Emergence of form-independent direction selectivity in human V3A and MT.

A fundamental challenge in motion perception lies in the fact that the global motion of any translating object will generate a heterogeneous set of local motion signals that vary depending on the oriented contour information within each local region. This so-called aperture problem illustrates how the visual system must integrate diverse types of motion signals to attain direction selectivity that is invariant to visual form. Here, we investigated how form-invariant motion selectivity emerges across the human visual pathway by using functional magnetic resonance imaging (fMRI) to measure direction-selective responses to drifting gratings and random-dot motion, and then testing for reliable generalization across stimulus types. All visual areas of interest showed highly reliable direction-selective classification performance for a given stimulus type, but early areas V1 and V2 showed chance-level generalization between motion types. Indeed, V1 responses tended to confuse random-dot motion as resembling orthogonal grating motion, implying that drifting random dots generated oriented motion-streak responses in V1. By contrast, motion-sensitive areas MT+ and V3A showed reliable cross-generalization performance in our fMRI experiments that tested both linear and spiral motion trajectories. Our findings provide compelling evidence that motion direction selectivity becomes more invariant to stimulus form in higher visual areas, particularly along the dorsal visual pathway.