Abnormal retinotopic representations in human visual cortex revealed by fMRI

The representation of the visual field in early visual areas is retinotopic . The point-to-point relationship on the retina is therefore maintained on the convoluted cortical surface. Functional magnetic resonance imaging (fMR I) has been able to demonstrate the retinotopic representation of the visua l field in occipital cortex of normal subjects. Furthermore, visual areas t hat are retinotopic can be identified on computationally flattened cortical maps on the basis of positions of the vertical and horizontal meridians. H ere, we investigate abnormal retinotopic representations in human visual co rtex with fMRI. We present three case studies in which patients with visual disorders are investigated. We have tested a subject who only possesses op erating rod photoreceptors. We find in this case that the cortex undergoes a remapping whereby regions that would normally represent central field loc ations now map more peripheral positions in the visual field. In a human al bino we also find abnormal visual cortical activity. Monocular stimulation of each hemifield resulted in activations in the hemisphere contralateral t o the stimulated eye. This is consistent with abnormal decussation at the o ptic chiasm in albinism. Finally, we report a case where a lesion to white matter has resulted in a lack of measurable activity in occipital cortex. T he activity was absent for a small region of the visual field, which was fo und to correspond to the subject's field defect. The cases selected have be en chosen to demonstrate the power of fMRI in identifying abnormalities in the cortical representations of the visual field in patients with visual dy sfunction. Furthermore, the experiments are able to show how the cortex is capable of modifying the visual field representation in response to abnorma l input. (C) 2001 Elsevier Science B.V. All rights reserved.