We have previously reported that there exist neurons that selectively responded to specific range of gloss in the inferior temporal (IT) cortex of the monkey (Nishio et al., 2012). These gloss selective neurons encode perceptual gloss parameters "c" and "d" as well as surface albedo where "c" corresponds to the contrast between surface regions with and without specular highlights, and "d" corresponds to the sharpness of highlight (Nishio et al., submitted). Gloss selective neurons were concentrated in a restricted region extending 2-3 mm in the lower bank of the superior temporal sulcus (STS) in the central IT cortex.
To understand the cortical processing related to the generation of gloss selective neurons, it is important to localize the source of inputs to these neurons. For this purpose, in the present study, we injected a small amount of retrograde tracer (CTB Alexa 555) in the lower bank of STS where gloss selective neurons were clustered. We observed retrogradely labelled neurons were distributed in several regions posterior to the injection site from V4 to the posterior IT cortex. Labelled neurons were most densely observed within the STS, but we also observed clustering of labeled cells in the lateral surface of the IT gyrus dorsal to the anterior end of the posterior middle temporal sulcus (PMTS), the ventral region near the occipitotemporal sulcus, and a small region around the central part of PMTS. We have previously reported that neurons selectively responsive to the luminance gradient were clustered in the region dorsal to the anterior end of the PMTS. Because "c" corresponds to the contrast between bright part and dark part on an object surface, there is possibility that this region may provide signals relevant to "c" to the gloss selective neurons in the STS. In the region at the central part of PMTS, cells sensitive to the luminance contrast were observed, and these neurons may be engaged in carrying the information of surface albedo. These data provide useful clues as to the understanding the cortical processing of gloss information.