Preattentive processing in vision

Visual analysis appears to be functionally divided between an early preattentive level of processing at which simple features are coded spatially in parallel and a later stage at which focused attention is required to conjoin the separate features into coherent objects.

Abstract

Visual analysis appears to be functionally divided between an early preattentive level of processing at which simple features are coded spatially in parallel and a later stage at which focused attention is required to conjoin the separate features into coherent objects. Evidence supporting this dichotomy comes from behavioral studies of visual search, from differences in the ease of texture segregation, from reports of illusory conjunctions when attention is overloaded, from subjects' ability to identify simple features correctly even when they mislocate them, and from the substantial benefit of pre-cuing the location of a relevant item when the task requires that features be conjoined but not when simple features are sufficient. Some further studies of search have revealed a striking asymmetry between several pairs of stimuli which differ in the presence or absence of a single part or property. The asymmetry depends solely on which of the pair is allocated the role of target and which is replicated to form the background items. It suggests that search for the presence of a visual primitive is automatic and parallel, whereas search for the absence of the same feature is serial and requires focused attention. The search asymmetry can be used as an additional diagnostic to help define the functional features extracted by the visual system.