Vision in subjects with hyperawareness of afterimages and “visual snow”

Abstract

AbstractPurpose Some patients complain of persisting visual noise, often described as “visual snow” (VS) and / or prolonged afterimages, but show no obvious clinical abnormalities. The purpose of this study was to examine the extent to which the processing of different stimulus attributes remains normal in VS patients.Methods Seven VS patients and nine control subjects were examined. Advanced vision and optometric tests were used to assess visual acuity (VA), red / green (RG) and yellow / blue (YB) colour sensitivity, rapid flicker sensitivity, chromatic afterimage strength and duration and pupil response amplitudes and latencies to chromatic stimuli.Results The VS patients exhibited normal VA, colour and rapid flicker sensitivity and chromatic afterimage strength. Both controls and four of the VS patients exhibited pupil constrictions to the onset of the coloured stimulus, followed by recovery during the stimulus and a further constriction at stimulus offset, normally attributed to perception of chromatic afterimages (Prog.Brain Res. 144:243‐259, 2004). The pupil responses in three of the VS patients lacked the rapid recovery phase following the initial constriction to stimulus onset.Conclusion The absence of recovery following the initial constriction of the pupil in three of the VS patients deviates from normal responses and suggests the presence of a more sustained retinal afferent signal that drives the pupil response. The latter may be linked to differences in retinal processing of visual signals that cause the perception of visual snow when the coloured stimulus is viewed against a uniform background.