Modifying experimental conditions of optokinetic nystagmus (OKN) result in different outcomes and may not optimally translate into clinical testing. The purpose of this study was to assess the influence of subject instruction on the anatomical correlates of OKN. The instructions were to voluntarily look or stare at the same moving grating with fixed contrast and spatial and temporal frequencies. Look and stare OKN were generated under identical stimulus "ON" conditions (vertical sine wave grating of 1.14c/deg drifting right to left at 11.4c/s with binocular viewing). FMRI was undertaken utilizing a 3.0T GE system and the BOLD technique. Subjects included 6 normal adults ranging in age from 18 to 54 years with normal visual acuity (20/20 or better) and normal stereoacuity (40s of arc or better). The results reveal that look OKN generated significantly more cortical FMRI activation than stare OKN. Look OKN areas included the culmen, parahippocampal, lingual, middle temporal gyri, inferior and superior parietal lobules and precuneus, all of which were unilaterally activated in the left hemisphere. The middle occipital gyrus was unilaterally activated in the right hemisphere while the cuneus was bilaterally activated. These results show that the activation sites for OKN studies are dependent on subject instruction which influence the type of OKN generated. Specifically, voluntary look OKN involved more brain sites than stare OKN. In so doing, we illustrate the importance of subject instruction and recommend that FMRI investigators of OKN be cognizant of these effects. The anatomical correlates of the look versus stare are discussed.
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