Electrophysiological evidence for an early (pre-attentive) information processing deficit in patients with right hemisphere damage and unilateral neglect

Patients with right hemisphere damage and contralesional neglect are often unaware of visual, auditory or tactile stimuli occurring on their left side , In an effort to understand the contribution of pre-attentive processes to this phenomenon, we examined the processing of the pitch, duration and spa tial location of auditory stimuli using an electrophysiological probe, the mismatch negativity (MMN). This event-related brain potential indexes the i ntegrity of cerebral processes that respond automatically to deviations fro m regularity in the acoustic environment, We compared the MMN elicited by r ight-and left-sided deviant stimuli in 10 patients with left unilateral neg lect and 10 age-matched healthy volunteers, exploring an anticipated dissoc iation between the processing of spatial localization of sounds and the pro cessing of the other auditory dimensions, Across dimensions, the MMN elicit ed by deviance occurring to the left of the patients was reduced relative t o that elicited by deviance occurring to the right, This effect was robust for spatial location, and less so for pitch, whereas the processing of stim ulus duration was not significantly affected by the side of stimulation, In healthy subjects, deviance in either side elicited similar MMN. We suggest that an early deficit in detecting changes in the environment hampers the involuntary triggering of attention in those patients and discuss the speci fic role of encoding spatial location in the establishment of conscious awa reness.