Post-lesional functionality of the visual system in hemianopic patients

Abstract

Hemianopic patients suffer for a loss of conscious vision in part of the visual field. The present work aimed to investigate the functionality of the visual system after lesions to visual cortices, by studying the spontaneous electrophysiological activity and the residual visual processing. The first three studies revealed the presence of alterations in the spontaneous alpha oscillatory activity during resting-state. Specifically, hemianopic patients showed a slowdown of the speed of alpha oscillations and a reduction of the amplitude of alpha activity in the lesioned hemisphere, resulting in an interhemispheric imbalance of the activity in the alpha range. Moreover, hemianopics showed also a reduction of alpha functional connectivity in the posterior regions of the lesioned hemisphere. However, the residual activity in the alpha range seemed functionally reactive, since hemianopics showed the typical alpha desynchronization in the transition from the eyes-closed to the eyes-open resting-state. More importantly, the spontaneous alpha activity predicted the visuospatial performance, suggesting that the resting-state activity in the alpha range, might be a biomarker for the functionality of the visual system. Notably, oscillatory patterns were more severely impaired in hemianopics with right lesions, suggesting a central role of the right posterior cortices in coordinating the spontaneous oscillatory activity. In the last study, unseen distractors presented in the blind visual field were able to interfere with the execution of saccades toward seen targets presented in the intact field, suggesting the presence of an implicit visual processing for stimuli presented in the blind visual field. However, only left-lesioned hemianopic patients showed implicit processing for the unseen distractors, suggesting that the right hemisphere might also contribute to this interference effect. Overall, the post-lesional oscillatory patterns and the implicit visual processing in the absence of awareness seem to reflect an impaired but residual functionality of the visual system in hemianopic patients.