Temporally distinct neural coding of perceptual similarity and prototype bias.

Psychological models suggest that perceptual similarity can be divided into geometric effects, such as metric distance in stimulus space, and non-geometric effects, such as stimulus-specific biases. We investigated the neural and temporal separability of these effects in a carry-over, event-related potential (ERP) study of facial similarity. By testing this dual effects model against a temporal framework of visual evoked components, we demonstrate that the behavioral distinction between geometric and non-geometric similarity effects is consistent with dissociable neural responses across the time course of face perception. We find an ERP component between the "face-selective" N170 and N250 responses (the "P200") that is modulated by transitions of face appearance, consistent with neural adaptation to the geometric similarity of face transitions. In contrast, the N170 and N250 reflect non-geometric stimulus bias, with different degrees of neural adaptation dependent upon the direction of transition within the stimulus space. These results suggest that the neural coding of perceptual similarity, in terms of both geometric and non-geometric representations, occurs rapidly and from relatively early in the perceptual processing stream.