Illusory shape representation in the monkey inferior temporal cortex.

Perceived boundaries without physical differences between shape and background are called illusory contours (ICs). ICs and real contours (RCs) activate the early processing stages of the macaque visual pathway and the occipitotemporal areas of the human visual system in a similar way. However, it is not known how these contours are processed further in the highest visual areas. We tested how the responses of inferior temporal cortical (IT) neurons of macaque monkeys change in relationship to figures with RCs or ICs. The same set of figures [coloured pictures, ICs and silhouettes (SILs)] was presented to awake, fixating rhesus monkeys while the single-cell activity was recorded in the anterior part of the IT. Most of the neurons responsive to RCs were also responsive to the same shapes presented as ICs. The average net firing rates, however, were significantly lower for the illusory stimuli than for the stimuli in the RC conditions, and the latency of the responses was significantly longer for the ICs than for the RCs. The shape selectivity was found to be different for coloured stimuli and ICs, and similar for SILs and ICs, suggesting the invariance of selectivity to shapes having the same contour but lacking internal surface information. These results suggest different modes of processing of RCs and ICs in the IT, which might explain the differences in their perception.