On the role of selective attention in visual perception

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   Figure 1

   (A) Common experimental design for neurophysiological studies of attention. The outline squares are continuously present and mark the two locations at which the solid square can be flashed. (B) Example occipital ERPs recorded in a paradigm of this nature (data from ref. 42). Note that the C1 wave (generated in area V1) shows no attention effect, whereas the P1 and the N1 waves (generated in extrastriate cortex) are larger for the attended stimuli. (C) Single-unit responses from area V4 in a similar paradigm (data from ref. 23). Note that the response is larger for attended compared with ignored stimuli. (D) Single-unit responses from area V1 (data from ref. 23) showing no effect of attention.

   
   
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   Figure 2

   (A) Example of the stimuli used in experiment 1. (B) ERPs from experiment 1, recorded at lateral occipital scalp sites ipsilateral and contralateral to the location of the target stimulus. The presence of an N2pc wave (shaded area) when the feature detection task was performed alone indicates that visual–spatial attention was allocated to the target in this condition. In contrast, although the observers were able to accurately detect the feature target while performing the concurrent letter discrimination task, no N2pc was observed, indicating that attention was not necessary for feature detection.

   
   
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   Figure 3

   (A) Example of the stimuli used in experiment 2. (B) ERPs from experiment 2, recorded at lateral occipital scalp sites ipsilateral and contralateral to the location of the target stimulus. A large N2pc wave (shaded area) can be observed for the conjunction task when it was performed alone (larger than for the feature task in experiment 1). When the conjunction task was combined with the central letter discrimination task, the N2pc was delayed, suggesting that the observers shifted attention to the iconic image of the target after completing the central letter task and could not perform this task without the use of spatially focused attention.