Department of Neurobiology and Anatomy and Center for Visual Science, University of Rochester, Rochester, New York 14642
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ABSTRACT |
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Zaksas, Daniel, James W. Bisley, and Tatiana Pasternak. Motion Information Is Spatially Localized in a Visual Working-Memory Task. J. Neurophysiol. 86: 912-921, 2001. We asked if the information about stimulus motion used in a visual working-memory task is localized in space. Monkeys compared the directions of two moving random-dot stimuli, sample and test, separated by a temporal delay and reported whether the stimuli moved in the same or in different directions. By presenting the two comparison stimuli in separate locations in the visual field, we determined whether information about stimulus direction was spatially localized during the storage and retrieval/comparison components of the task. Two psychophysical measures of direction discrimination provided nearly identical estimates of the critical spatial separation between sample and test stimuli that lead to a loss in threshold. Direction range thresholds measured with dot stimuli consisting of a range of local directional vectors were affected by spatial separation when a random-motion mask was introduced during the delay into the location of the upcoming test. The selective masking at the test location suggests that the information about the remembered direction was localized and available at that location. Direction difference thresholds, measured with coherently moving random dots, were also affected by separation between the two comparison stimuli. The separation at which performance was affected in both tasks increased with retinal eccentricity in parallel with the increase in receptive-field size in neurons in cortical area MT. The loss with transfer of visual information between different spatial locations suggests a contribution of cortical areas with localized receptive fields to the performance of the memory task. The similarity in the spatial scale of the storage mechanism derived psychophysically and the receptive field size of neurons in area MT suggest that MT neurons are central to this task.
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INTRODUCTION |
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Neurons in many visual
cortical areas are selective for such fundamental properties of visual
stimuli as their orientation and direction of motion. However, the role
of these areas in memory for these properties is poorly understood.
Physiological and imaging studies have shown that transient or
sustained activity is present during the performance of visual
working-memory tasks in some cortical areas processing visual
information (e.g., Ferrera et al. 1994; Fuster
and Jervey 1982
; Gnadt and Andersen 1988
;
Miller et al. 1993
). These findings suggest that
cortical areas, although known primarily for their selective roles in
sensory function, may also play a role in memory networks
(Fuster 1997
). The experiments presented in this paper
provide evidence for a role of an intermediate visual area in the
short-term storage of one of the fundamental dimensions of the visual
stimuli, the direction of visual motion.
The direction of stimulus motion is processed by directionally
selective neurons at several stages of cortical analysis (for review,
see Albright 1993). Such neurons, particularly in area MT, an intermediate cortical area, have been implicated in extracting direction from complex motion stimuli (Movshon et al.
1985
; Rodman and Albright 1989
), taking part in
perceptual decisions concerning the direction of stimulus motion
(Bisley et al. 2001
; Salzman et al.
1992
), and contributing to short-term retention of this information (Bisley and Pasternak 2000
; Bisley et
al. 2001
). Neurons in this area have localized receptive fields
representing a portion of contralateral visual field (Maunsell
and Van Essen 1987
), making it likely that the information they
provide to the rest of the brain will be retinotopic.
To determine whether the remembered direction of motion is spatially localized, we used a behavioral task in which macaque monkeys compared the directions of motion of two random-dot stimuli, sample and test, presented in different retinal locations and separated by a temporal delay. By placing the two comparison stimuli in different portions of the visual field, we examined whether information about stimulus direction was spatially localized during encoding, storage, and retrieval/comparison components of the task. We estimated the extent of local processing by the distance the sample and test stimuli could be separated without disrupting performance. Our results suggest that information about stimulus direction remains spatially localized during performance of the task. The spatial scale of this localization, determined with two different psychophysical measures, indicates that MT neurons play a critical role in both tasks.
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GENERAL METHODS |
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Subjects
Three adult macaque monkeys (Macaca nemestrina) were used, two males and one female. The monkeys received most of their daily fluid rations in the form of fruit juice during testing sessions, and food was freely available in the cages every day. Body weights were monitored on a regular basis to ensure normal health and to monitor growth.
Control of eye position and daily calibration
Magnetic search coils were used to monitor eye position. Each
monkey had an implanted scleral search coil, and its head was fixed
firmly during the testing session (see Rudolph and Pasternak 1999, for description). Daily calibration of the eye position took place prior to the testing session and consisted of rewarding the
monkey for maintaining fixation for 900 ms within a 1.5° window at
various locations of the display. This procedure generally required no
more than 10-15 trials and allowed for accurate calibration of the
vertical and horizontal offsets as well as signal gain.
Stimuli
The stimuli consisted of moving dots within a stationary
circular aperture repeatedly displaced in a direction of motion chosen randomly from a uniform distribution of directions. Each dot was displaced by a constant step size (x) and temporal
interval (
t = 13 ms). When the range of the
distribution of directions was 0°, all the dots were displaced in the
same direction. When the range of the distribution of directions was
360°, only local random motion of individual dots was present.
However, when the distribution was less than about 320-340°, the
dots appeared to flow in the direction of the mean of the distribution
(Williams and Sekuler 1984
). Each dot, viewed at a
distance of 42 cm, was 0.03° in diameter, and its luminance was set
about 3.5 log units above human-detection threshold. The lifetime of an
individual dot was equal to the duration of the stimulus presentation
(500 ms). The size of the aperture ranged from 1.5 to 6° in diameter,
and the dot density was kept constant at 4.7 dots/deg2 by adjusting the number of dots for
each stimulus size. Dot speed was set to 5, 10, or 20°/s by setting
x to 0.065, 0.13, or 0.26°, respectively, for the three
tested eccentricities. In a given experiment, the comparison stimuli
were always presented at equal eccentricities.
Behavioral procedures
The procedures, which were similar to those described in recent
papers from this laboratory (Bisley and Pasternak 2000;
Bisley et al. 2001
), are depicted in Figs. 1,
A-C, and 6, A and B.
During each testing session, the monkeys were seated in a primate chair equipped with two pushbuttons. Each trial began with the presentation of a small fixation spot. The monkeys were required to fixate the spot for a period of 1,000 ms before two stimuli, the sample and test, were presented in sequence. An auditory tone was presented for the duration of the trial, and the monkeys were required to maintain fixation throughout that period. The sample moved in one of eight directions chosen at random, and the test moved in a direction that was the same as or different from that of the sample. The monkeys were rewarded with a drop of juice for pressing the left button if the directions were different and the right button if the directions of the two stimuli were the same. Incorrect responses resulted in a 3- to 5-s tone and no reward. The duration of each stimulus was 500 ms, and the delay between them was 500 or 1,500 ms dependent on the experiment. In some experiments, a masking stimulus, 500 ms in duration, was presented during the middle portion of the delay. Failure to maintain fixation resulted in a distinct tone and immediate termination of the trial. Each session consisted of 400-800 trials, separated by a 3-s inter-trial interval.
Threshold Measurements.
Thresholds were measured in a staircase procedure once the monkey
performed a given the task at above 80% correct for four consecutive
sessions. In this procedure, three consecutive correct responses
resulted in a decrease of stimulus discriminability (i.e., increase in
the level of difficulty), while a single incorrect response resulted in
a decrease of difficulty. The data were fitted with a maximum
likelihood Weibull function (Weibull 1951), and the
threshold was defined as the stimulus value at which the animal performed at 75% correct. At least three to five thresholds
determinations were performed for each stimulus condition. The
significance of the effects of each manipulation was determined using
an ANOVA.
Spatial Separation Between Sample and Test. Localized processing was tested with sample and test stimuli presented in separate spatial positions but at a range of eccentricities and spatial separations. Initially, a comparison was made of within and between hemifields at an eccentricity of 10° and a target size of 3°. Subsequently, a variety of within hemifield separations were tested using stimulus parameters shown in Table 1. Only one experimental paradigm was applied per session, so the animal always knew where the stimuli were likely to appear. Within each experimental session, the locations of the sample and mask, if present, never changed, and the location of the test was constant except in the "spatial uncertainty" experiments described in the following text.
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EXPERIMENT 1 |
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In this experiment, the spatial specificity of the remembered direction of motion was examined with random-dot stimuli consisting of a range of local directional vectors and thus required motion integration. The monkeys were required to encode and retain the mean direction of such stimuli and then compare the remembered direction to the subsequently presented coherently moving test. We measured the maximal direction range in the stimulus that allowed the monkeys to reliably perform such discriminations (direction range threshold). To determine whether the remembered sample direction was spatially localized and remained at the spatial location where the encoding occurred, the sample and the test were presented at different spatial locations, and a random-motion mask was introduced during the middle portion of the delay. The intervening mask was placed either at the location of the recently presented sample or at the location of the future test. We reasoned that if the remembered direction is spatially localized, the random-motion mask may interfere with the representation of the remembered sample and that this interference is likely to depend on its spatial position.
Methods
On each trial, the sample moved in a direction randomly selected from eight possible directions, and the test moved in the direction that was the same as or opposite to the net direction of the sample. In most experiments, the width of the direction distribution in the sample was varied while the dots in the test moved coherently (0° range; Fig. 1A). In one control experiment, the conditions were reversed, and the sample moved coherently while the direction range was varied in the test stimulus. In all experiments, the length of the delay was 1,500 ms.
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A random-motion mask was introduced for 500 ms during the middle portion of the delay. The mask consisted of dots containing only random local motion with no net global motion vector, but otherwise retaining all the spatiotemporal properties of the sample and test stimuli. During each testing session, the mask was always presented at the same spatial position either at the location of the sample or at the location of the test (Fig. 1, D and E).
In "spatial uncertainty" experiments, the location of the sample stimulus was fixed while the test stimulus was presented randomly at one of two locations. On 50% of the trials the test appeared at the location of the sample and on 50% of the trials, the test appeared at a second preselected location (Fig. 1E). Thus the subject knew the location of the sample stimulus but could not predict at which of the two predetermined locations the test stimulus would appear. In cases where the masking and uncertainty paradigms were combined, the sample and mask stimuli appeared reliably in their respective positions while only the test position was unpredictable.
Results
EFFECT OF THE MASK INTRODUCED DURING THE DELAY. We first examined the effect of placing the test stimulus at a location remote from the sample without an intervening mask. This required the monkeys to encode stimulus direction at one spatial location and, after 1.5 s, compare the remembered direction with a test stimulus that appeared 10° away, either in the same hemifield or in the corresponding location in the opposite hemifield (see Fig. 1A). The results, shown in Fig. 2, revealed no detectable loss in thresholds when the animals were required to compare the remembered direction with that of a test placed 10° away, either within the same or in the opposite hemifield.
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SPATIAL UNCERTAINTY OF TEST LOCATION. The localized mask effect at the site of the test revealed in the preceding experiments suggests that the mask interfered with the representation of the remembered direction at the location of the test. In those experiments, the spatial location of the comparison stimuli and of the mask did not change from trial to trial and the monkeys always knew the location in which the future test stimulus would be presented. To determine whether a similar effect would be observed if the monkeys had less reliable information about the location of the test, we introduced uncertainty about its location. During these sessions, the sample and mask always appeared in their same respective positions in the visual field while the test was placed either in the same location as the sample or at the distance of 10° from the sample, either within the same hemifield or in opposite hemifields. For each trial, the position of the test was selected randomly between these two locations and thus could not be predicted. The results in Fig. 3B show that under the conditions of spatial uncertainty, the placement of the mask had an effect similar to that found when the position of the test remained unchanged but was in the opposite hemifield. Moreover under conditions of spatial uncertainty, the mask had an effect even when the comparison stimuli were placed in the same hemifield. Again the mask effect was present only at the location of the test. Thus under the conditions of spatial uncertainty the mask revealed that the retained directional signal is likely to be spatially localized within an area smaller than about 10°.
DOES THE EFFECT OF THE INTERVENING MASK DEPEND ON STIMULUS COMPLEXITY? We performed a control experiment aimed at determining whether the effect of the mask depended on the complexity of the remembered stimulus. In this experiment, the sample consisted of coherently moving dots and range thresholds were measured by varying the direction range in the test under conditions of uncertainty about the location of the test. We found that placement of the mask had no significant effect on range thresholds. The mean range thresholds measured with sample, mask, and test in the same location was not significantly different from thresholds measured with sample and test separated by 10° with the mask in either the sample or the test location (ANOVA, P > 0.1 for monkeys 1 and 2). These data confirmed that the effect of the mask depends on the complexity of the remembered stimulus.
SPATIAL SEPARATION BETWEEN SAMPLE AND TEST STIMULI. To examine the extent of spatial specificity of the remembered direction, we repeated the measurements over a range of spatial separations at an eccentricity of 7° with the sample and test placed within the same hemifield under conditions of spatial uncertainty of the test stimulus. The results in Fig. 4A show that the performance at 3° separation was identical to that measured when sample and test appeared in the same location. A modest but significant drop in performance first appeared with a separation between the sample and test of 5°. A subsequent increase in the separation to 10° produced no further loss in thresholds. To better visualize how the spatial separation affected performance, we re-plotted the data in terms of percent change in threshold relative to the threshold determined with sample and test at the same spatial location (0° separation). These data were fitted with a sigmoidal nonlinear regression function and are shown in Fig. 4B. The half-height point on the curve was taken as a measure of critical spatial separation.
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Discussion
EFFECT OF A RANDOM-MOTION MASK.
Using the direction range threshold, which provides a measure of the
integration of local motion signals (Watamaniuk and Sekuler 1992; Watamaniuk et al. 1989
; Williams
and Sekuler 1984
), we found that the detrimental effect of
spatial separation was present only when the mask was introduced into
the future location of the test.
SPATIAL LOCALIZATION OF THE REMEMBERED DIRECTION.
The effect of the mask was most pronounced when presented at the
location of the test. Furthermore the mask had no effect when it was
introduced into a site remote from sample and test stimuli, confirming
the spatial specificity of the effect. As we did not measure the loci
of the monkeys' attention, we cannot rule out the possibility that
this spatially specific effect of the mask could be associated with
shifts of attention from the location of the sample or mask. However, a
shift of attention to the location of the upcoming test is compatible
with the notion that during the performance of the task the remembered
direction may be represented at the test location, since the mechanisms subserving visual working memory and spatial attention are closely interrelated (Desimone et al. 1994).
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EXPERIMENT 2 |
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The measure used in the preceding experiment emphasized the
ability to integrate and retain complex motion information while requiring only the ability to discriminate between opposite directions of motion. Another commonly used measure of direction discrimination is
the accuracy with which direction can be discriminated
(Pasternak and Merigan 1994; Rudolph and
Pasternak 1999
; Watamaniuk et al. 1989
) and
preserved (Bisley and Pasternak 2000
). To determine the
generality of spatial specificity of the direction memory observed in
experiment 1, we measured the accuracy with which the
monkeys could discriminate differences in the direction of sample and
test presented in separate spatial locations (Fig. 6). We wanted to determine whether this
measure would reveal a similar degree of spatial localization of the
remembered direction to that measured with the task requiring motion
integration. Since we found that spatial separation between the
comparison stimuli led to an elevation of thresholds even without a
random-motion mask, no random-motion mask was used in this experiment.
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Methods
The behavioral procedure was largely the same as that used in the experiment 1. On each trial, the sample moved in one of eight randomly selected directions followed by a test moving in the same direction or in a different direction. If the test direction was different from that of the sample, the angle of deflection from the sample was randomly selected as either clockwise or counterclockwise along the radial direction distribution. The difference in direction was varied in a staircase procedure and the smallest angle between sample and test directions that allowed for discrimination at the 75% correct level was taken as direction difference threshold. The length of the delay was 750 ms. All the measurements were performed with coherently moving sample and test stimuli presented at predictable locations.
Results
The effect of spatial separation of the sample and test on the
accuracy of direction discrimination was initially measured with the
sample and test either at the same location or separated by 10°
within the same hemifield. The comparison of direction difference
thresholds measured under the two conditions is shown in Fig.
7. All three monkeys were more accurate
when the sample and test were in the same location. This suggests some
degree of spatial localization in the process underlying the storage and retrieval of directional information. To assess the limits of this
spatial specificity, we measured direction difference thresholds over a
range of spatial separations and eccentricities. Results from the three
monkeys are shown in Fig. 8. At all
eccentricities, an increase in spatial separation between sample and
test resulted in a loss of accuracy of discrimination. As in the
previous experiment, the separation at which this loss first appeared
doubled with a twofold increase in eccentricity. To better visualize
the changes in threshold with spatial separation, we computed the
percent change in threshold relative to the threshold measured with no spatial separation. These data are plotted in Fig.
9. We used the half-height point on the
curve as the value corresponding to the critical spatial separation
(). These values are remarkably similar to the corresponding values
obtained by measuring direction range thresholds (see Figs. 4 and 5).
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Discussion
While the emphasis in experiment 1 was on the ability
to integrate local motion vectors, the measure used in experiment
2 tested the ability to accurately judge small differences in
direction. The thresholds were lowest when the two comparison stimuli
were placed at the same spatial location. Under these conditions, the thresholds for all three animals ranged from 30 to 40°, and these values did not change with eccentricity. Such invariance of thresholds with eccentricity is likely to be due to the scaling of stimulus size
and speed with the distance from the fovea to approximate the changes
in the properties of receptive fields of MT neurons (Desimone
and Ungerleider 1986; Bisley and Pasternak, unpublished observations). We used properties of area MT receptive fields to scale
the stimuli because lesion studies have demonstrated an involvement of
this area in the discrimination of direction (Bisley and
Pasternak 2000
; Rudolph and Pasternak 1999
).
The accuracy of direction discrimination measured here was lower than
that previously measured in a paradigm where the monkey discriminated
between two simultaneously presented directions, one of which always
moved rightward (Pasternak and Merigan 1994). One likely
reason for this difference is that the present measurements were
performed under conditions of high uncertainty as to the direction of
motion in the sample, which changed from trial to trial (Ball
and Sekuler 1980
, 1981
; Magnussen et al. 1996
).
Another factor contributing to the low accuracy is the presence of a
temporal delay between the two comparison stimuli (Bisley and
Pasternak 2000
; Rudolph and Pasternak 1999
). In
fact, we used only a brief, 750-ms delay in this portion of the study
because in preliminary experiments we found that the performance of all
three monkeys was strongly affected by lengthening of the delay. For
instance with a 200-ms delay, their thresholds were around 30-35°,
with 1,500 ms they increased to around 50°, values too large to use as a baseline performance because that would prevent measuring larger
losses in performance with spatial separation.
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GENERAL DISCUSSION |
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We used two psychophysical measures to determine whether the information about motion direction is spatially localized during the encoding and storage phases of the working-memory task. By spatially separating the two comparison stimuli and determining the extent of separation leading to the loss in thresholds, we demonstrated that the remembered information about stimulus direction is spatially localized.
What is remembered?
The use of a masking stimulus in the first experiment shed some light on the way in which information about complex motion is stored. For instance, it is possible that the monkeys may be solving the task by retaining the stimulus that contains a broad range of directions as a template rather than extracting and storing a vector representing the mean direction. We tested the "template" model by presenting a coherently moving sample and measuring thresholds by varying the direction range in the test. The robust image of a coherently moving sample is more likely to be immune to the interference by the mask than the stimulus that contains a broad range of directions. On the other hand, if the monkey simply remembers a direction vector, the effect of the mask should be the same for a coherently moving sample as for a directionally complex stimulus. We found that the mask placed at the test location following a coherent sample, unlike that following a directionally complex sample, had no effect on performance. This suggests that information about sample direction is more likely to be retained as a template than as a simple vector.
Possible neural mechanisms
Our experiment was built on the premise that if the transfer of information between separate spatial locations results in a loss in threshold, the two locations are likely to belong to spatially distinct underlying mechanisms. By spatially separating the comparison stimulus from the site where the sample was first encoded and stored, we determined the shortest distance between the two stimuli that allowed the comparison to be performed without loss. A detrimental effect of spatially separating the two comparison stimuli was obtained with both psychophysical measures, and we computed the critical spatial separation that lead to the loss in thresholds (see Figs. 4, 5, and 9). These values gave us an indication of the scale of the mechanisms underlying the performance of the two types of working-memory tasks.
It should be pointed out that at each eccentricity thresholds were
measured for a relatively small number of spatial separations and
fitting this rather sparse data set with the sigmoid function may have
given us a relatively shallow slope. This could have a small effect on
the derived values of critical separation. However, the consistency of
the values obtained with this fitting procedure across animals and
tasks at each eccentricity gives us confidence in values of critical
separation estimated from these functions. The computed mean values of
critical separations derived from the data in Figs. 4, 5, and 9 were as
follows: 2.02 ± 0.13° at 3.5° eccentricity, 4.32 ± 0.34° at 7° eccentricity, and 8.67 ± 0.82° at 14°
eccentricity (means ± SD). These values are plotted in Fig.
10 (). For comparison, changes in
receptive field sizes with eccentricity for a number of visual cortical
areas are shown on the same plot. Area MT data (light gray circles)
were collected in this laboratory in monkeys 2 and
3 using recording procedures described previously
(Bisley et al. 2001
). The data for other cortical areas
were taken from the literature: V1 (Dow et al. 1981
); V3
(Felleman and Van Essen 1987
), and MST (Desimone
and Ungerleider 1986
). Only the spatial scale of receptive
fields in area MT is consistent with the psychophysically derived
estimates of spatial localization.
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The fact that both measures revealed a spatial scale and dependence on
eccentricity similar to the receptive fields in MT suggests that this
mechanism may, at least in part, depend on those neurons. During
fixation, visual information is encoded retinotopically in MT, so we
suggest that the directional information is also stored
retinotopically. However, because we did not have the monkeys perform
saccades or other movements during the task, we do not know if this
information would be dynamically transferred to other retinotopic
locations following eye, head, or body movements, a characteristic
found in other regions of parietal cortex (Duhamel et al.
1992; Galletti et al. 1993
). It should be noted
that receptive field sizes in area V4, another mid-level visual
cortical area, are similar to those of area MT. However, we do not
believe that these neurons played a significant role in the performance
of the tasks used in the present study. First of all, only a very small
proportion of neurons in area V4 are directionally selective (Felleman and Van Essen 1991
). Furthermore lesions of
area V4 did not produce deficits on the motion tasks used here
(Rudolph and Pasternak 1996
) or on other motion tasks
(Merigan 1996
; Schiller and Lee 1994
). On
the other hand, the role of MT neurons in the performance of our tasks
is supported by the finding that both measures were affected by lesions
of area MT (Bisley and Pasternak 2000
; Rudolph
and Pasternak 1999
). In the Bisley and Pasternak (2000)
study, the loss in range thresholds was present only
when the stimulus that required motion integration was placed in the affected portion of the visual field. This effect was particularly pronounced at longer delays, suggesting that the ability to store the
information about complex motion requires the presence of MT. The
nature of the lesion effect in the direction difference thresholds
task, measured with coherently moving dots, was different. This deficit
was present only when the test stimulus was placed in the
lesioned field, suggesting that for that task, the retrieval/comparison process was affected by the loss of MT. In that case, there was no
effect of the lesion on storage because the deficit did not increase
with delay. These results showed that while the successful execution of
both tasks requires area MT, its specific contribution depends on the
nature of the motion stimulus and the type of discrimination.
Further evidence supporting a role of MT in the discrimination and
storage of motion information is provided by a recent microstimulation study (Bisley et al. 2001) in which an identical
direction range task was used. Stimulation had a profound effect on
performance both when applied during the presentation of the sample and
during the delay, and this effect was specific to the location of the visual field represented by the stimulated portion of MT.
In summary, the present results provide psychophysical evidence supporting a role of MT in the motion discrimination task that involves processing and storage of visual motion information. In addition, they provide new insights into the way the nervous system briefly stores and retrieves directional information extracted from a complex motion stimulus. Our findings suggest that the stored direction of complex motion is localized to the site of the comparison with the upcoming test stimulus. Since this comparison can be performed without loss at the spatial scale of receptive fields of neurons in area MT, we conclude that these neurons are likely to play an important role in the performance of the tasks.
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ACKNOWLEDGMENTS |
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We thank D. Moore for excellent technical assistance and A. Bahirwani, M. Schear, and B. Singer for software development. We are grateful to D. Knill and B. Merigan for comments on the manuscript.
This work was supported by National Eye Institute Grants R01 EY-05911 (T. Pasternak) and, in part, by P30 EY-01319 (Center for Visual Science).
Present address of J. W. Bisley: Laboratory of Sensorimotor Research, National Eye Institute, 49 Convent Dr., Bldg. 49, Bethesda, MD 20892.
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FOOTNOTES |
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Address for reprint requests: T. Pasternak, Dept. of Neurobiology and Anatomy, Box 603, University of Rochester, Rochester, NY 14642 (E-mail: tania{at}cvs.rochester.edu).
Received 29 November 2000; accepted in final form 20 April 2001.
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REFERENCES |
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