Blue piglets? Electrophysiological evidence for the primacy of shape over color in object recognition.

The goal of the study was to investigate how the color and shape of visual stimuli are processed when they are conjointly presented and represent real, and familiar, entities for which normal individuals presumably have a specific 'object color knowledge' (e.g., piglets are pink, artichokes are green). There is evidence, from event related potential (ERP) literature on selective attention to color in conjunction with other, arbitrarily related, stimulus dimensions (e.g., geometrical shape), that color is processed faster than shape, and that the processing of shape depends on color relevance. In this study we recorded ERPs from 28 scalp sites in right-handed volunteers performing selective attention tasks to either color or shape of pictures representing familiar objects and animals. The results revealed that the selection of color was faster, and probably less demanding, than that of shape. However, it was also evidenced that the selection of color depended on object shape, but not vice versa. Indeed, in the attend-color condition, the N2 responses were significantly greater when stimulus shape was prototypically associated, rather than unassociated, with the color perceived. Topographical mapping of difference voltages identified the posterior occipito/temporal region of the left hemisphere as the possible locus of conjoined color and shape processing. Overall, the data support object-based attention models.