Global and local motion aftereffects from optic flow stimuli

As we move around the world, complex patterns of visual motion are produced that can in principle be informative about self-motion, or the motion of objects in our visual field. These fields of motion vectors, referred to as optic flow patterns, can be decomposed into four independent first order differential invariants, namely divergence (‘div’: related to expansion or contraction), curl (related to clockwise or anticlockwise rotation), and 2 components of deformation (‘def’: dilation and pure shear). Here, experiments are reported bearing on the first two of these, which could provide information about motion along, or rotations around the z-axis with respect to visible features. Such global flow patterns, it is held, are processed first by local direction selective units, before integration by global motion units. Although neurophysiological studies have presented evidence of units tuned to global flow patterns in MSTd, these mechanisms have proven difficult to isolate psychophysically. Here we quantify the magnitude of global motion aftereffects (MAEs) to a stimulus where local motions are entirely cancelled, along with measurements of local MAEs where global motions are cancelled. These results are discussed with reference to models of optic flow processing.