Vision in subjects with hyperawareness of afterimages and “visual snow” R Alissa1, W Bi1, A-C Bessero2, G Plant2, J L Barbur1 |
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1Applied Vision Research Centre, City University London, United Kingdom |
| Patients complain of persisting visual noise, often described as “visual snow” (VS), but show no obvious clinical abnormalities. The aim of this study was to investigate the extent to which the processing of different stimulus attributes remains normal in VS patients. Advanced vision tests were used to assess visual acuity (VA), colour sensitivity, chromatic afterimage strength and duration and pupil response amplitudes and latencies to chromatic stimuli on nine control subjects and eight VS patients. Preliminary results show that the VS patients exhibit normal VA, colour 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 (Prog.Brain Res. 144:243-259, 2004). However, the pupil responses measured in other four VS patients showed sustained recovery phase following the initial constriction to stimulus onset. The absence of pupil recovery suggests is consistent with a more sustained signal which may be either an input from retina or feedback signals from the cortex which can also drive pupil responses. This may be linked to differences in retinal processing of visual signals that cause the perception of visual snow. |
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