Abstract
Purpose: :
Unilateral V1 damage causes bilateral visual field defects, referred to as partial cortical blindness. Though homonymous, the deficits are never identical. Recently, our lab demonstrated that cortically blind patients who undergo intensive training on a motion discrimination task can recover visual motion perception to normal levels of discrimination at the trained locations (Huxlin et al. 2009). Also, the initial inter-ocular discordance was accentuated by visual training. We tested the hypothesis that the majority of visual gain induced by training occurs in the preferred eye.
Methods: :
Adults with visual defects secondary to V1 damage > 6 months underwent Humphrey visual field tests before (n=17 subjects) and after training (n=8). The border of the visual field defect was defined at baseline as a test point measure (x < -5dB) at the edge of the cortically blind field. The same border was compared post training using the Sign Test, a non-parametric paired t-test, for each of the laterality variables. Ocular preference was assigned by the Dolman Dominance Test. Task directed ocular preference and hand dominance was obtained by survey data (post training n=7).
Results: :
15 cortically blind patients completed the Dolman test (right eye preference n=9 subjects, left n=6). This largely agreed with the task-directed ocular preference, but not hand dominance. Pre-training, the mean+/-SD at the blind field border was -25.5+/-7.1dB for the dominant eye and -26.1+/-8.3dB for the non-dominant eye (n=15, p=0.30). Motion discrimination training improved visual performance, decreased the size of the blind field and significantly improved the mean pattern deviation along the original blind field border (n=8, p=0.0005). However, the greatest improvements did not consistently occur in the Dolman defined preferred eye (n=8, p=0.73), nor the task-directed preferred eye (n=7, p=1.00) and did not correlate with hand dominance (n=7, p=1.00).
Conclusions: :
Visual discrimination training significantly decreased the mean pattern deviation at the blind field border. However, there was no correlation between the laterality variables and the eye with the majority of the training-induced visual gain. Specifically, ocular preference did not explain the observed inter-ocular asymmetry of the visual field defects. We conclude that the inter-ocular asymmetry, regarding size and depth of the cortically blind fields, is most likely due to anatomical asymmetry in the representation of the left and right visual hemi-fields from the damaged V1 cortex and the intact visual areas mediating training-induced visual improvements.
Keywords: neuro-ophthalmology: cortical function/rehabilitation • visual fields • visual cortex