Experiments by Schiller et al. have suggested that non-directional edge-specific simple cells are constructed from two directionally selective subunits with opposite preferred direction. This hierarchical notion was based on the fact that the responses of such units to edges moving in opposite directions are spatially displaced with respect to each other.
An alternative explanation of the observed response separation is the delay between the responses of the center and surround mechanisms at the retinal level. Measurements of the response separation as a function of stimulus speed support this explanation and argues against the hierarchical notion of Schiller et al.