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F. F. Ghasia, P. E. Foeller, L. Tychsen; Cerebral Pathways for Vertical OKN: Effect of Callosotomy and Hemifield Stimuli. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2543.
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Periventricular leukomalacia (PVL) in preterm infants causes damage to visual fibers of the corpus callosum, particularly the splenium (i.e. posterior portion). Children with PVL tend to have vertical gaze deficits. We hypothesized that this might be due to damage to the splenium. To test the hypothesis we performed a callosotomy in a non-human primate.
The splenium of the callosum was sectioned in a 1 yr old monkey. One hemiretina in each eye was then laser ablated (to allow confinement of the OKN stimulus to a hemi-field in each eye). Horizontal and vertical OKN were recorded under conditions of monocular or binocular viewing, before and after callosotomy, using binocular magnetic search coils and positive reinforcement.
Vertical OKN was impaired markedly in the first week after callosotomy, but recovered partially in the ensuing weeks. Downward stimuli elicited a greater response than upward stimuli, as measured by calculating an up-down asymmetry index ( AI = mean OKN velocity upward/downward). The asymmetry index before callostomy = 1.02, and 1 month after callosotomy = 0.5 (one way ANOVA p<<0.05). An abnormal oblique component (horizontal added to vertical OKN) was also evident after callosotomy, with a horizontal slow-phase bias towards the cerebral hemisphere receiving V1 input.
These findings suggest that normal vertical gaze requires simultaneous activation of extrastriate visuomotor regions in both cerebral hemispheres. Damage to fibers of the callosal splenium impedes this activation, causing vertical gaze deficits which are initially profound, but recover.
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