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C Barber, Y Wen, L Wang, R Kakigi, Y Kaneoke; The Multifocal Visually Evoked Magnetic Field (mfVEF) Delineates Inter-subject Variation in Cortical Topography . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4744.
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Purpose: To use the mfVEF to investigate the cortical representation of foveal and extra-foveal visual field and its spatial frequency tuning characteristics; to compare the performance of checkerboard and dartboard patterns in this regard; to combine mVEF and multifocal VEP (mfVEP) recordings. Methods: A VERIS multifocal system generated a custom checkerboard pattern which was projected into a magnetically shielded room. The pattern comprised an 8x8 array of checkerboards, which subtended 1°, 2° or 4° of visual angle respectively in separate experiments. Individual check size was varied between 7.5' and 60' of arc. 24-channel magneto-encephalogram (MEG) signals were recorded using a Magnes (now 4-D Neuroimaging) system; 8 channels of the amplified and filtered signal being led back to the VERIS computer. Recordings were made from 6 volunteer subjects with normal or corrected-to-normal vision and compared with responses to the usual dartboard stimulus. Results: Clear mVEF waveforms were recorded in all subjects from stimulus elements as small as 1° subtense for those adjacent to the fixation point. Larger stimulus elements (2°, then 4°) were required for increasingly peripheral elements. Check size was optimised at 10' of arc for the inner elements, increasing to 60' at 8° eccentricity. The pattern of waveform variation with eccentricity did not agree well with the dartboard responses. Conclusion: The waveform of the mfVEF is similar between subjects, and the variation of spatial frequency tuning with eccentricity has good inter-subject consistency. The discrepancy between checkerboard and dartboard response behaviour may be due to orientation effects. Combining the mfVEF and mfVEP results yields insights regarding the curvature at the occipital pole, and its variation between individuals.
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