Proc. Natl. Acad. Sci. USA
Vol. 93, pp. 2664–2669, April 1996
Neurobiology

Activity of primate inferotemporal neurons related to a soughttarget in pair-association task
(visual cortexylong-term memoryyvisual imageryypaired associatesydelayed matching to sample)

YUJI NAYA, KUNIYOSHI SAKAI*, AND YASUSHI MIYASHITA
Department of Physiology, The University of Tokyo, School of Medicine, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan

Communicated by Mortimer Mishkin, National Institutes of Health, Bethesda, MD, November 29, 1995 (received for review March 24, 1995)

ABSTRACTVisual long-term memory in primates has
been assessed by using the pair-association (PA) task, inwhich a subject retrieves and chooses the paired associate ofa cue picture. Our previous studies on single neurons in theanterior inferotemporal (AIT) cortex suggested their roles in
representing paired associates in the mind. To test the possibility that the delay activity of AIT neurons is related to aparticular picture as a sought target, we devised the PA withcolor switch (PACS) task. In the PACS task, the necessity formemory retrieval and its initiation time were controlled by acolor switch in the middle of the delay period. A control task,
in which there is no color switch, corresponds to the conventional delayed matching-to-sample (DMS) task where themonkey chooses the same picture as a cue. We found that AITneurons started to respond just after the color switch in the
PACS task, when the cue-optimal picture’s associate waspresented as a cue. In contrast, they showed no responsechange in the DMS task. We confirmed that this effect is notdue to the visual response to colors. Furthermore, when thecue-optimal picture was presented as a cue, these neurons
showed suppression after the color switch in the PACS task.
These results suggest that the activity of AIT neurons mediates
gating mechanisms that preferentially pass information about
a sought target, even when the sought target is retrieved fromlong-term memory.

METHODS
Subjects. Two adult monkeys (Macaca fuscata; 8.0–9.5 kg)
were used. Four head bolts and a cylindrical chamber for
microelectrode recording were attached to the skull underaseptic conditions and general anaesthesia with Nembutal (30mgykg). The chamber was filled with sterile saline containing
gentamycin. The monkey was given antibiotics and allowed
sufficient rest for recovery after surgery. Training for headrestraint was achieved gradually, and the monkey’s operant
behavior was used as evidence of the absence of discomfort.
Recording sessions usually lasted 3 h, during which the animals
consumed 400–500 ml of fruit juice. The care and use of theanimals conformed with the current guidelines of the US
National Institutes of Health and of The Primate ResearchInstitute, Kyoto University, Japan.
Behavioral Tasks. The monkey was first trained in the PA
task. In a trial of the PA task, the monkey was presented 1 of
24 pictures as a cue and was required to retrieve the paired
associate of the cue picture from long-term memory and tochoose it (6). After the monkey learned the PA task, it wastrained in the PA with color switch (PACS) task (Fig. 1 A). As
a control task, the conventional delayed matching-to-sample(DMS) task was used (8). Single-unit recording was carried out
while the monkey was performing the PACS task and the DMS
task.
In the PACS task and the DMS task, 12 pairs of Fourier
descriptors (9) were used as visual stimuli (G1 and C1 to G12
and C12), each pair containing a green picture [Commission
International d’Eclairage (CIE) coordinates, x 5 0.27, y 5 0.58;
luminance, 30.5 cdym2) and a cyan picture (CIE coordinates,
x 5 0.21, y 5 0.31; luminance, 33.7 cdym2) (Fig. 1B). The formsof these stimuli and the pair combinations were the same as
those used in the PA task (6). In the PA task, all of the stimuli
were yellow pictures. The sequence of events in a trial of the
PACS task or the DMS task was as follows (Fig. 1 A). When themonkey started to keep pressing a lever in front of the video
monitor, a gray square (luminance, 19.3 cdym2) was presented
at the center of the screen for 1 s (warning). After the cue
presentation of 1 of 24 pictures for 0.5 s, a square was presented
during the delay period. The square’s color was the same as thecue’s color during the first part of the delay period (delay
period 1) for 2 s in the PACS task or for 5 s in the DMS task.
In the PACS task, the square’s color changed into the color of
the paired associate after delay period 1, signaling the initiation of retrieval, and the second part of the delay period (delay
period 2) for 3 s started. Delay period 2 was not included in the
DMS task. To balance the visual stimulus conditions in the twotasks, a gray square (luminance, 19.3 cdym2) was presented for1 s during the third part of the delay period (delay period 3).
After delay period 3, a choice of two stimuli was shown

The anterior inferotemporal (AIT) cortex has been proposed
to be the memory storehouse in object vision (1–4). Memorystorage mechanisms have been examined with the pairassociation (PA) task for monkeys, and the results havesuggested that an associative mechanism is involved in theestablishment of visual long-term memory (5, 6). Our previous
neurophysiological studies showed that responses of single
AIT neurons were tuned optimally to both paired associates
learned in the PA task (6, 7). Moreover, the activity of AIT
neurons may reflect retrieved information about paired associates, because we found pair-recall neurons in the AIT cortex,
whose delay activity is closely coupled with the pairedassociate retrieved through a cue stimulus (6). The purpose of
the present experiment was to clarify that this pair-recall effectis related to a particular picture retrieved from long-term
memory. We developed a novel task in which the necessity formemory retrieval and its initiation time were controlled by acolor switch, independently of the cue stimulus presentation.
By using this task, we examined whether delay responses of
AIT neurons are affected by a change of target stimuli soughtby the animal. We found significant effects of the color switch
on the picture-selective delay activity such as enhancement andsuppression, suggesting that these parallel mechanisms may be
critically involved in the dynamics of AIT neurons.

Abbreviations: AIT, anterior inferotemporal; PA, pair association;
PACS, PA with color switch; DMS, delayed matching to sample.
*Present address: Department of Radiology, Brigham and Women’s
Hospital and Harvard Medical School, Boston, MA 02115.

The publication costs of this article were defrayed in part by page charge
payment. This article must therefore be hereby marked ‘‘advertisement’’ inaccordance with 18 U.S.C. §1734 solely to indicate this fact.

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Proc. Natl. Acad. Sci. USA 93 (1996)

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FIG. 1. PACS task and DMS task used to assess the delay activity of AIT neurons. (A) Sequence of events in a trial of the PACS task or the
DMS task. Cue stimuli and squares were presented at the center of a video monitor. Choice stimuli were presented randomly in two of four positions
on the video monitor. Warning, gray square (1 s in both tasks); cue, 1 of 24 pictures in B as a cue stimulus (0.5 s); delay period 1, square that has
the same color as the cue picture (2 s in the PACS task; 5 s in the DMS task); delay period 2, square that has the same color as the paired associateof the cue picture (3 s in the PACS task); delay period 3, gray square (1 s in both tasks); choice, a choice of two stimuli (1.2 s in both tasks), thepaired associate of the cue (correct) and one from a different pair (error) in the PACS task, or the same picture as the cue (correct) and one froma different pair (error) in the DMS task. (B) Twelve pairs of Fourier descriptors used as stimuli in both tasks. The first pair is picture G1 (green)and picture C1 (cyan), the second pair is G2 and C2, etc. (C) Locations of recorded AIT neurons. (Left) Ventral view (anterior at the top) of amonkey brain. (Right) Coronal cross-section (dorsal at the top) indicated by a horizontal line on the ventral view. The stippled area represents therange of recording sites.

randomly in two of four possible positions (arranged in two
rows of two columns). The choice stimuli were the paired
associate of the cue (correct) and a distractor (error) in the
PACS task, while the choice stimuli were the same picture as
the cue (correct) and a distractor (error) in the DMS task. The
animal obtained a reward for touching the correct picture
within 1.2 s. If the animal released the lever before the choice,that trial was aborted. In earlier experiments, we tested bothtasks without delay period 3.
The response of a single neuron was first examined for its
response selectivity in the PACS task. Trials with a green cue
picture and those with a cyan cue picture were alternately
tested in a block manner. When a neuron showed delay
responses, we examined that neuron further in the DMS task.
Trials in the PACS task and those in the DMS task were also
alternately tested. The number of trials within each block was
randomized between 12 and 30. The animal could not have
known a priori which stimulus would need to be identified after
several trials of each block. In the recording sessions after
training, the performance of one monkey was 80–100% cor-

rect and the other’s was 70–90% correct in both tasks. We did
not find any differences in performance between the two tasks.
Recordings and Analyses. Extracellular discharges of single
neurons were recorded in three hemispheres with a glassinsulated tungsten microelectrode. The electrode was inserted
vertically into the target zone through the intact dura materalong a stainless steel guide tube, by means of a hydraulicmicrodrive manipulator. Standard chronic single-unit recording techniques were employed (10). The action potentials of
single cells were amplified and passed through high-pass(50–200 Hz) and low-pass (5 kHz) filter circuits and wereconverted into digital pulses by a time–window discriminator.
The isolation of each neuron was carefully monitored by using
two sets of storage oscilloscopes and sound monitors. The eye
position was monitored with a scleral search coil (11) in
separate experiments. Horizontal and vertical eye positions
were recorded every 80 ms while the monkey performed the
PACS or DMS task. The spatial resolution of the eye-position
monitoring system was ,0.1°.
Mean discharge rate was calculated over 1 s of warning
period as a baseline level. Cue and delay responses were

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Neurobiology: Naya et al.

evaluated by calculating the mean discharge rate for each
picture in each task. For the cue response, spike numbers werecollected over 80–480 ms from the beginning of cue period.
For the early delay response, spike numbers were collected
over 1.5 s, starting 0.5 s after the onset of delay period 1. For
the late delay response, the mean discharge rate was calculated
over 1 s of delay period 3.
To analyze the effects of the task on delay period 1 and delay
period 3 responses, we used an analysis of variance (ANOVA)
in which the data formed a trifactorial, with tasks and periods
as fixed factors and cells as random factors. In this ANOVA ofa split-plot experiment, task 3 cell interactions were derived
from main plot comparisons, and task 3 period interactions
were derived from subplot comparisons. After confirming thesignificance of task 3 period interactions, we examined theeffects of tasks and periods on delay responses further by using
the two-tailed paired t test, in which paired samples were
derived from the same cell.
For each cell in experiments using tasks without delay period
3, the effect of colors was examined by using cue responses
obtained in both tasks over a 240- to 480-ms period from the
beginning of cue period, in which there is behaviorally relevant
information and the animals actually viewed the cue stimuli(0.3° right, 0.1° down and 1.6° left, 0.3° down from the centerof the stimulus for the two animals, respectively, averaged over
288 trials). By using the Mann–Whitney test, we estimatedwhether one of the two colors elicited stronger cue responses
than the other. For color-insensitive cells in experiments using
tasks without delay period 3, spike numbers were collected
over 1.5 s just before the presentation of choice stimuli during
delay period 2 for late delay responses.

RESULTSIn this study, we used two memory tasks (Fig. 1 A). The PACS
task requires retrieval of a target stimulus from memory,whereas the DMS task does not require such a memory
retrieval. The results are based on a total of 103 AIT neurons(70 cells from one animal and 33 cells from the other) thatshowed cue responses in both tasks. The recording area was
localized in the inferotemporal gyrus and a part of the middle
temporal gyrus, including both banks of the anterior middle
temporal sulcus (Fig. 1C). The area was very similar to thatshown in the previous report (6). Out of the 103 cells, 15 cells
showing delay responses were held long enough to complete
the PACS task and the DMS task. The effective pictures for
eliciting responses from these 15 neurons covered the picturesin the set with no particular bias. When the paired associate of
the cue-optimal picture was presented as a cue, 8 of the 15
neurons exhibited the highest activity during delay period 3 in
the PACS task. In the PA task, we have reported (6) a similar
type of cell (pair-recall neuron), in which the paired associateof the cue-optimal picture elicited the highest delay activity.
We found a clear task difference in delay responses when thebest picture’s associate was used as a cue. In this paper, we callthe cue-optimal picture the best picture, irrespective of its
delay response. Fig. 2 shows data from a single AIT neuron.
One picture (G7) elicited the strongest response during cue
period from this neuron in both the PACS task and the DMS
task (Fig. 2 A and E). The paired associate (C7) of the best
picture (G7) elicited little response during the delay period in
the DMS task (Fig. 2F). In contrast, this neuron started torespond just after delay period 2 (d2) onset when the square’s
color changed from the cue’s color (C7; cyan) to that of thepaired associate (G7; green) in the PACS task (Fig. 2B). The
picture-selective activation after the color switch in the PACS
task is called here the ‘‘pair-recall’’ effect, according to thereport (6). These results suggest that this delay discharge thatis specific to the PACS task may be triggered by memory
retrieval.

Proc. Natl. Acad. Sci. USA 93 (1996)
We noted another task difference in delay responses when
the best picture (G7) was used as a cue (Fig. 2 A and E). In the
DMS task, this neuron exhibited sustained activation during
the delay period (Fig. 2E). In contrast, the response of thisneuron was suppressed after delay period 2 onset when thesquare’s color changed from the cue’s color (G7; green) to theassociate’s color (C7; cyan) in the PACS task (Fig. 2 A). The
picture-selective suppression after the color switch in the
PACS task is called here the ‘‘pair-suppression’’ effect.
We also observed that the effects of pair recall (Fig. 2B) andpair suppression (Fig. 2 A) continued from delay period 2 intodelay period 3 in which the square’s color was the same grayin both tasks. Therefore, these effects were not due to thesquare’s color, as confirmed by the following observations.
The pair-recall effect was observed in trials with cue C7 (Fig.
2B), but not in trials where other cyan pictures were used asa cue (Fig. 2D). Similarly, the pair-suppression effect wasobserved in trials with cue G7 (Fig. 2 A), but not in trials whereother green pictures were used as a cue (Fig. 2C). These results
suggest that the pair-recall effect and the pair-suppression
effect cannot be explained by the visual response to colors.
Fig. 3 summarizes the comparison between responses in the
PACS task and the DMS task for the 15 neurons. For eachneuron, we calculated average discharge rates in each of three
periods: warning, delay period 1, and delay period 3. Theseaverages were calculated across trials with the same cue picturein the same task. Fig. 3A shows the comparison between the
two tasks when the paired associate of the best picture was used
as a cue. The plotted points are the mean discharge rates foreach neuron, and they are joined by lines for individual cells.
An increased response from delay period 1 to delay period 3
was observed for 13 cells in the PACS task and for 5 cells in
the DMS task. Fig. 3B shows the comparison between the twotasks when the best picture was used as a cue. The decrease of
response from delay period 1 to delay period 3 was observed
for all 15 cells in the PACS task and for 6 cells in the DMS task.
We examined the activity of these 15 neurons further by
testing the overall significance of the pair-recall effect and thepair-suppression effect. From the average responses for eachneuron (Fig. 3), we obtained grand means and standard errors
across the 15 cells (Table 1). We analyzed the data of delay
period 1 and delay period 3 responses with ANOVA of a
split-plot experiment. To characterize the pair-recall effect, wecollected trials whose cue picture was the best picture’s associate. For the pair-suppression effect, we collected trials whosecue picture was the best picture. The effect of task on the delay
response was significant for the pair-recall effect [F(1, 14) 5
19.8; P , 0.001] and for the pair-suppression effect [F(1, 14)
5 24.5; P , 0.001]. Furthermore, there was an interaction
between task and period for the pair-recall effect [F(1, 28) 5
12.2; P , 0.005] and for the pair-suppression effect [F(1, 28)
5 20.8; P , 0.001].
For the pair-recall effect, we analyzed responses among
periods in each task and responses between the tasks in eachperiod. In both tasks, responses in delay period 1 remained
equal to warning responses (see Table 1). These delay period
1 responses were thus task-independent. In delay period 3 of
the PACS task, the responses were significantly stronger thanthose in delay period 1 (t 5 3.5, P , 0.005; see Table 1), while
the delay period 3 responses in the DMS task remained equal
to the delay period 1 responses. The delay period 3 responses
in the PACS task were significantly stronger than those in the
DMS task (t 5 5.2, P , 0.001). These results indicate that thepair-recall effect is triggered by the color switch.
For the pair-suppression effect, we performed the same
analyses. In both tasks, responses in delay period 1 weresignificantly stronger than warning responses (PACS, t 5 6.6,
P , 0.001; DMS, t 5 3.5, P , 0.005; see Table 1). The delay
period 1 responses were not significantly different between the
two tasks. In delay period 3, the responses were significantly

Neurobiology: Naya et al.

Proc. Natl. Acad. Sci. USA 93 (1996)

2667

FIG. 2. Differential delay responses of a single AIT neuron in the PACS task and the DMS task. Rastergrams of neural discharges in each trial
and spike density histograms are shown. These trials in each task were originally separated by intervening trials with other cue pictures and were
sorted by off-line computation. Bin width, 80 ms. (A–D) Responses in the PACS task. (E–H) Responses in the DMS task. (A and E) Responses
in trials with cue G7. (B and F) Responses in trials with cue C7. (C and G) Responses in trials where one picture (G) of G1–G12 except G7 wasused as a cue. (D and H) Responses in trials where one picture (C) of C1–C12 except C7 was used as a cue. Picture G7 elicited the strongest cue
response in both tasks (A and E). Note the suppressed response during delay period 2 (d2) and delay period 3 (d3) in the PACS task (A) but notin the DMS task (E). We called this phenomenon the pair-suppression effect. In trials with cue C7, little response was observed during cue period
in both tasks (B and F). Note the enhanced response during delay period 2 and delay period 3 in the PACS task (B) but not in the DMS task (F).
We called this phenomenon the pair-recall effect. In trials with cue G or C, no responses were observed in either task (C, D, G, and H), indicatingthat there was no significant color effect.

weaker than the delay period 1 responses only in the PACS task
(t 5 27.4, P , 0.001; see Table 1). The delay period 3
responses in the PACS task were significantly weaker thanthose in the DMS task (t 5 25.6, P , 0.001). These results
indicate that the pair-suppression effect is also triggered by thecolor switch.
In a separate experiment, we examined eye positions during
delay period 3 in each task. There were no significant differences in either horizontal or vertical eye positions between
trials of the PACS and DMS tasks for each animal (horizontal,
P . 0.1 for the two animals; vertical, P . 0.25 and 0.75 forindividual animals; t test, n 5 288). The mean eye positions
were 0.8° left and 2.2° up from the center of the stimulus in one
monkey and 3.4° right and 4.4° down, in the other. The
difference between neuronal responses in the PACS task andthose in the DMS task cannot be explained by retinal factors
related to differences in eye position.

Before the experiments using tasks with delay period 3, we
tested single neuron responses in the PACS and the DMS tasks
without delay period 3. In these earlier experiments, we
successfully examined 16 cells that were responsive during thedelay period, as well as the cue period, and insensitive to colors(P . 0.05; Mann–Whitney test). In these 16 cells, there was aninteraction between task and period for the pair-recall effect[F(1, 30) 5 4.5; P , 0.05] and for the pair-suppression effect[F(1, 30) 5 12.6; P , 0.005]. For the pair-recall effect, latedelay responses were significantly stronger than early delay
responses in the PACS task (t 5 4.3, P , 0.001), while latedelay responses remained equal to early delay responses in the
DMS task. For the pair-suppression effect, late delay responses
were suppressed compared to early delay responses in the
PACS task (t 5 22.5, P , 0.05), and late delay responses wereeven stronger than early delay responses in the DMS task.
These results of the experiments using tasks without delay
period 3 confirmed the conclusion described above.

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Proc. Natl. Acad. Sci. USA 93 (1996)

FIG. 3. Pair-recall and pair-suppression effects in the PACS task. (A) Discharge rates in trials where the best pictures’ associates were used asa cue in the PACS task and the DMS task. (B) Discharge rates in trials where the best pictures were used as a cue in these two tasks. Each circle
denotes the average firing rate for one of single AIT neurons recorded (n 5 15). The activity during warning period (w) reflects a condition thatprovides no specific information about paired pictures. The average firing rates in each condition are joined by lines for each neuron. For mostof the 15 cells in the PACS task, activities during delay period 3 (d3) were higher than those during delay period 1 (d1) for A (pair recall), whereas
activities during delay period 3 were suppressed compared to those during delay period 1 for B (pair suppression).

DISCUSSION
We have reported (6) pair-recall neurons, whose delay activity
is closely coupled with the paired associate that is not actuallyseen but retrieved through a cue stimulus. In the present study,
it was possible to extend the previous observation on pairrecall neurons to other AIT neurons with delay responses. We
have interpreted the pair-recall effect as a prospective code ofa sought target that is generated by the conversion of a cue into
its paired associate (4). We found that a single AIT neuron
started to respond just after the color switch (Fig. 2B) and thatit did not respond without the color switch (Fig. 2F), when thebest picture’s associate was presented as a cue. The pair-recall
effect could be induced if the signal provided by the color
switch changed a prospective code of an ineffective target intothat of an effective target for that cell. While there was no suchsignal, the prospective code of an ineffective target would bemaintained. Moreover, when the best picture was presented asa cue, the same neuron showed sustained activation while aprospective code of an effective target was maintained (Fig.
2E), and its response was suppressed after the signal forchanging the prospective code into that of an ineffective target(Fig. 2 A). This dual parallelism provides compelling evidence
that supports the prospective code hypothesis. Although aretrospective code, which is a persistent representation of a cue
stimulus itself, may be partially involved in the delay activity,
it does not account for the pair-recall effect in the PACS task.
Table 1.

Whereas the sustained activation is a well-known phenomenon in the DMS task (4, 8, 12, 13), the pair-suppression effectin the PACS task is a novel phenomenon, to our knowledge,
first reported in this study. This effect is not due to a simple
decay of responses over time, because the delay response in the
DMS task showed a gradual increase rather than attenuation(see Table 1). Furthermore, the pair-recall and pair-suppression effects cannot be explained by the visual response tocolors for the following reasons. First, we confirmed thestatistical significance of these effects in both of two separate
studies, where the contribution of color factors was carefully
excluded: In experiments with delay period 3, these effects (i)were analyzed during delay period 3, in which a gray square waspresented; (ii) in experiments without delay period 3, only cells
that did not show significant color preference were analyzed(see Results). Second, pair-recall and pair-suppression effects
were observed in trials with a particular cue, but not in trials
where other pictures with the same color were used as a cue.
Typical examples are shown in Fig. 2 A and C and Fig. 2 B and
D. Therefore, these PACS-task-dependent effects are related
to form information rather than color information.
Haenny et al. (14) proposed that the activity of some V4
neurons in monkeys performing an orientation matching task
is related to the orientation for which the animal is seeking.
This proposal has been supported by the finding that individual
V4 neurons show similar effects for samples of visual andtactile modality (15). Such a signal of the sought orientation

Significant effects of pair recall and pair suppression in the PACS task
t value

Response, no. spikes per s

Cue picture
Best picture’s
associate
Best picture

Warning

Cue

Delay 1

Delay 3

Delay 1vs.
warning

Delay 3
vs.
delay 1

2.4 6 0.6
2.0 6 0.6
1.6 6 0.4
3.0 6 0.9

4.9 6 1.8
3.6 6 1.5
27.9 6 4.5
27.0 6 3.5

2.8 6 0.7
2.1 6 0.7
4.9 6 0.8
5.2 6 0.9

5.3 6 0.8
1.8 6 0.5
1.8 6 0.5
7.0 6 1.0

1.9
0.4
6.6*
3.5*

3.5*
20.8
27.4*
1.8

Task
PACS
DMS
PACS
DMS

Data are the mean 6 SEM.
*P , 0.005, two-tailed paired t test (n 5 15; df 5 14).

Neurobiology: Naya et al.
indicates a prospective code, although those studies addressedchanges in responsivity to stimuli rather than delay activity.
Recently, Motter (16) reported ‘‘pop out’’ phenomenon suchthat single V4 neurons started to respond to an oriented barin the receptive field when the color of a fixation point changed
into the color of the bar signaling attention to that stimulus.
This attentional effect triggered by the color change might besimilar to the pair-recall effect in our paradigm. It should be
noted, however, that V4 neurons respond to bars that arephysically present in the receptive field, whereas AIT neurons
respond to pictures that are not physically present but retrieved
from long-term memory. Thus the pair-recall effect may beinterpreted as a result of one of the subprocesses involved ingenerating mental imagery of paired associates. We hypothesize that visual imagery is implemented by the interaction
between memory retrieval and focal attention mechanisms(17). According to our scheme, visual imagery is generated by
top-down activation of perceptual representations (18). Thismodel is consistent with our present finding that the pictureselective responses in AIT neurons are controlled dynamicallyby the color switch in the PACS task as the color switch
subserves a signal for changing prospective codes. Furthermore, the pair-suppression effect suggests gating mechanismsthat preferentially pass information about a sought target,
thereby suppressing the activity of neurons that respond to acue stimulus and enhancing the activity of neurons that
respond to its paired associate. The dynamics of AIT neurons
for memory retrieval appears to result in the parallel operationof these two mechanisms—one being enhancement and theother being suppression.

Proc. Natl. Acad. Sci. USA 93 (1996)

2669

We thank Dr. Isao Hasegawa for colaboration in early experiments.
This work was supported by a Grant-in-Aid for Specially Promoted
Research from the Japanese Ministry of Education, Science andCulture (No. 07102006) and a grant from the Uehara Memorial
Foundation to Y.M., and a grant from the Hayao Nakayama Foundation for Science and Technology and Culture to K.S.
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