Functional Neuroimaging of Cortical Plasticity in the Human Visual System

Abstract

Partial damage of the primary visual cortex (V1) and optic radiation lesions can cause visual eld de cits restricted to speci c regions of the contralateral visual hemi eld. This thesis has explored the functional properties of the visual cortex and its capacity to reorganize in patients with chronic V1 or optic radiation lesions resulting in partial or complete homonymous quadrantanopia. We used functional magnetic resonance (fMRI) methods and quantitative population receptive eld (pRF) analysis to investigate: i) how spared regions of the visual cortex cover the visual eld following V1 injury, and ii) whether the retinotopic organization of the spared visual cortex changes as a result of reorganization. We demonstrate that the spared part of area V1 has at best a limited-degree of reorganization that manifests in some patients with a small shift of the pRF centers towards the border of the scotoma and by a slight increase in V1 pRF sizes near the border of the scotoma. Importantly, we show that responses in early and higher visual cortex are not always congruent with visual perception in subjects with visual cortical lesions. Several patterns of mismatch were identi ed: 1) visual eld areas covered in both areas V1 and hV5/MT+, 2) visual eld areas covered in hV5/MT+ but not V1 suggesting the existence of functional pathways that bypass area V1. Interestingly these areas overlap with dense regions of the perimetric scotoma, suggesting that activity in these areas does not contribute to visual awareness. Nevertheless, identifying and characterizing the patterns of activation seen in the visual cortex may help choose visual eld locations with high potential for rehabilitation. Conversely, we found cases in which 3) spared area V1 failed to cover completely seeing visual eld locations in the perimetric map, suggesting the existence of V1-bypassing pathways that are able to mediate useful vision. Understanding how the properties of visual areas change after injury, and how this correlates with perception is important in the e ort to adopt new rational strategies for visual rehabilitation. Finally, we reviewed the literature and proposed a systematic approach to visual system rehabilitation using the combination of pRF mapping and real-time fMRI neuro-feedback methods.