Speed discrimination and its relation to involuntary eye movements in human vision.

In addition to a selective response to a narrow range of motion directions, a neural mechanism specialized for motion detection must also be able to discriminate between different speeds of target movement. Many psychophysical and electrophysiological investigations of motion perception have largely been concerned with identifying possible schemes or mechanisms capable of discriminating motion direction, but the ability to discriminate faster or slower movement in the same direction has so far received comparatively little attention. Two schemes capable of motion detection and speed discrimination are reported here, together with experimental data which show that the visual system employs both schemes, one for the slow speed range (i.e. less than 3 degrees/s) and the other for larger speeds of target movement. It is also shown that the use of both schemes ensures that retinal image displacements due to involuntary eye movements (i.e. slow drifts and microsaccades) are not detected as target movement.