Integration of perspective and disparity cues in surface-orientation-selective neurons of area CIP

We investigated the effects of linear perspective and binocular disparity, as monocular and binocular depth cues, respectively, on the response of sur face-orientation-selective (SOS) neurons in the caudal part of the lateral bank of the intraparietal sulcus (area CIP). During the single-unit recordi ng, monkeys were required to perform the delayed-matching-to-sample (succes sive same/different discrimination) of discriminating surface orientation i n stereoscopic computer graphics. Of 211 visually responsive neurons, 66 we re intensively tested using the solid-figure stereogram (SFS) of a square p late with both disparity and perspective cues (D+P condition), and 62 of th ese were identified as SOS neurons for responding selectively to the orient ation of stimuli. All these neurons were further tested using a solid figur e with perspective cues alone (P-only condition), and 58% (36/62) of these showed selective response to the orientation of the stimuli. Of the 62 SOS neurons, 35 neurons were also tested using SFS with disparity cues alone (D -only condition) in addition to the D+P and P-only conditions. We classifie d these 35 neurons into four groups by comparing the response selectivity u nder the P-only and D-only conditions. More than one-half of these (19/35) were sensitive to both perspective and disparity cues (DP neurons), and nea rly one-third (11/35) of these were sensitive to disparity cues alone (D ne urons), but a few (2/35) were sensitive to perspective cues alone (P neuron s). The remaining (3/35) neurons exhibited orientation selectivity only whe n both cues were present. In DP neurons, the preferred orientation under th e D+P condition was correlated to those under the D-only and P-only conditi ons, and the response magnitude under the D+P condition was greater than th ose under the D-only and P-only conditions, suggesting the integration of b oth cues for the perception of surface orientation. However, in these neuro ns, the orientation tuning sharpness under the D+P and D-only conditions wa s higher than that under the P-only condition, suggesting the dominance of disparity cues. After the single-unit recording experiments, muscimol was m icroinjected into the recording site to temporarily inactivate its function . In all three effective cases out of six microinjection experiments, discr imination of a three-dimensional (3D) surface orientation was impaired when disparity cues alone were present. In only one effective case, when a rela tively large amount of muscimol was microinjected, discrimination of a 3D s urface orientation was impaired even when both disparity and perspective cu es were present. These results suggest that linear perspective is an import ant cue for representations of a 3D surface of SOS neurons in area CIP, alt hough it is less effective than binocular disparity, and that both of these depth cues may be integrated in area CIP for the perception of surface ori entation in depth.