Visually evoked BOLD responses in human cortex modulated by gaze direction

Michela Tosetti, Stella Maris Scientific Institute, Pisa, Italy

Abstract
Gaze direction affects responses to visual stimuli in many cortical areas of the monkey, and in humans there is good psychophysical evidence for spatiotopic integration of motion stimuli. Here we demonstrate that human middle temporal cortex encodes motion in a spatiotopic fashion and therefore could mediate the spatiotopic integration of motion signals, converting retinotopic information into a spatiotopic reference frame. We first isolated the “retinotopic” portion of hMT complex in each subject then selected those voxels within this portion that responded to flow against speed-matched random motion. We then measured fMRI BOLD responses to coherent motion in four screen positions and for three different gaze directions. In V1 gaze did not alter the retinotopicity of the responses: the response curves for the three different fixations were almost identical when plotted in retinal coordinates, but widely spaced in screen coordinates. MT showed the opposite, with the curves were displaced in retinal coordinates, but aligned in spatial coordinates, showing that retinotopic selectivity changes with eye position to produce a clear spatiotopicity.

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