MAGNETOENCEPHALOGRAPHIC EVIDENCE FOR NON-GENICULOSTRIATE VISUAL INPUTTO HUMAN CORTICAL AREA V5

The aim of this study was to establish whether there is non-geniculost riate input to the extrastriate motion-sensitive area V5 in humans. Re sponses were measured with a SQUID neuro-magnetometer to motion stimul i presented within the blind hemifield of GY, a well-documented subjec t with a complete absence of the left primary visual cortical area V1. The motion stimulus was a 0.5 c/deg, rapidly drifting (16 Hz) achroma tic sinusoidal grating. With this stimulus, the magnetic responses rec orded over the temporo-parieto-occipital region in normals are well mo delled by localized current sources in areas V1 and V5 (Anderson, S. J . ct al., Proceedings of the Royal Society, London, Series B, 1996, 26 3, 423-431). As a control, evoked responses were measured to a 1.0 c/d eg, stationary, photometrically isoluminant red/green sinusoidal grati ng. With the chromatic stimulus, the principal component of the magnet ic responses recorded over the occipital pole in normals is well model led by a current source in area V1 (Fylan, F. et al., Investigative Op hthalmology and Visual Science, 1995, 36, s1053). Both stimuli subtend ed 4 deg vertically by 6 deg horizontally, positioned such that the st imulus extended beyond the area of macular sparing into the lower fiel d quadrant of the blind (or sighted) hemifield. Chromatic stimuli fail ed to evoked responses from GY's blind (contralateral) hemifield, cons istent with there being no V1 activity in his left cortical hemisphere . However, motion stimuli did evoke responses from GY's blind hemifiel d, originating from a location consistent with activity in area V5. We further observed that both colour and motion stimuli evoked responses from GY's sighted (ipsilateral) hemifield. We conclude that there is non-geniculostriate input to extrastriate motion-sensitive areas in th e human visual system, and that this pathway subserves the residual vi sual sensitivity to motion in the blind hemifield that has been demons trated psychophysically in observer GY. (C) 1997 Elsevier Science Ltd.