The neural basis of 3D rotation sensitivity from self-generated Optic Flow: a Transcranial Magnetic Stimulation Study

A stationary surface slanted around the vertical axis and observed during a head translation appears to be stationary during monocular, but not binocular viewing when the same image is projected to the two eyes (null disparity). This effect is likely caused by the perceptual interpretation of the optic flow affected by the null disparity information, rather than by extra-retinal signals coming from vergence and accommodation, which instead should lead to an unbiased perception (Fantoni, Domini & Caudek 2010). Here, we investigated the neural basis of this phenomenon with a rotation-detection task during active binocular and monocular viewing before and after offline inhibitory rTMS over early visual areas V2/V3 and the Middle Temporal area (hMT). At baseline subjects reported a rotating object significantly more often in the binocular relative to the monocular viewing condition. Stimulation overV2/V3 caused a reduction of the response bias and a general improvement of the sensitivity in the binocular viewing condition only. These findings were consistent with: I) a disruption of disparity information conflicting with motion and II) a residual effect of binocular summation occurring earlier in the visual hierarchy. Interestingly, stimulation of hMT led to a selective impairment of performance in the monocular viewing condition. Contrary to V2/V3 stimulation, binocular viewing condition was not affected by hMT stimulation. This might indicate that any potential performance gain due to the inhibition of hMT disparity neurons was constrained by the performance impairment due to the inhibition of hMT motion-sensitive neurons. Overall, the results suggest that: (1) the perception of 3D surfaces during active vision is likely mediated by the activity of cortical areas involved in the processing of retinal but not extra-retinal signals; (2) both hMT and V2/V3 are critically involved in the encoding and integration of motion and disparity signals generated during active vision