Abstract
Although the effectiveness of visual rehabilitation following post-chiasmal brain lesions is controversial, several visual training paradigms, including one developed in our laboratory, have recently shown promise in restoring both simple and more complex visual functions. A possible mechanism of such recovery is that spared striate cortex is re-activated by training. Alternatively, sub-cortical pathways that bypass striate cortex could transmit visual information into extra-striate visual areas. For example, direct projections from the dorsal lateral geniculate nucleus (dLGN) to V2, V4 and MT, as well as from the superior colliculus/pulvinar to MT have been postulated to mediate different aspects of blindsight. We postulate that V1-bypassing projections and the areas in which they terminate mediate training-induced recovery of visual motion discrimination following V1 damage. To test this hypothesis, we performed event-related functional magnetic resonance imaging (fMRI) in an adult subject with long-standing homonymous hemianopia following a unilateral stroke. fMRI was carried out before and after global direction discrimination training. Prior to training, performance at the chosen blind field location was at chance and fMRI showed an almost complete lack of responsiveness in the damaged hemisphere. Training consisted of performing daily sessions of a left-right direction discrimination task using random dot stimuli in the blind field. After training, direction range thresholds became normal at the retrained location and fMRI showed a robust response both in peri-lesional tissue (putative V2) and the ipsilateral MT+ complex. No “intact” islands of V1 were observed within the lesion. Thus, global motion discrimination training in the blind field of an adult human with long-standing, unilateral V1 damage induced significant functional plasticity in both lower- and higher-level visual areas ipsilateral to the lesion. Future studies will assess whether this effect is robust among different subjects and whether it is specific to global motion retraining in the blind field.