Direction selectivity in human visual perception, investigated with low contrast gratings.

Velocity thresholds (VT's) of direction selective mechanism were measured with coarse low contrast gratings, visible only when moving. direction of movement was detected only in a movement perception domain (MPD), limited by upper and lower VT's and a minimum exposure time of movement. MPD expanded with increasing contrast or increasing stimulus area. MPD shifted towards higher velocities with lower spatial frequencies or with larger stimulus eccentricities. Form perception in MPD was best at optimal velocities; the spatial structure of the gratings was not evident at the VT level. These observations suggest a velocity tuned sensitivity of direction selective mechanisms and an association of movement and form perception with the activity of movement analysing neurons.