The Hidden Details: Effects of Partial Occlusion on Response Dynamics in the Primate Inferotemporal Cortex

The primate brain can recognize objects even when partially concealed by occluders. To investigate the effect of occlusion on temporal dynamics of neuronal responses, we conducted experiments in two male macaques, recording single units in body-responsive regions in the posterior and anterior inferotemporal cortex (PIT & AIT) during fixation. Seven levels of occlusion were applied to static bodies, ranging from 5 to 60 percent occlusion. In both monkeys and regions, three key findings emerged: 1) average response strength decreased and 2) response onset and peak latency gradually increased by ~70 ms with degree of occlusion, PIT responses consistently preceding AIT. 3) The first response peak was followed by a trough and a stronger second peak under occlusion. To examine the role of visual information loss in the latency shifts, reduced responses, and response peaks, we presented, in addition to the partially occluded bodies, the same stimuli on top of the occluding pattern, and with an invisible occluding pattern, creating bodies with cut-outs. Interestingly, onset latency only shifted ~20 ms for the highest cut-out levels and remained unaffected by the background occluding pattern. Thus, onset latency shifts with occlusion may result from bottom-up occluder-related processing. Despite cut-out-induced response weakening, cut-outs with 60% information loss maintained selectivity similar to that observed during occlusion. However, the trough formation was pronounced when bodies were presented on top of the occluder. Intriguingly, the second peak did not align with response onset shifts but maintained latency differences between regions, occurring earlier in PIT. Thus, the second response peak in PIT is unlikely to arise from recurrent processing within the region or feedback from AIT. If generated by top-down feedback, one would expect it to appear earlier in AIT and may expect better body selectivity. Yet, it never surpassed early response selectivity based on neural decoding.