Analysis of pathways mediating preserved vision after striate cortex lesions

Abstract

This study investigated the neural substrates of preserved visual functioning in a patient with homonymous hemianopsia and Riddoch syndrome after a posterior cerebral artery stroke affecting the primary visual cortex (area V1). The limited visual abilities of this patient included above-chance verbal reports of movement and color change as well as discrimination of movement direction in the hemianopic field. Functional magnetic resonance imaging showed that motion and color-change stimuli presented to the hemianopic field produced activation in several extrastriate areas of the lesioned hemisphere that were defined using retinotopic mapping. Magnetoencephalographic recordings indicated that evoked activity occurred earlier in the higher-tier visual areas V4/V8 and V5 than in the lower-tier areas V2/V3 adjacent to the lesion. In addition, the functional magnetic resonance imaging analysis showed an increased functional connectivity between areas V4/V8 and V5 of the lesioned hemisphere in comparison with the same areas in the intact hemisphere during the presentation of color changes. These results suggest that visual perception after the V1 lesion in Riddoch syndrome is mediated by subcortical pathways that bypass V1 and project first to higher-tier visual areas V5 and V4/V8 and subsequently to lower-tier areas V2/V3.

The second face of blindness: Processing speed deficits in the intact visual field after pre- and post-chiasmatic lesions

“…Alterations in down-stream, higher cortical areas, not directly affected by the lesion, were already demonstrated by gene expression studies [42] and single-unit recordings in animal models [43,44], as well as MEG [45] and fMRI studies in humans [20,[46][47][48][49][50][51][52]. This is in line with findings from motor [53,54] and attentional systems [55] where lesion-induced network modifications have been increasingly recognized and appreciated.…”

“…After a unilateral visual cortex lesion the activity patterns of the seemingly ''unaffected'' hemisphere were found to be altered [50,52,58]. In addition, interhemispheric anatomical and functional connectivity undergoes modifications, as shown by two case studies [59,60].…”

Spatiotemporal profiles of visual processing with and without primary visual cortex

“…It is worth comparing our results with those of a combined fMRI and MEG study in another hemianopic patient with cortical blindness, admittedly of much younger age (Schoenfeld et al, 2002). The activity center for V5 from this study was~2.2 cm away from ours and the one for LOT~0.9 cm from our MOG activity.…”

Seeing with Profoundly Deactivated Mid-level Visual Areas: Non-hierarchical Functioning in the Human Visual Cortex

“…5) constraints such activation to be weak or of short duration nature. Finally, it is possible that such activity may be generated by subcortical connectivity such as thalamocortical connections, and reaches the visual system at all levels as suggested, for example, by the ''blindsight'' phenomenon (Weiskrantz 1990;Stoerig 2006) or the ''Riddoch syndrome'' (Riddoch 1917;Zeki and Ffytche 1998;Schoenfeld et al 2002). (Note, however, that the role of the primary visual cortex system in the account for blindsight phenomena is not finally settled; Stoerig 2006.)…”