In previous experiments we measured observers' performance in a rotation-detection task during active vision of structure from motion (SfM) displays. Observers performed a lateral head shift while viewing either monocularly or binocularly the same optic flows consistent with either static or rotating random-dot planar surfaces. An Optotrack Certus system was used to update in real-time the optic flows as a function of observer's head position and orientation. Results showed that the addition of a null disparity field increased the likelihood of perceiving surface rotation causing reduced rotation sensitivity for the binocular relative to the monocular viewing condition. A possible hypothesis for this phenomenon is that the introduction of a null disparity field creates an inconsistency among the depth cues forcing the visual system to interpret the optic flow in a way consistent with disparity (rotating surface far from the point of view) rather than vergence information (static surface located at the level of the screen). In order to test this hypothesis we used low-frequency rTMS over the early visual cortex. Neurophysiological inactivation studies (Ponce et al., 2008) have found that visual areas V2/V3 are selective for the recovery of depth from binocular-disparity information. Two groups of subjects performed the same rotation detection task before and after rTMS or Sham-TMS delivered offline (10min, 1Hz) over V2/V3 targeting binocular disparity-sensitive neurons. Consistent with our hypothesis rTMS induced an improvement in the rotation sensitivity that was selective for binocular condition, while monocular performance remained intact. We conclude that low-frequency rTMS over V2/V3 inhibits binocular disparity-sensitive neurons allowing the visual system to interpret a binocularly viewed optic flow as consistent with retinal motion information and vergence regardless of disparity information.
