The role of selective attention in the perception of multiple-element visual arrays: Cognitive and neural mechanisms

The identification of objects embedded within complex, multiple-element visual stimulus arrays poses a significant computational problem. In order to solve this problem, the primate visual system appears to employ mechanisms of spatial selective attention that restrict processing to a subset of the visual input. These attentional mechanisms were examined by means of event-related potential (ERP) recordings that were obtained while subjects performed visual search tasks.;When subjects searched for a target defined by the absence of a particular feature, the amplitude and latency of the P3 component varied in a manner that was consistent with the serial application of an attentional process to each item in the stimulus array. When targets were defined by the presence of this feature, in contrast, the pattern of results was consistent with a parallel detection process. Some investigators have proposed that this parallel detection process does not involve spatially focused attention, but several signs of spatial selective attention were observed during feature detection tasks in additional experiments, indicating that some spatially selective processes are used regardless of target type.;In these search experiments, several ERP components were found to reflect various aspects of visual processing. One of these components, called "N2pc", was observed only for stimulus arrays containing both an item that required discrimination and competing distractor items, suggesting that the N2pc reflects a process that selects one item for identification and filters out information from surrounding distractor items. This component appears to be generated in posterior visual cortex and is contingent upon a preliminary, task-controlled analysis of the stimulus array. Together, these findings suggest that spatial attention operates by sending feedback signals from higher cortical areas to lower cortical areas in order to select one item and suppress the others. Consistent with this hypothesis, ERPs elicited by task-irrelevant probe stimuli were found to be enhanced for probes presented at the location of a target item and suppressed for probes presented at the locations of distractor items. These results provide support at the neural level for guided search models of visual search performance and for early selection models of visual-spatial selective attention.