| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
The Journal of Neurophysiology Vol. 80 No. 6 December 1998, pp. 2918-2940
Copyright ©1998 by the American Physiological Society
1 Laboratory of Neuropsychology, National Institute of Mental Health, Bethesda, Maryland 20892-4415; 2 Dipartimento di Scienze Neurologiche e della Visione, Sezione di Fisiologia, University of Verona, Verona 37134, Italy; 3 Medical Research Council Applied Psychology Unit, Cambridge CB2 2EF, United Kingdom; 4 Department of Brain and Cognitive Sciences and The Center for Learning and Memory, MIT, Cambridge, Massachusetts 02139
 |
ABSTRACT |
|---|
 Abstract
 Introduction
Methods
Results
Discussion
References
|
|---|
Chelazzi, Leonardo, John Duncan, Earl K. Miller, and Robert Desimone. Responses of neurons in inferior temporal cortex during memory-guided visual search. J. Neurophysiol. 80: 2918-2940, 1998. A typical scene will contain many different objects, few of which are relevant to behavior at any given moment. Thus attentional mechanisms are needed to select relevant objects for visual processing and control over behavior. We examined this role of attention in the inferior temporal cortex of macaque monkeys, using a visual search paradigm. While the monkey maintained fixation, a cue stimulus was presented at the center of gaze, followed by a blank delay period. After the delay, an array of two to five choice stimuli was presented extrafoveally, and the monkey was rewarded for detecting a target stimulus matching the cue. The behavioral response was a saccadic eye movement to the target in one version of the task and a lever release in another. The array was composed of one "good" stimulus (effective in driving the cell when presented alone) and one or more "poor" stimuli (ineffective in driving the cell when presented alone). Most cells showed higher delay activity after a good stimulus used as the cue than after a poor stimulus. The baseline activity of cells was also higher preceding a good cue, if the animal expected it to occur. This activity may depend on a top-down bias in favor of cells coding the relevant stimulus. When the choice array was presented, most cells showed suppressive interactions between the stimuli as well as strong attention effects. When the choice array was presented in the contralateral visual field, most cells initially responded the same, regardless of which stimulus was the target. However, within 150-200 ms of array onset, responses were determined by the target stimulus. If the target was the good stimulus, the response to the array became equal to the response to the good stimulus presented alone. If the target was a poor stimulus, the response approached the response to that stimulus presented alone. Thus the influence of the nontarget stimulus was eliminated. These effects occurred well in advance of the behavioral response. When the array was positioned with stimuli on opposite sides of the vertical meridian, the contralateral stimulus appeared to dominate the response, and this dominant effect could not be overcome by attention. Overall, the results support a "biased competition" model of attention, according to which 1) objects in the visual field compete for representation in the cortex, and 2) this competition is biased in favor of the behaviorally relevant object by virtue of "top-down" feedback from structures involved in working memory.
A typical visual scene contains many different objects, not all of which can be fully processed by the visual system at any given time. Thus attentional mechanisms are needed to limit processing to items that are currently relevant to behavior (Broadbent 1958 Three adult male rhesus monkeys weighing 7.5-9 kg were used. The general methods were described previously (Miller et al. 1993b Stimuli
The stimuli consisted of a set of 24 complex, multicolored pictures presented on a computer graphics display. The stimuli ranged from 1 × 1° to 2 × 2° in size and were digitized from magazine pictures; some were of identifiable objects, and some were simply colored textures and patterns.
Saccade task
TWO-STIMULUS ARRAYS.
The basic task is schematically shown in Fig. 1. Each trial began with the presentation of a fixation target (white spot, 0.1° diam) at the center of the display, which the monkey was required to fixate. After an interval of 700-1,000 ms, a cue stimulus was presented over the fixation target for 300 ms, followed by a 1,500-ms blank delay period. The fixation target remained on during the delays, and the animal was required to maintain fixation within a 1° diam window from the beginning of the trial until the end of the 1,500-ms delay. Eye movements at any time from the onset of fixation to the end of the delay period were counted as errors, and the trial was aborted.
ONE-STIMULUS ARRAYS.
On some trials, the search array was replaced by a single stimulus, which was either the good or poor stimulus for the cell. The stimulus appeared randomly at each of the positions used for the two-stimulus arrays. The stimulus was equally often a target and a nontarget, depending on the preceding cue. These trials were treated as target-present and target-absent trials, respectively, which are described in the previous section. All other conditions of the task were the same as in the task with two-stimulus arrays.
FIXATION-ONLY TASK.
On some trials, a cue stimulus was followed by a single matching or nonmatching test stimulus at the center of gaze. The cue was on for 300 ms, the delay was 1,500 ms, and the final stimulus was on for 300 ms. On these trials, the animal was rewarded for simply maintaining fixation until the end of the stimulus sequence. The purpose of these trials was to test for any differences in delay activity when memory of the cue stimulus was not required.
THREE- AND FIVE-STIMULUS ARRAYS.
For some cells, the search arrays were composed of either three or five stimuli. In this condition, stimuli were evenly spaced along a hemicircle within the contralateral hemifield. One of the stimuli was a good stimulus for the cell, and the remaining stimuli, all different from one another, were poor ones. All other conditions were the same as with the two-stimulus arrays, and there was an equal number of target-present and target-absent trials.
BLOCKING AND INTERLEAVING OF TRIALS.
Cells were typically studied with 400-480 trials, which allowed for 10-12 correct trials for each trial type. For cells studied with the combination of two-stimulus arrays, one-stimulus arrays, and the fixation-only control task (i.e., excluding cells studied with 3- and 5-stimulus arrays), trials were divided among these three tasks in the ratio of 6:3:1, respectively. Trials for the three different tasks were run in separate blocks. A given block typically contained 10-30 trials, and each block was typically repeated 2-3 times, randomly interleaved, during the recording of an individual cell.
Lever release task
Cells studied in the lever release task were tested using a combination of two-stimulus arrays, single-stimulus arrays, and a foveal task. However, we did not test three- or five-stimulus arrays in the lever release task.
TWO-STIMULUS ARRAYS.
The search task with saccades described above required both an eye movement and the explicit localization of the target. To test whether these two factors were necessary for any neuronal effects of attention in the search task, we tested some cells in a variation of the task in which the behavioral response was a lever release rather than a saccade. The monkey grasped a lever to initiate the trial. A fixation target then appeared at the center of the display, which the monkey was required to fixate for the remainder of the trial. If the animal broke fixation, the trial was terminated. After an interval of 700-1,000 ms, a cue stimulus was presented over the fixation target for 300 ms, followed by a 1,500-ms blank delay period. At the end of the delay, a search array was presented for 500 ms at the same extrafoveal locations tested in the saccade version of the task. On half the trials (target-present, or match, trials), the array contained a stimulus that matched the initial cue, and the monkey was rewarded for releasing the lever within 700 ms of array onset. On the other half of the trials (target-absent, or nonmatch, trials), neither stimulus in the array matched the initial cue, and the monkey had to hold the lever until after an additional 1,000-ms delay from array offset and successive presentation of a single matching stimulus.
ONE-STIMULUS ARRAYS.
On some trials, the search array contained only a single stimulus. If the stimulus matched the previous cue, the trial was treated as a target-present trial, whereas if it did not match the previous cue, it was treated as a target-absent trial (see above).
Data analysis
Attentional effects were evaluated at both the single cell and population level using analyses of variance (ANOVAs) and t-tests. When statistical tests were conducted individually on every cell in a population, a P < 0.05 criterion was used to evaluate whether the test was significant. In such cases, the number of cells with significant effects in the population was evaluated using the binomial theorem and was always found to be different from chance except where otherwise stated. Cells were assessed for visual responsiveness by conducting paired t-tests on the response to each cue stimulus in a time window from 50-300 ms poststimulus onset, compared with the firing rate in a 300-ms prestimulus period. Visual selectivity was assessed by conducting an ANOVA and post hoc t-tests on the responses to the different cue stimuli. Population response histograms were created by averaging the responses of all neurons, with time bins of 10-50 ms. It made virtually no difference whether the histograms were averaged from actual firing rates or from responses normalized to the peak rate; therefore the figures show the unnormalized responses.
Histology
At the conclusion of the experimental sessions, fluorescent dyes were injected through a cannula at the boundaries of the recording area. A few days later, after an overdose of pentobarbital sodium, the animals were perfused transcardially with formalin. Sections were cut every 50 µm, stained with thionin, and examined for electrode tracks and dye marks. Although older tracks could not be visualized, recording sites could be inferred from the identifiable tracks and the location of the dye marks.
Fixation errors (before the presentation of the choice stimuli) were made on fewer than 10% of the trials, and these trials were excluded from the following performance scores. In the saccade version of the task, the animals made a saccade to the correct stimulus on 86 and 84% of the trials, with two- and five-stimulus choice arrays, respectively. On target-absent trials of this task, the animals inappropriately made a saccadic eye movement to one of the nontarget stimuli on almost 40% of the trials. Performance on the bar-release version of the task was 82% correct.
Search task with saccades (1- and 2-stimulus arrays): cue-related activity
The results from 83 stimulus-selective cells were analyzed in the search task with saccades (an additional 17 stimulus-selective cells studied with 3- and 5-stimulus arrays will be described in a later section). All of them were studied in the blocked cue design, and 28 of the 83 cells were also studied in the random cue design, in which the cue varied randomly from trial to trial.
RESPONSES TO THE CUES AND IN THE SUBSEQUENT DELAY PERIOD.
As expected, the responses to the good cue in the search task were invariably greater than to the poor cue. In addition, many cells had higher maintained activity in the blank delay period after the offset of the good cue than in the delay after the offset of the poor cue. This maintenance of cue-specific activity throughout the delay period could be evidence of the "bias" in favor of the cells representing the target stimulus, as predicted by the biased competition model. A related possibility is that the activity during the delay is part of the mechanism for maintaining the memory of the cue.
ACTIVITY PRECEDING ONSET OF THE CUE.
Although we interpreted the higher maintained activity in the delay following the good cue as evidence for a bias in favor of cells coding the cue-target stimulus, we also considered whether it might be simply a long-lasting aftereffect of the response to the good cue. Two lines of evidence argue strongly against this. One line of evidence comes from the fixation-only trials, which are discussed in the next section. The other evidence is that for cells studied in the blocked design, the higher maintained activity following the good cue was paralleled by a similar increase in maintained activity preceding the onset of the good cue. This can be clearly seen in the single cell and population histograms of Figs. 3 and 4. In these figures, cells studied in the blocked design had higher maintained activity both preceding and following the good cue, whereas the cells studied in the random cue design had higher activity only in the interval following the good cue. The increase in activity preceding the good cue in the blocked design could not be an artifact of nonspecific changes in cell activity across the session because the blocks of trials with the good and poor cue were interleaved. We also considered the possibility that the higher activity preceding the good cue in the block design may have been due to a lingering response to the good stimulus as target on the previous trial. To test for such lingering responses, we examined the activity preceding the cue in the random design, comparing the precue activity on trials immediately after a trial with the good stimulus as the target with the precue activity on trials immediately after a trial with the poor stimulus as target. There was no difference in activity across the two types of trials, ruling out the possibility that the activity preceding the cue was due to a lingering response to the target. Thus the maintained activity preceding the cue in the blocked design was apparently a purely "cognitive" phenomenon related to expectation of a specific cue and could not be a sensory response.
FIXATION-ONLY TRIALS.
Fixation-only trials were included in the saccade task as a control condition. These trials provided a second line of evidence against the possibility that the cue-specific activity before and after the cue in the saccade task was some type of sensory phenomenon. On these trials, the cue stimulus was followed, after a 1,500 ms delay, by a matching or nonmatching stimulus at the center of gaze. The animal was rewarded for simply maintaining fixation throughout the trial. As in the blocked cue design for the saccade trials, the particular cue stimulus used at the start of the trial was kept constant for a block of trials. Thus the sensory conditions both before and after the cue (until the end of the delay) were identical to those of the saccade trials, but the cue had no behavioral relevance for the animal. All of the cue-specific activity that had been found both preceding and following the cue in the saccade task was eliminated in this condition. Thus the cue-specific shifts in baseline firing rates found in the saccade task must have been caused by the animal actively using the cues in the task.
Search task with saccades (1- and 2-stimulus arrays): arrays in contralateral field
A second major goal of the study was to test for a role of IT neurons in selecting the target stimulus out of the search array. Of the 83 cells studied in the blocked cue design, 58 gave a significant response to at least 1 of the 2-stimulus array configurations tested, as did 22 of the 28 cells studied in the random design. The analyses of responses to the search array were restricted to these cells.
EFFECTS OF TARGET SELECTION ON TWO-STIMULUS ARRAYS.
Because the results differed depending on whether the stimuli were located in the contralateral or ipsilateral field, we will present the contralateral field data first. In this configuration, one stimulus was in the upper quadrant and one in the lower quadrant, and we pooled the data across the two possible spatial configurations of the good and poor stimuli. For simplicity, we will focus on the data in the blocked cue design, which were very similar to the data from the random cue design.
EFFECTS OF TARGET SELECTION ON ONE-STIMULUS ARRAYS.
To help interpret the results with two stimuli inside the RF, we examined the effects of target selection with only a single stimulus inside the RF. Competition between the good and poor stimulus was obviously eliminated when there was only a single stimulus inside the RF. In target-present trials, the test stimulus matched the previous cue, and the animal was rewarded for making a saccade to it. In target-absent trials the test stimulus did not match the previous cue, and the monkey was required to simply maintain fixation until a third, matching, stimulus appeared at the end of the trial (see METHODS). To assess the effects of selection on the response to an individual stimulus inside the RF, we therefore compared the response to the good and poor stimuli presented alone on target-present trials with the response to the same stimuli presented alone on target-absent trials.
SENSORY INTERACTIONS BETWEEN GOOD AND POOR STIMULI.
Once we established that the effects of target selection were much larger when two stimuli competed within the RF, we next examined the nature of that competition. The first step was to determine the sensory response to the good stimulus alone, the poor stimulus alone, and the combined good and poor stimulus in the array. It would have been optimal to obtain these sensory responses in the absence of both target selection and attention. Unfortunately, it is not possible to require monkeys to attend to "nothing" while stimuli are presented. However, the target-absent trials afforded the opportunity to measure responses to the different sensory conditions in the array with target selection eliminated as a factor and attention presumably held constant. Trials in which the third, neutral, stimulus appeared as a cue were used to obtain the sensory response to the array composed of both the good and the poor stimulus (see Fig. 1). Although the monkey must have attended to the array, there was no basis for the monkey to attend selectively to one stimulus rather than the other, because neither stimulus was a target. Only the data from the blocked cue design were used for these analyses because not all trial types were run in the random cue design.
EFFECT OF TARGET SELECTION ON SENSORY INTERACTIONS.
The next step was to ask how target selection affected the sensory interactions (described in the previous section) between the good and poor stimuli in the array. To this aim, we compared the responses to 1) the two-stimulus array with the good stimulus as the target, 2) the two-stimulus array with the poor stimulus as the target, 3) the good stimulus presented alone, and 4) the poor stimulus presented alone. For the latter two responses, we used the data from the one-stimulus arrays in target-present trials, although as indicated above, the responses on these trials were the same as on the target-absent trials.
Search task with saccades (1- and 2-stimulus arrays): arrays across the VM
The results described in the previous sections were obtained when the search array was confined entirely to the contralateral field. However, the results changed significantly when one of the stimuli in the array was located within the contralateral field and one was in the ipsilateral field. Figure 15 shows the response, averaged across the population, to the two-stimulus array when the good versus poor stimulus was the target. Figure 15A shows the results from the configuration in which the good stimulus was in the contralateral field and the poor stimulus was in the ipsilateral field, whereas B shows the results from the opposite spatial configuration. Only the data from the blocked cue design are used in these analyses, because some of the comparisons described in the following sections were not run in the random cue design.
SENSORY INTERACTIONS BETWEEN GOOD AND POOR STIMULI.
Given that the target effects were so different in the cross-midline condition compared with within-hemifield, we asked whether there would be comparable differences in the sensory interactions between the two stimuli in the array. We therefore determined the sensory response to the good stimulus alone, the poor stimulus alone, and the combined good and poor stimulus in the array in the cross-midline condition. We used the responses in the target-absent trials as the best measure of sensory responses independent of target selection, just as we did when the array was confined to the contralateral field.
EFFECTS OF TARGET SELECTION ON SENSORY INTERACTIONS.
The last step was to ask how, in the target-selection trials, the response to the array with the good or poor stimulus as the target compared with the response to those same stimuli presented alone. For this purpose, we compared the responses to 1) the array with the good stimulus as the target, 2) the array with the poor stimulus as the target, 3) the good stimulus presented alone, and 4) the poor stimulus presented alone. For the latter two responses, we used the data from the target-present trials, although as indicated above, the responses on these trials were the same as on the target-absent trials.
Retinal information
Because the target effects differed according to the spatial configuration of stimuli in the array, we asked whether the cells showed any spatial selectivity in their responses to the individual stimuli presented at different retinal locations. For this analysis, we used the responses to the good stimulus presented alone, pooling target-present and target-absent trials. Responses were averaged over a 50- to 200-ms time window after stimulus onset. According to a t-test computed on the responses to the good stimulus in the upper contralateral quadrant versus lower contralateral quadrant, 6/58 cells showed a significant difference in response at the 2 locations (which is not different from the number of significant effects expected by chance according to a binomial test, P > 0.05). When we computed the same test on responses to the good stimulus in the lower contralateral quadrant versus the lower ipsilateral quadrant, 6/58 cells responded significantly better with the stimulus in the contralateral field, and 4/58 cells responded significantly better with the stimulus in the ipsilateral field. Thus, although most IT neurons gave similar responses to an individual stimulus in any quadrant, a few cells seemed to convey significant information about whether the stimulus was in the contralateral or ipsilateral visual field.
Three- and five-item search array experiment
Under natural conditions, there will frequently be more than two objects in the visual field. To test whether the target effect would generalize to larger search arrays, 20 additional cells were studied in 1 monkey using the same general task but with search arrays consisting of 3 or 5 stimuli arranged in a hemicircle within the hemifield contralateral to the recorded hemisphere. One of the stimuli was a good stimulus for the cell, and the remainder were poor stimuli. The poor stimulus that caused the least response was treated as the single "poor stimulus" in all of the experimental manipulations and analyses to be presented below. Different cue stimuli were presented in separate blocks.
Lever release task
It is possible that both the cue-related activity and the target effects described in the previous sections are specific to tasks that require an eye movement to a target. To test the generality of the target effects, we measured IT responses in an additional monkey taught a version of the search task with a lever release as the behavioral response. The monkey was presented with a cue stimulus at the start of the trial. After a delay period, a search array of one to two stimuli appeared, and the animal was rewarded for releasing a lever if any stimulus in the array matched the previous cue (see METHODS). To facilitate comparison with the data from the saccade task, we will refer to the matching stimulus as the target and the nonmatching stimulus as the nontarget, even though the animal did not make a response directed to the location of the matching stimulus.
DELAY ACTIVITY.
As was found for cells in the saccade task, the level of delay activity following the cue depended on which stimulus was used as the cue. Across the population of 61 selective cells, the average firing rate in the last 500 ms of the delay was 3.9 spikes/s following the good cue and 2.9 spikes/s following the poor cue, which was a significant difference according to a paired t-test (P < 0.001). For 28/61 cells that individually showed a significant difference in delay activity following the good versus poor stimulus as cue, the average firing in the delay was 3.7 versus 1.7 spikes/s, respectively. Only 2 of 61 cells showed significantly higher activity in the delay following the poor stimulus used as a cue than following the good stimulus.
EFFECTS OF TARGET SELECTION ON TWO-STIMULUS ARRAYS.
Of the 61 stimulus selective cells considered in the previous section, only 44 gave a significant response to any of the extrafoveal 2-stimulus arrays, according to a paired t-test computed on the pre- and poststimulus firing rates. These 44 cells are the subject of the analyses presented below.
EFFECTS OF TARGET SELECTION ON ONE-STIMULUS ARRAYS.
As in the saccade task, we asked whether target selection had any effect on responses when there was a single stimulus within the RF, i.e., in the absence of competition between the stimuli in the array. We therefore compared the response to the good and poor stimuli presented alone on target-present trials versus target-absent trials. This analysis showed that, as in the saccade task, there was no evidence for the type of target effects found when the good and poor stimuli were presented together in the array.
SENSORY INTERACTIONS BETWEEN GOOD AND POOR STIMULI.
Because the effects of target selection were different when two stimuli competed within the RF than when there was a single stimulus, we next examined the nature of that competition. As in the saccade task, we compared the sensory response to the good stimulus alone, the poor stimulus alone, and the combined good and poor stimulus in the array in target-absent trials.
EFFECTS OF TARGET SELECTION ON SENSORY INTERACTIONS.
The next step was to ask how the response to the array with the good or poor stimulus as the target compared with the response to those same stimuli presented alone. For this comparison, we compared the responses to 1) the two-stimulus array with the good stimulus as the target, 2) the two-stimulus array with the poor stimulus as the target, 3) the good stimulus presented alone, and 4) the poor stimulus presented alone. For the latter two responses, we used the data from the one-stimulus arrays in target-present trials, although as indicated above, the responses on these trials were about the same as on the target-absent trials.
RESPONSES TO TWO-STIMULUS ARRAYS PRESENTED ACROSS THE MIDLINE.
Positioning the two stimuli in the search array across the midline of the visual field virtually eliminated the target effect, as was found in the saccade task. Indeed, the effects of this manipulation were so similar in the saccade and lever release tasks that they will not be further described here.
A typical scene will contain many different objects, few of which are relevant to behavior at any given moment. Thus attentional mechanisms are needed to select relevant objects for visual processing and control over behavior. The results of the present study, taken in conjunction with the results of previous studies of attention in IT cortex, suggest that attentional selection is accomplished by suppressing the responses of IT neurons to irrelevant stimuli within their RF (Chelazzi et al. 1993 Bias and delay activity
The stimulus-selective activity found during the delay period of the search task is evidence for the predicted bias in favor of the behaviorally relevant stimulus, i.e., the stimulus used as the cue and target. During the delay, most cells had a higher maintained firing rate when the good stimulus was the cue than when the poor stimulus was the cue. The change in maintained rate might come about as a result of either increased excitability or decreased inhibition.
Resolution of competition
If cells had linear input-output response functions, a cell's response to two stimuli, A and B, presented simultaneously within the RF should equal the sum of the responses to A and B presented alone. According to the biased competition model, however, two stimuli presented within the RF should compete for the cell's response. Therefore, in the absence of any attentional bias in favor of one stimulus, the response to the pair should, on average, fall between the responses to A and B presented alone. If either one has a competitive advantage because it is a "stronger" sensory stimulus (e.g., because of higher contrast, etc.) (see Reynolds and Desimone 1997 SENSORY COMPETITION.
The competition model predicts that two stimuli within the same RF should, on average, have a mutually suppressive interaction. This was tested in the target-absent condition, in which neither stimulus in the array was a target and, thus, there was presumably no attentional bias in favor of either one. As predicted, the response to the two-stimulus arrays in the contralateral field was, on average, intermediate between the responses to the good and poor stimuli presented alone. Consistent with this, several previous studies also found that IT responses to two stimuli presented simultaneously were less than responses to the preferred stimulus presented alone (Miller et al. 1993a EFFECTS OF ATTENTION.
The results revealed a powerful effect of attention on the competition between stimuli in the array, at least when the two stimuli were in the contralateral visual field. When the good stimulus was the target, the response to the array was similar to the response to the good stimulus presented alone, whereas when the poor stimulus was the target the response approached the response to the poor stimulus alone. Thus the major effect of attending to a stimulus was to eliminate the excitatory or suppressive influence of the unattended stimulus, as predicted by the biased competition model.
Lever release versus saccade task
The basic effects of attention in visual search are independent of the specific motor response to the target, as we found qualitatively similar effects of attention in both the saccade and lever release versions of the task. The magnitude of the attentional effect, however, was much larger in the saccade version of the task. It is possible that the difference is due to individual differences among animals, because there were only two animals studied in the saccade task and one animal in the lever release task. Alternatively, the attentional effects may have been larger in the saccade task either because the task was more difficult or because the task required the explicit spatial localization of the target, or because of both factors. In area V4, attentional effects become larger as task difficulty is increased (Spitzer et al. 1988 Serial versus parallel processing
A long-standing issue in the psychology of attention has been whether a target object in a complex visual display is found using serial or parallel neural mechanisms (for reviews, see Desimone and Duncan 1995
INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References
; Bundesen 1990; Desimone and Duncan 1995; Duncan 1996; Neisser 1967; Treisman 1969; Tsotsos 1990). Mechanisms for spatially directed attention have been described not only in the dorsal stream that mediates spatial perception (e.g., Bushnell et al. 1981; Colby et al. 1996; Lynch et al. 1977; Robinson et al. 1978, 1995; Steinmetz et al. 1994; Treue and Maunsell 1996) but also in the ventral stream that mediates object recognition (Connor et al. 1996; Luck et al. 1997; Moran and Desimone 1985; Motter 1993), including areas V2, V4 and the inferior temporal (IT) cortex. When two or more stimuli are located within the receptive field (RF) of cells in V2 or V4, and the animal attends to one of them, the cell's response is predominantly determined by the attended stimulus (Luck et al. 1997; Moran and Desimone 1985). The response to the unattended stimulus may be completely blocked, even though it is an otherwise optimal sensory stimulus within the RF. Comparable effects are found in IT cortex, although the RFs of IT cells are much bigger and the attentional effects generalize over a much larger spatial range than in V2 and V4 (Moran and Desimone 1985). By contrast, more subtle and variable effects of spatially directed attention are found when only a single stimulus is located within the RF of a V2 or V4 neuron (Haenny et al. 1988; Luck et al. 1997; Maunsell et al. 1991; Moran and Desimone 1985).
; Desimone and Duncan 1995; Duncan 1996; Luck et al. 1997). The model is based on the notion that objects in the visual field activate neural representations in the cortex in a parallel fashion, and that the cells participating in these representations engage in competitive interactions. Frequently, these interactions are evidenced by suppresive effects of one stimulus on the response to another (Miller et al. 1993a; Reynolds et al. 1994, 1995; Rolls and Tovee 1995; Sato 1989), and the effects are typically strongest when nearby stimuli activate nearby cells in the cortex, such as cells with similar RFs. Further, these competitive interactions are biased in favor of cells participating in one object representation versus another by many different mechanisms, including both bottom-up, or stimulus-driven mechanisms (e.g., high relative contrast) and top-down, or attentional, mechanisms (Luck et al. 1997; Moran and Desimone 1985; Reynolds and Desimone 1997; Reynolds et al. 1996). According to the model, the changes in baseline firing rates (Luck et al. 1997) as well as the modest increases in sensory-evoked responses found in some studies when attention is directed to the RF stimulus (e.g., Spitzer et al. 1988) are effects of the top-down bias. When two stimuli are present within the RF, an attentional bias in favor of one of them will drive the competition in favor of that stimulus, resulting in the suppression of responses to the unattended stimulus.
; Treisman and Gelade 1980; Wolfe et al. 1989). When searching for a "face in a crowd," for example, search is guided by information about the features of the face stored in long or short-term memory.
). The animal was rewarded for making an eye movement to the target stimulus in the array that matched the cue. In an initial analysis of the results, we found that the baseline activity of cells was higher during the delay following a preferred cue, which could be due to a bias in favor of cells coding the relevant object features. Furthermore, when the array was presented, responses were determined primarily by the target stimulus; responses to nontarget stimuli were suppressed. In the present study, we have conducted a full analysis of the results from the visual search task, including the nature of the competitive bias during the delay, the time course of the attention effects, the role of sensory competition between target and distracter stimuli, and the special role of the vertical meridian (VM). We also present new results from task conditions directed at the nature of the competitive bias during the delay as well as results from a new search task that did not require an eye movement to the target. The latter task was designed to test whether the attentional effects depended on the specific motor response made to the target.
METHODS
Abstract
Introduction
Methods
Results
Discussion
References
) and will only be briefly described here. Under aseptic conditions, a head post, recording chamber, and scleral eye coil for monitoring eye position (Robinson 1963) were implanted while the monkeys were under isofluorane anesthesia. One animal was implanted with chambers over both hemispheres, and the other two were implanted with a single chamber.
View larger version (62K):
[in a new window]
FIG. 1.
Stimulus sequences for representative trials in the task with 2-stimulus arrays, with the array confined to the hemifield contralateral to the recording site.
to study spatial attention in area V4 and IT cortex. In this technique, two stimuli are placed inside the RF of the recorded neuron, one of which is effective (good) in driving the cell by itself and one of which is ineffective (poor). The poor stimulus can then be treated as though it were "outside" the RF. One can then measure the effects of attention on the response to the good stimulus by comparing the firing rate in trials where the monkey attends to the good stimulus versus trials where the monkey attends to the poor stimulus.
RESULTS
Abstract
Introduction
Methods
Results
Discussion
References
View larger version (15K):
[in a new window]
FIG. 2.
Location of recording regions (closed curves) in the ventral temporal cortex of the 3 monkeys. The regions containing the recording sites were marked on ventral reconstructions of the hemispheres in each of the 3 monkeys and were then transferred to a standard ventral view. All sites were either on the ventral convexity of the temporal cortex or in the banks of the anterior middle temporal sulcus. amt, anterior middle temporal sulcus; rh, rhinal sulcus; st, superior temporal sulcus; la, lateral sulcus; orb, orbitofrontal suclus.
View larger version (19K):
[in a new window]
FIG. 3.
Responses to the cue stimuli by an individual neuron studied in both the random-cue (A) and blocked-cue (B) design. The horizontal bar indicates the 300-ms duration of the cue stimulus. Binwidth is 50 ms.
View larger version (19K):
[in a new window]
FIG. 4.
Population histograms showing responses to the cue stimuli. A: average from 28 cells studied in the random-cue design. B: average from 83 cells studied in the blocked-cue design. Horizontal bar indicates duration of the cue stimulus (300 ms). In this and all subsequent figures, SE is the standard error of the mean firing rate in the population, averaged from the standard error of the mean in all individual bins. Binwidth is 50 ms.
Activity following Poor Cue)/(Activity following Good Cue + Activity following Poor Cue).
View larger version (19K):
[in a new window]
FIG. 5.
A: frequency distribution of the delay activity index (DAI) for 83 cells studied in the blocked-cue design. The dashed vertical line separates cells with a DAI larger or smaller than 0. B: relationship between the stimulus selectivity index and the delay activity index. 
, 83 cells with significant stimulus selectivity, studied in the blocked design. 
, cells that did not have significant stimulus selectivity.
Response to Poor Cue)/(Response to Good Cue + Response to Poor Cue).
View larger version (32K):
[in a new window]
FIG. 6.
Response of an individual neuron to the 2-stimulus array in the contralateral hemifield. A: responses time locked to the onset of the array. Vertical bar indicates average saccadic latency to the target. B: responses time locked to the onset of the saccade. Binwidth is 25 ms. Below the histograms in A and B are rasters from the good-target and poor-target trials. Each tick in the rasters represents an action potential from the neuron, and each row corresponds to a different trial.
View larger version (20K):
[in a new window]
FIG. 7.
Population histograms showing the average response of 58 neurons to the 2-stimulus search array confined to the contralateral hemifield. A: responses time locked to array onset. Vertical bar indicates average latency of the saccade to the target. B: responses time locked to eye movement onset. Binwidth is 10 ms.
Activity on Poor-Target Trials)/(Activity on Good-Target Trials + Activity in Poor Target Trials).
View larger version (27K):
[in a new window]
FIG. 8.
A: relationship between the target effect index (TEI) computed in the early time window (70-170 ms postarray onset) and the TEI computed in the late time window (last 100 ms before saccade onset). Numbers at the 2 sides of the top right corner of the box indicate the relative number of cells falling to the right and left of the diagonal (- - -). B: histograms showing the response of an individual cell to the search array confined to the contralateral hemifield. Responses are time locked to onset of the search array, and the vertical bar indicates the average saccadic latency to the target. Binwidth is 25 ms. Below the histograms are rasters from the good-target and poor-target trials.
View larger version (21K):
[in a new window]
FIG. 9.
A: population histograms averaged from 58 cells showing the response to the good and poor stimuli presented alone, both in target-present and target-absent trials. Stippled vertical bar indicates average saccadic latency in target-present trials. Empty circles on the "No Target, Good Stim. Alone" curve indicate bins in which the response differed significantly from that in the "Target = Good Stim. Alone" condition. Binwidth is 20 ms. B: relationship between response to the good stimulus alone in target-present and target-absent trials. Numbers at the top right corner of the plot indicate relative number of cells falling to the right and to the left of the diagonal (- - -).
View larger version (23K):
[in a new window]
FIG. 10.
A: population histograms averaged from 58 cells. Responses to the good and poor stimuli presented alone are compared with the response to the search array. In all 3 conditions responses were measured in target-absent trials. Empty circles on the "No Target, Good Stim. Alone" curve indicate bins in which the response differed significantly from that in the "No Target, 2-Stim. Array" condition. Empty circles on the "No Target, Poor Stim. Alone" curve indicate bins in which the response differed significantly from that in the "No Target, 2-Stim. Array" condition. B: population histograms averaged from 58 cells. Responses to the search array when the good or poor stimulus was the target are compared with responses to the same array in target-absent trials. Empty circles on the "Target = Good Stim. in 2-Stim. Array" curve indicate bins in which the response differed significantly from that in the "No Target, 2-Stim. Array" condition. Empty circles on the "Target = Poor Stim. in 2-Stim. Array" curve indicate bins in which the response differed significantly from that in the "No Target, 2-Stim. Array" condition. Binwidth is 20 ms.
View larger version (18K):
[in a new window]
FIG. 11.
A: relationship between the response to the good stimulus presented alone and the response to the 2-stimulus array. B: relationship between the response to the poor stimulus presented alone and the response to the 2-stimulus array. In both plots, figures near the top right corner indicate the number of cells falling to the right and left of the diagonal (- - -).
View larger version (26K):
[in a new window]
FIG. 12.
Population histograms from 58 cells comparing the response to the 2-stimulus array with the responses to the component stimuli presented alone. A: responses time locked to array onset. Stippled and solid vertical bars indicate average saccadic latency for 1-stimulus and 2-stimulus arrays, respectively. Empty circles on the "Target = Good Stimulus Alone" curve indicate bins in which the response differed significantly from that in the "Target = Good Stim. in 2-Stim. Array" condition. Empty circles on the "Target = Poor Stimulus Alone" curve indicate bins in which the response differed significantly from that in the "Target = Poor Stim. in 2-Stim. Array" condition. B: same as in A, but responses are time locked to saccade onset. Binwidth is 20 ms.
View larger version (18K):
[in a new window]
FIG. 13.
Responses to the target stimulus presented alone compared with when the same target was presented in the 2-stimulus array. A: responses to the good stimulus as target, measured in a time window spanning the last 100 ms before saccade onset. B: responses to the poor stimulus as target, in the same time window as in A.
View larger version (16K):
[in a new window]
FIG. 14.
Relationship between the TEI and the stimulus selectivity index.
View larger version (36K):
[in a new window]
FIG. 15.
Population histograms averaged from 58 cells, showing the response to the search array in which the stimuli were in opposite hemifields. In A and C the good stimulus in the array was positioned in the hemifield contralateral to the recording site, whereas the poor stimulus was positioned in the ipsilateral hemifield. In B and D the good stimulus in the array was positioned in the hemifield ipsilateral to the recording site, whereas the poor stimulus was positioned in the contralateral hemifield. In A and B responses are time locked to array onset (black vertical bars indicate average saccadic latency), whereas in C and D responses are time locked to the eye movement. Binwidth is 20 ms.
View larger version (26K):
[in a new window]
FIG. 16.
Population histograms averaged from 58 cells, comparing the response to the good and poor stimuli presented alone with the response to the 2-stimulus array. In all 3 conditions responses were measured in target-absent trials. In A the good stimulus was in the contralateral hemifield and the poor stimulus was in the ipsilateral hemifield, whereas in B the good stimulus was in the ipsilateral hemifield and the poor stimulus was in the contralateral hemifield. Empty circles on the "No Target, Good Stim. Alone" curve indicate bins where the response was significantly different from that in the "No Target, 2-Stim. Array" condition. Empty circles on the "No Target, Poor Stim. Alone" curve indicate bins where the response was significantly different from that in the "No Target, 2-Stim. Array" condition. Binwidth is 20 ms.
View larger version (48K):
[in a new window]
FIG. 17.
Responses to the 2-stimulus array in trials with the good and poor stimuli used as targets are compared with the responses to the same stimuli presented alone in target-present trials. Population histograms are averaged from 58 cells. A and C: responses where the good stimulus was in the contralateral hemifield and the poor stimulus was in the ipsilateral hemifield. B and D: responses where the good stimulus was in the ipsilateral hemifield and the poor stimulus was in the contralateral hemifield. A and B: responses are time locked to array onset (stippled and solid vertical bars indicate average saccadic latency for 1-stimulus and 2-stimulus arrays, respectively). Empty circles on the "Target = Good Stim. Alone" curve indicate bins where the response differed significantly from that in the "Target = Good Stim. in 2-Stim. Array" condition. Empty circles on the "Target = Poor Stim. Alone" curve indicate bins where the response differed significantly from that in the "Target = Poor Stim. in 2-Stim. Array" condition. C and D: same as in A and B, but responses are time locked to saccade onset. Binwidth is 20 ms.
View larger version (41K):
[in a new window]
FIG. 18.
A and B: examples of responses of an individual neuron to the 5-stimulus array. Responses are shown separately for trials in which the good stimulus was the target ( 
) and trials in which the poor stimulus was the target (- - -). C and D: same as in A and B but histograms are averaged from the responses of 10 neurons. In A and C responses are time locked to array onset (solid vertical bars indicate average saccadic latency to the target stimulus). B and D: responses are time locked to the onset of the saccade. Binwidth is 20 ms.
0.05. By contrast, in a later time window covering the last 100 ms before saccade onset, 6 cells had a significantly higher firing rate on trials with the good stimulus as target compared with the poor stimulus as target, and the mean TEI for all 10 cells was 0.28. Thus, on average, the target effect occurred in the late phase of the response, as we found with two-stimulus arrays. The pattern of sensory interactions on target-absent trials was also very similar to what was found with two-stimulus arrays confined to the contralateral field.
View larger version (37K):
[in a new window]
FIG. 19.
Data from the lever release task. A and B: responses of an individual neuron to the 2-stimulus array confined to the contralateral hemifield. C and D: same as in A and B but averaged across the population of 44 neurons. In A and C responses are time locked to array onset (solid vertical bars indicate average latency of the lever release from array onset). B and D: responses are time locked to the lever release. Binwidth is 25 ms in A and B, and 20 ms in C and D.
View larger version (25K):
[in a new window]
FIG. 20.
Population histograms averaged from 44 cells, comparing the response to the good and poor stimuli presented alone with the response to the 2-stimulus search array. All responses were measured in target-absent trials. Empty circles on the "No Target, Good Stim. Alone" curve indicate bins where the response was significantly different from that in the "No Target, 2-Stim. Array" condition. Empty circles on the "No Target, Poor Stim. Alone" curve indicate bins where the response was significantly different from that in the "No Target, 2-Stim. Array" condition. Binwidth is 20 ms.
View larger version (26K):
[in a new window]
FIG. 21.
Response to the 2-stimulus arrays with the good and poor stimuli as target are compared with the response to the good and poor stimuli presented alone in target-present trials. Population histograms are averaged from 44 cells. A: responses time locked to array onset (stippled and solid vertical bars indicate average latency of the lever release for 1-stimulus and 2-stimulus arrays, respectively). Empty circles on the "Target = Good Stimulus Alone" curve indicate bins where the response was significantly different from that in the "Target = Good Stim. in 2-Stim. Array" condition. Empty circles on the "Target = Poor Stimulus Alone" curve indicate bins where the response was significantly different from that in the "Target = Poor Stim. in 2-Stim. Array" condition. B: same as in A, but responses are time locked to the lever release. Binwidth is 20 ms.
DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References
; Moran and Desimone 1985). Furthermore, the detailed pattern of results in the present study are consistent with a model of attentional selection that we have termed "biased competition" (Desimone 1996; Desimone and Duncan 1995; Duncan 1996; Luck et al. 1997).
; Reynolds et al. 1996) and "top-down" feedback mechanisms (e.g., one stimulus has greater behavioral relevance than another) (Luck et al. 1997; Moran and Desimone 1985). According to the model, these feedback mechanisms are closely associated with neural mechanisms for working memory. A computational model of attention related to this biased competition scheme has been developed by Usher and Niebur (1996). We will first consider the evidence for the biasing inputs in the present study and then consider the evidence for the competition.
; Fuster and Jervey 1981; Miller et al. 1993b; Miyashita and Chang 1988; Vogels and Orban 1994). In our view, the delay activity found in both short-term memory tasks and in visual search reflects the same underlying process. In both cases, there is a bias in favor of cells representing the features of a behaviorally relevant stimulus that is actively held in working memory. Indeed, there is little formal difference between the visual search task and the DMS task, except that the choice stimuli are distributed across space in the former task and distributed across time (typically) in the latter task. One might say that the contents of working memory guide both visual search and the selection of a matching stimulus in a top-down fashion.
), whereas delay activity in prefrontal cortex is maintained under the same conditions (Miller et al. 1996). Prefrontal cortex, then, is likely to be a major source of stimulus-specific delay activity in both working memory and visual search (Fuster 1973; Fuster et al. 1982; Miller et al. 1996; Rao et al. 1997; Wilson et al. 1993).
). The animal performed a target discrimination task at one location in the visual field and ignored distracters at different locations. When the animal was cued to attend to a location within the recorded cell's RF, the cell's maintained activity increased compared with when the animal's attention was directed to a location outside the RF (Luck et al. 1997). As in the blocked-cue conditions of the present study, no physical stimulus was necessary to trigger the activity: the maintained activity started at the beginning of each trial before any stimulus was presented, as long as the animal knew that a location within the RF was the relevant one. Again, these results suggest that visual cortex receives feedback from structures involved in working memory, and this feedback biases activity in favor of those cells representing the behaviorally relevant stimulus.
; Reynolds et al. 1996), the response to the pair should shift to the response to the stronger stimulus presented alone. Likewise, an attentional bias in favor of either A or B should have a similar effect, namely that the response to the pair should be similar to the response to the attended stimulus when it is presented alone (Luck et al. 1997; Moran and Desimone 1985). We will first consider the inhibitory interactions between the two stimuli, in the absence of any attentional bias in favor of one or the other.
; Rolls and Tovee 1995; Sato 1989). Similarly, Richmond et al. (1983) found that a spot presented at fixation had an inhibitory effect on the responses of IT cells to an extrafoveal stimulus.
, 1989) also reported that the responses of IT neurons to two stimuli in opposite hemifields is nearly equal to the response to the contralateral stimulus alone. These results suggest that stimuli within the contralateral visual field have a very strong competitive advantage over stimuli in the ipsilateral field in IT cortex.
has reported that spatially directed attention to a stimulus in the ipsilateral visual field has little or no effect on the response of IT neurons to a stimulus in the contralateral field. Likewise, in a previous study of spatial attention in IT cortex (Moran and Desimone 1985), we found smaller effects of attention when competing stimuli were located in opposite hemifields than when both were located within the contralateral field (unpublished data).
; Schall et al. 1995; Thompson et al. 1996). This suppression is found even when the target and nontarget stimuli are located in opposite hemifields. Even this far into the oculomotor system, however, suppression of the nontarget response is larger when the target and nontarget stimuli are located near one another rather than far apart (Schall and Hanes 1993; Schall et al. 1995).
).
; Duncan 1996; Duncan and Humphreys 1989). This issue arises when the location of the target object is not known in advance (and thus must be found on the basis of nonspatial features) and when the object is difficult to find, i.e., it does not "pop out." According to one account of serial search, the features of an object are not bound together unless the object is within the focus of spatially directed attention (Treisman and Gelade 1980; Treisman and Sato 1990; Treisman and Schmidt 1982). To find a target object defined by a conjunction of features in a search display, a roving "spotlight" of attention must be rapidly switched from object to object in the display until the target is found. The processing of objects outside the spotlight is suppressed. In this case, the search for an object based on a conjunction of features rather than its location is, nonetheless, accomplished by a spatial attention mechanism.
; Bundesen 1990; Duncan 1996; Duncan and Humphreys 1989). To find a target object, an "attentional template" of the target is distributed throughout the visual field representations of the visual processing areas. Objects that match the template are allowed to pass through, whereas nonmatching objects are suppressed.
View larger version (37K):
[in a new window]
FIG. 22.
Schematic representation of the search task with 2-stimulus arrays confined to the contralateral hemifield, and of the pattern of activity in a representative population of inferior temporal (IT) neurons. Bottom diagrams illustrate the visual displays during the relevant portions of the task. Each dot in the top diagrams represents an individual neuron, and the size of the dot indicates relative firing rate. A specific cue (here exemplified by the flower) activates the subpopulation of IT cells tuned to any of the various features of the cue. During the delay period, this subpopulation maintains a higher level of sustained activation, relative to other cells that are tuned to the properties of the distracter. When the search array is 1st presented, both the target and the nontarget initially activate neurons for which they represent effective sensory stimuli. Later, cells tuned to the properties of the target stimulus remain active, whereas cells tuned to the properties of the distracter are suppressed. This late divergence in activation may depend on competitive interactions within IT cortex, here schematically depicted by inhibition of cells tuned to the distracter (cup) by cells tuned to the target (flower). We hypothesize that the competitive interactions are biased by top-down feedback projections from prefrontal cortex. In a given trial these projections give a competitive advantage (positive bias) to cells in IT coding the cue-target stimulus in that trial, at the expenses of cells coding the distracter.
in area V4. Animals were presented with an array of red and green stimuli distributed over a wide extent of the central visual field. When the animals were cued to attend to stimuli of one color, cells preferring that color gave enhanced responses to the appropriately colored stimuli at any locations in the array. Apparently the feedback biasing input to cortex can target cells with the appropriate feature preference at all locations in the visual field simultaneously.
| |
ACKNOWLEDGEMENTS |
|---|
We thank J. Sewell and T. Galkin for help with the histology and J. Hart and R. Hoag for help in training the monkeys.
This research was supported in part by a grant from the Human Frontier Science Program Organization to R. Desimone and J. Duncan, by grants from Office of Naval Research (N00014-91-J-1347) and Air Force Office of Scientific Research (AFOSR-90-0043) to J. Duncan, and by a fellowship from the Human Frontier Science Program to L. Chelazzi.
| |
FOOTNOTES |
|---|
Address for reprint requests: R. Desimone, Laboratory of Neuropsychology, Bldg. 49, Rm. 1B80, 49 Convent Dr., MSC 4415, Bethesda, MD 20892-4415.
Received 24 October 1997; accepted in final form 17 August 1998.
| |
REFERENCES |
|---|
|
Abstract
Introduction
Methods
Results
Discussion
References
|
|---|
This article has been cited by other articles:
|
|
 |
|
  M. Stokes, R. Thompson, A. C. Nobre, and J. Duncan Shape-specific preparatory activity mediates attention to targets in human visual cortex PNAS, November 17, 2009; 106(46): 19569 - 19574. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. B. Sala and S. M. Courtney Flexible working memory representation of the relationship between an object and its location as revealed by interactions with attention Atten Percept Psychophys, October 1, 2009; 71(7): 1525 - 1533. [Abstract] [PDF]
|
|
|
|
 |
|
 A. P. Weible, D. C. Rowland, R. Pang, and C. Kentros Neural Correlates of Novel Object and Novel Location Recognition Behavior in the Mouse Anterior Cingulate Cortex J Neurophysiol, October 1, 2009; 102(4): 2055 - 2068. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G. T. Buracas and T. D. Albright Modulation of neuronal responses during covert search for visual feature conjunctions PNAS, September 29, 2009; 106(39): 16853 - 16858. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
N. Li, D. D. Cox, D. Zoccolan, and J. J. DiCarlo What Response Properties Do Individual Neurons Need to Underlie Position and Clutter "Invariant" Object Recognition? J Neurophysiol, July 1, 2009; 102(1): 360 - 376. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 B.-C. Kuo, A. Rao, J. Lepsien, and A. C. Nobre Searching for Targets within the Spatial Layout of Visual Short-Term Memory J. Neurosci., June 24, 2009; 29(25): 8032 - 8038. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 I. A. Muzzio, C. Kentros, and E. Kandel What is remembered? Role of attention on the encoding and retrieval of hippocampal representations J. Physiol., June 15, 2009; 587(12): 2837 - 2854. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. Hollingworth and S. J. Luck The role of visual working memory (VWM) in the control of gaze during visual search Atten Percept Psychophys, May 1, 2009; 71(4): 936 - 949. [Abstract] [PDF]
|
|
|
|
 |
|
 I. C. Fiebelkorn, J. J. Foxe, and S. Molholm Dual Mechanisms for the Cross-Sensory Spread of Attention: How Much Do Learned Associations Matter? Cereb Cortex, April 24, 2009; (2009) bhp083v1. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 K. H. Nuechterlein, S. J. Luck, C. Lustig, and M. Sarter CNTRICS Final Task Selection: Control of Attention Schizophr Bull, January 1, 2009; 35(1): 182 - 196. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. BELKE, G. W. HUMPHREYS, D. G. WATSON, A. S. MEYER, and A. L. TELLING Top-down effects of semantic knowledge in visual search are modulated by cognitive but not perceptual load Atten Percept Psychophys, November 1, 2008; 70(8): 1444 - 1458. [Abstract] [PDF]
|
|
|
|
 |
|
 I. Arend, R. Rafal, and R. Ward Spatial and temporal deficits are regionally dissociable in patients with pulvinar lesions Brain, August 1, 2008; 131(8): 2140 - 2152. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. SOTO and G. W. HUMPHREYS Stressing the mind: The effect of cognitive load and articulatory suppression on attentional guidance from working memory Atten Percept Psychophys, July 1, 2008; 70(5): 924 - 934. [Abstract] [PDF]
|
|
|
|
|
|
R. VanRullen, T. Carlson, and P. Cavanagh The blinking spotlight of attention PNAS, December 4, 2007; 104(49): 19204 - 19209. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. Zoccolan, M. Kouh, T. Poggio, and J. J. DiCarlo Trade-Off between Object Selectivity and Tolerance in Monkey Inferotemporal Cortex J. Neurosci., November 7, 2007; 27(45): 12292 - 12307. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. A. McMains, H. M. Fehd, T.-A. Emmanouil, and S. Kastner Mechanisms of Feature- and Space-Based Attention: Response Modulation and Baseline Increases J Neurophysiol, October 1, 2007; 98(4): 2110 - 2121. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 T. Stafford and K. N Gurney Biologically constrained action selection improves cognitive control in a model of the Stroop task Phil Trans R Soc B, September 29, 2007; 362(1485): 1671 - 1684. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. R. Allred and B. Jagadeesh Quantitative Comparison Between Neural Response in Macaque Inferotemporal Cortex and Behavioral Discrimination of Photographic Images J Neurophysiol, September 1, 2007; 98(3): 1263 - 1277. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G. F. Woodman, S. J. Luck, and J. D. Schall The Role of Working Memory Representations in the Control of Attention Cereb Cortex, September 1, 2007; 17(suppl_1): i118 - i124. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. Vuilleumier and J. Driver Modulation of visual processing by attention and emotion: windows on causal interactions between human brain regions Phil Trans R Soc B, May 29, 2007; 362(1481): 837 - 855. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. Jacques and B. Rossion Electrophysiological Evidence for Temporal Dissociation between Spatial Attention and Sensory Competition during Human Face Processing Cereb Cortex, May 1, 2007; 17(5): 1055 - 1065. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. Ledberg, S. L. Bressler, M. Ding, R. Coppola, and R. Nakamura Large-Scale Visuomotor Integration in the Cerebral Cortex Cereb Cortex, January 1, 2007; 17(1): 44 - 62. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. J Tate, H. Fischer, A. E Leigh, and K. M Kendrick Behavioural and neurophysiological evidence for face identity and face emotion processing in animals Phil Trans R Soc B, December 29, 2006; 361(1476): 2155 - 2172. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J.-M. Hopf, S. J. Luck, K. Boelmans, M. A. Schoenfeld, C. N. Boehler, J. Rieger, and H.-J. Heinze The neural site of attention matches the spatial scale of perception. J. Neurosci., March 29, 2006; 26(13): 3532 - 3540. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. Soto and G. W. Humphreys Seeing the content of the mind: Enhanced awareness through working memory in patients with visual extinction PNAS, March 21, 2006; 103(12): 4789 - 4792. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. E. Rollenhagen and C. R. Olson Low-Frequency Oscillations Arising From Competitive Interactions Between Visual Stimuli in Macaque Inferotemporal Cortex J Neurophysiol, November 1, 2005; 94(5): 3368 - 3387. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
L. Zago, M. J. Fenske, E. Aminoff, and M. Bar The Rise and Fall of Priming: How Visual Exposure Shapes Cortical Representations of Objects Cereb Cortex, November 1, 2005; 15(11): 1655 - 1665. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. Zoccolan, D. D. Cox, and J. J. DiCarlo Multiple Object Response Normalization in Monkey Inferotemporal Cortex J. Neurosci., September 7, 2005; 25(36): 8150 - 8164. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. Silver and P. Feldman Evidence for Sustained Attention and Working Memory in Schizophrenia Sharing a Common Mechanism J Neuropsychiatry Clin Neurosci, August 1, 2005; 17(3): 391 - 398. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
F. H. Hamker The Reentry Hypothesis: The Putative Interaction of the Frontal Eye Field, Ventrolateral Prefrontal Cortex, and Areas V4, IT for Attention and Eye Movement Cereb Cortex, April 1, 2005; 15(4): 431 - 447. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. A. Buffalo, G. Bertini, L. G. Ungerleider, and R. Desimone Impaired Filtering of Distracter Stimuli by TE Neurons following V4 and TEO Lesions in Macaques Cereb Cortex, February 1, 2005; 15(2): 141 - 151. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. B. Rowe, K. E. Stephan, K. Friston, R. S.J. Frackowiak, and R. E. Passingham The Prefrontal Cortex shows Context-specific Changes in Effective Connectivity to Motor or Visual Cortex during the Selection of Action or Colour Cereb Cortex, January 1, 2005; 15(1): 85 - 95. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. T. Serences, J. Schwarzbach, S. M. Courtney, X. Golay, and S. Yantis Control of Object-based Attention in Human Cortex Cereb Cortex, December 1, 2004; 14(12): 1346 - 1357. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. K. Seth, J. L. McKinstry, G. M. Edelman, and J. L. Krichmar Visual Binding Through Reentrant Connectivity and Dynamic Synchronization in a Brain-based Device Cereb Cortex, November 1, 2004; 14(11): 1185 - 1199. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
W. H. Merigan and R. C. Saunders Unilateral Deficits in Visual Perception and Learning after Unilateral Inferotemporal Cortex Lesions in Macaques Cereb Cortex, August 1, 2004; 14(8): 863 - 871. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
T. Ogawa and H. Komatsu Target Selection in Area V4 during a Multidimensional Visual Search Task J. Neurosci., July 14, 2004; 24(28): 6371 - 6382. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. F.X. DeSouza and S. Everling Focused Attention Modulates Visual Responses in the Primate Prefrontal Cortex J Neurophysiol, February 1, 2004; 91(2): 855 - 862. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. Graboi and J. Lisman Recognition by Top-Down and Bottom-Up Processing in Cortex: The Control of Selective Attention J Neurophysiol, August 1, 2003; 90(2): 798 - 810. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. J. DiCarlo and J. H. R. Maunsell Anterior Inferotemporal Neurons of Monkeys Engaged in Object Recognition Can be Highly Sensitive to Object Retinal Position J Neurophysiol, June 1, 2003; 89(6): 3264 - 3278. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. R. Roelfsema, P. S. Khayat, and H. Spekreijse Subtask sequencing in the primary visual cortex PNAS, April 29, 2003; 100(9): 5467 - 5472. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. R. Friedman-Hill, L. C. Robertson, R. Desimone, and L. G. Ungerleider Posterior parietal cortex and the filtering of distractors PNAS, April 1, 2003; 100(7): 4263 - 4268. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Iba and T. Sawaguchi Involvement of the Dorsolateral Prefrontal Cortex of Monkeys in Visuospatial Target Selection J Neurophysiol, January 1, 2003; 89(1): 587 - 599. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. T. Rolls, N. C. Aggelopoulos, and F. Zheng The Receptive Fields of Inferior Temporal Cortex Neurons in Natural Scenes J. Neurosci., January 1, 2003; 23(1): 339 - 348. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. C. Wiener, M. W. Oram, Z. Liu, and B. J. Richmond Consistency of Encoding in Monkey Visual Cortex J. Neurosci., October 15, 2001; 21(20): 8210 - 8221. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
L. Chelazzi, E. K. Miller, J. Duncan, and R. Desimone Responses of Neurons in Macaque Area V4 During Memory-guided Visual Search Cereb Cortex, August 1, 2001; 11(8): 761 - 772. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
B. Jagadeesh, L. Chelazzi, M. Mishkin, and R. Desimone Learning Increases Stimulus Salience in Anterior Inferior Temporal Cortex of the Macaque J Neurophysiol, July 1, 2001; 86(1): 290 - 303. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. L. Sheinberg and N. K. Logothetis Noticing Familiar Objects in Real World Scenes: The Role of Temporal Cortical Neurons in Natural Vision J. Neurosci., February 15, 2001; 21(4): 1340 - 1350. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J.-M. Hopf, S. J. Luck, M. Girelli, T. Hagner, G. R. Mangun, H. Scheich, and H.-J. Heinze Neural Sources of Focused Attention in Visual Search Cereb Cortex, December 1, 2000; 10(12): 1233 - 1241. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
R. P. Hasegawa, M. Matsumoto, and A. Mikami Search Target Selection in Monkey Prefrontal Cortex J Neurophysiol, September 1, 2000; 84(3): 1692 - 1696. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
