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S. Mei, F. Proudlock, I. Gottlob, R. Mclean; Horizontal Asymmetry of Optokinetic Nystagmus in Strabismus: An fMRI Study . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2935.
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Purpose:An asymmetry in horizontal optokinetic nystagmus (hOKN) in favour of nasalward motion has been described in strabismics patients. The reason for this asymmetry is not well understood. We have used functional magnetic resonance imaging (fMRI) to investigate asymmetries of neural activation in the cerebral cortex and cerebellum in response to nasalward and temporalward hOKN stimuli. Methods: Five strabismics patients, selected because of previously demonstrated hOKN asymmetry, were compared to 7 healthy volunteers using fMRI. The patients viewed either horizontal nasalward or temporalward moving OKN stimuli, interleaved with a fixation task, under monocular conditions using the non–dominant eye. Eye movements were recorded from one eye in all sessions. Functional brain images were acquired on a clinical 1.5T MR scanner (Siemens, Symphony Sonata). Thirty transverse slices (matrix size = 128 x 128) covering the whole brain and the cerebellum were acquired using T2* weighted gradient–echo EPI multislice sequences (TE=59ms, TR=4280ms, voxel size = 2x2x3mm). Each scan session comprised 4 successive time series, two with nasalward OKN and two with temporalward OKN in randomized order. The data were processed using Brainvoyger (Brain Innovation BV, Maastricht). A delayed box car function was used as the predictor to model the expected haemodynamic response to the stimulus. The statistic analysis was performed and the obtained the statistic map was obtained. The activation and deactivation clusters were defined with Talairach coordinates. Results: The healthy subjects showed consistent activation (in decreasing order) of V1/V2, MT+ and cerebellum and the FEF areas. Activation was independent of the direction of hOKN stimulation and was symmetrical. The strabismic patients also showed activity in V1/V2, MT+ and cerebellum and the FEF areas, which was independent of the direction of hOKN stimulation. Three of the five strabismics subjects showed greater activation in the MT+ complex of one hemisphere (contralateral to the non–dominant eye). In one strabismic subject tested with either eye, the activation in MT+ was more symmetrical when tested using the dominant eye. Discussion: This results we observed in the seven control subjects are consistent with earlier findings (Bucher et al., 1997, Dieterich et al., 1998, 2003). The preliminary findings suggest abnormal activation patterns in MT+ activity resulting from strabismus. The asymmetry in response in MT+ in the two hemispheres may underlie the abnormal responses to hOKN observed in strabismics.
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