Neuropsychological disorders
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In conjunction with studies of visual attention in normal observers, one other important constraint on understanding the mechanisms of visual selection comes from neuropsychology-the study of disorders of cognition. One example of such a disorder is visual agnosia, a term applied to patients with a selective impairment in the visual recognition of common objects (e.g. Farah, 1990; Humphreys, 1999; Humphreys & Riddoch, 1993). Patients with visual agnosia can have difficulties in selecting information from visual displays, due to problems in grouping together relevant visual features and in segmenting apart irrelevant features. This becomes particularly apparent in visual search tasks in which target selection is aided by the efficient grouping and rejection of distractors (e.g. when distractors are homogeneous). Normal subjects show relatively flat search functions under these conditions, and absent responses can even be faster than present (due to fast rejection of distractors) (Duncan & Humphreys, 1989; Humphreys, Quinlan, & Riddoch, 1989; Quinlan & Humphreys, 1987). In contrast, agnosic patients can show inefficient search and they may fail to demonstrate any evidence of fast absent responses (Humphreys, Riddoch, Quinlan, Price, & Donnelly, 1992). This is not due to a breakdown in visual attention per se, since such patients can conduct quite efficient serial search, when grouping processes do not act to facilitate search (Humphreys et al., 1992). Thus, the study of such patients can inform us about the contribution of grouping processes to selection. Theories hoping to provide a full account of how visual attention relates to the processes involved in grouping and object recognition need to be able to account for deficits of this sort.