Previous single-unit studies of visual cortex have reported that spatial attention modulates responses to different orientations and directions proportionally, such that it does not change the width of tuning functions for these properties. Other studies have suggested that spatial attention causes a leftward shift in contrast response functions, such that its effects on responses to stimuli of different contrasts are not proportional. We have further explored the effects of attention on stimulus-response functions by measuring the responses of 131 individual V4 neurons in two monkeys while they did a task that controlled their spatial attention. Each neuron was tested with a set of stimuli that spanned complete ranges of orientation and contrast during different states of attention. Consistent with earlier reports, attention scaled responses to preferred and nonpreferred orientations proportionally. However, we did not find compelling evidence that the effects were best described by a leftward shift of the contrast response function. The modulation of neuronal responses by attention was well described by either a leftward shift or proportional scaling of the contrast response function. Consideration of differences in experimental design and analysis that may have contributed to this discrepancy suggests that it was premature to exclude a proportional scaling of responses to different contrasts by attention in favor of a leftward shift of contrast response functions. The current results reopen the possibility that the effects of attention on stimulus-response functions are well described by a single proportional increase in a neuron's response to all stimuli.

The effects of attention on neuronal responses in visual cortex have been likened to a change in stimulus contrast. Attention and stimulus contrast both modulate the magnitude of neuronal responses. However, changes in stimulus contrast also affect the latency of visual responses. Although many neurophysiological studies have examined how attention affects the strength of neuronal responses, few have considered whether attention affects neuronal latencies. To compare directly the effects of stimulus contrast and attention, we recorded responses from individual neurons in area V4 of macaque monkeys while they performed a task that independently controlled spatial attention and stimulus contrast. As expected, changes in stimulus contrast affected both the magnitude and latency of neuronal responses. Although attention had the expected effects on the magnitudes of neuronal responses, we did not detect statistically reliable changes in neuronal latency. A direct comparison of the effects of contrast and attention revealed a reliable difference. When a shift in spatial attention decreased response magnitude, response latency increased much less than when the same magnitude change was caused by reducing stimulus contrast. Thus, attention is distinct from contrast in the way it affects the relationship between neuronal response magnitude and latency.

sell. Effects of task difficulty and target likelihood in area V4 of macaque monkeys. J Neurophysiol 96: 2377-2387, 2006. First published July 19, 2006 doi:10.1152/jn.01072.2005. Spatial attention improves performance at attended locations and correspondingly modulates firing rates of cortical neurons. The size of these behavioral and neuronal effects depends on the difficulty of the task performed at the attended location. Psychological theorists have attributed this to a tighter focus of a fixed amount of processing resource at the attended location, but the effects of task difficulty on the distribution of neuronal effects of attention across the visual field have not been fully explored. We trained rhesus monkeys to do a detection task in which difficulty and spatial attention were manipulated independently. Probe stimuli were used to measure behavioral performance in different conditions of attention and difficulty. Animals performed better at attended locations and this advantage increased with difficulty, consistent with data from human psychophysics. Neuronal modulation by spatial attention was larger with greater difficulty. In two animals, increasing difficulty caused a modest increase in neuronal responses to visual stimuli regardless of the locus of spatial attention. In a third animal, which was previously trained to ignore multiple distracting stimuli, increasing task difficulty increased responses at the focus of attention and suppressed responses away from the focus of attention. The results show that difficulty can modulate effects of spatial attention in V4; it can alter the distribution of sensory responses across the visual scene in ways that may depend on the subject's behavioral strategy. I N T R O D U C T I O NHuman psychophysical studies have demonstrated that attentional load, defined as either the number of relevant items or the complexity of the processing required for the task, can have a profound impact on behavioral effects of spatial attention (Lavie and Tsal 1994;Sade and Spitzer 1998;Urbach and Spitzer 1995). For example, orientation-discrimination thresholds for peripherally presented stimuli are elevated when subjects perform a demanding task at fixation, as opposed to merely fixating (Lee et al. 1997). Increasing the difficulty of a central task reduces interference from peripheral distractors (LaBerge et al. 1991;Lavie and Cox 1997).Because high attentional load can reduce interference from distractors outside the attentional focus, it has been proposed that spatial attention has a fixed capacity and that increasing load at an attended location concentrates the fixed amount of attentional resource at that location, necessarily withdrawing it elsewhere (Lavie 1995). This attentional load hypothesis accounts for increases in behavioral performance at the focus of attention, as more attentional resource is directed there. In addition, it dictates a decline in performance away from the attentional focus as the resource is removed. What neurophysiological changes correspond to the...