When subjects are explicitly cued to focus attention on a particular location in visual space, targets presented at that location have been shown to elicit enhanced sensory-evoked activity in recordings of event-related brain potentials (ERPs). The present study sought to determine if this type of sensory facilitation also occurs during visual search tasks in which a feature conjunction target must be identified, presumably by means of focal attention, within an array of distractor items. In this experiment, subjects were required to discriminate the shape of a distinctively colored target item within an array containing 15 distractor items, and ERPs were elicited by task-irrelevant probe stimuli that were presented at the location of the target item or at the location of a distractor item on the opposite side of the array. When the delay between search-array onset and probe onset was 250 msec, the sensory-evoked responses in the latency range 75-200 msec were larger for probes presented at the location of the target than for probes presented at the location of the irrelevant distractor. These results indicate that sensory processing is modulated in a spatially restricted manner during visual search, and that focusing attention on a feature conjunction target engages neural systems that are shared with other forms of visual-spatial attention.

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