Electrophysiological evidence for an early(pre-attentive) information processing deficit in patients with right...

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 spatial location of auditory stimuli using an electrophysiological probe, the mismatch negativity (MMN). This event-related brain potential indexes the integrity of cerebral processes that respond automatically to deviations from regularity in the acoustic environment. We compared the MMN elicited by right- and left-sided deviant stimuli in 10 patients with left unilateral neglect and 10 age-matched healthy volunteers, exploring an anticipated dissociation between the processing of spatial localization of sounds and the processing of the other auditory dimensions. Across dimensions, the MMN elicited by deviance occurring to the left of the patients was reduced relative to that elicited by deviance occurring to the right. This effect was robust for spatial location, and less so for pitch, whereas the processing of stimulus 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 specific role of encoding spatial location in the establishment of conscious awareness.