Visual adaptation is highly localised in the human retina

Purpose. Visual adaptation is a retinal process that decreases visual sensitivity as ambient illumination increases. It is well known that increased illumination in a small area of the visual field decreases sensitivity only in the neighbourhood of that area. The aim of our experiments was to determine the size of the neighbourhood. Methods. Steady sinusoidal luminance gratings were used to increase adaptation: a small test flash was superimposed on a peak of the grating to measure sensitivity at that point. Results. At low spatial frequencies, detection sensitivity for the test was reduced in approximately inverse proportion to peak luminance. At high spatial frequencies, sensitivity approached the value found in the grating's absence. The model fitted to these data assumed that adapting influences were summed over a pool with a Gaussian weighting. The best-fitting adaptation pool radius in two subjects was slightly greater than 1 minute of visual angle. Much smaller changes in sensitivity were found when the grating was delivered to one eye and the test flash to the other; most of the sensitivity changes described above must therefore be occurring prior to the site of binocular interaction, and are presumably retinal. Conclusions. The adaptation mechanism has a spatial resolution similar to the minimum angle of resolution.