1998 Annual Meeting, Los Angeles, Calif. Nov. 7-12, 1998

T.Uka, H.Tanaka, M.Kato and I.Fujita

  Neurons in the inferior temporal cortex (IT) are known to segment two overlapping shapes which can be perceptually segregated by cues such as color or exture (Missal et al., 1997).  In the present study, we examined whether IT neurons can signal the presence of shapes segmented by disparity or contours.  Extracellular neuronal responses to a vertical bar, a horizontal bar and a cross were recorded from the IT of two Japanese monkeys (Macaca fuscata).  If a neuron responded to any one of these figures, we analyzed its response to a cross segmented by disparity or real contours into two bars, one in front of the other.  If it can be assumed that neurons which respond more strongly to a bar than to a cross signal the presence of a bar, it can also be assumed that such neurons possess the potential to signal the presence of a segmented bar by responding more strongly to a cross segmented into a two-bar configuration than to a simple cross.  We addressed this hypothesis by calculating the segmentation index using the following formula

         Segmentation Index =  1 -   maxbar ? segmented cross    

                                      maxbar ? cross

in which maxbar is the larger response magnitude of a neuron to a horizontal or vertical bar, segmented cross is the response magnitude to a cross segmented into a two-bar configuration, and cross the response magnitude to a cross.  The distribution of segmentation index for the 18 neurons studied peaked around 0.53 and 0.21 (significantly different from 0), for crosses segmented by disparity and real contours, respectively.  This indicates that IT neurons can signal the presence of segmented shapes, although more weakly than when the shapes are presented in isolation.  We suggest that contour by itself, whether real or illusory, is sufficient for the segmentation of shapes in IT neurons.

(Supported by CREST)