Biological visual systems can detect positional changes that are finer
than these systems' acuity to sine-wave gratings, a property known as
hyperacuity. Some systems can even detect changes finer that the phot
oreceptor spacing. We report here that rabbit's directionally selectiv
e ganglion cells not only detect positional changes in the hyperacuity
range, but also discriminate the direction of their motion. Our exper
iments show that directional selectivity occurs for edges of light mov
ing as little as 1.1 mu m (26'' of visual angle) across the retina. Th
is distance corresponds to a hyperacuity, since the acuity to sine-wav
e gratings of rabbit's On-Off DS ganglion cells is about 125 mu m (50'
). In addition, this distance is smaller than the minimal spacing betw
een rabbit photoreceptors (1.9 mu m or 46''), as estimated from cell-d
ensity studies (Young and Vaney, 1991). Such a hyperacuity suggests lo
w-noise high-gain signal transmission from photoreceptors to ganglion
cells and that directional selectivity can arise in small portions of
retinal dendritic processes.