Sustained extrastriate cortical activation without visual awareness revealed by fMRI studies of hemianopic patients

Patients with lesions in the primary visual cortex (V1) may show processing of visual stimuli presented in their field of cortical blindness even when they report being unaware of the stimuli. To elucidate the neuroanatomical basis of their residual visual functions, we used functional magnetic resonance imaging in two hemianopic patients, FS and GY. In the first experiment, a rotating spiral stimulus was used to assess the responsiveness of dorsal stream areas. Although no response was detectable within denervated or destroyed early visual cortex, motion-sensitive areas (hMT+/V5) ipsilateral to the lesion showed a strong sustained hemodynamic response. In GY, this activation was at least as strong as that of his contralesional hMT+/V5 to the stimulus in the normal hemifield. In the second experiment, coloured images of natural objects were used to assess the responsiveness of ventral stream areas. Again, no activity was detectable in ipsilesional early visual areas, but extrastriate areas in the lateral occipital cortex (hMT+/V5 and LO) and within the posterior fusiform gyrus (V4/V8) showed a robust sustained hemodynamic response. In both experiments, we observed that ipsilesional areas responded to stimuli presented in either hemifield, whereas the normal hemisphere responded preferentially to stimuli in the sighted hemifield. As only one subject occasionally noticed the onset of stimulation in the impaired field, the unexpectedly strong sustained activity in ipsilesional dorsal and ventral cortical areas appears to be insufficient to generate conscious vision.