Shape interactions in macaque inferior temporal neurons

Missal et al. observed that the responses of inferior temporal (IT) neurons to a shape were reduced markedly when this shape partially overlapped a la rger second shape, suggesting that shape interactions determine IT response s. In the present study. we compared the responses of IT neurons with combi nations of two shapes which did or did not overlap and studied the effect o f the relative and absolute positions of the two shapes. In a first test, a preferred shape (figure) was presented at the fixation point while a secon d, nonpreferred, shape was displayed either in the background of the figure (overlap) or at one of four peripheral positions (nonoverlap). Controls co nsisted of presentations of either shape in isolation at each of the file p ositions. The stimuli were presented during a fixation task. The responses to these combinations of two shapes were, on average, reduced compared with those elicited by the preferred shape presented in isolation. This suppres sion occurred whether or not the two shapes overlapped, but the degree of s uppression in the overlap and nonoverlap conditions did not correlate. Thes e interactions were stronger when the interacting stimulus was located in t he contralateral compared with the ipsilateral hemifield. The position of t he interacting stimulus within a hemifield significantly affected the suppr ession associated with combined shapes in some neurons. The strength of the interactions of the two nonoverlapping shapes depended on the shape of the interacting stimulus in half of the neurons. In a second test, the preferr ed shape and interacting stimulus could appear either at the fixation point or at one eccentric position. Here we found that the suppression was, on a verage, strongest when the interacting stimulus, rather than the preferred shape, was presented at the fixation position. Also, in 40% of the neurons, the response reduction was similar in overlap and nonoverlap conditions if effects of stimulus position were taken into account. In both tests, we al so measured the responses to combinations of a nonpreferred shape and the i nteracting stimulus and showed that the response to a combination of two no npreferred shapes was, in general, smaller than the response to a combinati on of the preferred and nonpreferred shape. Overall the results indicate th at stimulus interactions in the receptive fields of IT neurons can be posit ion and shape selective: this can contribute to the coding for the relation ships between object parts.