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INTRODUCTION |
Neurons at the higher stages of the primate visual system typically have very large receptive fields (RFs), and this may be important for identifying objects in a position-independent manner (Gross and Mishkin 1977
; Ito et al. 1995; Lueschow et al. 1994). However, large RFs frequently contain several objects, and the information communicated by a cell about each object is presumably degraded as the number of objects is increased (Miller et al. 1993a; Wise and Desimone 1988). A possible solution to this problem was reported by Moran and Desimone (1985), who found that neurons in area V4 and inferotemporal (IT) cortex were influenced by attention such that their responses primarily reflected the features of attended stimuli. Specifically, when two stimuli were presented inside the RF of the neuron being recorded and the monkey was instructed to attend to one of the stimuli, the neuron's response to the attended stimulus appeared to be of normal magnitude whereas its response to the ignored stimulus was suppressed. However, no attentional modulation was observed in V4 when only one stimulus was located inside the RF, presumably because this eliminated the ambiguities that arise when multiple stimuli fall within a cell's RF (RFs in IT cortex were too large to test the effects of placing 1 stimulus outside the RF).
The finding that attentional modulations occur only when multiple stimuli compete for access to a cell's RF appears to conflict with numerous behavioral and electrophysiological studies of attention in humans, in which attention effects have been observed under conditions that presumably did not lead to the simultaneous presence of attended and ignored stimuli inside a single RF in prestriate cortex (e.g., Luck 1995; Posner 1980; Prinzmetal et al. 1986). Recent single-unit studies have also indicated that spatial attention can modulate V4 responses when only one stimulus is located inside the RF (Connor et al. 1996; Motter 1993). The primary purpose of the present study was to address some of these discrepancies by recording single-unit responses in areas V1, V2, and V4 with a behavioral paradigm that was derived from human event-related potential (ERP) studies.
In this paradigm, attention was directed to one of two locations for each trial block, and sequences of stimuli were presented at the attended and ignored locations. The direction of attention was alternated between trial blocks, which allowed the response elicited by a given stimulus to be measured when it was attended and when it was actively ignored. The to-be-attended location was indicated to the monkey by means of instruction trials at the beginning of each block, and the monkey was required to remember which location was to be attended without the aid of any visible cues. This procedure avoids the stimulus-stimulus interactions that may occur when the to-be-attended location is indicated by the presentation of a cue stimulus at that location at the beginning of each trial.
In several previous studies, attending to a nonspatial feature was found to influence spontaneous (baseline) firing rates in addition to modulating stimulus-evoked responses (e.g., Ferrera et al. 1994; Haenny et al. 1988). In a study by Chelazzi et al. (1993), for example, inferior temporal cells that were selective for a given stimulus exhibited elevated baseline firing rates when that stimulus was the to-be-detected target in a visual search task. Changes in baseline activity such as this may reflect top-down bias signals that provide attended stimuli with an advantage over ignored stimuli, and these signals may play an important role in selective attention. However, effects of this nature have not been reported in prestriate cortex in studies of spatial selective attention. A secondary goal of the present study was therefore to determine whether baseline firing rates are influenced when attention is directed to spatial locations just as they are when attention is directed to nonspatial features.
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METHODS |
Subjects and surgical techniques
Many of the details of the recording techniques have been described previously (Miller et al. 1993b). Briefly, two adult male rhesus monkeys (Macaca mulatta) were surgically implanted with a headpost, a scleral eye coil, and recording chambers. These monkeys will be denoted monkey A and monkey B. Surgery was conducted under aseptic conditions with isofluorane anesthesia, and antibiotics and analgesics were administered postoperatively. The V4 recording chamber was placed over the prelunate gyrus (left hemisphere of monkey A and right hemisphere of monkey B), which was located in stereotaxic coordinates on the basis of a preoperative magnetic resonance imaging (MRI) scan. An additional recording chamber was used for V1 and V2 recordings, centered 15 mm lateral and 15 mm dorsal to the occipital pole in the right hemisphere; V1 and V2 recordings were obtained only from monkey B. However, recordings that were obtained from area V2 of a second monkey after the present study was completed have confirmed the major findings from monkey B (Reynolds et al. 1994). The skull remained intact during the initial surgery, and small holes (~3 mm diam) were later drilled within the recording chambers under ketamine anesthesia to expose the dura for electrode penetrations.
Confirmation of recording sites
Before the main study, several penetrations were made in each chamber to ensure that the electrode was in the appropriate visual area. This was determined by assessing RF sizes, topographic organization, and feature preferences at each site. Neurons in area V2 were recorded by passing the electrode through V1 on the opercular surface, through the underlying white matter, and into the portion of V2 that lies on the posterior bank of the lunate sulcus.
Monkey B had originally been implanted with plastic recording chambers, titanium screws, and a brass headpost. In this monkey, tungsten electrodes were inserted into the centers of the V1/V2 and V4 recording sites at the conclusion of the study, and a new MRI scan was obtained to verify the electrode placements. The electrodes were clearly visible in the scans. Monkey A was originally implanted with a stainless steel chamber and screws, which were removed at the conclusion of the study. This monkey was then rescanned with MRI to confirm the location of the recording hole in the bone over the prelunate gyrus.
Recording techniques
Recordings were obtained from a tungsten microelectrode that was controlled by a hydraulic microdrive. In most cases, two neurons could be recorded simultaneously and differentiated on the basis of the size and shape of the spike waveform, and an online spike-sorting computer was used to classify these spikes by means of a template-matching procedure. Although this system allowed the concurrent recording of two neurons, spikes arising from both neurons simultaneously (within a 1-ms interval) could not be detected. Over all conditions, 86% of the recordings were from two simultaneous cells that were both usable (i.e., significantly responsive and appropriately selective for the condition being run) and the remaining 14% were from a single usable cell.
Cells were isolated while the monkey performed the standard attention task (described below) with the use of stimuli presented in the same RF location that was measured in previous recordings at the same site. After one or two responsive neurons were isolated, we determined the RF borders (minimum response field method) and stimulus preferences of the neurons by moving and flashing colored bars on the screen under manual control while the monkey performed a fixation task. The RF borders were used to help place the stimuli in appropriate locations for the attentional manipulations but were not quantified. Depending on which experimental conditions were to be tested in a given session, we sometimes required that a cell be selective for orientation or color to proceed with the attentional manipulations.
Stimuli and task
The basic attention task is diagrammed in Fig. 1. Stimuli were presented at two locations, and the monkey had to attend to one of the locations and ignore the other to detect a target stimulus at the attended location. Attention was directed to one location in some trial blocks and to the other location in other blocks. This was achieved by the use of "instruction" trials at the beginning of each block, as detailed below. There were no visible cues indicating the attended location during each trial, and the monkey was therefore required to remember which location was to be attended.
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| FIG. 1.
Example stimulus sequences for sequential and simultaneous trials. The receptive field (RF) of the cell being recorded was mapped before data collection and is represented by the region enclosed by a dashed line. When both sequential and simultaneous trials were used with the inside/inside configuration (as shown here), different orientations and colors were used at the 2 locations, 1 of which was effective at driving the cell and 1 of which was ineffective. For many cells, however, all trials were sequential and the same stimuli were used at both locations. The same stimuli were typically used at both locations in the inside/outside configuration.
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The monkey initiated a trial by grasping and maintaining contact with a response bar. A fixation spot then appeared on the screen, and the monkey was required to fixate this spot (within a window of 0.5°) for the remainder of the trial. Location markers, consisting of white outline boxes (1.4 × 1.4°), appeared at both the attended and ignored locations 300 ms after fixation was achieved and remained visible throughout the trial (see Fig. 1). The sequence of task-relevant stimuli began 500 ms after the onset of the marker boxes. The sequences began with a series of nontarget stimuli (rectangles, typically 0.3 × 1.2°), which were presented at both attended and ignored locations. Each sequence ended with a target stimulus (a square, typically 1 × 1°) presented at the attended location. The monkey was required to respond by releasing the response bar when the target square appeared. Any loss of fixation or release of the response bar before the presentation of the target led to immediate termination of the trial, and data from such trials were excluded from all neural response analyses.
In one variant of this task, pairs of nontarget stimuli were presented simultaneously at the attended and ignored locations. On these "simultaneous" trials, each sequence consisted of between zero and five pairs of nontargets, followed by the simultaneous presentation of a target at the attended location and a nontarget at the ignored location. In another variant of the task, the stimuli were presented asynchronously at the two locations. On these "sequential" trials, the sequence began with between zero and five individual nontargets, presented in a randomized sequence at the two locations, followed by the presentation of the target at the attended location (see Fig. 1). For example, a typical sequential trial might consist of an attended-location nontarget, two ignored-location nontargets, and finally an attended-location target. The monkey was rewarded with a drop of orange juice for releasing the response bar within 500 ms of the onset of the attended-location target (which was always presented as the final item in the stimulus sequence). Each of the six sequence lengths (1-6 total stimuli) was equally likely, and all of the possible sequential orders at each sequence length were equiprobable in the sequential presentation conditions. Each stimulus was presented for 50 ms, and the interval between successive stimulus onsets varied randomly between 300 and 450 ms.
In both the sequential and simultaneous conditions, 20% of trials were "catch trials." On a catch trial, a square "pseudotarget" stimulus was presented at the ignored location before the end of the sequence; bar releases to these pseudotargets were considered errors and resulted in a time out. As on normal trials, catch trials ended with a true target presented at the attended location, and responses to this target were rewarded with a drop of juice. The presence of these catch trials was intended to force the monkeys to discriminate both the location and the shape of the target. Bar releases for squares presented at the to-be-ignored location were used as an indication that the monkey was not restricting attention to the appropriate location. The neural activity recorded on catch trials was not included in any of the analyses described below, with the exception of comparisons between attended-location and ignored-location targets.
The first few trials at the start of each block served as instruction trials that indicated which location was to be attended for that block (data from these trials were not included in any of the analyses presented below). On these instruction trials, the to-be-attended location was indicated by a brighter location marker box. After it became clear from the animal's performance that attention was being directed to the appropriate location, the brightened box was returned to the same brightness as the box at the to-be-ignored location. Thus there were no visible cues indicating which location was to be attended, and the monkey therefore had to rely on memory. The monkeys occasionally forgot which location was to be attended and began responding to the square stimulus at the to-be-ignored location on the catch trials. When this occurred, the block was interrupted and another sequence of instruction trials was initiated. Each block consisted of ~150 correctly completed trials. The attended and ignored locations alternated between blocks, and in most cases 6-10 blocks of data were obtained from each neuron under a given set of stimulus parameters. Neurons were not included in the analyses below unless data were obtained from at least two blocks of trials for each direction of attention (3-4 blocks per direction of attention was typical).
Stimulus conditions
We tested several different stimulus configurations to determine the effects of the spatial positions of the attended and ignored locations with respect to the RF borders, as summarized in Table 1. For some neurons, both locations were placed inside the classical excitatory RF, and this was termed the "inside/inside" configuration. For other neurons, an "inside/outside" configuration was used in which one location was inside the RF and the other was at one of three possible locations outside the RF: 1) just outside the RF, in the same visual quadrant as the inside-RF location; 2) in a symmetrical position across the vertical meridian from the inside-RF location; or 3) in a symmetrical position across the horizontal meridian from the inside-RF location. Most neurons were tested with only one of these spatial configurations, but some neurons were tested in two conditions.
For many neurons, only sequential or only simultaneous trials were presented. For other neurons, sequential and simultaneous trials were randomly intermixed, although sequential or simultaneous stimuli were never mixed within a trial. In other words, the sequence of one to six stimuli presented on a given trial consisted entirely of stimulus pairs on simultaneous trials or individual stimuli on sequential trials (see Fig. 1). In some cases, the stimuli at the two locations were identical, and the color and orientation of the stimuli were chosen to be the most effective features for driving the neuron. In other cases, the stimuli at one location were chosen to be the most effective for driving the cell and the stimuli at the other location were chosen to be the least effective. For example, if a cell responded well only to red stimuli, we used red stimuli (effective stimuli) at one location and green stimuli (ineffective stimuli) at the other. The nontarget rectangle and target square at a given location always had the same color and orientation. Except as noted below, the relative locations of the effective and ineffective stimuli remained constant across trial blocks, but they were varied randomly across neurons. The conditions in which different features were used were particularly important when stimuli were presented simultaneously at two locations within the RF, because the use of different features provided a means of observing different responses as a function of which stimulus was attended (e.g., the neuron might produce a greater response when attention was directed to the location of the effective stimulus). The various conditions are summarized in Table 1, which indicates 1) the number of cells recorded in each condition; 2) the visual area from which the recordings were obtained; 3) the positions of the stimuli with respect to the RF border; 4) whether the attended and ignored stimuli were presented sequentially, simultaneously, or both; and 5) whether the same or different stimulus features were used at the two locations.
Monkey A was slightly strabismic. Behavioral testing indicated that this monkey could perform the task equally well with either eye, and its left eye was occluded during all recordings (recordings were obtained only from area V4 in the left hemisphere in this monkey). Binocular presentation was used for the V1, V2, and V4 recordings in monkey B.
Data analysis
Baseline activity was measured as the firing rate during the 100-ms period preceding stimulus onset, and the sensory response was measured as the average firing rate from 50 to 175 ms poststimulus for V4 cells and from 30 to 130 ms poststimulus for V1 and V2 cells. The baseline activity that preceded a stimulus was subtracted from the stimulus-evoked response for that stimulus in all analyses, except as noted below. Because nontarget stimuli greatly outnumbered target stimuli, especially at the ignored location, the nontarget responses could be measured more reliably than the target responses and were therefore the focus of most analyses. Similarly, many more stimuli were presented at the early sequential positions than at the later sequential positions, and our analyses therefore focused on the first three nontarget stimuli in each sequence to avoid the statistical problems that can arise with widely varying sample sizes.
The effects of attention on neural activity were assessed by comparing the activity elicited by a particular stimulus when it was attended versus when it was ignored (i.e., when attention was directed to the other location). Statistical analyses were conducted on each neuron individually by means of an analysis of variance (ANOVA) on the population of single-trial firing rate measurements from that neuron. Each ANOVA had two factors: attention (whether the stimulus was attended or ignored) and sequential position (1st, 2nd, or 3rd position in the sequence). Any variance due to the order of trial blocks was ignored in these ANOVAs. In most cases, each single-cell ANOVA was based on data from 
150 trials. We also measured the mean firing rate across trials for each neuron and computed statistics on the population of neurons to assess the presence of consistent attention effects across neurons. A criterion level of P < 0.05 was used in all statistical analyses.
Some of the neurons produced larger responses at early positions in the stimulus sequence and others produced larger responses at late positions in the sequence. However, no consistent pattern was observed across cells, and the effects of attention did not vary across sequential positions. As a result, all of the data presented below were averaged across sequential position.
To quantify the size of the attention effects, we computed an attentional modulation index (AMI) in which the size of the attention effect was scaled by the size of the sensory response. Specifically, the firing rate during the prestimulus baseline period was subtracted from the mean firing rate during the sensory response (with the use of the time intervals described above), and the AMIwas then computed as: AMI = (attended 
ignored) 
(attended +ignored). The AMI can range between 1.0 (complete suppression of response to the attended stimulus) and +1.0 (complete suppression of the response to the ignored stimulus), with a value of 0 indicating no effect of attention. The AMI values can be transformed into a percent change measurement, in which the difference between attended and ignored responses is scaled by the size of the ignored response by the following formula: percent change = 100 × 2AMI (1 AMI).
We also examined the effects of attention across populations of cells by computing poststimulus histograms for each cell for every combination of stimulus type and direction of attention and then creating averaged poststimulus histograms across the population of cells within a given experimental condition. Several of these averaged histograms are displayed in the figures below. Although these averaged histograms provide a good measure of the central tendency in a population, they may, in principle, differ considerably from the histograms obtained for any of the individual cells. We therefore present histograms of this type only when the averaged histogram is qualitatively similar to the histograms obtained for a large number of individual cells.1
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RESULTS |
We begin this section by summarizing the behavioral performance of the monkeys and the recording sites, and then describe the responses of the neurons during task performance. The description of the neural responses begins with area V4 and then proceeds to area V2 and finally to area V1. Within each area, we describe the effects of attention on both sensory responses and baseline firing rates for the inside/inside and inside/outside conditions (where appropriate).
Behavioral performance
Trials were terminated because of eye movements on 5.7% of standard trials and 8.8% of catch trials. Excluding these trials, the monkeys responded correctly on 93.8% of standard trials and 87.1% of catch trials, with mean reaction times of 323 and 304 ms, respectively. The behavioral errors consisted mostly of false alarms (responses to 1 of the nontargets preceding the target) rather than misses (lack of any response). False alarms were more frequent on catch trials (11.8%) than on standard trials (4.3%), which almost certainly reflects occasional errors in focusing attention onto the correct location. Misses occurred relatively infrequently on both catch trials (1.1%) and standard trials (1.9%). Overall, the monkeys performed the task with a high level of speed and accuracy and exhibited selective processing of attended-location stimuli. There were no obvious differences in performance as a function of the different stimulus conditions except for the differences between standard and catch trials.
Number and locations of neurons
We collected complete data sets from 253 neurons in V4, 73 neurons in V2, and 79 neurons in V1; these numbers exclude neurons that lacked a significant excitatory response or appropriate stimulus selectivity. The recording sites are illustrated in Fig. 2. Neurons in all three areas had RFs centered in the lower quadrant of the contralateral field. The mean RF eccentricities in V4, V2, and V1 were ~4.5, 6, and 5°, respectively.
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| FIG. 2.
Locations of recording sites (shaded ovals) in monkey A (top) and monkey B (bottom) as determined from magnetic resonance imaging (MRI) scans. The recordings were obtained from the right hemisphere in monkey B, but the drawing has been reflected horizontally to facilitate comparison with monkey A. The V4 recording sites in both animals were located on the prelunate gyrus, between the lunate and superior temporal sulci. In the bottom drawing, the oval on the right indicates the location of both the V1 recording sites on the cortical surface and the sites of entry for the V2 recordings, which were located in the immediately underlying posterior bank of the lunate sulcus. io, Inferior occipital sulcus; lu, lunate sulcus; sts, superior temporal sulcus.
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Area V4, inside/inside configuration (condition A)
SIMULTANEOUS VERSUS SEQUENTIAL STIMULI (CONDITION A1).
In the first set of recordings, we attempted to replicate the results of Moran and Desimone (1985) by measuring the effects of attention on sensory responses in area V4 when the attended and ignored stimuli were presented simultaneously inside the RF. In addition, we also compared sequential and simultaneous stimulus presentation conditions to determine whether the effects of attention depend on the simultaneous presentation of attended and ignored stimuli. To measure the effects of attention on simultaneous trials, different colors and orientations were used at the two locations, as shown in Fig. 1. The stimuli were chosen so that one stimulus would be effective in driving the cell when presented by itself and the other would be ineffective (this configuration was used for both sequential and simultaneous trials). When effective and ineffective stimuli were presented simultaneously, the effects of attention were assessed by determining whether the cell's response was determined primarily by the features of the attended stimulus and not by the features of the ignored stimulus. This would yield a larger response when the effective stimulus was attended and a smaller response when the ineffective stimulus was attended.
We obtained data for both sequential and simultaneous trials from 34 feature-selective cells in this condition (condition A1 in Table 1). In each case, the cell's response was at least twice as large for the effective stimulus presented alone as it was for the ineffective stimulus presented alone (on average, the response elicited by the effective stimulus was ~10 times greater than the response elicited by the ineffective stimulus).
Consistent with the findings of Moran and Desimone (1985), attention had a large and consistent effect on simultaneous trials, with 85% of cells (29 of 34) showing a significantly larger response when attention was directed to the effective stimulus compared with when attention was directed to the ineffective stimulus (see METHODS for description of statistical tests). This can be seen in Fig. 3A, which shows poststimulus histograms averaged across all of the cells that showed a significant attention effect. It is important to note that the sensory stimulus was identical no matter which location was attended: the only difference was the monkey's internal attentional state.
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| FIG. 3.
A: poststimulus histograms for nontarget stimuli on simultaneous trials in condition A1, averaged over the 29 V4 neurons that showed a significant attention effect for these trials. Solid line: trials on which attention was directed to the effective stimulus. Dashed line: trials on which attention was directed to the ineffective stimulus. The responses shown here and in all figures below were collapsed across all sequential positions of the stimuli in the sequence, weighted by the total number of stimuli that occurred at each position. The histograms were calculated with 20-ms bins centered at 0, 20, 40 ms, etc. B: poststimulus histograms for the effective stimuli on sequential trials in the same condition, averaged over the 14 cells that showed a significant attention effect for these stimuli. C: poststimulus histograms for the ineffective stimuli on sequential trials, averaged over the 12 cells that showed a significant attention effect for these stimuli. D: probability distribution of the attentional modulation index (AMI) for sequential trials (effective stimuli) and simultaneous trials (effective + ineffective) over all 34 cells run in this condition. E: difference between the attended and ignored histograms shown in A-C.
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We also found an effect of attention on the sequential trials, as shown in Fig. 3, B and C. For both the effective and ineffective stimuli, the response tended to be larger when the stimulus was attended than when it was ignored. This effect was statistically significant for the effective stimulus in 41% of the cells (14 of 34) and for the ineffective stimulus in 35% of the cells (12 of 34; primarily in cells with some excitatory response to the ineffective stimulus). Only 9% of the cells (3 of 34) showed significant attention effects for both the effective and ineffective stimuli, but all except 2 of the cells with significant effects for sequential stimuli
whether effective or ineffectivewere among the 29 cells that showed significant effects for simultaneous stimuli. Effects in the opposite direction (i.e., larger responses for the ignored stimulus) were significant in only two cells for the effective stimuli and in none of the cells for the ineffective stimuli or for the simultaneous stimuli. Example results from an individual cell in this condition are shown in Fig. 4.
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| FIG. 4.
Data from a single V4 neuron in condition A1. A: poststimulus histograms for the effective stimulus on sequential trials. B: poststimulus histograms for the ineffective stimulus on sequential trials. C: poststimulus histograms for the effective-ineffective pair on simultaneous trials.
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There were several notable differences between the results for the sequential and simultaneous trials. First, the overall response to the simultaneous presentation of an effective stimulus and an ineffective stimulus inside the RF (simultaneous trials) was smaller than the response to the effective stimulus when presented alone (sequential trials). On average, the response was 77% larger for the effective stimulus presented alone than for the effective and ineffective stimuli presented together, and a difference in this direction was observed in 33 of the 34 cells that were recorded under these conditions.
A second important difference was that the effects of attention were considerably larger for simultaneous trials than for sequential trials. The difference in the magnitude of the attention effects for cells with significant effects can be seen by comparing Fig. 3, A and B. The size of the effect over the entire population can be seen in Fig. 3D, which shows the probability distribution of the AMI for simultaneous and sequential trials. Almost all of the AMI values were positive for simultaneous trials, and the mean AMI across the population for these trials was 0.24, which corresponds to a 63% increase in the response when the effective stimulus was attended compared with when it was ignored. In contrast, the AMI was frequently negative for the effective stimuli on sequential trials, and the population average was only 0.03, which corresponds to a 6% average increase in response when the stimulus was attended compared with when it was ignored (the AMI was an inappropriate measure for the ineffective stimuli because many cells produced no response to these stimuli, leading to a denominator of 0 in the AMI calculation). In addition, significant attention effects were found in approximately twice as many cells for simultaneous trials as for sequential trials. Thus the effect of attention on the sensory response was both stronger and more consistent when the attended and ignored stimuli were presented simultaneously rather than sequentially.
It should be noted that this pattern of small attention effects on sequential trials and larger attention effects on simultaneous trials was observed in both monkeys, with no obvious differences between monkeys in the size of the effects or the probability of obtaining a significant effect. It should also be noted that, although we have used a criterion of P < 0.05 for classifying attention effects as significant, many of the single-neuron attention effects described above were significant at the P < 0.001 level (~45% of the cells with significant effects on sequential trials and ~75% of the cells on simultaneous trials).
The time course of the attentional modulation also appeared to differ between simultaneous and sequential trials. This can be seen in Fig. 3E, which shows the difference between the attended and ignored histograms. On sequential trials, the attention effect began sharply at the onset of the sensory response for both the effective and ineffective stimuli (~60 ms poststimulus), with a second peak at ~200 ms after stimulus onset for the effective stimuli. In contrast, the attention effect for the simultaneous trials rose gradually from 60 ms poststimulus to a peak at ~150 ms. It should be noted that part of the late portion of the attention effect fell outside the 50- to 175-ms measurement window used in the analyses described above, but extending the end of the measurement window to 250 ms had only a minor effect on the outcome of these analyses (and led to difficulties in the analysis of the data from the inside/outside configuration because of shifts in baseline firing rates, as described below).
The poststimulus histograms shown in Fig. 3 were created with a binwidth of 20 ms, which makes it difficult to use them in determining the exact time at which attention began to modulate the response. A more precise analysis of the time course was therefore conducted on the basis of a statistical analysis that examined the entire population of 34 cells (similar results were obtained when only the cells with significant attention effects were examined). In this analysis, the average firing rate was measured for each cell in successive5-ms time bins following stimulus onset and a series ofANOVAs was then conducted that compared the mean firing rate across cells during a given time bin with the baseline firing rate. The onset latency of the sensory response was then assessed by finding the first time bin at which the mean firing rate across cells was significantly greater than the mean baseline firing rate. Similarly, the onset of the attention effect was assessed in a second set of ANOVAs, which was used to determine the first time bin at which the mean firing rate across cells was significantly greater when attention was directed to the effective stimulus than when attention was directed to the ineffective stimulus. Separate analyses were conducted for the simultaneous trials and for the effective stimuli on sequential trials (the ineffective stimuli were not analyzed in this manner). On sequential trials, both the sensory response and the attention modulation of the response first reached the P < 0.05 criterion level at the time bin centered at 60 ms poststimulus, which corresponds well with the time course shown in the histograms in Fig. 3. The sensory response also began at 60 ms poststimulus on simultaneous trials, but the attention effect on those trials did not become significant until 75 ms poststimulus. Thus attention influenced responses from the very beginning of the sensory response on sequential trials, but there was a short delay in the onset of the attentional modulation on simultaneous trials.2
SAME FEATURES CONDITION (CONDITION A2).
The data described above were obtained with different stimulus features at the attended and ignored locations. This differs from the procedure of most ERP studies of attention, which have typically used identical stimuli at both locations, and makes it possible that the attention effects described above were not purely due to spatial attention. We therefore tested 75 additional cells with the same stimulus features at both locations (condition A2 in Table 1). We again used the inside/inside spatial configuration, with an average spatial separation of 3.1° between the stimuli (center to center). Because there would be no straightforward way to measure the effects of attention for identical stimuli presented simultaneously inside the RF, all trials in this condition were sequential.
As in the experiments described above, many cells exhibited a larger response to a stimulus when it was attended than when it was ignored. ANOVAs calculated for each individual neuron indicated that attended stimuli produced significantly larger overall responses than ignored stimuli in approximately half of the cells (37 of 75). Only two cells showed a significantly smaller response for attended stimuli in this condition, which is approximately the number expected by chance. Figure 5A displays average poststimulus histograms for the 37 cells that exhibited significantly larger responses for attended stimuli, and histograms from a representative individual cell are shown in Fig. 5B.
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| FIG. 5.
A: average poststimulus histograms from the 37 V4 cells that showed a significant attention effect with the inside/inside configuration and identical stimuli at the attended and ignored locations (condition A2). Solid line: response of the neurons to a nontarget stimulus when it was attended. Dashed line: response of the neurons to the same stimulus when the other location was attended. B: poststimulus histograms from an individual neuron in this condition. C: difference between the attended and ignored histograms shown in A. D: probability distribution of the AMI over all 75 cells run in this condition.
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The size of the attentional modulation was quantified with the AMI, as shown in Fig. 5D. Although the majority of cells exhibited positive AMI values, indicating a larger response for attended stimuli than for ignored stimuli, the effects tended to be rather weak. The average effect across the entire population was only 0.04 (corresponding to an 8% modulation), and only three cells exhibited an AMI of >0.15 (i.e., an increase of 35%). This was approximately the same magnitude as observed for sequential trials in the previous condition (condition A1).
As shown in Fig. 5C, the time course of the attention effect in this condition was very similar to the time course observed for effective stimuli on sequential trials in condition A1. Specifically, the attention effect began sharply at ~60 ms after stimulus onset, with a second peak at ~200 ms poststimulus. A statistical analysis of the time course of the sensory response and the attentional modulation was conducted with 5-ms time bins, as described above for condition A1, and this analysis indicated that both the sensory response and the attentional modulation first became significant at the time bin centered at 60 ms poststimulus. Thus both the magnitude of the attention effect and its time course in this condition were highly similar to the sequential-trial results from the previous condition, which suggests that these effects reflect spatial attention rather than feature-based selective processing.
The data discussed above were obtained from nontarget stimuli, but similar results were also obtained for the target stimuli. Figure 6 shows poststimulus histograms for target stimuli, averaged across the same set of cells shown in Fig. 5A. As was observed for nontargets, target stimuli elicited larger responses when presented at the attended location than when presented at the ignored location (which occurred only on catch trials). Moreover, the magnitude of this effect was comparable with the effects observed for nontarget stimuli. Thus the effects of spatial attention in this condition appeared to be independent of the form of the stimulus and its meaning within the behavioral task.
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| FIG. 6.
Average poststimulus histograms from the same 37 V4 cells shown in Fig. 5 (condition A2), but showing the responses elicited by targets rather than nontargets. Note that targets were presented at the ignored location only on catch trials.
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Area V4, inside/outside configuration (condition B)
EFFECT OF ATTENTION ON SENSORY RESPONSES (CONDITIONS B1 AND B2).
To test the hypothesis that spatial attention influences V4 responses primarily when both the attended and ignored locations are inside the RF, 74 V4 cells were tested with the inside/outside configuration (conditions B1 and B2). In these cells, one location was centered inside the RF and the other location was placed at a mirror-image location across either the horizontal meridian or the vertical meridian; the average interlocation distance was 9.6°. No consistent differences in attention effects were observed between these two inside/outside configurations. Thirty-six cells were run only with sequential trials (condition B1, monkey A only), and 38 were run with both simultaneous and sequential trials (condition B2, monkey B only). In general, the effects of attention on the sensory response in these conditions were small and inconsistent in comparison with the inside/inside conditions, and the effects that were observed were complicated by substantial attention-related shifts in baseline firing rates.
We consider the sequential data first. As can be seen in the population-average poststimulus histograms shown in Fig. 7A, there was no consistent effect of attention on the peak stimulus-evoked response. Of the 74 cells tested in conditions B1 and B2, 7 gave a significantly larger response to attended stimuli than to ignored stimuli (positive attention effect) and 3 gave a significantly smaller response to attended stimuli (negative attention effect). This is a far smaller proportion of cells with significant attention effects than was found in the inside/inside conditions. There was, however, a clear effect of attention on the baseline firing rate: higher baseline firing rates were observed when attention was directed inside rather than outside the RF (see Fig. 7A). These baseline effects are described in more detail in the next section; here we only consider their possible influence on the measurements of firing rates in the poststimulus period.
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| FIG. 7.
A: poststimulus histograms for nontarget stimuli presented inside the RF on sequential stimulus presentation trials, averaged over all 74 V4 cells in the inside/outside configuration with the same features at both locations (conditions B1 and B2 combined). One location was inside the RF and the other was at a mirror-symmetrical position across either the horizontal or vertical meridian. B: probability distribution of the standard AMI and the AMI computed without baseline subtraction for the cells shown in A. C: poststimulus histograms for sequential trials, averaged over 38 cells in which data were obtained for both sequential and simultaneous stimulus presentation trials (condition B2 alone). D: poststimulus histograms for simultaneous trials, averaged over the same cells shown in C.
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Inspection of the single-cell histograms suggested that the sustained shift in baseline firing rate when attention was directed inside the RF may have carried over into the time interval of the stimulus-evoked response in some cells, and this may have artificially elevated firing rates during the sensory response period. If so, this might account for the few cells with significant positive attention effects in this condition. To compensate for any effects of differential baseline activity on the responses, an additional analysis was conducted in which the firing rate preceding a stimulus was subtracted from the firing rate measured during the stimulus-evoked response. When the single-neuron ANOVAs were recalculated with these adjusted firing rates, the number of cells with significant positive attention effects dropped from seven to two.
However, subtracting the baseline activity from the sensory response increased the number of cells with significant negative attention effects (i.e., smaller responses to a stimulus when it was attended compared with when it was ignored) from 3 to 14, because a higher baseline firing rate was subtracted from the responses when attention was directed inside the RF. This bias in favor of negative attention effects is ambiguous, however, because it is not clear whether these effects were due to a real decrease in the sensory response for attended stimuli or to an inappropriate subtraction of the baseline activity from the sensory responses of some cells. To ensure that none of the significant effects were caused by the baseline shift, it is possible to accept positive attention effects as valid only when they are significant with the baseline subtracted away and to accept negative attention effects as valid only when they are significant without the baseline subtracted away. With this criterion, there were two cells with significant positive effects of attention in V4 not accounted for by baseline shifts and three cells with negative effects, which is not significantly different from the number expected by chance (binomial theorem, P = 0.165). However, this conservative criterion probably underestimates the true number of cells with real effects of attention on the sensory response.
The difference in baseline activity when attention was directed inside versus outside the RF also affected the AMI values in the same way. Because the baseline firing rate was subtracted from the sensory response when the AMI was computed, the higher baseline firing rate on trials in which attention was directed inside the RF caused a negative shift in the AMI values for this condition (Fig. 7B, solid line). The mean AMI value was 0.10, which corresponds to a 22% decrease in the response when the stimulus inside the RF was attended compared with when it was ignored. To reduce the effect of the baseline activity changes, we computed an alternative AMI value without subtracting the baseline (Fig. 7B, dashed line). The AMI without baseline subtraction had a mean of 0.01, which is consistent with the lack of an attention effect on the peak stimulus-evoked response that can be seen in the poststimulus histograms for this condition (Fig. 7A). In sum, there was relatively little overall effect of attention on the stimulus-evoked response in the inside/outside configuration, and the modest effects that were observed could be interpreted as either positive or negative depending on how the baseline firing rate was treated.
Of the 74 total cells tested with the inside/outside configuration, 38 were tested with both simultaneous and sequential trials, and poststimulus histograms are shown for this subset of cells in Fig. 7, C (sequential trials) and D (simultaneous trials). Although simultaneous presentation greatly increased the size of the attention effect in the inside/inside configuration (condition A1 above), the effects of attention were virtually identical for simultaneous and sequential trials in the inside/outside configuration.
EFFECTS OF ATTENTION ON BASELINE FIRING RATES (CONDITIONS B1 AND B2).
As indicated above, attention had a consistent effect on the baseline firing rate of the cells in the inside/outside configuration even though there was little or no effect on the peak stimulus-evoked response. Specifically, individual ANOVAs computed on the data from each cell on sequential trials indicated that 40 of the 74 cells in conditions B1 and B2 had a significantly higher firing rate during the prestimulus interval when attention was directed inside the RF compared with when attention was directed outside the RF (similar results were obtained on simultaneous trials). Only two cells showed a significant effect in the opposite direction. For those cells showing a significant positive effect of attention in the baseline period, the baseline firing rate was 42% higher when attention was directed inside rather than outside the RF (14.4 vs. 10.1 spikes/s, respectively).
As shown in Fig. 8A, this shift in the baseline firing rate began at the start of the trial, ~175 ms after the onset of the location markers that appeared at the beginning of each trial (time 0 on the abscissa). This effect was also present in the time period during which a stimulus was presented outside the RF, as shown in Fig. 8B. The increase in baseline firing therefore began before the first task-relevant stimulus was presented and appeared to continue throughout the entire trial, but was not present at the very beginning of the trial.
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| FIG. 8.
A: activity at the beginning of the trial for the 40 V4 cells that showed a significant attention effect in the baseline period in conditions B1 and B2. Time 0: onset of the location markers that appeared 500 ms before the start of the task-relevant stimulus sequence. Note that the increase in firing that can be seen between 50 and 100 ms poststimulus reflects the onset of the location marker boxes. B: activity for the same cells shown in A during the time periods in which a nontarget stimulus was presented outside the RF. Time 0: onset of the outside-RF stimulus. C: probability distribution of the baseline shift index for all 74 cells in the inside/outside configuration with the same features at both locations (conditions B1 and B2 combined).
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To quantify the magnitude of this baseline shift across the entire population of cells, a baseline shift index (BSI) was computed in a manner similar to the AMI: BSI = (baseline when attending inside the RF baseline when attending outside the RF) (baseline when attending inside theRF + baseline when attending outside the RF). Baseline activity was quantified as the mean firing rate in the 100 ms preceding stimulus onset. The distribution of the BSI over the population is shown in Fig. 8C. This index was >0 for 80% of the 74 cells in this population, and the mean value was 0.13, which corresponds to a 30% higher firing rate when attention was directed inside the RF compared with outside the RF. Thus, although the baseline shift effect consists of an increase of only a few spikes per second in a given cell, it represents a substantial effect when the entire population is considered. Additional experiments concerning this effect are described in a later section.
CONTROL FOR STIMULUS SEPARATION (CONDITIONS B3 AND B4).
Although the effects of attention on sensory responses in the inside/inside and inside/outside configurations appeared to be very different, the spatial separation between the attended and ignored stimuli was larger in the inside/outside configuration; if attention effects simply decrease as the distance increases, this might explain the difference in results between the two configurations. We therefore tested an additional 59 V4 cells with the use of one location inside the RF and one location that was just outside the classical excitatory RF (conditions B3 and B4). All 59 cells were tested with sequential trials, and 29 were also tested with simultaneous trials. The outside location was often in the inhibitory surround of the RF, as indicated by an inhibition of the cell's baseline firing rate when a stimulus was presented alone at this location (significant inhibitory responses were observed in 17 of the 59 cells). The average separation between the two locations was 3.6°, which was approximately equal to the separation used in the inside/inside configuration.
As is evident in the population-average poststimulus histograms shown in Fig. 9A, bringing the two locations closer together did not lead to an increase in positive attention effects. Indeed, the average peak response was slightly smaller for attended stimuli than for ignored stimuli (i.e., a negative attention effect). An increase in baseline activity was observed when attention was directed to the location inside the RF, just as in the cells tested with a greater separation between the attended and ignored locations (conditions B1 and B2). The mean BSI across the population of cells in the present condition was 0.17, corresponding to a 41% change in baseline firing rates. Similar effects were observed on both simultaneous and sequential trials (see Fig. 9, Cand D).
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| FIG. 9.
A: poststimulus histograms for nontarget stimuli on sequential trials, averaged over all 59 V4 cells recorded with 1 location inside the RF and the other location in the same quadrant, just outside the RF (conditions B3 and B4 combined). B: probability distribution of the standard AMI and the AMI without baseline subtraction for the cells shown in A. C: poststimulus histograms for sequential trials, averaged over 28 cells in which data were obtained for both sequential and simultaneous trials (condition B4 alone). D: poststimulus histograms for simultaneous trials, averaged over the same cells shown in C.
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To assess these effects quantitatively, we first computed the firing rates in the poststimulus period, without subtracting baseline activity, for the stimuli presented inside the RF on sequential trials. Of the 59 cells tested, 5 showed significantly larger responses when the stimulus was attended compared with when it was ignored (positive attention effects) and 6 showed significant effects in the opposite direction (negative attention effects). As was the case with larger stimulus separations, the proportion of cells with significant attention effects was substantially smaller than it was when both stimuli were inside the RF. When the baseline activity was subtracted away to compensate for the shift in baseline, the number of cells with positive attention effects dropped to 1 and the number of cells with apparently negative attention effects increased to 25. Thus, if any possible effects of the baseline are eliminated by counting positive effects with the baseline subtracted and negative effects with the baseline included, there were seven cells with significant effects of attention; this is somewhat more than would be expected by chance (binomial theorem, P = 0.009).
The shift toward negative effects caused by subtracting the baseline activity can also be seen in the AMI population histogram (see Fig. 9B), which was computed with the baseline activity subtracted. The majority of cells had a negative AMI, and the mean AMI across the population was 0.14, which corresponds to a 33% decrease in the response to a stimulus when it was attended compared with when it was ignored. However, the average AMI was 0.00 when the AMI was computed without subtracting the baseline activity. Thus, depending on how the baseline was treated, the effects of attention on the sensory response in this condition were either small or predominantly negative.
Area V4, multiple-item displays (condition C)
A recent study by Motter (1993) suggested that cells in V4 may show attention effects with a single stimulus inside the RF, but only when many stimuli are presented simultaneously outside the RF. It is therefore possible that the absence of large attention effects in the inside/outside conditions described above was due to the use of only one stimulus outside of the RF. To examine this possibility, we conducted an experiment in which stimuli were presented at one location inside the RF and either one or four additional locations outside the RF, positioned as shown in Fig. 10A (condition C, monkey B only). Although this is fewer stimuli than used by Motter, they were positioned to create a relatively high density within the same hemifield as the RF, without actually encroaching on the RF. The two or five stimuli were always presented simultaneously and were presented in the same color and orientation. Trials with two stimuli were run in separate blocks from trials with five stimuli. Except for these differences, the conditions were unchanged from the recordings described above (e.g., condition A1).
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| FIG. 10.
A: stimulus configuration used to test the effects of the number of stimuli in area V4 (condition C). All 5 locations were used on 5-item trials, whereas only the locations labeled b and d were used on 2-item trials. For both trials types, however, attention was always directed toward either location b or location d, thus equating the spatial demands of the task across configurations. The stimuli were always presented simultaneously, and the same stimulus features were used at all locations. B: poststimulus histograms for nontarget stimuli on 2-item trials, averaged over 5 cells showing a significant attention effect. C: poststimulus histograms for nontarget stimuli on 5-item trials, averaged over 6 cells showing a significant attention effect. D: same as B, except that the average firing rate in the 100-ms prestimulus interval was subtracted away from the histograms to eliminate any effects of baseline differences. E: same as C, except that the average firing rate in the 100-ms prestimulus interval was subtracted away from the histograms to eliminate any effects of baseline differences.
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In general, the effects of attention were not strongly influenced by increases in the number of stimuli. Figure 10 shows average poststimulus histograms for the 15 cells studied under these conditions. When the responses were measured without subtracting the baseline, five cells showed significantly larger responses when attention was directed to the stimulus inside the RF on two-item trials, and six cells showed such effects on five-item trials (4 of these cells showed significant effects on both 2- and 5-item trials). One cell showed significantly smaller responses when attention was directed to the stimulus inside the RF for both two-item and five-item trials. As was the case for the inside/outside data in conditions B1-B4, there was a substantial shift in baseline activity when attention was directed inside the RF for both the two- and five-item trials in condition C. When we subtracted this baseline difference from the sensory responses, there was a decrease in the number of positive attention effects (3 significant cells for each array size), accompanied by an increase in the number of significant negative attention effects (2 significant cells for each array size).
To test whether the attention effects were significantly different for the two-item compared with the five-item trials, we measured the mean sensory response across trials for each cell (with the baseline firing rate subtracted away) and entered these values into a single two-factor ANOVA with direction of attention and number of stimuli as within-subjects factors. Although there was a tendency for the attention effects to be larger on the five-item trials, the interaction between the attention effect and the number of stimuli did not approach statistical significance [F(1,14) = 2.35, P = 0.15]. This analysis was also repeated with a longer measurement window of 50-250 ms to include the offset of the sensory response, but the interaction again failed to reach significance [F(1,14) = 2.83, P = 0.11]. Thus increasing the number of stimuli in the display did not substantially influence the attention effects under the present task and stimulus conditions.
Explorations of the baseline shift effect in area V4 (conditions B5 and D)
ROLE OF THE LOCATION MARKER BOXES (CONDITION B5).
There are many potential explanations for the increase in baseline activity that was observed when attention was directed inside the RF, and we explored several of these possibilities. We first tested the hypothesis that this shift reflected a change in the sustained sensory response elicited by the location marker boxes, which were present continuously throughout the entire trial. Specifically, if the sustained sensory response to the location marker box located inside the RF was larger when attention was directed inside the RF, then this would have produced an apparent increase in the baseline firing rate.
To test this hypothesis, we eliminated the location marker boxes in recordings from 26 V4 cells, with the use of the inside/outside configuration and sequential stimulus presentation (condition B5, monkey A only). One location was centered inside the RF and the other was in the mirror-symmetrical position across the vertical meridian. At the beginning of each trial, a 500-ms blank delay period replaced the 500-ms period during which the location markers were normally presented before the onset of the task-relevant stimulus sequence. The task-relevant stimuli were then presented on a screen that was entirely blank except for the fixation point (which was always located outside the RF). The to-be-attended location was indicated to the monkey by means of instruction trials at the beginning of each trial block in which a location marker box was presented only at the to-be-attended location; once the monkey began responding selectively to targets at this location, the location marker was eliminated and the recording for that block began.
As shown in Fig. 11, the baseline shift effect was indeed present under these conditions, beginning in the blank period that preceded the onset of the task-relevant stimuli and continuing throughout the entire course of the trial. Of the 26 cells, 18 showed significantly greater firing rates in the prestimulus interval when attention was directed inside the RF compared with when attention was directed outside the RF, and none of the cells showed a significant effect in the opposite direction. The distribution of the BSI across the population of cells in this condition was similar to the distribution observed when location markers were present (compare Figs. 8C and 11C), and the mean BSI value of 0.13 in the present condition was similar to the mean value of 0.12 that was obtained when location markers were present. Thus the baseline shift effect can occur in the absence of continuous stimulus presentation and presumably reflects a top-down input to the cells rather than a modulation of sensory processing.
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| FIG. 11.
A: poststimulus histograms averaged over all 26 V4 cells tested in the inside/outside configuration with no location markers (condition B5). These histograms show activity at the beginning of the trial, and time 0 represents the time point at which the location markers normally appeared, 500 ms before the start of the task-relevant stimulus sequence. This is analogous to Fig. 8A, except no location markers were present. B: poststimulus histograms for the same cells shown in A, but showing activity during the periods in which nontarget stimuli were presented outside the RF. C: probability distribution of the baseline shift index for the cells shown in A.
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ROLE OF TARGET FEATURES (CONDITION D).
A second possible explanation for the baseline shift effect is that it reflects an internal memory or template of the target stimulus, achieved by means of activating the cells that would normally respond to the target when it is actually presented. For example, a target consisting of a green square in the lower left quadrant of the display might be represented in short-term memory by an increased spontaneous firing rate in cells that are responsive to green squares and have RFs that include the lower left quadrant. If so, this would lead to the increase in baseline activity that was observed when attention was directed inside the RF, because all of the cells described above were responsive to the target stimulus when presented inside the RF. We tested this hypothesis by recording baseline activity in trial blocks in which the target stimulus was an effective sensory stimulus for the cell being recorded and comparing this with the baseline activity recorded in trial blocks in which the target stimulus features were ineffective in driving the cell (condition D, monkey A only). We predicted that, if the baseline shift effect reflects activation of cells that code the expected target stimulus features, then this activity would be found primarily in those neurons that would normally respond well to the target.
In this condition, stimuli were presented simultaneously at two locations, one centered inside the RF and the other at the mirror-symmetrical position across the vertical meridian. The stimuli were selected so that both the nontarget and target stimuli at one location were effective at driving the cell (when presented alone inside the RF), whereas both the nontarget and target stimuli at the other location were ineffective. As in the previous conditions, the nontarget and target stimuli at a given location shared the same color and orientation. The positions of the effective and ineffective stimuli remained fixed during each 3-min trial block, and each block was preceded by instruction trials to indicate the color, orientation, and location of the target for that block. The positions of the effective and ineffective stimuli were switched between blocks, and all combinations of stimulus location and attended location were tested in each cell.
We were able to test only eight highly selective cells from a single monkey in this condition, but the results were very clear in these cells, as summarized in Fig. 12. Specifically, the baseline shift was approximately equal in magnitude whether the target was an effective or ineffective stimulus, and no statistically significant differences were observed between these cases. In addition, significant baseline shift effects were observed equally often when the target was effective and when it was ineffective. Thus directing attention inside the RF leads to an increase in baseline activity even when the cell does not respond to the target stimulus presented inside the RF. This finding casts doubt on the hypothesis that the baseline shift effect reflects the activation of a template or memory that includes the color and orientation of the target stimulus, although it could reflect a memory that specifies only the location of the target.
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| FIG. 12.
Poststimulus histograms averaged over 8 highly selective V4 cells (condition D), tested with an effective stimulus inside the RF (A) or with an ineffective stimulus inside the RF (B). These histograms show the average of the nontarget trials, because these were the most numerous and therefore had the highest signal-to-noise ratio. The cells were selected on the basis of showing a large response for the effective target stimulus and little or no response for the ineffective target stimulus (<3 spikes/s on average), but these cells were also typically highly selective for the corresponding nontarget stimuli, as shown here.
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BASELINE SHIFTS IN THE INSIDE/INSIDE CONFIGURATION (CONDITION A2).
Although the baseline shift effect was observed with several different spatial configurations in the inside/outside configuration, this effect cannot ordinarily be observed in the inside/inside configuration because attention is always directed inside the RF in this configuration. However, we noticed that even when both stimulus locations were inside the RF, stimuli at one location elicited larger responses than stimuli at the other location for many cells, presumably because one location was closer to the center of the RF. This suggested that the baseline shift might actually be observable in the inside/inside configuration and that we could measure it if we compared the baseline activity when attention was directed to the more responsive versus the less responsive location.
To test this possibility, a regression analysis was conducted that quantified the relationship between a measure of location preference and the BSI across the 75 cells tested in the inside/inside sequential condition with the same features at both locations (condition A2). In this analysis, the two stimulus locations were arbitrarily labeled location 1 and location 2, and a location preference index (LPI) was computed by measuring the response to a stimulus at each location, irrespective of the direction of attention, with the baseline firing rate subtracted. The difference between the responses at the two locations was then scaled by the sum of the responses, and the LPI was computed as the scaled difference in response size: LPI = (location 1 response location 2 response) (location 1 response + location 2 response). This index ranges between +1.0 for complete location 1 preference and 1.0 for complete location 2 preference. The BSI also ranged between +1.0 for greater baseline activity when location 1 was attended and 1.0 for greater baseline activity when location 2 was attended, and was computed for this condition as follows: BSI = (baseline when attending to location 1 baseline when attending to location 2) (baseline when attending to location 1 + baseline when attending to location 2).
Figure 13A displays the results of this analysis and shows that directing attention to the more effective location led to a higher baseline firing rate than directing attention to the less effective location. The correlation between the LPI and BSI was strong and highly significant (r = 0.51, P < 0.001), and the slope of the regression line was fairly steep (0.68).
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| FIG. 13.
A: scatterplot of the relationship between the baseline shift index and the location preference index (LPI), based on all 75 V4 cells from the original inside/inside configuration (condition A2). For these cells, all trials were sequential and the same features were used at both locations. B: activity at the beginning of the trial, averaged over 16 cells that showed substantial preference for 1 of the 2 locations (LPI less than |
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Abstract
Introduction
Abstract
