Abstract
The human visual system is intimately interconnected with effector circuitries that mediate visually-driven behavior. fMRI studies have shown that damage early in the visual stream, e.g. in V1, decreases responsiveness throughout higher-level visual circuitry. However, we know little about how such damage affects activity in effector circuitries. Interestingly, most patients with long-standing V1 damage are “high-functioning” in terms of visually-guided behavior, suggesting that the system must compensate somehow for the perceptual deficit. To identify whether intact brain areas exhibit changes in BOLD signal during performance of a visually-driven behavior, 3 stroke patients with right V1 damage causing left homonymous hemianopia and 4 age-matched controls underwent whole brain fMRI. Subjects performed a global, left-right direction discrimination task using random dot stimuli presented at 100% and near-threshold coherence at two locations in each of their visual hemifields. Global direction discrimination performance in the intact hemifield of hemianopes was similar to that of control subjects. In the blind field however, performance was at chance levels. The BOLD response was modeled at the individual subject level and statistical parametric maps were analyzed at the group level using a 2 (Group) × 2 (Hemifield) mixed factorial design. There was no main effect of hemifield and no interaction between group and hemifield. However, there was a main effect of group. As expected, control subjects exhibited greater activity than hemianopes in the right areas 17/18, the site of brain damage in hemianopes. In contrast, hemianopes exhibited greater activity than controls in the precuneus, and in somatosensory and somato-motor cortices bilaterally. The greater BOLD response in the hemianopic precuneus may indicate greater reasoning and visuo-spatial imagery required to process visual information. Together with increased somato-motor activity, this could represent evidence of compensatory activity within effector circuitries following damage early in the visual stream.