Visual sensitivity in search tasks depends on the response requirement

This paper reports a comparison between two tasks of visual search. Two observers carried out, in separate blocks, a saccade-to-target task and a manual-target-detection task. The displays, which were identical for the two tasks, consisted of a ring of eight equally spaced Gabor patches. The target could be defined by a difference from the distractors along four possible dimensions: orientation, spatial frequency, contrast or size. These four dimensions were used as variables in separate experiments. In each experiment, performance was measured over an extensive range of values of the particular dimension. Thresholds were thus obtained for the saccade and the manual response tasks. The nature of the response was found to modify the relative visual sensitivity. For orientation differences, manual response performance was better than saccade-to-target performance. The reverse was true for spatial frequency and contrast differences, where saccade-to-target performance was better than manual response performance. We conclude that saccade-selection in a search task draws on different visual information from that used for manual responding in the equivalent task. The two tasks thus differ in more than the different response systems used: the results suggest the action of different underlying neural visual mechanisms as well as different neural motor mechanisms.