Unconscious activation of visual cortex in the damaged right hemisphere ofa parietal patient with extinction

Visual extinction is a sign classically associated with right parietal dama ge. The patient can see a single stimulus presented in the ipsilesional or contralesional visual field, but is characteristically unaware of the same contralesional stimulus during simultaneous stimulation of both fields. The ipsilesional stimulus is said to 'extinguish' the contralesional stimulus from awareness during bilateral stimulation, perhaps due to a pathological bias in attention towards the ipsilesional side. Recent psychophysical evid ence suggests that, although extinguished stimuli are not consciously seen, they may undergo residual processing and exert implicit effects on perform ance. However, the neural structures mediating such residual processing for extinguished stimuli remain unknown. Here we studied the neural activity e voked by an extinguished visual stimulus, using event-related functional MR I (fMRI), in a patient with circumscribed right inferior parietal damage an d profound left-sided extinction. Monochrome objects (faces or houses) were presented in the left or right field, either unilaterally or bilaterally o n each trial, with the patient indicating by button press whether he saw an object on the left, the right or on both sides, He usually sa rv only the right object on bilateral trials, yet the fMRI data shelved activation of v isual cortex contralateral to the extinguished left stimulus on these trial s (compared with right-only stimulation), in both striate and early extrast riate areas of the right hemisphere. This activity had a similar location a nd timecourse to that resulting from a single stimulus in the left versus r ight visual field. Cortical pathways involved in the normal processing of a single seen stimulus can thus still be activated by an unseen, extinguishe d stimulus after right parietal damage. Comparison of fMRI responses for fa ces versus houses revealed some category-specific activation for extinguish ed stimuli in right fusiform regions, but only at low statistical threshold . These results are discussed in terms of theoretical accounts for parietal extinction and, more generally, for the neural substrates of visual awaren ess.