The role of attention in visual processing

Attention to a visual stimulus typically increases the responses of cortical neurons to that stimulus. Because many studies have shown a close relationship between the performance of individual neurons and behavioural performance of animal subjects, it is important to consider how attention affects this relationship. Measurements of behavioural and neuronal performance taken from rhesus monkeys while they performed a motion detection task with two attentional states show that attention alters the relationship between behaviour and neuronal response. Notably, attention affects the relationship differently in different cortical visual areas. This indicates that a close relationship between neuronal and behavioural performance on a given task persists over changes in attentional state only within limited regions of visual cortex.

Physiological correlates of perceptual learning in monkey V1 and V2

Performance in visual discrimination tasks improves with practice. Although the psychophysical parameters of these improvements have suggested the involvement of early areas in visual cortex, there has been little direct study of the physiological correlates of such perceptual learning at the level of individual neurons. To examine how neuronal response properties in the early visual system may change with practice, we trained monkeys for more than 6 mo in an orientation discrimination task in which behaviorally relevant stimuli were restricted to a particular retinal location and oriented around a specific orientation. During training the monkeys' discrimination thresholds gradually improved to much better than those of naive monkeys or humans. Although this improvement was specific to the trained orientation, it showed little retinotopic specificity. The receptive field properties of single neurons from regions representing the trained location and a location in the opposite visual hemifield were measured in V1 and V2. In most respects the receptive field properties in the representations of the trained and untrained regions were indistinguishable. However, in the regions of V1 and V2 representing the trained location, there were slightly fewer neurons whose optimal orientation was near the trained orientation. This resulted in a small but significant decrease in the V1 population response to the trained orientation at the trained location. Consequently, the observed neuronal populations did not exhibit any orientation-specific biases sufficient to explain the orientation specificity of the behavioral improvement. Pooling models suggest that the behavioral improvement was accomplished with a task-dependent and orientation-selective pooling of unaltered signals from early visual neurons. These data suggest that, even for training with stimuli suited to the selectivities found in early areas of visual cortex, behavioral improvements can occur in the absence of pronounced changes in the physiology of those areas.

Attentional modulation of behavioral performance and neuronal responses in middle temporal and ventral intraparietal areas of macaque monkey

Although many studies have demonstrated that neuronal responses are modulated by attention, the significance of this modulation for behavior is poorly understood. We recorded from neurons in the middle temporal (MT) and ventral intraparietal (VIP) areas in the visual cortex of two macaque monkeys while they performed a motion detection task under two conditions of spatial attention. The ability of the animals to detect the motion was reduced when they withdrew attention from the stimulus. Withdrawing attention also reduced neuronal responses to the motion in both the MT and VIP areas. To compare the neuronal and behavioral effects of attention, the amount of attentional modulation was expressed in units of stimulus strength. On average, attention modulated neuronal responses in MT less than needed to account for the attentional effect on behavior. The opposite was observed in VIP, where the average effect of attention on neuronal responses was greater than that needed to account for behavior. Similar results were obtained when the effects of attention on neuronal response and behavioral performance were compared using a parametric function of stimulus strength. Across neurons in both areas, attentional modulation of neuronal responses was more variable than, and uncorrelated with, attentional modulation of behavioral performance. These findings suggest that attention can alter the average relationship between neuronal activity in visual cortex and behavioral performance. Where this relationship is preserved may indicate which cortical regions are most closely associated with the behavior in a given task.