Parietal neurons represent surface orientation from the gradient of binocular disparity

In order to elucidate the neural mechanisms involved in the perception of t he three-dimensional (3D) orientation of a surface, we trained monkeys to d iscriminate the 3D orientation of a surface from binocular disparity cues u sing a Go/No-go type delayed-matching-to-sample (DMTS) task and examined th e properties of the surface-orientation-selective (SOS) neurons. We recorde d 57 SOS neurons from the caudal part of the lateral bank of the intraparie tal sulcus (area CIP) of three hemispheres of two Japanese monkeys (Macaca fuscata). We tested 29 of 57 SOS neurons using the square plate of a solid figure stereogram (SFS) and random-dot stereogram (RDS) without perspective cues; almost all of the tested neurons (28/29) showed surface orientation selectivity for the SFS and/or the RDS without perspective cues. Eight of t hese 28 neurons (28.6%) showed selectivity for both the RDS and SFS, 7 (25. 0%) were dominantly selective for the RDS, and 13 (46.4%) were dominantly s elective for the SFS. These results suggest that neurons that show surface orientation tuning for the RDS without perspective cues compute surface ori entation from the gradient of the binocular disparity given by the random-d ot across the surface. On the other hand, neurons that show surface orienta tion tuning for the SFS without perspective cues may represent surface orie ntation primarily from the gradient of the binocular disparity along the co ntours. In conclusion, the SOS neurons in the area CIP are likely to operat e higher order processing of disparity signals for surface perception by in tegrating the input signals from many disparity-sensitive neurons with diff erent disparity tuning.