The visual system separates processing of an object's form and color ("what") from its spatial location ("where"). In order to direct action to objects, the identity and location of those objects must somehow be integrated. To examine whether this process occurs within the prefrontal (PF) cortex, the activity of 195 PF neurons was recorded during a task that engaged both what and where working memory. Some neurons showed either object-tuned (what) or location-tuned (where) delay activity. However, over half (52 percent, or 64/123) of the PF neurons with delay activity showed both what and where tuning. These neurons may contribute to the linking of object information with the spatial information needed to guide behavior.

We examined neural activity in prefrontal (PF) cortex of monkeys performing a delayed paired associate task. Monkeys were cued with a sample object. Then, after a delay, a test object was presented. If the test object was the object associated with the sample during training (i.e., its target), they had to release a lever. Monkeys could bridge the delay by remembering the sample (a sensory-related code) and/or thinking ahead to the expected target (a prospective code). Examination of the monkeys' behavior suggested that they were relying on a prospective code. During and shortly after sample presentation, neural activity in the lateral PF cortex primarily reflected the sample. Toward the end of the delay, however, PF activity began to reflect the anticipated target, which indicated a prospective code. These results provide further confirmation that PF cortex does not simply buffer incoming visual inputs, but instead selectively processes information relevant to current behavioral demands, even when this information must be recalled from long-term memory.

Responses to subjective contours in visual cortical areas V1 and V2 in adult cats were investigated by optical imaging of intrinsic signals and single-unit recording. Both V1 and V2 contain maps of the orientation of subjective gratings that have their basis in specific kinds of neuronal responses to subjective orientations. A greater proportion of neurons in V2 than in V1 show a robust response to subjective edges. Through the use of subjective stimuli in which the orientation of the luminance component is invariant, an unmasked V1 response to subjective edges alone can be demonstrated. The data indicate that the processing of subjective contours begins as early as V1 and continues progressively in higher cortical areas.

Neurons in primary visual cortex (area 17) respond vigorously to oriented stimuli within their receptive fields; however, stimuli presented outside the suprathreshold receptive field can also influence their responses. Here we describe a fundamental feature of the spatial interaction between suprathreshold center and subthreshold surround. By optical imaging of intrinsic signals in area 17 in response to a stimulus border, we show that a given stimulus generates activity primarily in iso-orientation domains, which extend for several millimeters across the cortical surface in a manner consistent with the architecture of long-range horizontal connections in area 17. By mapping the receptive fields of single neurons and imaging responses from the same cortex to stimuli that include or exclude the aggregate suprathreshold receptive field, we show that intrinsic signals strongly reveal the subthreshold surround contribution. Optical imaging and single-unit recording both demonstrate that the relative contrast of center and surround stimuli regulates whether surround interactions are facilitative or suppressive: the same surround stimulus facilitates responses when center contrast is low, but suppresses responses when center contrast is high. Such spatial interactions in area 17 are ideally suited to contribute to phenomena commonly regarded as part of "higher-level" visual processing, such as perceptual "popout" and "filling-in."A prominent feature of nearly every region of the mammalian cortex is a dense network of patchy, long-range horizontal connections within the superficial cortical layers (1). In primary visual cortex, these connections arise primarily as axonal branches of pyramidal cells in layers 2/3 (2, 3), and link neurons located at distances up to several millimeters away in the superficial cortical layers. Long-range horizontal connections make excitatory synapses on their target neurons (4), but those postsynaptic neurons can be excitatory (spiny stellate or pyramidal cells) or inhibitory (smooth stellate cells, see ref. 5).Although these anatomical features of horizontal connections are by now well established, their physiological role remains only partially understood. Long-range connections in area 17 are clustered into regions with similar orientation preference (6), and form a likely substrate for mediating influences on neurons from outside their "classical" receptive field. (We define the classical receptive field as the region over which a stimulus can evoke a suprathreshold spike response from the cell.) These influences can include modulation of orientation specific responses in area 17 neurons (7,8). Consistent with the anatomy of long-range connections, the effect of electrically stimulating lateral connections in cortical slices can be both excitatory and inhibitory, although the balance between the two can be modified (9). Reducing thalamocortical excitation, either in the long term by a retinal lesion (10) or in the short term by an artificial scotoma (11,12), causes changes in ...

Although many studies have explored the neural correlates of visual attention and selection, few have examined the reliability with which neurons represent relevant information. We monitored activity in the frontal eye field (FEF) of monkeys trained to make a saccade to a target defined by the conjunction of color and shape or to a target defined by color differences. The difficulty of conjunction search was manipulated by varying the number of distractors, and the difficulty of feature search was manipulated by varying the similarity in color between target and distractors. The reliability of individual neurons in signaling the target location in correct trials was determined using a neuron-anti-neuron approach within a winner-take-all architecture. On average, approximately seven trials of the activity of single neurons were sufficient to match near-perfect behavioral performance in the easiest search, and ϳ14 trials were sufficient in the most difficult search. We also determined how many neurons recorded separately need to be evaluated within a trial to match behavioral performance. Results were quantitatively similar to those of the single neuron analysis. We also found that signal reliability in the FEF did not change with task demands, and overall, behavioral accuracy across the search tasks was approximated when only six trials or neurons were combined. Furthermore, whether combining trials or neurons, the increase in time of target discrimination corresponded to the increase in mean saccade latency across visual search difficulty levels. Finally, the variance of spike counts in the FEF increased as a function of the mean spike count, and the parameters of this relationship did not change with attentional selection. Key words: oculomotor; visual cortex; vision; attention; eye movements; selection; modelNeural correlates of visual selection and attention have been observed in nearly all visual and visual-association brain areas that have been examined (Bushnell et al., 1981;Moran and Desimone, 1985;Mountcastle et al., 1987;Motter, 1993;Zipser et al., 1996;Luck et al., 1997;Treue and Maunsell, 1999) (for review, see Desimone and Duncan, 1995;Maunsell, 1995). In a majority of these studies, the average activity of a neuron during one behavioral state was compared with the average activity of the same neuron during another behavioral state. However, it is not clear from these results how reliably neurons signal changes in behavioral state. This is because analyses have usually been confined to average discharge rate in specific time intervals of interest and have not examined the variability in discharges of cortical neurons under identical conditions. Most analyses of neural reliability have compared the variance of responses with their magnitude (Henry et al., 1973;Tolhurst et al., 1983;Britten et al., 1993;McAdams and Maunsell, 1999), commonly finding that the variance of spike counts is proportional to the mean number of spikes produced by the neuron.Only a few studies have looked at neural reliability f...