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Takashi Owaki, Tsunehiro Takeda; The first whole-head recordings of multifocal visually evoked magnetic field (VEF). Journal of Vision 2002;2(7):102. doi: 10.1167/2.7.102.
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Purpose: Although a few multifocal VEF studies have been reported previously, the number of the sensor channels was limited to 8 or 16 which covered only a part of a subject's head because of the constraint of their multifocal analysis systems. In order to record the whole-head multifocal VEF, we developed our original multifocal stimulus generator and multifocal analysis software. Their performance was tested. Methods: A PC-based multifocal stimulus generation system developed by our laboratory was used for stimulus presentation. The stimulus pattern consisted of 12 checkerboards around a fixation point: Four checkerboards were placed centrally and the rest of them were placed peripherally. We used achromatic luminance pattern reversals (condition L) and isoluminant red/green pattern reversals (condition I). Each checkerboard subtended 5 degrees of visual angle and the whole checkerboards subtended 22 deg of visual angle. Individual check size was 75 min of visual angle. The multifocal VEFs were recorded from four normally sighted subjects using a 64-channel whole-head MEG system (CTF Inc., Model-100). The first slices of the second-order Volterra kernels were extracted from the recordings with our original multifocal analysis software. Results: Clear responses were obtained in all subjects in the both conditions of L and I. The central checkerboard pattern reversals elicited larger responses than the peripheral ones. The sources of the responses evoked by the central checkerboard pattern reversals were localized around the calcarine sulcus according to the well-known retinotopic cortical representation. The difference in the locations of the sources between in conditions L and I was not clear, but the first responses in condition I started about 20 ms later than those in condition L. Conclusions: The fact that the results were consistent with those of previous multifocal VEF studies indicates the whole-head multifocal VEFs were measured appropriately.
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