Effects of Task Difficulty and Target Likelihood in Area V4 of Macaque Monkeys

Abstract

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...

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