Independent first- and second-order motion energy analyses of optic flow.

The human visual system contains a functional sub-system that is specialized to extract image motion. The sensitivities of neurons change as one moves higher in the pathway. Initially cells collect responses from small retinal areas but later those local signals are combined to extract global motion; either frontoparallel or radial motion relative to the center of the visual field. This sequence of processing is conducted in parallel by pathways sensitive to the motion of either the first- or second-order luminance statistics of the image. Previously it had been shown that these two pathways were independent at the level at which local motion signals and frontoparallel global motion signals are extracted. In this study independence is tested during the extraction of radial global motion; a process strongly associated with cortical area MST (or V6) and the next logical level in the motion pathway. We find that the two pathways do provide independent estimates of radial motion and are, therefore, independent at all levels of the motion pathway that have been tested to date.