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W.V. Good, C. Hou; In Children With Cortical Visual Impairment Sweep VEP Grating Acuity Thresholds Are Improved Under Low Luminance . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5651.
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Purpose: The leading cause of bilateral vision impairment in children in the US is cortical visual impairment (CVI); i.e., loss of vision secondary to damage to the visual cortex or optic radiations. Children with this condition are often photophobic and appear to see better in dim lighting. We tested the validity of this clinical observation by measuring sweep VEP grating acuity under conditions of low (20 cd./m2) and normal (109 cd./m2) luminance to determine whether luminance affects grating acuity thresholds. Methods: Sweep VEP grating thresholds were recorded from 20 CVI patients ranging in age from 6 month to 5 year old and in 17 age–matched normal infants and young children. The grating acuity stimulus was a cosine–wave vertical grating presented at 80% contrast in an on/off configuration and temporal frequency of 3.76 Hz. Grating acuity thresholds were measured under 2 different luminance conditions: high luminance (109 cd/m2) and low luminance (20 cd/m2). The spatial frequency sweep ranged from 1 to 12 cyc/deg for CVI subjects, and 2 to 24 cyc/deg for the normal subjects. Multiple trials were averaged for each stimulus condition and a threshold computed. An average for each condition was computed across all subjects for each of the 2 groups (CVI and control). Results: Overall, grating acuity of CVI patients was substantially worse than normal subjects, as expected. CVI patients showed better grating acuity at low luminance than at high luminance, while normal subjects showed no difference. There was a main effect of subject category, due to higher thresholds for normal subjects, F(1,26) = 96.249, p < 0.001. There was also an interaction between subject category and luminance, with CVI patients and normals showing opposite effects of luminance, F(1,26) = 9.604, p = 0.005. Response amplitudes were higher under low luminance conditions, at increasing spatial frequency ranges in CVI. Conclusions: Children with CVI have improved grating acuity thresholds under low luminance conditions. This finding is unexpected, and could have ramifications for rehabilitation strategies for these children.
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