Feature analysis in early vision: evidence from search asymmetries.

In this article we review some new evidence relating to early visual processing and propose an explanatory framework. A series of search experiments tested detection of targets distinguished from the distractors by differences on a single dimension. Our aim was to use the pattern of search latencies to infer which features are coded automatically in early vision. For each of 12 different dimensions, one or more pairs of contrasting stimuli were tested. Each member of a pair played the role of target in one condition and the role of distractor in the other condition. Many pairs gave rise to a marked asymmetry in search latencies, such that one stimulus in the pair was detected either through parallel processing or with small increases in latency as display size increased, whereas the other gave search functions that increased much more steeply. Targets denned by larger values on the quantitative dimensions of length, number, and contrast, by line curvature, by misaligned orientation, and by values that deviated from a standard or prototypical color or shape were detected easily, whereas targets defined by smaller values on the quantitative dimensions, by straightness, by frame-aligned orientation, and by prototypical colors or shapes required slow and apparently serial search. These values appear to be coded by default, as the absence of the contrasting values. We found no feature of line arrangements that allowed automatic, preattentive detection; nor did connectedness or containment—the two examples of topological features that we tested. We interpret the results as evidence that focused attention to single items or to groups is required to reduce background activity when the Weber fraction distinguishing the pooled feature activity with displays containing a target and with displays containing only distractors is too small to allow reliable discrimination.