Functional and histological properties of caudal intraparietal area of macaque monkey

In our previous studies, we found that cells in the caudal intraparietal (CIP) area of the macaque monkey selectively responded to three-dimensional (3D) features, such as the axis and surface orientations, and we suggested that this area played a crucial role in 3D vision. In this study, we investigated (1) whether cells in CIP respond to other 3D features, such as curvature, and (2) whether CIP has any histological property to distinguish it from neighboring areas. Curvatures defined by a random-dot stereogram were presented on a display while the monkey performed a fixation task. The shape and amount of curvature were manipulated by two independent variables, shape index and curvedness, respectively. Two-way ANOVA showed that 19 out of 56 visually responsive cells (34.0%) showed the main effect of shape index. We tentatively designated these cells as 3D curvature-selective (3DCS). Of these, six 3DCS cells showed the main effects of shape index and curvedness, whereas 13 showed the main effect of shape index only. In both types of 3DCS cells, preferred shape indices calculated from tuning curves at two levels of curvedness matched well. These results indicate that the majority of 3DCS cells responded equally to a particular shape of curvatures with different curvedness levels. An immunohistochemical study showed that the recording sites of 3DCS cells were in a cortical region characterized by a dense SMI-32 immunoreactivity in the caudal portion of the lateral intraparietal sulcus (IPS), which suggests that this region is comparable to the lateral occipital parietal (LOP) designated in the caudal IPS previously. Further investigations showed that this region was separated from LIPv, the ventral subdivision of lateral intraparietal (LIP) located rostral to CIP/LOP. These results suggest that CIP is a cortical area distinct from LIP histologically as well as functionally.