkonen et al., 1995).
Using this method, induced activities can be analyzed. So-called
induced activities appear with a jitter in latency from one trial to the next; thus, they tend to disappear on the classic averaged
evoked potential. When the TF energy is computed on each single trial
and averaged (TF energy averaged across single trials), induced
activities can be analyzed, provided their signal-to-noise ratio is
high enough. The TF transformation can also be applied to the averaged
evoked potential (TF energy of the evoked potential). Induced
activities tend to disappear on these plots, whereas information on
oscillatory bursts that are phase-locked to stimulus onset (e.g.,
appearing exactly at the same latency in each trial) can be
obtained.
The signal was convolved with complex Morlet's wavelets
w(t, f0)
(Kronland-Martinet et al., 1987) having a Gaussian shape both in the
time domain (SD  t) and in the frequency domain
(SD f) around its central frequency
f0: w(t,
f0) = A.exp(t2/2t2) × exp(2if0t),
with f = 1/2t. Wavelets were
normalized so that their total energy was 1, the normalization factor A
being equal to (t ) 1/2. A wavelet
family is characterized by a constant ratio
(f0/f), which should be chosen in practice greater than ~5 (Grossmann et al.,
1989). The wavelet family we used was defined by
f0/f = 7, with
f0 ranging from 8 to 100 Hz in 1 Hz steps. At 8 Hz, this leads to a wavelet duration (2t) of 278 msec and to a spectral bandwidth (2f) of 2.3 Hz;
at 20 Hz, to a wavelet duration of 111.4 msec and to a spectral
bandwidth of 5.8 Hz; and at 100 Hz, to a duration of 22.2 msec and a
bandwidth of 28.6 Hz. The time resolution of this method thus increases
with frequency, whereas the frequency resolution decreases. The
time-varying energy E(t, f0) of the signal in a frequency band
around f0 is the squared norm of the result of
the convolution of a complex wavelet w(t, f0) with the signal
s(t): E(t,
f0) = |w(t,
f0) × s(t)|2. A family of wavelets will
provide a TF representation of the energy of the signal. The mean TF
energy of the prestimulus (between 300 and 50 msec) is considered
as a baseline level and subtracted from the prestimulus and
poststimulus TF energy. This correction is done separately in each
frequency band.
Data analysis. On average, approximately 140 correct no-go
trials and 50 correct go trials were included in the analysis in each
condition after artifact rejection. Because the subjects' performance
was maintained at about 90%, there were not enough incorrect trials to
analyze them. Go and no-go trials were pooled together to improve the
signal-to-noise ratio for the analysis of the responses to S1 and of
the activities during the delay. Go and no-go trials were processed
separately to analyze the responses to S2 in the memory condition.
Because the TF energy values were far from having a Gaussian
distribution, the nonparametric test of Wilcoxon for matched pairs was
used for all statistical analysis.
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RESULTS |
Behavioral results
Performance was on average 88% in both conditions. Reaction times
measured on correct go trials from S1 onset were shorter in the memory
(1992 ± 23 msec) than in the dimming condition (2049 ± 54 msec). Although small, this difference is significant
(p = 0.039; Wilcoxon test for matched
pairs).
Induced -band responses
The TF energy averaged across single trials, where oscillatory
activities induced by the stimulus (e.g., appearing with a jitter in
latency) can be analyzed, is presented in Figure
2A for both the memory
and dimming condition. Two peaks of enhanced activity around 30 Hz can
be observed in both conditions: a first one at ~280 msec after S1
onset, called the ON response, and a second one at ~680 msec (e.g.,
280 msec after S1 offset) that we called an OFF-induced response. A
third area of enhanced -band activity can also be observed during
the delay in the memory condition (700-1000 msec). These three peaks
of -band activity disappear on the TF representation of the averaged
evoked potential (Fig. 2B, white boxes). Hence, they
do correspond to induced activities. The frequency band between 24 and
60 Hz was chosen for further analysis of these phenomena. The choice of
the lower limit of this frequency band (24 Hz) was made to avoid mixing
these enhancements of -band energy with the decreases in energy that
can be observed at lower frequencies (Fig. 2A).
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Figure 2.
A, Time-frequency
(TF) representation of the energy averaged across
single trials at electrode C3, grand average across subjects, in both
conditions. Time is presented on the x-axis. Frequency
is presented on the y-axis on a logarithmic scale. The
energy level is coded on a color scale: yellow areas
show an enhancement of energy compared with prestimulus level, and
red areas show decrease. Three areas of enhanced
high-frequency activity can be observed (white boxes):
(1) an ON response, peaking at ~280 msec and 30 Hz, higher in the
memory than in the dimming condition; (2) an OFF response, peaking at
~680 msec (e.g., 280 msec after S1 offset), similar in both
conditions; and (3) a -band activity during the delay, in the memory
condition only. B, Time-frequency
(TF) representation of the energy of the averaged
evoked potential at electrode C3, grand average across subjects, in
both conditions. Only activities phase-locked to stimulus onset (e.g.,
appearing at a fixed latency from one trial to the next) can be
observed on these representations. The three areas of enhanced
high-frequency activity in A thus correspond to induced
activities (e.g., appearing with a latency jitter from one trial to the
next).
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S1 ON and OFF -band responses
The first peak of -band activity appears at ~280 msec. It was
analyzed by averaging the 24-60 Hz TF energy (Fig.
3A) between 230 and 330 msec:
the ON response is higher in the memory than in the dimming condition.
This effect is significant at electrodes OM1, O1, and P3
(p < 0.04). Although there is a difference in energy, the ON response is most prominent at posterior electrodes in
both conditions (Fig. 3B). Still, it is lateralized on the left in the memory condition only. When the 24-60 Hz, 230-330 msec
energy across OM1, O1, P7, and P3 was averaged and compared with the
energy averaged across the corresponding electrodes over the right
hemisphere (OM2, O2, P8, and P4), a significant difference was revealed
in the memory condition (p = 0.039) but
not in the dimming condition (p = 0.39).
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Figure 3.
A, Energy of the mean activity
between 24 and 60 Hz, grand average across subjects, in the memory
(thick line) and in the dimming (thin
line) conditions. A first peak of enhanced -band activity
appears at ~280 msec (ON). It is significantly stronger in the memory
than in the dimming condition. An OFF response can be observed at
~680 msec (e.g., 280 msec after S1 offset); it does not show any
significant difference between conditions. Later on, during the delay,
a sustained -band activity appears in the memory condition only,
mainly at left posterior electrodes and bilaterally at frontal
electrodes. It tends to decrease before the end of the delay.
B, Topographical maps of the 24-60 Hz energy (left,
back, and right views of the head) averaged between 230 and 330 msec
(ON response), and between 630 and 730 msec (OFF response). In both
conditions, the ON response is maximum over occipital electrodes and
decreases smoothly until the frontal sites. It is not only enhanced but
also lateralized over the left hemisphere in the memory condition. The
topography of the OFF response is less clear-cut; it tends to be
maximum over occipital sites also. No tendency for lateralization of
the OFF response could be found in any condition.
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The OFF-induced -band response appears at ~680 msec, e.g., 280 msec after S1 offset (Fig. 2A). Comparing the 24-60
Hz energy averaged between 630 and 730 msec at each electrode does not
reveal any significant differences between conditions
(p > 0.46). The topography of this OFF response
(Fig. 3B) seems more widespread than the topography of the
ON response, and no significant effect of lateralization can be
found.
S1 ON and OFF evoked 0-25 Hz responses
After S1 onset, the P1 (105 msec) and N1 (155 msec) components of
the averaged evoked potential filtered between 0 and 25 Hz can be
observed in both conditions (Fig. 4).
They are not affected by condition type, neither in peak latency nor in
peak amplitude. The first difference in the evoked potential can be
observed in the 200-240 msec time window (P2 component), where the
mean amplitude is significantly enhanced in the dimming condition
compared with the memory condition at electrodes OM1
(p = 0.013), Iz (p = 0.046), OM2 (p = 0.033), and P7
(p = 0.046). Later on, between 300 and 550 msec,
a sustained activity is higher in the memory condition at electrodes
POz and Pz (p < 0.04).
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Figure 4.
Filtered (0-25 Hz) averaged evoked potentials,
grand average across subjects, in the memory (thick
line) and in the dimming (thin line) conditions.
The first significant difference (*1) between the two
conditions occurs at 200-240 msec: the posterior P2 component is more
pronounced in the dimming condition. Later on (300-550 msec), a
sustained positivity appears at electrodes POz (not shown) and Pz
(*2) in the memory condition. During the delay, a
negativity appears at left occipitotemporal sites (O1,
*3) in the memory condition and tends to decrease in the
end of the delay, whereas a parietocentral negativity rises
(Cz, *4). See also the maps of the
0-25 Hz evoked potential during the delay in Figure
5B.
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An N1 OFF component appears in the evoked potential 155 msec after S1
offset, at 555 msec (Fig. 4). It does not vary with condition type at
any electrode (amplitude averaged between 500 and 600 msec;
p > 0.25).
Induced -band responses during the delay
As depicted in Figure 3A, sustained 24-60 Hz activity
is observed in the memory condition, whereas in the dimming condition, the level of 24-60 Hz energy is often negative (e.g., below
prestimulus level). It should be noted that this sustained activity in
the memory condition tends to decrease before the end of the delay. Table 1 shows the electrodes at which the
difference between the two conditions during the delay is significant,
on 200-msec time windows shifted by steps of 100 msec. The enhancement
in the memory condition is highly significant from 750 to 1050 msec at
several electrodes; at the end of the delay (950-1150 msec), it
remains significant only at electrode C3.
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Table 1.
Electrodes at which a significant effect between the two
conditions can be observed, in the 24-60 Hz energy (left), in the
evoked potential (middle), and in the 15-20 Hz band (right) in 200 msec time windows shifted by steps of 100 msec.
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The topography of the 24-60 Hz activity during the delay is depicted
in Figure 5A in both
conditions. In the memory condition, two regions of enhanced -band
activity can be observed, one at left occipitotemporal electrodes and
another one bilaterally at frontocentral electrodes. The
occipitotemporal maximum is lateralized on the left side in the memory
condition (comparison of the 24-60 Hz energy averaged across
electrodes OM1, O1, P7, and P3 vs OM2, O2, P8, and P4; 750-950 msec
time window: p = 0.003 in the memory condition, 0.91 in
the dimming condition; 850-1050 msec: p = 0.008 in the
memory condition, 0.25 in the dimming condition; 950-1150 msec: not
significant in both conditions). The frontocentral activity shows no
significant preference for an hemisphere (comparison of the 24-60 Hz
energy averaged across electrodes C3 and F3 vs C4 and F4 in the time
windows 750-950 msec, 850-1050 msec, and 950-1150 msec:
p > 0.25 in both conditions).
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Figure 5.
Summary of the activities observed during the
delay. A, -band (24-60 Hz) energy in the memory
(top row) and in the dimming condition (bottom
row), averaged on three overlapping 200-msec time windows
(750-950, 850-1050, and 950-1150 msec). Topographical maps of left,
back, and right views of the head are displayed for these three time
windows. Enhanced -band activity appears in the memory condition at
left posterior electrodes, and bilaterally at more frontal sites
(arrows). Differences between conditions are more
significant at the beginning of the delay (750-950 and 850-1050 msec
time windows) than in the end (950-1150 msec). B,
Topographical maps of the evoked potential in both conditions, at three
latencies (700, 900, and 1050 msec). Two negativities with different
time courses and topographies can be observed in the memory condition
(arrows): a left posterior negativity is prominent at
the beginning of the delay and decreases in the end of the delay,
whereas a parietocentral negativity rises. C, Energy
(15-20 Hz) averaged in three 200 msec time windows in both conditions.
An occipital enhancement, with a tendency for being lateralized on the
right, is observed in the memory condition. Another peak of 15-20 Hz
activity is observed in the memory condition at the midline frontal
electrode. D, Energy in the -band (8-12 Hz),
averaged between 750 and 1150 msec. No difference between the two
conditions could be detected in this frequency band during the
delay.
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Evoked (0-25 Hz) responses during the delay
The evoked potential during the delay was studied on 200 msec time
windows shifted by steps of 100 msec. Electrodes at which a significant
difference between the two conditions could be found appear in Table 1.
Two components are enhanced in the memory condition (Fig. 4): a
sustained left-sided negativity, restricted to the posterior electrodes
and more prominent at the beginning of the delay, and a parietocentral
negativity that rises later on. On topographical maps (Fig.
5B), the balance between these two components results in a
shift of the maximum of voltage from left posterior electrodes (750 msec) to parietocentral sites (1050 msec). It should be noted that no
significant difference between conditions could be found at frontal
electrodes in the 0-25 Hz evoked potential during the delay.
Induced activity in other frequency bands during
the delay
As can be seen in Figure
6A, in addition to the
increase in the -band, an enhancement of the energy at lower
frequencies (15-20 Hz) is observed in the memory condition at
occipital electrodes during the delay. This peak of enhanced energy
disappears on the TF representation of the evoked potential (Fig.
6B); it thus corresponds to induced activity. The
15-20 Hz band was chosen for further analysis.
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Figure 6.
A, Time-frequency energy averaged
across single trials at electrode O2, in the memory
(top) and in the dimming (bottom)
conditions. Sustained 15-20 Hz activity is observed during the delay
in the memory condition only (arrows). B,
Time-frequency energy of the averaged evoked potential at electrode
O2, in the memory condition. The 15-20 Hz energy observed during the
delay disappears: it is an induced activity.
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The 15-20 Hz energy during the delay was averaged on 200 msec time
windows shifted by steps of 100 msec. Electrodes at which a significant
difference between the two conditions could be found are shown in Table
1. Significant effects are found at both occipital and frontal sites.
The occipital enhancement of the 15-20 Hz activity in the memory
condition remains significant through all the delay, although the
energy level in the memory condition tends to decrease in the end of
the delay. The frontal enhancement in the memory condition is
significant at Fz and F4 only within the 850-1050 msec time interval.
Topographical maps of this frequency band are shown in Figure
5C.
As opposed to the 24-60 Hz energy in the same latency range, the
occipital component of the 15-20 Hz activity is not lateralized on the
left side. A tendency for being lateralized on the right side in the
memory condition can even be observed (Fig. 5C), although it
does not reach the significance level [comparison of the averaged 15-20 Hz energy at electrodes O1 (left) and O2 (right); time window 850-1050 msec: p = 0.13 in the memory condition and
0.86 in the dimming condition; 950-1150 msec: p = 0.1 in the memory condition and 0.55 in the dimming condition]. The
frontal 15-20 Hz activity is not lateralized (comparison of F3 vs F4;
time interval 850-1050 msec: p = 0.22 in the memory
condition and 0.80 in the dimming condition; 950-1150 msec:
p > 0.35 in both conditions).
Finally, we studied the variations in the band (8-12 Hz) during
the delay. TF energy values averaged between 8 and 12 Hz and 750 and
1150 msec did not reveal any significant difference between conditions
during the delay at any electrode (p > 0.34). The topography of the band in the 750-1150 msec time interval is
shown in Figure 5D.
Baseline level
Up to now, the TF energy values used for statistical analysis were
baseline-corrected by subtracting the averaged value of the prestimulus
level between 300 and 50 msec. For each of the frequency bands
studied (24-60, 15-20, and 8-12 Hz), we thus examined whether there
was any significant difference between conditions of the prestimulus
level. Comparison of the averaged energy between 300 and 50 at each
electrode did not reveal any significant difference between conditions
in any frequency band (24-60 Hz band: p > 0.25;
15-20 Hz: p > 0.27; 8-12 Hz: p > 0.31).
Responses to S2
Go and no-go trials were pooled together in the analysis of the
responses to S1 and the responses during the delay to improve the
signal-to-noise ratio. To study the response to S2 in the memory
condition, we separated the trials with matching S2 (20%) from the
trials with nonmatching S2 (80%). The averaged evoked potentials for
matching and nonmatching S2 at electrode O1 are depicted in Figure
7A: matching and nonmatching
S2 differ at ~1410 msec, e.g., 210 msec after S2 onset. The mean
amplitude, measured in a 40 msec time window centered at 1410 msec,
differs significantly between matching and nonmatching S2 at electrode
O1 and O2 (p = 0.013) and P8 and POz
(p = 0.039). Before this latency, neither the P1
nor the N1 components vary, estimated on a 40 msec time window centered
at their peaking latency (P1: p > 0.2 at all
electrodes; except O1 and OM1: p > 0.1; N1:
p > 0.2).
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Figure 7.
Responses to the matching and nonmatching S2 in
the memory condition. A, Evoked (0-25 Hz) potentials at
electrode O1, for matching (thin line) and nonmatching
(thick line) S2. The first significant difference occurs
around 1410 msec (*), e.g., 210 msec after S2 onset. B,
Time-frequency representation of the energy averaged across single
trials at electrode O1, in the memory condition, trials with
nonmatching S2. A small -band response to S2 can be observed at
~1500 msec (300 msec after S2 onset); it is much smaller than the ON
response to S1. C, Topographical maps at 28 Hz (back
views) of the ON response to S1 (top), of the ON
response to nonmatching S2 (bottom left), and of the ON
response to matching S2 (bottom right). In both cases
(matching or nonmatching S2), the ON response to S2 is much smaller
than the response to S1. Nevertheless, it shows the same left occipital
maximum.
|
|
In the -band, a small ON induced response to S2 can be observed at
~1500 msec (Fig. 7B), e.g., 300 msec after S2 onset and later than the difference between matching and nonmatching S2 in the
evoked potentials. It is much smaller than the ON response to S1, but
with a similar topography (Fig. 7C). Whether or not S2
matches, S1 does not seem to influence this small -band
response.
| |
DISCUSSION |
-band activity during the delay
This experiment was designed to test the existence of induced
-band activity when one has to hold active an object representation in short-term memory. Such an activity was indeed observed in the
memory condition. It appears at left occipitotemporal electrodes and
bilaterally at left and right frontal sites. In the dimming condition,
where there is no need to memorize S1, no -band activity is found.
This finding meets with the hypothesis of a representation of visual
objects through the oscillatory synchronization of a distributed neural
assembly.
This enhancement of -band activity during the delay in the memory
condition appears at both occipitotemporal and frontal electrodes. The
occipitotemporal component seems to be already present in the ON response to the first stimulus. This ON occipital response in
previous studies (Tallon-Baudry et al., 1996, 1997) was associated with
the building of an object representation. A possible interpretation of
our data is that during the delay the occipital S1 representation is
being rehearsed. The additional frontal -band activity could be
necessary to maintain the occipitotemporal activity. Indeed, there may
be some cortical interplay between the ventral visual areas and the
prefrontal areas that have both been involved in stimulus retention in
short-term memory in animals (for review, see Goldman-Rakic, 1995;
Desimone, 1996; Fuster, 1997) and humans (for review, see Ungerleider,
1995) (also see Swartz et al., 1995; Courtney et al., 1996). Although
the idea of a synchronized cortical network involving prefrontal and
ventral visual areas fits with current hypotheses both on short-term
visual memory and on synchronized cell assembly, this interpretation goes beyond our results. Indeed, because scalp recordings do not allow
one to localize directly the active neural sources, we cannot be sure
that the -band activity appearing at left occipitotemporal and
frontal electrodes is generated within the underlying cortical areas.
Furthermore, we do not know whether the -band activities appearing
at frontal and occipitotemporal electrodes are synchronized or whether
they behave as two independently oscillating ensembles.
We thus interpret our data in terms of neural activity related to
memory processes. We cannot rule out a possible contribution of muscle
activity to our results. Nevertheless, it seems unlikely that muscle
activity alone can explain our data. Indeed, it would mean that muscle
activity is task-dependent (memory/dimming condition), has a different
topography according to the processing stage of the stimulus in the
memory condition (occipitotemporal when the stimulus is being encoded,
occipitotemporal plus frontal when it is being rehearsed), and is
lateralized on the left. Another argument is the frequency distribution
of the induced -band activity compared with the frequency
distribution of muscle activity. Indeed, a control experiment in one
subject (Fig. 8) shows that the ON induced response observed in the memory condition is confined to
the 24-46 Hz band, whereas muscle activity extends up to 100 Hz.
View larger version (16K):
[in this window]
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|
Figure 8.
Power spectrum of the ON induced response
compared with muscle activity (one subject, electrode O1 referenced to
the nose). The subject was first recorded while performing the memory
task. Comparison of the spectrum of the prestimulus (350 to 50
msec) to the spectrum of the ON induced -band response (270-320
msec) reveals a difference around 35 Hz (gray area), whereas above 55 Hz, both spectra are similar. The ON induced response is thus
confined to a narrow frequency band. The subject was then recorded
while simply fixating on the screen, both at rest and when contracting
different muscles (neck, jaw, eyebrow). The power spectrum at rest was
lower than during the prestimulus of the memory condition, but with a
similar profile. To get rid of scaling effects, the power spectrum at
rest was normalized with respect to the 24-46 Hz band of the power
spectrum of the prestimulus. Similarly, the power spectrum of muscle
activity was normalized with respect to the 24-46 Hz band of the ON
induced response power spectrum. It clearly appears that the effect of
muscle activity is not restricted to this 24-46 Hz band, but extends
up to 100 Hz. The same type of spectrum is observed when the different
muscles (neck, jaw, eyebrow) are activated separately.
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|
Because our subjects had to produce a motor response, one can also
wonder whether our results are not related to activity elicited by
motor preparation (Pfurtscheller and Neuper, 1992; Kristeva-Feige et
al., 1993; Nashmi et al., 1994; Pfurtscheller et al., 1994). This does
not seem to be the case because (1) motor responses were given both in
the memory and the dimming conditions, whereas we observe -band
activity during the delay in the memory condition only; and (2) if the
activity we observe was related to motor preparation, one would
expect it to increase until movement onset, whereas it tends to
decrease even before the end of the delay.
The temporal course of the -band activity during the delay is indeed
surprising: the energy level decreases before S2 onset. Why does not it
remain high until the end of the delay? The duration of the delay was
constant for all the trials, so it may be that S2 onset was anticipated
by the subject. Such an anticipation phenomena was already described by
Klimesch (1996) in the band.
A number of other activities were analyzed during the delay, namely the
averaged evoked potential and induced activities in the 15-20 Hz band
and in the band. Do all these activities belong to the same
functional ensemble?
Evoked potentials (0-25 Hz)
The 0-25 Hz filtered evoked potential shows two enhanced
components in the memory condition: a left posterior negativity and a
parietocentral negativity. It should be emphasized that no differences between conditions were detected at frontal electrodes in the evoked
potential. The left occipitotemporal negativity may be related to the
sustained firing of inferotemporal neurons observed in similar tasks in
animal (Fuster and Jervey, 1981; Miyashita and Chang, 1988; Fuster,
1990; Miller and Desimone, 1993). It may be tightly linked to the
-band activity appearing at the same left occipitotemporal
electrodes. A possible functional role for this component is to prepare
a speedy matching process between S1 and S2, because later on the
evoked potentials reflect the result of this process by indicating a
difference between matching and nonmatching S2. This difference between
matching and nonmatching S2 in the evoked potential was already
observed in other studies in the same latency range (Begleiter et al.,
1993; Sugita, 1994). Furthermore, if this difference means that the
comparison process between S1 and S2 is completed 210 msec after S2
onset, it may account for the reduced -band response 300 msec
after S2 onset: there is no need for building a representation of S2
because the matching task is over.
The other component in the evoked potential during the delay is a
rising parietocentral negativity. It is greater in the memory than in
the dimming condition. Both the topography (parietocentral) and the
time course (increases until S2 onset) of this negativity suggest that
it belongs to the family of contingent negative variation waves, which
have been observed repeatedly in humans in paired S1-S2 paradigms,
especially when the contingency between the two stimuli is strong (for
review, see Tecce and Cattanach, 1987).
Activity in other frequency bands during the delay
During the delay, an enhancement of induced activity in the 15-20
Hz band in the memory condition can also be observed at some
electrodes. It thus shows the same functional variations with
conditions as the activity in the -band. Furthermore, its temporal
course is quite similar to the temporal course of the -band activity
during the delay, with a tendency to decrease before S2 onset. Still,
the topographies of these two activities are different: at least
partially different areas would be involved in the generation of 24-60
and 15-20 Hz activities. The difference between the 24-60 and 15-20
Hz topographies may also partly justify the choice of these two
frequency bands: they do not reflect the same phenomena.
Activity in the -band (8-12 Hz) during the delay did not show any
significant difference between conditions during the delay, which seems
to rule out the possibility (Jürgens et al., 1995) that either
the -band or the 15-20 Hz activities are simply harmonics of activity, at least in this experiment. In other studies (Gevins et al.,
1997; Klimesch et al., 1997), energy in the -band was found to
correlate with memory load. Still, both the protocols (continuous
matching task or words learning and recognition) and the long time
windows that were studied (>1 sec) in these experiments are very
different from what we used.
Responses to S1
Responses to S1 differ in the memory and in the dimming tasks.
Evoked potentials show a decrease of the P2 component in the memory
condition, which could be associated with feature selection processes
(for review, see Anllo-Vento and Hillyard, 1996). It is followed by a
positive wave that may be related to the memory encoding process. In
the -band, the ON induced response appearing at ~280 msec after S1
onset is reduced in the dimming condition, where S1 probably acts only
as a warning stimulus. Because our experimental protocol was designed
to study the delay, during S1 presentation many behavioral parameters
differ between conditions (spatial attention more focal in the dimming
task, feature selective attention to shape and memory encoding in the
memory task, etc.). It is thus difficult to give a precise
interpretation of the responses to S1.
Finally, OFF responses to S1 have been observed not only in the evoked
potential but also in the -band. To our knowledge, such a response
had not been described so far in the -band. Its latency (280 msec
after S1 offset) seems to indicate that it is indeed an induced
OFF response. Because it does not vary between conditions in
this experiment, its functional significance remains unclear.
Conclusion
Both the existence and the topography of the -band activity
during the delay in the memory condition fit the hypothesis of an
oscillatory network centered on prefrontal and visual areas that would
ensure the rehearsal of the first stimulus representation in memory.
This experiment does not prove definitively that this interpretation is
the right one. In particular, we have not ruled out definitively a
possible contribution of muscle activity to our data. In any case,
oscillatory -band activity seems to be a specific component of the
response: it can be distinguished from the averaged evoked potential
and from activities in other frequency bands (8-12, 15-20 Hz) either
by its topography or by its variations with conditions. Finally, the
topography of -band activity is modified depending on whether S1 is
being encoded (left occipitotemporal) or rehearsed (left
occipitotemporal plus frontal). This suggests that depending on the
task to be performed, different functional areas can be recruited to
participate in an oscillatory -band ensemble.
| |
FOOTNOTES |
Received Nov. 25, 1997; revised March 10, 1998; accepted March 13, 1998.
This work was supported by grants from the Human Frontier Science
Program and the Rhône-Alpes region. We thank J. F. Echallier and P. E. Aguerra for helpful assistance.
Correspondence should be addressed to Catherine Tallon-Baudry, INSERM
U280, 151 cours Albert Thomas, F-69003 Lyon,
France.
| |
REFERENCES |
-
Anllo-Vento L,
Hillyard SA
(1996)
Selective attention to the color and direction of moving stimuli: electrophysiological correlates of hierarchical feature selection.
Percept Psychophys
58:191-206[Web of Science][Medline].
-
Begleiter H,
Porjesz B,
Wang W
(1993)
A neurophysiologic correlate of visual short-term memory in humans.
Electroenceph Clin Neurophysiol
87:46-53[Web of Science][Medline].
-
Brosch M,
Bauer R,
Eckhorn R
(1997)
Stimulus-dependent modulations of correlated high-frequency oscillations in cat visual cortex.
Cereb Cortex
7:70-76[Abstract/Free Full Text].
-
Courtney SM,
Ungerleider LG,
Keil K,
Haxby JV
(1996)
Object and spatial visual working memory activate separate neural systems in human cortex.
Cereb Cortex
6:39-49[Abstract/Free Full Text].
-
Desimone R
(1996)
Neural mechanisms for visual memory and their role in attention.
Proc Natl Acad Sci USA
93:13494-13499[Abstract/Free Full Text].
-
Echallier JF,
Perrin F,
Pernier J
(1992)
Computer assisted placement of electrodes on the human head.
Electroenceph Clin Neurophysiol
82:160-163[Web of Science][Medline].
-
Eckhorn R,
Bauer R,
Jordan W,
Brosch M,
Kruse W,
Munk M,
Reitboeck HJ
(1988)
Coherent oscillations: a mechanism of feature linking in the visual cortex?
Biol Cybern
60:121-130[Web of Science][Medline].
-
Engel AK,
König P,
Singer W
(1991)
Direct physiological evidence for scene segmentation by temporal coding.
Proc Natl Acad Sci USA
88:9136-9140[Abstract/Free Full Text].
-
Engel AK,
Roelfsema PR,
Fries P,
Brecht M,
Singer W
(1997)
Role of the temporal domain for response selection and perceptual binding.
Cereb Cortex
7:571-582[Abstract/Free Full Text].
-
Freiwald WA,
Kreiter AK,
Singer W
(1995)
Stimulus dependent intercolumnar synchronization of single unit responses in cat area 17.
NeuroReport
6:2348-2352[Web of Science][Medline].
-
Fuster JM
(1990)
Inferotemporal units in selective visual attention and short-term memory.
J Neurophysiol
64:681-697[Abstract/Free Full Text].
-
Fuster JM
(1997)
Network memory.
Trends Neurosci
20:451-459[Web of Science][Medline].
-
Fuster JM,
Jervey JP
(1981)
Inferotemporal neurones distinguish and retain behaviorally relevant features of visual stimuli.
Science
212:952-955[Abstract/Free Full Text].
-
Gevins A,
Smith ME,
Mcevoy L,
Yu D
(1997)
High-resolution EEG mapping of cortical activation related to working memory: effects of task difficulty, type of processing, and practice.
Cereb Cortex
7:374-385[Abstract/Free Full Text].
-
Goldman-Rakic PS
(1995)
Cellular basis of working memory.
Neuron
14:477-485[Web of Science][Medline].
-
Gray CM,
DiPrisco GV
(1997)
Stimulus-dependent neuronal oscillations and local synchronization in striate cortex of the alert cat.
J Neurosci
17:3239-3253[Abstract/Free Full Text].
-
Gray CM,
König P,
Engel AK,
Singer W
(1989)
Oscillatory responses in cat visual cortex exhibit inter-columnar synchronization which reflects global stimulus properties.
Nature
338:334-337[Medline].
-
Grossmann A,
Kronland-Martinet R,
Morlet J
(1989)
Reading and understanding continuous wavelets transforms.
In: Wavelets, time-frequency methods and phase space (Combes JM,
Gossmann A,
Tchamitchian P,
eds), pp 2-20. Berlin: Springer.
-
Jürgens E,
Rösler F,
Hennighausen E,
Heil M
(1995)
Stimulus-induced gamma oscillations: harmonics of alpha activity?
NeuroReport
6:813-816[Web of Science][Medline].
-
Klimesch W
(1996)
Memory processes, brain oscillations and EEG synchronization.
Int J Psychophysiol
24:61-100[Web of Science][Medline].
-
Klimesch W,
Doppelmayr M,
Schimke H,
Ripper B
(1997)
Theta synchronization and alpha desynchronization in a memory task.
Psychophysiology
34:169-176[Web of Science][Medline].
-
Kreiter AK,
Singer W
(1992)
Oscillatory neuronal responses in the visual cortex of the awake macaque monkey.
Eur J Neurosci
4:369-375[Web of Science][Medline].
-
Kreiter AK,
Singer W
(1996)
Stimulus-dependent synchronization of neuronal responses in the visual cortex of the awake macaque monkey.
J Neurosci
16:2381-2396[Abstract/Free Full Text].
-
Kristeva-Feige R,
Feige B,
Makeig S,
Ross B,
Elbert T
(1993)
Oscillatory brain activity during a motor task.
NeuroReport
4:1291-1294[Web of Science][Medline].
-
Kronland-Martinet R,
Morlet J,
Grossmann A
(1987)
Analysis of sound patterns through wavelet transforms.
Int J Patt Recogn Art Intell
1:273-302.
-
Lutzenberger W,
Pulvermüller F,
Elbert T,
Birbaumer N
(1995)
Visual stimulation alters local 40 Hz responses in humans: an EEG study.
Neurosci Lett
183:39-42[Web of Science][Medline].
-
Makeig S
(1993)
Auditory event-related dynamics of the EEG spectrum and effects of exposure to tones.
Electroenceph Clin Neurophysiol
86:283-293[Web of Science][Medline].
-
Miller EK,
Li L,
Desimone R
(1993)
Activity of neurons in anterior inferior temporal cortex during a short-term memory task.
J Neurosci
13:1460-1478[Abstract].
-
Milner PM
(1974)
A model for visual shape recognition.
Psychol Rev
81:521-535[Web of Science][Medline].
-
Miyashita Y,
Chang HS
(1988)
Neuronal correlate of pictorial short-term memory in the primate temporal cortex.
Nature
331:68-70[Medline].
-
Müller MM,
Bosch J,
Elbert T,
Kreiter AK,
Sosa MV,
Sosa PV,
Rockstroh B
(1996)
Visually induced gamma-based responses in human electroencephalographic activity: a link to animal studies.
Exp Brain Res
112:96-102[Web of Science][Medline].
-
Müller MM,
Junghofer M,
Elbert T,
Rochstroh B
(1997)
Visually induced gamma-band responses to coherent and incoherent motion: a replication study.
NeuroReport
8:2575-2579[Web of Science][Medline].
-
Nashmi R,
Mendonça AJ,
Mackay WA
(1994)
EEG rhythms of the sensorimotor region during hand movements.
Electroenceph Clin Neurophysiol
91:456-467[Web of Science][Medline].
-
Pfurtscheller G,
Neuper C
(1992)
Simultaneous EEG 10 Hz desynchronization and 40 Hz synchronization during finger movements.
NeuroReport
3:1057-1060[Web of Science][Medline].
-
Pfurtscheller G,
Flotzinger D,
Neuper C
(1994)
Differentiation between finger, toe and tongue movement in man based on 40 Hz EEG.
Electroenceph Clin Neurophysiol
90:456-460[Web of Science][Medline].
-
Singer W,
Gray CM
(1995)
Visual feature integration and the temporal correlation hypothesis.
Annu Rev Neurosci
18:555-586[Web of Science][Medline].
-
Sinkkonen J,
Tiitinen H,
Näätänen R
(1995)
Gabor filters: an informative way for analyzing event-related brain activity.
J Neurosci Methods
56:99-104[Web of Science][Medline].
-
Sugita Y
(1994)
Electrophysiological correlates of human visual recognition memory.
Neurosci Lett
178:151-154[Medline].
-
Swartz BE,
Halgren E,
Fuster JM,
Simpkins F,
Gee M,
Mandelkern M
(1995)
Cortical metabolic activation in humans during a visual memory task.
Cereb Cortex
5:205-214[Abstract/Free Full Text].
-
Tallon-Baudry C,
Bertrand O,
Delpuech C,
Pernier J
(1996)
Stimulus specificity of phase-locked and non-phase-locked 40 Hz visual responses in human.
J Neurosci
16:4240-4249[Abstract/Free Full Text].
-
Tallon-Baudry C,
Bertrand O,
Delpuech C,
Pernier J
(1997)
Oscillatory -band (30-70 Hz) activity induced by a visual search task in humans.
J Neurosci
17:722-734[Abstract/Free Full Text].
-
Tecce JJ,
Cattanach L
(1987)
Contingent negative variation (CNV).
In: Electroencephalography, Ed 2 (Niedermeyer E,
Lopes da Silva F,
eds), pp 657-679. Baltimore: Urban & Schwarzenberg.
-
Ungerleider LG
(1995)
Functional brain imaging studies of cortical mechanisms for memory.
Science
270:769-775[Abstract/Free Full Text].
-
von der Malsburg C,
Schneider W
(1986)
A neural cocktail-party processor.
Biol Cybern
54:29-40[Web of Science][Medline].
Copyright © 1998 Society for Neuroscience 0270-6474/98/18114244-11$05.00/0
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February 1, 2008;
18(2):
386 - 396.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
C. Haenschel, R. A. Bittner, F. Haertling, A. Rotarska-Jagiela, K. Maurer, W. Singer, and D. E. J. Linden
Contribution of Impaired Early-Stage Visual Processing to Working Memory Dysfunction in Adolescents With Schizophrenia: A Study With Event-Related Potentials and Functional Magnetic Resonance Imaging
Arch Gen Psychiatry,
November 1, 2007;
64(11):
1229 - 1240.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
P. A. McCoy and L. L. McMahon
Muscarinic Receptor Dependent Long-Term Depression in Rat Visual Cortex Is PKC Independent but Requires ERK1/2 Activation and Protein Synthesis
J Neurophysiol,
October 1, 2007;
98(4):
1862 - 1870.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. Kaiser, T. Lennert, and W. Lutzenberger
Dynamics of Oscillatory Activity during Auditory Decision Making
Cereb Cortex,
October 1, 2007;
17(10):
2258 - 2267.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
W. P. Medendorp, G. F.I. Kramer, O. Jensen, R. Oostenveld, J.-M. Schoffelen, and P. Fries
Oscillatory Activity in Human Parietal and Occipital Cortex Shows Hemispheric Lateralization and Memory Effects in a Delayed Double-Step Saccade Task
Cereb Cortex,
October 1, 2007;
17(10):
2364 - 2374.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. Mishra, A. Martinez, T. J. Sejnowski, and S. A. Hillyard
Early Cross-Modal Interactions in Auditory and Visual Cortex Underlie a Sound-Induced Visual Illusion
J. Neurosci.,
April 11, 2007;
27(15):
4120 - 4131.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
D. Jokisch and O. Jensen
Modulation of Gamma and Alpha Activity during a Working Memory Task Engaging the Dorsal or Ventral Stream
J. Neurosci.,
March 21, 2007;
27(12):
3244 - 3251.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. Siegel, T. H. Donner, R. Oostenveld, P. Fries, and A. K. Engel
High-Frequency Activity in Human Visual Cortex Is Modulated by Visual Motion Strength
Cereb Cortex,
March 1, 2007;
17(3):
732 - 741.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
S. Yuval-Greenberg and L. Y. Deouell
What You See Is Not (Always) What You Hear: Induced Gamma Band Responses Reflect Cross-Modal Interactions in Familiar Object Recognition
J. Neurosci.,
January 31, 2007;
27(5):
1090 - 1096.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
P. J. Uhlhaas, D. E. J. Linden, W. Singer, C. Haenschel, M. Lindner, K. Maurer, and E. Rodriguez
Dysfunctional Long-Range Coordination of Neural Activity during Gestalt Perception in Schizophrenia
J. Neurosci.,
August 2, 2006;
26(31):
8168 - 8175.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. Pavlova, N. Birbaumer, and A. Sokolov
Attentional Modulation of Cortical Neuromagnetic Gamma Response to Biological Movement
Cereb Cortex,
March 1, 2006;
16(3):
321 - 327.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. J. Kahana
The Cognitive Correlates of Human Brain Oscillations
J. Neurosci.,
February 8, 2006;
26(6):
1669 - 1672.
[Full Text]
[PDF]
|
|
|
|
|
|
|
N. H. Woo and B. Lu
Regulation of Cortical Interneurons by Neurotrophins: From Development to Cognitive Disorders
Neuroscientist,
February 1, 2006;
12(1):
43 - 56.
[Abstract]
[PDF]
|
|
|
|
|
|
|
C. van der Togt, S. Kalitzin, H. Spekreijse, V. A.F. Lamme, and H. Super
Synchrony Dynamics in Monkey V1 Predict Success in Visual Detection
Cereb Cortex,
January 1, 2006;
16(1):
136 - 148.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. Rose and C. Buchel
Neural Coupling Binds Visual Tokens to Moving Stimuli
J. Neurosci.,
November 2, 2005;
25(44):
10101 - 10104.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. Kaufman, G. Csibra, and M. H. Johnson
Oscillatory activity in the infant brain reflects object maintenance
PNAS,
October 18, 2005;
102(42):
15271 - 15274.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
C. S Carter
Applying New Approaches From Cognitive Neuroscience to Enhance Drug Development for the Treatment of Impaired Cognition in Schizophrenia
Schizophr Bull,
October 1, 2005;
31(4):
810 - 815.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. Kaiser, I. Hertrich, H. Ackermann, K. Mathiak, and W. Lutzenberger
Hearing Lips: Gamma-band Activity During Audiovisual Speech Perception
Cereb Cortex,
May 1, 2005;
15(5):
646 - 653.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
C. Tallon-Baudry, O. Bertrand, M.-A. Henaff, J. Isnard, and C. Fischer
Attention Modulates Gamma-band Oscillations Differently in the Human Lateral Occipital Cortex and Fusiform Gyrus
Cereb Cortex,
May 1, 2005;
15(5):
654 - 662.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. M. Palva, S. Palva, and K. Kaila
Phase Synchrony among Neuronal Oscillations in the Human Cortex
J. Neurosci.,
April 13, 2005;
25(15):
3962 - 3972.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
S. D. Slotnick
Visual Memory and Visual Perception Recruit Common Neural Substrates
Behav Cogn Neurosci Rev,
December 1, 2004;
3(4):
207 - 221.
[Abstract]
[PDF]
|
|
|
|
|
|
|
R. Rodriguez, U. Kallenbach, W. Singer, and M. H. J. Munk
Short- and Long-Term Effects of Cholinergic Modulation on Gamma Oscillations and Response Synchronization in the Visual Cortex
J. Neurosci.,
November 17, 2004;
24(46):
10369 - 10378.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
A. Lutz, L. L. Greischar, N. B. Rawlings, M. Ricard, and R. J. Davidson
Long-term meditators self-induce high-amplitude gamma synchrony during mental practice
PNAS,
November 16, 2004;
101(46):
16369 - 16373.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. K. Belmonte, G. Allen, A. Beckel-Mitchener, L. M. Boulanger, R. A. Carper, and S. J. Webb
Autism and Abnormal Development of Brain Connectivity
J. Neurosci.,
October 20, 2004;
24(42):
9228 - 9231.
[Full Text]
[PDF]
|
|
|
|
|
|
|
C. Tallon-Baudry, S. Mandon, W. A. Freiwald, and A. K. Kreiter
Oscillatory Synchrony in the Monkey Temporal Lobe Correlates with Performance in a Visual Short-term Memory Task
Cereb Cortex,
July 1, 2004;
14(7):
713 - 720.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. Pavlova, W. Lutzenberger, A. Sokolov, and N. Birbaumer
Dissociable Cortical Processing of Recognizable and Non-recognizable Biological Movement: Analysing Gamma MEG Activity
Cereb Cortex,
February 1, 2004;
14(2):
181 - 188.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. Kaiser and W. Lutzenberger
Induced Gamma-Band Activity and Human Brain Function
Neuroscientist,
December 1, 2003;
9(6):
475 - 484.
[Abstract]
[PDF]
|
|
|
|
|
|
|
M. W. Howard, D. S. Rizzuto, J. B. Caplan, J. R. Madsen, J. Lisman, R. Aschenbrenner-Scheibe, A. Schulze-Bonhage, and M. J. Kahana
Gamma Oscillations Correlate with Working Memory Load in Humans
Cereb Cortex,
December 1, 2003;
13(12):
1369 - 1374.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
P. B. Sederberg, M. J. Kahana, M. W. Howard, E. J. Donner, and J. R. Madsen
Theta and Gamma Oscillations during Encoding Predict Subsequent Recall
J. Neurosci.,
November 26, 2003;
23(34):
10809 - 10814.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
F. Wendling, F. Bartolomei, J. J. Bellanger, J. Bourien, and P. Chauvel
Epileptic fast intracerebral EEG activity: evidence for spatial decorrelation at seizure onset
Brain,
June 1, 2003;
126(6):
1449 - 1459.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
N. Cotillon-Williams and J.-M. Edeline
Evoked Oscillations in the Thalamo-Cortical Auditory System Are Present in Anesthetized but not in Unanesthetized Rats
J Neurophysiol,
April 1, 2003;
89(4):
1968 - 1984.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J O Willoughby, S P Fitzgibbon, K J Pope, L Mackenzie, A V Medvedev, C R Clark, M P Davey, and R A Wilcox
Persistent abnormality detected in the non-ictal electroencephalogram in primary generalised epilepsy
J. Neurol. Neurosurg. Psychiatry,
January 1, 2003;
74(1):
51 - 55.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
A. Bibbig, R. D. Traub, and M. A. Whittington
Long-Range Synchronization of gamma and beta Oscillations and the Plasticity of Excitatory and Inhibitory Synapses: A Network Model
J Neurophysiol,
October 1, 2002;
88(4):
1634 - 1654.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
W. Lutzenberger, B. Ripper, L. Busse, N. Birbaumer, and J. Kaiser
Dynamics of Gamma-Band Activity during an Audiospatial Working Memory Task in Humans
J. Neurosci.,
July 1, 2002;
22(13):
5630 - 5638.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. R. Wolpaw
Memory in neuroscience: rhetoric versus reality.
Behav Cogn Neurosci Rev,
June 1, 2002;
1(2):
130 - 163.
[Abstract]
[PDF]
|
|
|
|
|
|
|
J. Kaiser, W. Lutzenberger, H. Ackermann, and N. Birbaumer
Dynamics of Gamma-band Activity Induced by Auditory Pattern Changes in Humans
Cereb Cortex,
February 1, 2002;
12(2):
212 - 221.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
A. Bibbig, H. J. Faulkner, M. A. Whittington, and R. D. Traub
Self-Organized Synaptic Plasticity Contributes to the Shaping of gamma and beta Oscillations In Vitro
J. Neurosci.,
November 15, 2001;
21(22):
9053 - 9067.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. Bhattacharya, H. Petsche, and E. Pereda
Long-Range Synchrony in the {gamma} Band: Role in Music Perception
J. Neurosci.,
August 15, 2001;
21(16):
6329 - 6337.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. B. Caplan, J. R. Madsen, S. Raghavachari, and M. J. Kahana
Distinct Patterns of Brain Oscillations Underlie Two Basic Parameters of Human Maze Learning
J Neurophysiol,
July 1, 2001;
86(1):
368 - 380.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. M. Müller, C. S. Herrmann, A. D. Friederici, G. Csibra, and M. H. Johnson
Object Processing in the Infant Brain
Science,
April 13, 2001;
292(5515):
163a - 163.
[Full Text]
|
|
|
|
|
|
|
M. A. Whittington, H. C. Doheny, R. D. Traub, F. E. N. LeBeau, and E. H. Buhl
Differential Expression of Synaptic and Nonsynaptic Mechanisms Underlying Stimulus-Induced Gamma Oscillations In Vitro
J. Neurosci.,
March 1, 2001;
21(5):
1727 - 1738.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
G. Csibra, G. Davis, M. W. Spratling, and M. H. Johnson
Gamma Oscillations and Object Processing in the Infant Brain
Science,
November 24, 2000;
290(5496):
1582 - 1585.
[Abstract]
[Full Text]
|
|
|
|
|
|
|
N. Kopell, G. B. Ermentrout, M. A. Whittington, and R. D. Traub
Gamma rhythms and beta rhythms have different synchronization properties
PNAS,
February 15, 2000;
97(4):
1867 - 1872.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
S. Herculano-Houzel
Yves Delage: Neuronal Assemblies, Synchronous Oscillations, and Hebbian Learning in 1919
Neuroscientist,
September 1, 1999;
5(5):
341 - 345.
[Abstract]
[PDF]
|
|
|
|
|
|
|
A. Keil, M. M. Muller, W. J. Ray, T. Gruber, and T. Elbert
Human Gamma Band Activity and Perception of a Gestalt
J. Neurosci.,
August 15, 1999;
19(16):
7152 - 7161.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
O. Jensen and J. E. Lisman
An Oscillatory Short-Term Memory Buffer Model Can Account for Data on the Sternberg Task
J. Neurosci.,
December 15, 1998;
18(24):
10688 - 10699.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
Top
Abstract
Introduction
