The Sizable Difficulty in Recognizing Unfamiliar Faces Differing Only Moderately in Orientation in Depth is a Function of Image Dissimilarity

Attempting to match unfamiliar faces at moderate differences in orientation in depth is surprisingly difficult. No general account of these costs has been offered. We assessed the effects of orientation disparity in a match-to-sample paradigm of a triangular display of three faces. Two lower test faces, a matching face and a foil, were always at the same orientation and differed by 0° to 20° from the sample on top. The similarity of the images was scaled by a model based on simple cell tuning that correlates almost perfectly with psychophysical similarity. Two measures of face similarity accounted for matching performance: a) the decrease in similarity between the images of the matching and sample faces produced by increases in their orientation disparity, and b) the similarity between the matching face and the selection of a particular foil. The two images of the same face at a 20° difference in orientation revealed a previously unappreciated marked increase in dissimilarity that was so high that it could be equivalent to the image dissimilarity between two faces at the same orientation, but differing in race, sex, and expression. The 20° orientation disparity was thus sufficient to yield a sizeable 301 msec increase in reaction time