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EEG and fMRI correlates of non-retinotopic motion processing in the human visual system.

While most of the visual brain areas are organized retinotopically, perception is typically non-retinotopic. For example, as a bicycle passes by, we perceive a reflector on its wheel to move in a circular or prolate cycloidal orbit. By contrast, the true retinotopic trajectory traced out by the reflector is a curtate cycloid. We cannot see the retinotopic motion because the horizontal motion of the bicycle is discounted from the motion of the reflector. The bicycle thus serves as a moving, non-retinotopic reference system, within which the motion of the reflector is computed. Despite the important role of non-retinotopic processing in visual perception, almost nothing is known about its neural correlates. Here, we used the Ternus-Pikler display to contrast retinotopic processing in a stationary reference system against non-retinotopic processing in a moving one. In a 7T fMRI experiment, we found hMT+ to be the first area in the visual processing stream where non-retinotopic percepts were reflected in the BOLD signals. The signals in the early visual areas (V1-V3) instead reflected the retinotopic stimulation. We propose that hMT+ computes the motion of the reference system and immediately discounts this motion in order to compute relative motions. This is in line with our EEG study, where we found neural correlates of non-retinotopic perception already from the earliest evoked peak around 120 ms after stimulus onset and throughout the rest of the visual processing. Meeting abstract presented at VSS 2015.