Cortical correlates of stereoscopic depth produced by temporal delay

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Stereoscopic depth processing for static objects depends on retinal disparities between the two eyes and has been shown in previous functional imaging (fMRI) work to involve widely distributed activity in the human visual cortex, including both dorsal and ventral streams. Stereoscopic depth processing of moving objects, on the other hand, can be produced by purely temporal lags between the eyes, and the cortical basis for this kind of stereopsis has received less attention. Using fMRI in human subjects, we measured the activations produced by dynamic visual noise both when it was in phase between the eyes and appeared two-dimensional (2D) and when an interocular delay made it appear like a 3D rotating cylinder. When observers attended to the depth, the stimulus with the interocular delay produced more activity than the 2D stimulus in a large variety of cortical areas, including V1, V3A, caudal intraparietal sulcus (cIPS), and MT. When, on the other hand, observers attended to a digit counting task in the fovea, the stimulus with the interocular delay tended to decrease the BOLD response in V1 while still increasing it significantly in area cIPS. The areas that are activated by interocular delay even when not attending to depth (MT, cIPS) are similar to those previously described for traditional stereoscopic stimuli, and we conclude that the dorsal-stream mechanisms for processing interocular delay are not different at the level of spatial resolution of this study.