SELECTIVE ATTENTION TO CONJUNCTIONS OF COLOR AND SHAPE OF ALPHANUMERIC VERSUS NON-ALPHANUMERIC STIMULI - A COMPARATIVE ELECTROPHYSIOLOGICALSTUDY

We compared multi-dimensional selection on the basis of the color, the global shape and the local shape of alphanumeric (letters) and non-al phanumeric (non-letters) stimuli. We investigated whether letters are selected on the basis of name codes or on the basis of highly familiar local shape codes. Participants responded to a single conjunction of color, global shape and local shape occurring in a randomized stream o f other conjunctions of these attributes. Dependent variables were rea ction time and measures derived from event-related brain potentials (o nset latencies and peak amplitudes of the occipital selection negativi ty, SN). The SN results showed that, for both letters and non-letters, color and global shape were selected first and local shape was select ed later. Reaction times were faster, and SN to the local shape occurr ed earlier for letters than for non-letters. The SN to the local shape of letters was larger than the SN to the local shape of non-letters. In contrast, the SN to the global shape of letters was smaller than th e SN to the global shape of non-letters. Selection of the global shape of letters, but not of non-letters, depended on whether they occurred in the relevant color. Selection of the color of both letters and non -letters was independent of shape relevance, and selection of the loca l shape of both letters and non-letters was independent of color relev ance. These results suggest that, (1) both letter and non-letter shape s are initially analyzed in a feature-specific manner; and (2) letters are selected for task-directed processing on the basis of highly fami liar local shape codes and not on the basis of name codes. (C) 1997 El sevier Science B.V.