The Spatial specificity of adaptation underlying the direction aftereffect

Research into illusions and after effects of motion direction (“direction repulsion”) has produced conflicting evidence with regard to the site of underlying neural activity. We present data designed to indicate whether more than one population of cells can produce a direction after effect (DAE). We adapted and tested observers with drifting dot stimuli and with drifting grating stimuli in a repeated measures design. DAEs were measured when the adapting and test stimuli were spatially superimposed and when they were spatially separated. If the grating and dot DAEs have a common neural substrate, the degree of spatial specificity of the two DAEs should be the same. A significant difference in spatial specificity would cast doubt on the usual assumption that gratings and dots can be employed interchangeably to measure direction repulsion.