Inferior temporal neurons show greater sensitivity to nonaccidental than to metric shape differences

It has long been known that macaque inferior temporal (IT) neurons tend to fire more strongly to some shapes than to others, and that different IT neu rons can show markedly different shape preferences. Beyond the discovery th at these preferences can be elicited by features of moderate complexity, no general principle of (nonface) object recognition had emerged by which thi s enormous variation in selectivity could be understood. Psychophysical, as well as computational work, suggests that one such principle is the differ ence between viewpoint-invariant, nonaccidental (NAP) and view-dependent, m etric shape properties (MPs). We measured the responses of single IT neuron s to objects differing in either a NAP (namely, a change in a geon) or an M P of a single part, shown at two orientations in depth. The cells were more sensitive to changes in NAPs than in MPs, even though the image variation (as assessed by wavelet-like measures) produced by the former were smaller than the latter. The magnitude of the response modulation from the rotation itself was, on average, similar to that produced by the NAP differences, a lthough the image changes from the rotation were much greater than that pro duced by NAP differences. Multidimensional scaling of the neural responses indicated a NAP/MP dimension, independent of an orientation dimension. The present results thus demonstrate that a significant portion of the neural c ode of IT cells represents differences in NAPs rather than MPs. This code m ay enable immediate recognition of novel objects at new views.