The photopic hill: a new phenomenon of the light adapted electroretinogram.

Utilizing a high intensity photographic flash unit, electroretinograms were recorded from normal adults under fully light adapted conditions over a 5 log unit range of stimulus luminance (-1.35 to 3.34 log cd-s/m2). At lower luminance levels b-wave amplitude increased with increased luminance until it reached a maximum (Vmax of the Naka-Ruston equation) in agreement with previous work. At higher luminance levels, the b-wave amplitude decreased to 33% of Vmax and then plateaued. This previously unreported phenomenon has been named the photopic hill. There was no appreciable change in b-wave amplitude with increased interstimulus intervals from 15 sec to 5 min and luminance-response functions serially recorded for increasing and for decreasing stimulus luminance were very similar. These latter results indicate that the photopic hill is not due to light adaptation. The reason for the photopic hill and possible clinical implications are discussed.