Neuroscience Research 39 (2001) 123 – 131
www.elsevier.com/locate/neures

Unmasking of silent ‘task-related’ neuronal activity in the monkey
prefrontal cortex by a GABAA antagonist
Toshiyuki Sawaguchi *Department of Beha6ioral and Brain Sciences, Primate Research Institute, Kyoto Uni6ersity, Inuyama, Aichi 484 -8506, Japan
Received 01 May 2000; accepted 28 September 2000

Abstract
To examine the role of GABA on prefrontal neuronal activity in the control of behavior, a GABAA receptor antagonist,bicuculline methiodite (BMI), was iontophoretically applied to prefrontal neurons while monkeys performed a visual reaction time
task. Iontophoretic application of BMI uncovered ‘task-related’ activity of silent neurons (n= 40), which did not show any activity
during performance of the task. The distribution, by type, of these silent ‘task-related’ neurons differed from that of standard (i.e.
active) task-related neurons (N= 95), and a particular type of silent ‘task-related’ neuron was found most frequently. Thesefindings suggest that GABA continuously and preferentially suppresses neuronal activity via GABAA receptors to limit the
population of prefrontal neurons related to behavior, thereby organizing neuronal activities for behavior mediated by theprefrontal cortex. © 2001 Elsevier Science Ireland Ltd and Japan Neuroscience Society. All rights reserved.
Keywords: Prefrontal cortex; GABA; GABAA receptor; Iontophoresis; Visual reaction time task; Monkey

1. Introduction
The prefrontal cortex (PFC) of primates is responsible for various behaviors that require cognitive functions such as working memory and the association of
sensory information with motor performance (for reviews, see Goldman-Rakic, 1995; Fuster, 1997).
Whereas neuronal activity in the PFC has been studied
extensively in a variety of behavioral tasks (e.g. delayed-response, delayed-alternation, visual discrimination go/no-go, and visual reaction time tasks; seeFuster, 1997), its neurochemical basis remains to berevealed. In the PFC of primates, there are numerous
neurons that contain an inhibitory neurotransmitter,gamma-aminobutyric acid (GABA) (Schwartz et al.,1988), and the primate cerebral cortex contains a highlevel of GABAA receptors (Jones, 1993). Previous studies in monkeys have demonstrated that local injectionof a GABAA antagonist, bicuculline methiodite (BMI),* Corresponding author. Present address: Laboratory of Neurobiology, Hokkaido University Graduate School of Medicine, N15 W7,
Kita-ku, Sapporo 060-8638, Japan. Tel.: + 81-11-706-6899; fax: +81-11-706-6899.
E-mail address: toshi-sw@med.hokudai.ac.jp (T. Sawaguchi).

into the dorsolateral PFC induces abnormal bursting
activity at the injection site and induces deficits inbehavioral tasks, such as a delayed-response task
(Sawaguchi et al., 1988, 1989a) and a go/no-go visualdiscrimination task (Oishi and Kubota, 1990). Therefore, it is likely that GABA might continuously inhibit
prefrontal neuronal activity via GABAA receptors toorganize it for controlling various behaviors mediatedby the PFC.
To address this hypothesis, we examined the effects
of BMI on neuronal activity in the PFC in monkeys
that performed a visual reaction time task, using iontophoretic techniques. A subset of neurons in the monkey PFC are involved in this task and show different
types of activities related to its different phases(Sawaguchi, 1987a; Sawaguchi et al., 1989b). We report
here that iontophoretic application of BMI uncoveredthe ‘task-related’ activity of neurons that are normallysilent during task performance and that a particular
type of silent ‘task-related’ neuron was found mostfrequently. Our findings suggest that the continuousand preferential suppression by GABA via GABAA
receptors plays a role in organizing prefrontal neuronal
activity during behaviors mediated by the PFC.

0168-0102/01/$ - see front matter © 2001 Elsevier Science Ireland Ltd and Japan Neuroscience Society. All rights reserved.
PII: S 0 1 6 8 - 0 1 0 2 ( 0 0 ) 0 0 2 0 4 - 2

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T. Sawaguchi / Neuroscience Research 39 (2001) 123–131

2. Materials and methods

2.2. Recording, iontophoresis and data analysis

2.1. Subjects and beha6ioral task

Multibarreled glass micropipettes were used for extracellular recording of neuronal activity and iontophoretic application of drugs. The central barrel ofthe micropipette contained a carbon-fiber filament (7mm in diameter; T300, Toray, Tokyo, Japan), and wasfilled with 0.1 M NaCl and was used for the recordingof neuronal activity. Surrounding barrels were used for
iontophoretic application of the following drugs: BMI(0.01 M, dissolved in saline, pH= 5–6; Sigma, St.
Louis, MO); GABA (0.01 M, dissolved in saline, pH=
5–6; Sigma); and 0.1 M NaCl (to balance the current).
BMI was used in all penetrations of the micropipette(n= 17), but GABA was used in only four penetrations:since exactly the same results were obtained in either
case, we pooled the data obtained with thesepenetrations.
While the monkeys performed the task, the micropipette was inserted into the dorsolateral PFC contralateral to the hand used for task performance, andthe activities of single neurons were recorded. The
micropipette was inserted using a pulse-motor-driven
micromanipulator (MO-90, Narishige, Tokyo, Japan).
To prevent leakage of drugs, a backing current (2–5nA) was always applied to each barrel before andbetween drug applications. Neuronal activity was converted from analog to digital by a window-discriminator (DIS-1, BAK Electronics, Germantown, MD, USA)and the discharge rate during each phase of the task
was compared with the background discharge rate during the waiting phase on a trial-by-trial basis. When the
discharge rate of a neuron during at least one taskphase differed significantly from the background discharge rate (Mann-Whitney’s U-test, PB 0.05), theneuron was considered to be task-related. When taskrelated neuronal activity was encountered, it wasrecorded during more than eight successive trials.
Drugs were then applied with currents of 5–30 nA
during more than eight successive trials to examine theinfluence of the drugs on task-related neuronal activity.
Drug applications were separated by more than 2–3
min to allow for the recovery of neuronal activity.
When the discharge rate of a neuron changed significantly during the application of a drug (Mann-Whitney’s U-test, PB 0.05), the neuron was judged to be
responsive to the drug. The amplitude of spikes of aneuron was similar between the predrug control period
and during BMI application (see Fig. 3); for 10 typicalneurons whose activities were increased by the iontophoretic (10 nA) application of BMI, the average
amplitude did not significantly differ between these
periods; 104.2926.1 (SD) mV for the control period
and 106.59 25.1 mV for the period with BMI application (paired t-test, P\ 0.05, NS). As mentioned below,
during some penetrations, the tip of the micropipette

The details of the various techniques, the behavioral
task, and the instrumentation have been described previously (Sawaguchi, 1987a,b; Matsumura et al., 1992).
The experiment was carried out in accordance with the
Guide for Care and Use of Laboratory Animals (National Institutes of Health) and the Guide for Care andUse of Laboratory Primates (Primate Research Institute, Kyoto University). Two male rhesus monkeys(Macaca mulatta, weighing 3.5 – 4.5 kg) were trained toperform a visual reaction time task (Fig. 1A). The
monkeys faced a panel on which a (two-color lightemitting-diode (LED) was located at eye level, and alever was located at waist level. The task was initiatedby the monkey pressing the lever. After a fixed periodof 3.0 s, the LED was turned on to green, indicating thewarning phase. After a variable period of time (1.5–3.5s), the LED turned from green to red, indicating the gophase. The monkey released the lever within 600 ms
after the onset of the go signal to receive a drop of juice
as a reward. The reward phase lasted for 1.5 s after
delivery of the reward. Eye movements that were consistently related to the task events were not observed by
EOG recording. The monkeys performed 1500 trials
in each recording session of 2 – 3 h. The reaction timefrom the go signal to the onset of lever release was 350–400 ms with a SD of  40 – 60 ms for one
subject and 320– 380 ms with a SD of  50 – 70 ms
for the other subject.

Fig. 1. (A) Time sequence of the visual reaction time task. The task
was initiated by the monkey pressing a lever. After a 3.0 s waiting
phase, a light-emitting diode (LED) was turned on (green) to indicate
the onset of the warning phase. After a variable period of 1.5 – 3.5 s,
the LED turned from green to red, indicating the go signal (gophase), he was rewarded with a drop of juice 0.3 s after lever release(reward phase). (B) Sites of penetration by the electrodes, illustrated
on the surface of the left prefrontal cortex. A total of 17 penetrations
were made in four hemispheres of two monkeys. The sites on theright hemisphere were homotopically transferred to the left side. PS,
principal sulcus. AS, arcuate sulcus.

T. Sawaguchi / Neuroscience Research 39 (2001) 123–131

125

was positioned at various depths in the PFC, at which
no neuronal activity was observed for more than 3 min,and BMI was applied with currents of 5 – 30 nA to
uncover the activity of certain ‘silent’ neurons thatshowed no activity for more than 3 min during performance of the task. In addition, the activity of silent
neurons was sometimes uncovered by the application of
BMI at sites where the activity of task-related neurons
was recorded.

2.3. Histology
At the end of the recording sessions, the monkeys
were deeply anesthetized with sodium pentobarbital
and perfused with saline followed by 10% formalin. The
points of penetration by electrodes were detected on thesurface of the formalin-fixed brains. Fig. 1B shows the
17 sites of penetration on the surface of the left prefrontal cortex; the sites on the right hemisphere were homotopically transferred to the left side. The points were
scattered throughout the periprincipal sulcal area andthe immediately adjacent cortex in the PFC, which has
cytoarchitectural features of Walker’s area 46 (Walker,1940).

Fig. 2. Types of neuronal activity related to the task. Averaged
histograms, aligned at the onset of the warning signal (WS), go signal(GS) and reward delivery (RD), are shown (sampling rate, 20 ms).
From top to bottom: a neuron that showed a change in activity
during the warning phase (W-type) (average of 23 trials); a neuron
that showed a change in activity during the go phase (GO-type)(average of 17 trials); a neuron that showed changes in activity during
the warning and go phases (WG-type) (average of 25 trials); and aneuron that showed a change in activity during the reward phase(RE-type) (average of 19 trials).

3.2. Unmasking of silent ‘task-related’ neuronal acti6ity
by BMI

3. Results

3.1. Types of task-related neurons and the effects of
BMI on their acti6ities
We recorded the activities of a total of 95 task-related neurons, and these neurons showed changes in
activity during the warning phase (n =25), the go phase(n= 19), both the warning and the go phases (n =41),
and the reward phase (n = 10) of the visual reaction
time task. These neurons were classified, as in previous
studies (Sawaguchi, 1987a,b; Sawaguchi et al., 1989b),
as W (warning)-, GO (go)-, WG (warning and go)-, andRE (reward)- type neurons, respectively. Fig. 2 shows
examples of each type of neuron. These task-related
neurons were spontaneously active, and the averagefrequency of their background discharge during thewaiting phase was 8.19 6.9 (SD) spikes/s (n = 95). Iontophoretic application of BMI increased the activity of
all of the task-related neurons. Fig. 3 shows an exampleof the effects of BMI and GABA on the activity of aW-type neuron, in which averaged histograms for theactivity before (control) and during the application of
GABA (with 20-nA current) and BMI (with 10-nA
current) are shown separately. As shown in Fig. 3B, theiontophoretic application of GABA reduced the activity, whereas BMI increased both the task-related andbackground activities. The properties of task-related
activities themselves did not change during BMI application except that overall discharge rates were increased, as shown in the example in Fig. 3B.

While examining the effects of the drugs on task-related neuronal activity, we recognized that normallysilent neurons became active during the application of
BMI, and that these neurons were invariably task-related. Therefore, during eight penetrations of the micropipette, its tip was moved away from the site wheretask-related neurons were recorded to positions at various depths in the PFC at which no neuronal activity
was observed for 3 min or more, and BMI was appliedwith currents of 5–30 nA. At a total of 19 such sites,
the application of BMI uncovered the activities of silent
neurons, which were related to the task. Fig. 3C shows
an example of this result. The neuron in Fig. 3C, whose
activity was recorded at a site  180 mm deeper thanthat at which the activity shown in Fig. 3B wasrecorded (see Fig. 3A-b), showed no activity duringperformance of the task for more than 3 min. However,it began to show activity during the application of
BMI, and this activity was related to the warning phaseof the task (i.e. W-type). Such originally silent but
subsequently task-related neurons, i.e. silent ‘task-related’ neurons, during the application of BMI wereusually found near the standard task-related neurons
during the same penetration of the electrode, as seen in
Fig. 3.
Silent ‘task-related’ neurons were also found at sites
where the activities of standard task-related neurons
were recorded. Fig. 4 shows an example of this situation. The activity of one neuron was observed before

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T. Sawaguchi / Neuroscience Research 39 (2001) 123–131

the application of BMI (Fig. 4A-a). Another neuron,
which was silent for more than 3 min prior to theapplication of BMI, showed activity during the application of BMI (Fig. 4A-b, arrow). The initial active
neuron showed a tonic change in activity during thewarning phase (i.e. W-type, Fig. 4B-a), and the previously silent neuron also showed a tonic change inactivity during the warning phase (Fig. 4B-b).
The effect of BMI on silent task-related neurons
appeared to be dose-dependent as far as we examined
this point for six neurons. A typical example is shown

Fig. 4. An example of the effects of the application of BMI on silent
neurons that were located at the same sites as where task-related
neurons were recorded. (A) Superimposition of traces of spikes ofneuron(s) prior to (a, Control) and during the application of BMI
with 20 nA (b, BMI20). Note that spikes, which did not appear in thecontrol, appeared during the application of BMI (arrow in b). (B)Averaged histograms of the activities of neurons whose spikes areshown in A. (a) Activity of a neuron that showed activity both in thecontrol state and during the application of BMI (arrowhead in A-b,
18 trials with the application of BMI). (b) Activity of a neuron thatwas silent in the control state and began to show activity during theapplication of BMI (arrow in A-b, 18 trials with the application of
BMI). Conventions are as in Fig. 2.

Fig. 3. Effects of iontophoretically applied GABA and bicuculline
methiodite (BMI) on neuronal activity during performance of thetask. (A) Recorded sites. (a) Penetration point of the electrode. The
oblique line shows the coronal plane at which the section shown in b
was cut. (b) Recording sites illustrated on the coronal section. The
oblique line shows the track of the electrode. The filled circle andfilled triangle show sites at which the neuronal activities shown in B
and C were recorded, respectively. The asterisk indicates the deepest
point of the electrode, marked by carbon deposits of the electrode(see Sawaguchi et al., 1989a,b). The space between the dotted lines
indicates layer IV. (B) The effects of GABA and BMI on the activityof a task-related neuron. CON, neuronal activity without any drugs(average of 10 trials). GABA20, activity during the application of
GABA with 20 nA (8 trials). BMI10, activity during the applicationof BMI with 10 nA (10 trials). (C) An example of the effects of theapplication of BMI on a silent ‘task-related’ neuron. The neuron did
not show any activity prior to the application of BMI (CON, 25
trials), but showed activity related to the task during the applicationof BMI with 10 nA (BMI10, 18 trials). Conventions are as in Fig. 2.

in Fig. 5, in which raster displays and averaged histograms of the activity of a silent ‘task-related’ neuron
are shown for trials during the predrug control period
lasting for  4 min and during BMI application withcurrent intensities of 10 and 20 nA. As shown, thisneuron showed no activity during the predrug control
period, whereas it began to show a warning phase-related activity during the application of BMI with a 10nA current. After the termination of BMI application,
the activity gradually disappeared. When BMI was thenapplied with a larger current of 20 nA, the activity
appeared again, and the discharge rate was significantly
larger than that during BMI application with a 10 nA
current (mean 9 SD spikes/s; 3.9 92.4 for 10 nA current vs. 7.6 93.9 for 20 nA current, PB 0.01, U-test).
Similar results were obtained for all of the six neurons
examined with different intensities for the iontophoretic
application of BMI.

3.3. Distribution, by type, of silent ‘task-related’
neurons
A total of 40 silent ‘task-related’ neurons were found.
Of these, 19 were found at sites different from, and 21
were found at the same sites as active task-related

T. Sawaguchi / Neuroscience Research 39 (2001) 123–131

127

Fig. 5. Dose-dependent effect of the application of BMI on a silent neuron. Raster display (A) and averaged histograms (B) are shown for theactivity of a silent neuron that showed no activity during the predrug control period (CON) and showed warning phase-related activity during
the application of BMI with current intensities of 10 and 20 nA. Conventions are as in Fig. 2.

neurons (of these, 34 neurons were found during penetrations without GABA). Similar to the task-related
neurons, the silent ‘task-related’ neurons consisted of
W-, GO-, WG-, and RE-type neurons. Table 1 compares the distribution, by type, of standard task-related
neurons and silent ‘task-related’ neurons. The distribution of silent ‘task-related’ neurons differed significantly
from that of standard task-related neurons (x2 =14.42,
df= 3, PB0.001), and the most common silent ‘taskrelated’ neurons were W-type. The proportion of silent
W-type neurons among the silent ‘task-related’ neurons(21/40, 53%) differed significantly from that among thestandard task-related neurons (25/95, 26%) (x2 =7.47,
df= 1, PB 0.01). This was the case for silent ‘task-related’ neurons that were found where active task-related neurons were found (11/21, 52% vs. 25/95, 26%;x2 = 4.31, df= 1, PB 0.05).

3.4. Similarities of silent ‘task-related’ neurons to
nearby task-related neurons
The activities of silent ‘task-related’ neurons tended
to be similar to those of nearby task-related neurons, asseen in the examples in Fig. 3 and Fig. 4. To confirm
this finding, we examined the distribution of task-related neurons at various distances from sites at whichsilent ‘task-related’ neurons were found during eachpenetration of the electrode. Table 2 shows the distribu-

tion of each type of task-related neuron, found at
various distances from the silent ‘task-related’ neurons.
In Table 2, the distances have been arbitrarily divided
into four classes: 0 mm (the same site), 1–200 mm,
201–400 mm, and over 400 mm ( \ 400 mm). Each of thetask-related neurons contributed to two or more data
points when two or more silent ‘task-related’ neurons
were found during the same penetration. As shown in
Table 2, the task-related neurons that were found near
the silent ‘task-related’ neurons more frequently showed
Table 1
Distribution, by type, of active task-related neurons and silent ‘taskrelated’ neurons
Type

Task related
neurons

Silent ‘task-related’ neurons

Same sitesa

Nearby sitesb

Total

W-typeGO-type
WG-typeRE-type

25
19
41
10

11*
2
5
3

10*
1
3
5

21**
3
8
8

Total

95

21

19

40

a
Number of silent ‘task-related’ neurons that were found at thesame sites as task-related neurons.
b
Number of silent ‘task-related’ neurons that were found at sites
different from task-related neurons. * PB0.05, ** PB0.01: significantly different from the distribution of standard task-related neurons(x2-test, df = 1).

T. Sawaguchi / Neuroscience Research 39 (2001) 123–131

128

Table 2Distribution of task-related neurons at various distances from each type of silent ‘task-related’ neuron during the same penetration of the electrodeTypes of silent neurons

W-type (n =21)

GO-type (n = 3)

WG-type (n =8)

RE-type (n = 8)aDistances (mm)

0a
1–200
201–400
Subtotal
\400
0a
1–200
201–400
Subtotal
\400
0a
1–200
201–400
Subtotal
\400
0a
1–200
201–400
Subtotal\400

Types of active task-related neurons

Total

W

GO

WG

RE

9
15
7
31
22
0
1
1
2
6
1
2
2
5
16
0
1
1
2
11

0
0
2
2
10
2
3
1
6
4
0
0
3
3
5
0
2
0
2
14

2
6
0
8
28
0
1
1
2
11
4
10
6
20
24
0
0
1
1
31

0
1
2
3
4
0
0
0
0
1
0
0
2
2
4
3
5
2
10
1

11
22
11
44
64
2
5
3
10
22
5
12
13
30
49
3
8
4
15
57

Numbers of active neurons that were found at the same sites as silent ‘task-related’ neurons.

the same type of activity as that of the nearby silent‘task-related’ neurons. Most of the task-related neurons
showed the same type of activity as that of the silent‘task-related’ neurons that were recorded at the samesites (18/21, 86%). Two-thirds of the task-related neurons that were recorded at sites less than 400 mm(including 0 mm) from those of silent ‘task-related’
neurons showed the same type of activity as silent
neurons (67/99, 68%). This ratio (67/99, 68%) differedsignificantly from that (51/192, 27%) for task-related
neurons that were more than 400 mm from silent ‘taskrelated’ neurons (x2 =44.11, df= 1, P B0.001).

3.5. Cortical distribution of silent ‘task-related’ neurons
Silent ‘task-related’ neurons were found during all
penetrations of the micropipette, and there was no
obvious clustering or ordering of their cortical distributions in the dorsolateral PFC. However, with regard to
depth distribution, silent ‘task-related’ neurons appeared to be more numerous in the superficial layers(II – IV) than in the deep layers (V and VI). Among
silent ‘task-related’ neurons (n = 32) whose laminar distributions could be estimated with the methods described previously (Sawaguchi and Matsumura, 1985;
Sawaguchi et al., 1986, 1989b), most were found in the
superficial layers (n=24) with a relatively small numberin the deep layers (n =8). This corresponds to the factthat most of the silent ‘task-related’ neurons were Wtype (see above), and W-type neurons are relatively

numerous in the superficial layers, as demonstrated
previously (Sawaguchi et al., 1989b).

4. Discussion
Iontophoretic application of BMI uncovered the‘task-related’ activity of silent neurons in the dorsolateral PFC which did not show any activity during theperformance of a visual reaction time task for over 3
min. It is unlikely that these silent ‘task-related’ neurons are actually involved in, or responsible for, performance of the task, since they did not show any activity
during several trials (i.e. more than  20 trials) of thetask. Although these silent ‘task-related’ neurons might
be considered an artifact due to the leakage of GABA
from the tip of the electrode, this possibility is unlikely
since (1) a backing current was always applied toprevent leakage of the drugs; (2) most of the silent‘task-related’ neurons were found during penetrations
without GABA; and (3) the mean discharge rate of the
background activity of the task-related neurons in thepresent study (8.1 spikes/s) was similar to that examined in a previous study of prefrontal neurons (8.6
spikes/s) which used the same type of electrode andtask but not GABA or BMI (Sawaguchi, 1987a). Similar (7.6–8.1 spikes/s) or slightly lower values (4–6
spikes/s) for the mean discharge rate of the background
activity in prefrontal neurons of monkeys have beenreported in studies using metal electrodes and tasks

T. Sawaguchi / Neuroscience Research 39 (2001) 123–131

different from that in the present study (Quintana et al.,
1988; Funahashi et al., 1989; Koch and Fuster, 1989).
Thus, the present data suggest that the ‘task-related’
activity of some prefrontal neurons is continuously
suppressed by GABA so that they are silent during task
performance for several minutes, although it is unknown how long they are actually silent during task
performance or whether or not they are active in relation to the performance of tasks other than the present
task.
The silent neurons found in the present study should
receive substantial excitatory input during task performance, since, with the reduction of GABA inhibition
with the application of BMI, they showed backgroundas well as task-related activities. Thus, more neurons
are spontaneously active and responsive to the task
events when GABA inhibition is reduced during task
performance. GABA may limit the population of prefrontal neurons that are involved in a behavioral taskover several minutes, thereby organizing the prefrontal
neuronal activity associated with the behavioral task.
This possible role of GABA in the PFC should be
mediated by GABAA receptors, since BMI is a GABAA
antagonist and the primate cerebral cortex contains ahigh level of GABAA receptors (Schwartz et al., 1988).
GABA could also play a similar role in other cerebral
cortical areas for organizing neuronal activities, sinceboth GABA neurons and GABAA receptors are widelydistributed in the cerebral cortex (Hendry et al., 1987;
Schwartz et al., 1988) and local injection of BMI induces abnormal bursting activity in other cortical areas,such as the premotor and motor cortices of monkeys(Matsumura et al., 1991; Sawaguchi et al., 1996). Indeed, local injections of BMI into the motor or premotor cortex induced a disorder of behavior related toeach of these cortices (Matsumura et al., 1991;
Sawaguchi et al., 1996); these findings also support thepresent notion that GABA limits and organizes neuronal activities associated with a behavioral task.
The distribution, by type, of the silent ‘task-related’
neurons differed from that of neurons related to thetask. This was true for both silent ‘task-related’ neurons
found at the same sites as standard task-related neurons
and those found at sites different from where standard
task-related neurons were recorded. Among the silent‘task-related’ neurons, W-type neurons were foundmost frequently. These findings indicate that the suppression of neuronal activity by GABA is selective or
preferential, and that W-type neurons are the mostfrequently suppressed by GABA during performance ofthe visual reaction time task. W-type neurons are relatively numerous in the superficial layers (II – IV) of thedorsolateral PFC (Sawaguchi et al., 1989b) and GABA
neurons in this area are distributed more densely in the
superficial layers (II– IV) than in the deep layers (V andVI) (Jones, 1993). Therefore, it is likely that the inhibi-

129tion of prefrontal neurons by GABA may be stronger
in the superficial layers than in the deep layers, and, asa result, W-type neurons may be the most frequently
suppressed during task performance. Furthermore, theactivities of silent ‘task-related’ neurons were in mostcases similar to those of task-related neurons that were
found nearby or at the same site. Anatomical studies ofthe monkey have demonstrated that neurons that project their axons to the same regions form clusters orcolumns in the PFC (Arikuni et al., 1983; Schwartz andGoldman-Rakic, 1984). Taken together, these findings
suggest that some particular neurons within neuronalclusters, which have similar physiological properties
and project their axons to similar regions, are continuously suppressed by GABA via GABAA receptors andare therefore silent during behavioral tasks over several
minutes. This continuous and preferential suppression
by GABA via GABAA receptors within neuronal clusters may be critical for organizing neuronal activities
for behavior mediated by the PFC.
Thus, the present study is the first to demonstratethat a substantial number of prefrontal neurons are
silent during a behavioral task and are latent related tothe task over several minutes, and appear to be continuously and preferentially suppressed by GABA via
GABAA receptors. Since GABA neurons and GABAA
receptors are widely distributed in the cerebral cortex(Hendry et al., 1987; Schwartz et al., 1988) and columnar/modular organization is common across various
neocortical areas, the present findings and conclusions
may be applicable to neocortical areas other than the
PFC; the continuous and preferential/selective suppression by GABA within neuronal clusters/columns islikely to be a critical/general mechanism for the organization of neocortical neuronal activities that are associated with various behavioral tasks/information
processing. Indeed, GABAergic inhibitory control in
the formation of receptive fields is well known in somatosensory and visual cortices (Dykes et al., 1984;
Sillito, 1984; Crook et al., 1996, 1997, 1998), and, inmotor cortices, GABAergic inhibition plays a role inorganizing movement-related activities (Matsumura etal., 1992).
The PFC is involved in the learning of behaviors, andneural activities of the PFC show flexible changes depending on the behavioral context (Petrides, 1982;
Sasaki and Gemba, 1982; Gemba and Sasaki, 1984;
Kubota and Komatsu, 1985; Watanabe, 1990; Gembaet al., 1995; Petrides, 1997; Asaad et al., 1998). The
present findings may allow us to hypothesize that preferential/selective suppression by GABA underlies thisflexibility of the PFC, in particular, fast and reversible
learning mediated by the PFC. In this regard, a previous finding with in vivo optical imaging demonstrated
that modular activation of neurons by ICMS was accompanied by modular suppressions in the frontal cor-

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T. Sawaguchi / Neuroscience Research 39 (2001) 123–131

tex of monkeys, which suggests that some functional
modules are suppressed when a functional module isactivated (Sawaguchi, 1994). It is likely that different
populations (or ‘modules’) of PFC neurons are deployed in different behavioral contexts during which
many other neurons are silent due to GABAergic inhibition; many functional modules are already present
and some are selected with GABAergic inhibition in agiven context. This hypothesis is worth considering for
further studies of the neuronal mechanism of the flexible control and learning of behaviors, which are mediated by the PFC.

Acknowledgements
The author thanks Dr K. Kubota for his usefulcomments on an early version of this paper. The author
also thanks Dr M. Matsumura for his technical assistance and useful comments.

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