1Department of Physiology, Juntendo University, Tokyo 113-0033; 2Brain Science Research Center, Tamagawa University, Tokyo 194-8610; 3Department of Physiology, Nihon University, Tokyo 173-8610; and 4Department of Neurology, University of Tokyo, Tokyo 113-8655, Japan
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ABSTRACT |
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Lauwereyns, Johan, Masamichi Sakagami, Ken-Ichiro Tsutsui, Shunsuke Kobayashi, Masashi Koizumi, and Okihide Hikosaka. Responses to Task-Irrelevant Visual Features by Primate Prefrontal Neurons. J. Neurophysiol. 86: 2001-2010, 2001. The primate brain is equipped with prefrontal circuits for interpreting visual information, but how these circuits deal with competing stimulus-response (S-R) associations remains unknown. Here we show different types of responses to task-irrelevant visual features in three functionally dissociated groups of primate prefrontal neurons. Two Japanese macaques participated in a go/no-go task in which they had to discriminate either the color or the motion direction of a visual target to make a correct manual response. Prior to the experiment, the monkeys had been trained extensively so that they acquired fixed associations between visual features and required responses (e.g., "green = go"; "downward motion = no-go"). In this design, the monkey was confronted with a visual target from which it had to extract relevant information (e.g., color in the color-discrimination condition) while ignoring irrelevant information (e.g., motion direction in the color-discrimination condition). We recorded from 436 task-related prefrontal neurons while the monkey performed the multidimensional go/no-go task: 139 (32%) neurons showed go/no-go discrimination based on color as well as motion direction ("integration cells"); 192 neurons (44%) showed go/no-go discrimination only based on color ("color-feature cells"); and 105 neurons (24%) showed go/no-go discrimination only based on motion direction ("motion-feature cells"). Overall, however, 162 neurons (37%) were influenced by irrelevant information: 53 neurons (38%) among integration cells, 71 neurons (37%) among color-feature cells, and 38 neurons (36%) among motion-feature cells. Across all types of neurons, the response to an irrelevant feature was positively correlated with the response to the same feature when it was relevant, indicating that the influence from irrelevant information is a residual from S-R associations that are relevant in a different context. Temporal and anatomical differences among integration, color-feature and motion-feature cells suggested a sequential mode of information processing in prefrontal cortex, with integration cells situated toward the output of the decision-making process. In these cells, the response to irrelevant information appears as a congruency effect, with better go/no-go discrimination when both the relevant and irrelevant feature are associated with the same response than when they are associated with different responses. This congruency effect could be the result of the combined input from color- and motion-feature cells. Thus these data suggest that irrelevant features lead to partial activation of neurons even toward the output of the decision-making process in primate prefrontal cortex.
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INTRODUCTION |
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There is no doubt that primate prefrontal cortex is involved in
the executive control of behavior (Fuster 1997;
Goldman-Rakic 1987
; Passingham 1993
). A
large body of evidence from single-unit studies indicates that
prefrontal neurons interpret visual information to determine the
correct hand movement (Hoshi et al. 2000
; Niki 1974
; Rainer et al. 1998
; Sakagami and
Niki 1994a
; Sakagami and Tsutsui 1999
;
Sakagami et al. 2001
; Watanabe 1986
;
White and Wise 1999
) or eye movement (Asaad et
al. 1998
, 2000
; Funahashi et al. 1993
;
Hanes and Schall 1996
; Kim and Shadlen
1999
; Schall et al. 1995a
). However, it remains
unknown how such prefrontal neurons behave when they are presented with
conflicting relevant and irrelevant visual information.
Recent evidence from an electrical-stimulation study in frontal eye
field (Gold and Shadlen 2000) suggests that the brain's decision-making process to determine the required action consists of a
gradual commitment toward a choice based on the accumulation of sensory
evidence (see also Sakagami and Tsutsui 1999
;
Schall and Thompson 1999
). In line with this view, it is
possible that conflicting sensory evidence disturbs the development of
a prefrontal neuronal code favoring one action over another. To
investigate this prediction, we devised a conflict paradigm in which
the monkey should discriminate one visual feature while ignoring
another to make the appropriate behavioral response.
Depending on the discrimination condition, the monkey had to interpret either the color or the motion direction of the visual target. The monkey had been trained extensively prior to the experiment to acquire fixed associations between stimulus features and required responses (e.g., "purple = go"; "rightward motion = no-go"). In this design, as shown in Fig. 1A, the monkey was confronted with a visual target from which it had to extract relevant information (e.g., color in the color-discrimination condition, henceforth "color condition") while ignoring potentially confusing information (e.g., motion direction in the color condition).
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In a previous behavioral study with a reaction-time version of this
task, we found that the monkeys' manual responses were slower and less
accurate when the irrelevant feature primed a different response than
the relevant feature as compared with when both features primed the
same response (Lauwereyns et al. 2000). The data
suggested that irrelevant target information automatically activates
hard-wired but presently inappropriate S-R associations. If this is
true, such inappropriate S-R associations could be represented in
prefrontal neuronal activity. To test this hypothesis, we conducted a
single-unit study in prefrontal cortex while the monkey performed the
same type of visual multidimensional discrimination. We opted for a
nonspeeded version of the task, introducing a delay between target
presentation and manual response to prevent confounding with motor
processes in the single-unit activity.
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METHODS |
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Behavioral paradigm
The monkey was required to discriminate either the color or the motion direction of a visual target to make a correct go or no-go manual response. The behavioral meaning (go or no-go) of each target feature was fixed for each monkey during both training and experiments but could be either relevant or irrelevant depending on the discrimination condition. The color of the fixation spot indicated which of the target features the monkey should discriminate. Throughout a block of trials, the monkey had to select the appropriate behavior based on the same visual dimension (i.e., the color of the fixation spot remained constant). The correct response to a particular multidimensional target depended on only the relevant stimulus feature and so could vary across discrimination conditions (see Fig. 1A). Throughout an experimental block, we used one of two stimulus sets on which the monkey had been trained separately: set 1, the colors purple/yellow and the motion directions left/right; set 2, the colors red/green and the motion directions up/down.
With this design, in each trial irrelevant stimulus information could
be either congruent or incongruent with the required response. Using a
speeded version of the discrimination task, we confirmed that this
paradigm leads to interference effects in the behavior of the two
monkeys used in the present study with longer response times and
increased error rates when the irrelevant feature was incongruent with
the required response (Lauwereyns et al. 2000).
To prevent confounding between go/no-go discrimination and motor execution processes in the neuronal activity, we introduced a delay period between stimulus and response in the discrimination task for the neurophysiological recordings. In all other respects, the task and stimuli were exactly the same as for the behavioral test. In the paradigm with the delay period, the sequence of events in each trial was as follows (see Fig. 1B). The monkey initiated each trial with a lever press. The fixation spot (0.3° diam) appeared in the center of the CRT. After a variable period (1-2 s), the target stimulus was presented for 200 ms, the center of the target appearing at 4.1° either to the left or right of the center of the fixation spot. Following a variable delay (0.5-2 s), the fixation spot dimmed. The monkey then had to release the lever within 0.8 s (for a correct go response) or refrain from releasing the lever for at least 1.2 s (for a correct no-go response). In a no-go trial, the monkey could release the lever at any time after the 1.2-s no-go period. A drop of fruit juice was delivered on lever release as reward for every correct go or no-go response.
Eye movements were restricted to within 1° of the fixation spot by means of an infrared camera and associated equipment (R-21C-A, RMS) from 500 ms before until 500 ms after the onset of the target stimulus (with a sampling rate of 250 kHz). Trials in which an eye movement was detected outside the fixation window were aborted and counted as errors.
The monkey viewed a dynamic random dot pattern through a virtual square aperture (6.2 × 6.2°) as a target stimulus. All dots were of the same color and moved unidirectionally and coherently. Approximately 280 dots moving at 6°/s were used to cover 11% of the virtual aperture area. Apparent motion was produced by successive frame replacement (4 frames). All stimuli were presented on a 20-in CRT (HC39PEX, Mitsubishi) controlled by personal computers (PC386V, Epson). A lever consisting of a small plastic disk, with a diameter of 2.0 cm, was used for the manual responses. The lever was attached to the monkey chair in front of the right hand at the height of the elbow in such a way that the monkey could reach it with the right hand only.
Electrophysiological recording
Recording was done in at least two blocks of 32 to 64 trials,
one block using either stimulus set 1 or 2 in the
motion condition and one block using the same stimulus set in the color
condition, in random order. The stimulus set was determined randomly;
for some neurons, the entire experiment was repeated with the
alternative stimulus set. Since many prefrontal cells show a spatial
preference similar to the receptive fields found in visual cortices
(Sakagami and Niki 1994b), we presented the target
stimuli either ipsilaterally or contralaterally, where the cell showed
the largest change in activity during preliminary investigation.
We recorded from four hemispheres in two Japanese monkeys (Macaca
fuscata): monkeys EC and FR. The training
history of the monkeys is described in Lauwereyns et al.
(2000). After completion of the training, a head-holding device
and a unit-recording chamber were implanted with standard surgical
techniques under pentobarbital sodium anesthesia. During single-unit
recording, the monkey's head was restrained, and a hydraulic
microdrive (Narishige, MO-90) was attached to the chamber. A
glass-coated elgiloy microelectrode with 10- to 15-µm tip exposure
was used for unit recording. Action potentials were identified using a
dual-voltage, time-window discriminator and were stored on computer at
1-kHz sampling rate. For detailed information on the histological
procedures, see Sakagami and Tsutsui (1999)
.
All surgical and experimental protocols were approved by the Animal Care and Use Committees at Juntendo University and were in accordance with the Guidelines for the Care and Use of Laboratory Animals of the National Institutes of Health.
Data analysis
Trials in which the monkey made an incorrect manual go or no-go response were eliminated from analyses. To analyze cell activity, two-factor ANOVA (color × motion direction) was applied to the responses of each neuron (100- to 400-ms period from target onset) separately for each discrimination condition. This time window was chosen as it showed the highest discrimination between go and no-go trials for the entire population of recorded neurons; the limit of 400 ms also ensured that the neuronal responses were not confounded with eye movement or retinal eccentricity (as the monkey's gaze was restricted to the fixation point up to 500 ms after target onset).
Based on the ANOVA results, we selected cells that could discriminate between go and no-go targets based on the relevant feature in at least one discrimination condition. If the neuron was recorded with both stimulus sets, we used the set to which the neuron showed the largest differential response. Specifically, we selected cells that produced a statistically reliable main effect (P < 0.05) of color in the color condition (color-feature cells), of motion in the motion condition (motion-feature cells), or both (integration cells). Cells were considered to show interference from an irrelevant feature (e.g., motion direction in the color condition) if there was a significant main effect of the irrelevant feature or if there was a significant two-way interaction effect between the irrelevant and relevant feature. Post hoc tests consisted of two-tailed t-tests.
To characterize the direction of the neuronal responses, we compared
the discrimination of one type of visual information (e.g., color) when
the monkey was required to process this information (e.g., color in the
color condition) versus when it was required to ignore this information
(e.g., color in the motion condition). This comparison allows us to
evaluate the similarity between the neuronal representation of relevant
and irrelevant information. We calculated a relevant color index based
on data from the color condition as follows
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To examine the temporal properties of the neuronal responses to
irrelevant information, we made sliding population histograms from 200 ms before to 400 ms after target onset. The sliding histograms were
computed separately for color-feature, motion-feature, and integration
cells. We calculated the three-point smoothed population average of the
(go - ng) discrimination values in time epochs of 10 ms. To combine
the data from cells showing a "go preference" with data from cells
showing a "no-go preference," we reversed the sign of the index for
the latter type of cells. From this population average, we subtracted
the average (go - ng) value (per 10 ms) during the precue period, that
is, at a time when differential values cannot reflect perceptual
discrimination. This was done by way of control because there were
different precue levels among the color-feature, motion-feature, and
integration cells. For each population of cells, the precue level was
estimated per 10 ms based on the data from 500 to 0 ms before target
onset. To determine the onset latency of the irrelevant-feature
discrimination, we used running one-tailed t-tests to check
at which moment there were two consecutive time epochs in which the
corrected (go - ng) value reliably exceeded zero.
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RESULTS |
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Behavioral performance
Both monkeys performed the manual go/no-go task with very high accuracy, reaching a grand average of over 94% correct responses, with better performance (P < 0.01) in the color condition (94.9%) than in the motion condition (93.4%). With the delay between target presentation and cue to respond, and so without time pressure, the monkeys were able to optimize their decision strategy, making correct responses despite the irrelevant stimulus-response associations: The congruency effect between relevant and irrelevant features (i.e., error rates on incongruent versus congruent trials) was 0.2% in the color condition (not significant) and 1.2% in the motion condition (P < 0.01). As a consequence, for any given neuron, there were not enough error trials to compute a correlation between the neuron's response to irrelevant information and the probability of making an error. We therefore decided to exclude error trials from further analyses.
Database of neurons
We explored the lateral part of prefrontal cortex (see Fig. 1C for indications of electrode penetrations in one hemisphere). A total of 436 neurons discriminated reliably (P < 0.05, ANOVA) between go- and no-go-indicating targets based on color or motion direction or both. Among these, 162 neurons (37.2%) were influenced by irrelevant information, as indicated by a significant main effect of, or interaction with, the target feature that the monkey should ignore (see METHODS). That is to say, "irrelevant information" refers to the color feature in the motion condition, and the motion feature in the color condition.
Specifically, 139 neurons (31.9% of the total population) showed go/no-go discrimination in both discrimination conditions (integration cells); among these, 53 neurons (38.1%) were influenced by irrelevant information. There were 192 neurons (44.0% of the total population) that showed go/no-go discrimination only based on color (color-feature cells); among these, 71 neurons (37.0%) were influenced by irrelevant information. There were 105 neurons (24.1% of the total population) that showed go/no-go discrimination only based on motion direction (motion-feature cells); among these, 38 neurons (36.2%) were influenced by irrelevant information.
Thus irrelevant target information is processed quite extensively in lateral prefrontal cortex even though the monkey succeeds in making correct go or no-go responses. The effects of irrelevant target information, however, are different for the three functionally dissociated groups of prefrontal neurons. In this article, we aim to characterize these different neuronal responses to irrelevant visual features.
Congruency effect from irrelevant information in integration cells
Integration cells are able to interpret information from different visual dimensions. Figure 2 presents histograms and rasters of two prefrontal neurons that were classified as integration cells because they showed reliable differential responses to go- and no-go-indicating stimuli in both discrimination conditions. The cell shown at the top (Fig. 2A) is a good representation of the majority (n = 86) of integration cells, firing differentially for go- and no-go-indicating stimuli in the color condition (main effect of color, P < 0.01) as well as in the motion condition (main effect of motion, P < 0.01) without reliable effects from the irrelevant feature. Looking more closely at the firing rates especially in go trials, however, there did appear a tendency for stronger activity in congruent go trials than in incongruent go trials (P < 0.10).
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Figure 2B presents an integration cell in which the influence from irrelevant information was much more pronounced (n = 53). Overall, this unit discriminated reliably between go and no-go trials in the color condition (main effect of color, P < 0.05) as well as in the motion condition (main effect of motion, P < 0.05), but the unit was influenced also by the congruency of the irrelevant feature. In the color condition, there was a significant main effect of the irrelevant motion feature (P < 0.01); in the motion condition, color caused a nonsignificant trend (P < 0.10).
The neuron's activity was observed while the monkey succeeded in making a correct manual response in each trial regardless of the irrelevant information. Yet, in both conditions, the neuronal go/no-go discrimination was degraded in case the two target features were incongruent. Thus the activity of this unit shows a blurred or suboptimal discrimination in case of conflict between the required response and the response primed by the irrelevant feature. As such, the direction of the influence from irrelevant information on the visual activity of this neuron reflects a general tendency of the population of neurons (see the population analyses in the following text).
Nonadaptive responses to one visual dimension in feature cells
In addition to integration cells, we found many neurons that were able to distinguish between go- and no-go-indicating stimuli in only one discrimination condition (color- and motion-feature cells). As with integration cells, the majority of color- and motion-feature cells showed no significant response to irrelevant information. Figure 3A, for instance, shows a motion-feature cell that discriminated reliably between go and no-go trials in the motion condition (main effect of motion, P < 0.01) but not in the color condition (no effect of color; F < 1). The irrelevant target features did not lead to significant effects in the 2 × 2 ANOVA, neither in the color nor in the motion condition (we observed 67 cells of this type).
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Although this motion-feature cell generally transmitted task-relevant information about motion, there was a small trend of influence from irrelevant information in its firing rate in the color condition (tendency toward a main effect of motion, P < 0.10). Specifically, the firing rate in congruent go trials was slightly higher than in incongruent no-go trials (P < 0.05). Thus the motion-feature cell did not remain entirely neutral in the color condition, leaking some information pertaining to a task-irrelevant visual dimension. Such leakage of irrelevant information led to significant effects of irrelevant information in about one-third of the population of motion-feature cells.
Figure 3B presents an example of a motion-feature cell influenced by irrelevant information (n = 38). The cell fired differentially for go- and no-go targets in the motion condition (main effect of motion, P < 0.01), but not in the color condition (no effect of color; F < 1). In the color condition, however, the firing rate of this neuron was still determined by the motion direction of the target (main effect of motion, P < 0.01), even though the monkey successfully disregarded the motion direction to make the appropriate manual response. This neuron, then, seems to encode the target's motion direction regardless of the discrimination condition and so fails to adapt to the requirements of the task.
Similarly, Fig. 4, A and B, shows color-feature cells that are unable to fully adapt to the task requirements. Both cells reliably discriminated color (main effects of color, P < 0.01) in the color condition but also in the motion condition. We observed 71 cells of this type, whereas 121 color-feature cells responded to color information only when the monkey was required to interpret color. The cells illustrate very well the large range of irrelevant responses we found in color-feature cells.
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Specifically, the cell in Fig. 4A showed some task-dependent
modulation. In the color condition, this cell fired for yellow targets
regardless of the motion direction (i.e., no reliable difference
between ng IN and ng CO trials), whereas in the motion condition this
cell fired more (P < 0.05) for yellow rightward moving
targets (ng CO trials) than for yellow leftward moving targets (ng IN
trials). Thus in the motion condition this cell suppressed its activity
for yellow in case the irrelevant color-based S-R association
(yellow = no-go) primed a different response than the relevant
motion-based S-R association (leftward motion = go; ng IN trials
in the motion condition). The fact that this cell changed its behavior
depending on the task is further underscored by its activity in the
waiting period right before the appearance of the target: the cell had
a stronger anticipatory or background firing rate in the color
condition than in the motion condition (P < 0.01).
Subtle condition-dependent changes in background activity were quite
common (25-30% of task-related neurons) (see also Sakagami and
Niki 1994a). Typically, such cells changed their background activity slightly when the task required discriminating the preferred visual dimension. The cell shown in Fig. 4B, on the other
hand, fired phasically for a yellow color, regardless of the
discrimination condition, and without any changes in the background activity.
Direction of the responses to irrelevant information
The effects of irrelevant information in the neurons shown in Figs. 2B, 3B, and 4, A and B, exhibit a common direction. The neurons' firing rate to a particular irrelevant feature shows the same go/no-go preference as when that feature is relevant to the monkey's task. Specifically, the neurons shown in Figs. 2B and 3B delivered more spikes in response to a go- than to a no-go-indicating motion direction (i.e., "go preference") when motion was relevant (in the motion condition) but also when motion was irrelevant (in the color condition). The neurons shown in Fig. 4, A and B, on the other hand, consistently preferred the no-go-indicating color across conditions.
To confirm this observation at the population level, we computed color and motion indices in both discrimination conditions (see METHODS). With these indices, we could examine the relation between a neuron's sensitivity to a particular visual dimension when this dimension was relevant versus irrelevant to the task.
Figure 5, top, presents the color indices (RC and IC) for three populations of neurons: color-feature cells (left), motion-feature cells (middle), and integration cells (right). Each point represents one neuron; the horizontal coordinate is determined by the color index in the color condition (RC, when color constitutes relevant information, indicated by a thick black bar), whereas the vertical coordinate is determined by the color index in the motion condition (IC, when color is irrelevant).
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There was a positive correlation between RC and IC for color-feature cells (r2 = 0.244; significantly different from 0, P < 0.01; Fig. 5, top left), confirming that the effect of irrelevant color on the neurons' activity showed the same direction as the effect of color when it was the relevant dimension. There was also a significant positive correlation between RC and IC with integration cells (r2 = 0.200; significantly different from 0, P < 0.01; Fig. 5, top right) but not with motion-feature cells (r2 = 0.057; not different from 0; Fig. 5, top middle).
In the same way as for color, Fig. 5, bottom, presents the motion indices (RM and IM) for the three populations of cells. There was a positive correlation between RM and IM for motion-feature cells (r2 = 0.337; significantly different from 0, P < 0.01; Fig. 5, bottom middle), confirming once again that the effect of the irrelevant feature on the neurons' activity showed the same direction as the effect of the same feature when it was relevant. There was also a significant positive correlation between RM and IM with Integration cells (r2 = 0.348; significantly different from 0, P < 0.01; Fig. 5, bottom right), and, be it less pronounced, with color-feature cells (r2 = 0.142; significantly different from 0, P < 0.01; Fig. 5, bottom left).
Together, these positive correlations indicate that responses to irrelevant information can be characterized as a residual of S-R associations from a different context. In other words, the influence seems to be due to the neurons' inability to entirely suppress their go/no-go preference of features that are presently irrelevant to the monkey's task.
Temporal properties of responses to irrelevant information
To understand how the responses to irrelevant information develop over time, we made sliding population histograms, separately for different types of cells. The histograms are based on the running population average of irrelevant-discrimination indices (see METHODS).
Figure 6, top, shows the histograms of color discrimination when it is irrelevant, that is, in the motion condition. The histograms are shown for the two populations of cells that are responsive to color information, that is, color-feature cells and integration cells. Color-feature cells show a relatively sharp and fast discrimination, with a latency of 80 ms after target onset and a peak between 150 and 250 ms after target onset, followed by a gradual dissipation of the response. The curve of the irrelevant discrimination of integration cells, on the other hand, shows a slower and generally smaller response, with an onset latency of 90 ms after target onset and with no discernable peak in the response.
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Similarly, Fig. 6, bottom, shows the histograms of responses to irrelevant motion in the color condition. The curve of the irrelevant responses of motion-feature cells appears relatively phasic, with an onset latency of 120 ms after target onset and a peak at 170 ms after target onset, followed by a gradual dissipation of the response. And again, the curve of the Integration cells shows a slower and smaller response, with an onset latency of 130 ms after target onset and with no discernable peak in the response.
Anatomical locations of different types of cells
Figure 1C indicates the locations of electrode
penetrations in the primate prefrontal cortex (example from the left
hemisphere of monkey EC; similar distributions were obtained
in the other hemispheres, not shown here). Considering cells that are
unaffected by irrelevant information (red dots), we found an anatomical
segregation: color-feature cells appeared mainly ventral to the
principal sulcus in areas 46 and the upper part of area 12, whereas
motion-feature cells tended to be located dorsal to the principal
sulcus in areas 46 and 8A. These results are consistent with anatomical
data on the connections between color-sensitive areas in inferotemporal cortex and ventrolateral prefrontal cortex (Barbas 1988;
Ungerleider et al. 1989
) and the connections between
motion-sensitive areas in parietal cortex and peri-arcuate prefrontal
cortex (Andersen et al. 1990
; Schall et al.
1995b
). Integration cells appeared in area 8A, both in the
dorsal and ventral sectors, as well as ventral to the principal sulcus.
Considering cells that are affected by irrelevant information (blue circles), it appears that the same anatomical segregation holds true for color-feature cells and for integration cells. With motion-feature cells, the segregation was less clear.
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DISCUSSION |
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Our data are the first to show prefrontal representations of irrelevant information in a feature-discrimination task. Using a manual go/no-go task with two Japanese monkeys, we found many prefrontal neurons that were able to code the behavioral meaning of targets based on color and/or motion direction. More than one-third of these neurons, however, also coded S-R associations with presently irrelevant information even though the monkey successfully ignored the irrelevant information in its behavior.
Typically, the responses to irrelevant information appeared as the
neuron's inability to completely adapt to the changed requirements of
the task. For instance, color-feature cells (see Fig. 4, A and B) were able to discriminate between go and no-go
targets in the color condition, but tended still to leak information
about color even when this feature was irrelevant, that is, in the
motion condition. Thus relevant and irrelevant S-R features appear to run in parallel in the brain even up to the stage of decision making,
for which prefrontal cortex is presumed to be responsible (Kim
and Shadlen 1999; Sakagami and Tsutsui 1999
).
Among the neurons that were influenced by irrelevant information, the effects ranged from complete interaction between relevant and irrelevant information (such as with the neuron shown in Fig. 2B) to nonadaptive responses to only one visual dimension, regardless of whether this dimension is relevant to the monkey's discrimination task (such as with the neurons shown in Figs. 3B and 4B). These different types of neuronal activity may reflect distinct stages of visual interpretation.
Specifically, nonadaptive responsiveness to one visual dimension seems to be derived from a purely sensory neural code and so could reflect the input from extrastriate and/or association visual areas such as V4 and IT for color information and MT and MST for motion direction. The interaction between relevant and irrelevant information in integration cells, on the other hand, can be characterized as a congruency effect toward the output side of the decision-making process.
Sequential mode of information processing in prefrontal cortex
The conceptual scheme in Fig. 7
shows how responses to irrelevant features could be gated through
prefrontal cortex. We propose that the conversion of visual information
into appropriate behavior is a hierarchically organized decision-making
process (Sakagami and Tsutsui 1999), in which
feature-selective cells (i.e., color- and motion-feature cells)
generate behavioral significance based on specific sensory properties
and send their output to integration cells, which in turn encode the
appropriate behavioral action. This process is represented in Fig. 7
with information flowing from independent sensory modules through
feature units toward integration units, which in turn influence motor
preparation.
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In this scheme, it is presumed that long-term training enables fixed or
automatic associations between representations in independent visual
modules and feature-selective representations of behavioral meaning. By
this interpretation, experience with the task leads to automation of
stimulus-response associations (Logan 1988). The notion
that experience can alter visual processing in prefrontal cortex was
established already in visual-interpretation tasks to guide manual
behavior (Niki et al. 1990
; Rainer and Miller 2000
) or oculomotor behavior (Bichot and Schall
1999
; Bichot et al. 1996
). Such training effects
can explain why feature-selective cells respond to one visual dimension
(e.g., color) even when it is presently irrelevant to the task. As a
consequence, these learning mechanisms would lead to a competition
between relevant and irrelevant S-R associations.
Depending on the attentional demands, we assume that information processing is selectively enhanced for only the relevant visual pathway (e.g., in Fig. 7, the color pathway). Through the fixed associations, however, both relevant and irrelevant information travels from the feature modules to the integration module. Because of the attentional modulation (either enhancement or suppression), relevant information on average outweighs the irrelevant information. In this way, prefrontal circuits appear to be organized so that they can filter out irrelevant information during the decision-making process, leading to smaller responses to irrelevant features in the later than in the earlier stages of decision making (i.e., smaller irrelevant responses by integration cells, see Fig. 6). Yet the irrelevant information influences even integration cells to some extent, leading to less efficient discrimination when the irrelevant feature primes a different answer than the relevant feature (bottom) as compared with when both features prime the same answer (top).
The notion that decision-making in prefrontal cortex is indeed
organized hierarchically finds support in the anatomical segregation of
different types of cells (Fig. 1C) as well as the temporal differences between color- and motion-feature cells on the one hand and
integration cells on the other hand (Fig. 6). Color- and motion-feature
cells showed a more phasic and slightly faster response to the
irrelevant feature, consistent with their presumed function toward the
perceptual or input side of the decision-making process, whereas
integration cells showed a more sustained and slower response,
consistent with their presumed function toward the motor-preparation or
output side of the decision-making process (see also Sakagami and
Tsutsui 1999).
Another finding that supports the notion of a hierarchical organization is that integration cells show a high consistency in their preference for either go- or no-go-indicating features across both visual dimensions: Out of 139 integration cells, 75 cells (53.9%) consistently preferred go-indicating stimuli; 51 cells (36.7%) consistently preferred no-go-indicating stimuli; and only 13 cells (9.4%) showed a different preference in the color than in the motion dimension. This observation suggests that integration cells do indeed integrate behaviorally relevant information from multiple dimensions rather than showing a random combination of tuning curves to independent sensory features.
In sum, the activity of the neural population as a whole includes both
sensory-derived input and behaviorally relevant output signals as
should be expected from a neural substrate of decision making
(Kim and Shadlen 1999; Leon and Shadlen
1998
; Zhang et al. 1997
). In this population,
however, irrelevant stimulus representations compete with relevant
representations throughout the entire decision-making process. Even
toward the output side of this process, the irrelevant stimulus
representations are still strong enough to influence the activity of
Integration cells. Thus the cells showing partial responses to
irrelevant features appear to be part of a network that takes sensory
input and turns it into a decision output. As such, these cells could
be merely intermediate in the computation process
that is, they could
be hidden units in a multilayer neural network. Alternatively, the
partial responses to irrelevant features could represent interference,
which in turn may disturb the monkey's decision-making behavior.
In this regard, further research is needed to investigate to what
extent the representations of irrelevant information in prefrontal
neuronal codes cause interference on behavioral performance. The two
monkeys in the present study showed clear interference effects in their
behavioral reaction times in a speeded version of the task with
otherwise exactly the same experimental set-up, whereas with the
delayed version of the task we found interference effects in the error
rates in the motion condition but not in the color condition. Reaction
times are a more sensitive behavioral measure of interference effects
than error rates because they can be related to the decision process
rather than the decision outcome (MacLeod 1991). Given
that the present study establishes that there exist responses to
irrelevant features in prefrontal cortex at a cognitive stage
dissociable from motor control, the next step in this research should
be to estimate the influence of such irrelevant prefrontal neuronal
codes on behavior. This can be done, for instance, by recording
single-cell activity during a speeded discrimination task
(Lauwereyns et al. 2000
) so that trial-by-trial neuronal
signals can be correlated to behavioral reaction times.
A related matter is the question of the relationship of the prefrontal
neuronal code to different types of motor control. We used fixed
one-to-one mapping between visual features and manual responses in the
present task. Consequently, we cannot indicate whether the irrelevant
neuronal activity pertains to the behavioral meaning of the visual
features (go or no-go) or whether the irrelevant activity is more
tightly linked to the response dimension (manual lever release). To
tease apart these two possibilities, future research should examine the
influence of irrelevant features on neuronal activity in situations
with variable instead of fixed stimulus-response mapping. This issue
can be resolved, for instance, by comparing the perceptual
decision-making process of the same neurons in manual versus oculomotor
tasks. In this respect, it is interesting to note that the present
manual go/no-go task revealed neurons with multidimensional
discriminative activity that could not be reduced to oculomotor
activity in the frontal eye field. Our data, which were obtained while
the monkey gazed at a fixation spot, suggest that the process of
perceptual decision-making could be more independent from motor control
than has been suggested recently (Gold and Shadlen
2000).
Partial prefrontal activation by irrelevant features
Partial activation by task-irrelevant information in prefrontal
cortex might simply be regarded as the corollary of the notion that
there are prefrontal representations of relevant information in tasks
with complex stimuli (e.g., Asaad et al. 2000;
Bichot and Schall 1999
; Hoshi et al.
2000
; Rainer et al. 1998
, 1999
; Sakagami
and Niki 1994a
; White and Wise 1999
).
However, in previous studies, which were not designed to study
responses to task-irrelevant features, decision-making was performed in
situations where there was no irrelevant visual dimension that could
imply an alternative S-R association. For instance, the study by
Bichot and Schall (1999)
showed history effects from
previous S-R associations in a situation where the monkey performed a
conjunction task, for which it had to consider both the color and the
shape of the target. The partial activation by a previous S-R
association, then, was derived from a task-relevant visual dimension.
In other studies, there were no competing S-R associations from
different dimensions. For instance, in the study by Asaad et al.
(2000)
, the monkey performed different tasks such as a spatial
task and an object task, but the stimuli changed with the task as well
so that the monkey was presented with simple dots in the spatial task
(with no possibility of interference from object information) or with a
reference object at the center of the screen (with no possibility of
interference from spatial information). Hoshi et al.
(2000)
used a similar design.
One intriguing study by White and Wise (1999) did show
partial activation of irrelevant features in a conflict situation
between two rules. Yet one of the two rules involved spatial
information, which may not be ideal as a task-irrelevant dimension,
especially when the monkey has to process visual information at the
same position in space. Specifically, under the conditional
rule in the White and Wise study, the monkey was required to identify an object at a particular position, while the same position implied an
alternative behavioral meaning. It could be argued that the monkey had
to allocate attention to the object's position to be able to identify
the object, implying that spatial information was not entirely
irrelevant to the monkey's task even under the conditional
rule. The same argument can be applied to Sakagami and Niki
(1994a)
or to the interference effects observed with the
anti-saccade paradigm (e.g., Funahashi et al. 1993
).
In contrast, in the present study, the monkey was required to discriminate one of two visual dimensions, either color or motion, while ignoring the alternative visual dimension because it carried an alternative S-R association. Thus we could examine how prefrontal cortex responds to visual information that was clearly task-irrelevant but could prime the same or a different manual response than the task-relevant information. We found phenomenally different responses to irrelevant information in three functionally segregated groups of prefrontal neurons. These responses could all be characterized as residuals of S-R associations from the alternative discrimination condition (Fig. 5). Color-feature cells leaked information about color in the motion condition, whereas motion-feature cells still responded to motion in the color condition. In integration cells, relevant and irrelevant input from different visual dimensions appeared to be combined, leading to a congruency effect in the go/no-go discrimination.
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ACKNOWLEDGMENTS |
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We thank H. Niki for advice on the research.
This work was supported by Grant RFTF96L00204 from the Japanese Society for the Promotion of Science; Grants 08279106, 08279208, 09268207, and 09710045 from the Ministry of Education, Science, Sports, and Culture of Japan; and a graduate fellowship from the Belgian National Fund for Scientific Research.
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FOOTNOTES |
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Address for reprint requests: M. Sakagami, Brain Science Research Center, Tamagawa University, Tamagawa-gakuen 6-1-1, Machida, Tokyo 194-8610, Japan (E-mail: sakagami{at}lab.tamagawa.ac.jp).
Received 5 March 2001; accepted in final form 21 May 2001.
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