Abnormal retinotopic representations in human visual cortex revealed by fMRI

Abstract

The representation of the visual ®eld 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 (fMRI) has been able to demonstrate the retinotopic representation of the visual ®eld in occipital cortex of normal subjects. Furthermore, visual areas that are retinotopic can be identi®ed on computationally¯attened cortical maps on the basis of positions of the vertical and horizontal meridians. Here, we investigate abnormal retinotopic representations in human visual cortex with fMRI. We present three case studies in which patients with visual disorders are investigated. We have tested a subject who only possesses operating rod photoreceptors. We ®nd in this case that the cortex undergoes a remapping whereby regions that would normally represent central ®eld locations now map more peripheral positions in the visual ®eld. In a human albino we also ®nd abnormal visual cortical activity. Monocular stimulation of each hemi®eld resulted in activations in the hemisphere contralateral to the stimulated eye. This is consistent with abnormal decussation at the optic 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. The activity was absent for a small region of the visual ®eld, which was found to correspond to the subject's ®eld defect. The cases selected have been chosen to demonstrate the power of fMRI in identifying abnormalities in the cortical representations of the visual ®eld in patients with visual dysfunction. Furthermore, the experiments are able to show how the cortex is capable of modifying the visual ®eld representation in response to abnormal input. Ó

Reorganization of Visual Processing in Macular Degeneration

“…Furthermore, although the authors report a region of cortex with no visual activation, there is nonetheless some activation in cortical locations corresponding to the scotomata. Although previous studies (Morland et al, 2001;Baseler et al, 2002) have reported activation of foveal cortex in humans with congenital rod monochromacy, vision loss is difficult to assess reliably in such subjects, is confined to a very small area at the center of the fovea (Ͻ1°), and any changes in visual processing in these subjects could reflect early developmental plasticity.…”

Functional and cortical adaptations to central vision loss

“…Baseler, Morland, and colleagues reported on retinotopic maps in three people with abnormal retinal inputs (Morland et al, 2001;Baseler et al, 2002). Baseler et al measured retinotopic organization in three rod monochromats and two participants with normal vision.…”

Large-scale remapping of visual cortex is absent in adult humans with macular degeneration

“…Abnormal visual development can modify these retinotopic maps [2][3][4][5] . Under circumstances where individuals acquire early visual experience in the presence of a lesion to the center of the retina, reorganization of the visual representation occurs 6 .…”