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Altered Functions of Visual Brain Areas: Review of Recent Evidence from Blind Human Subjects

We investigated the functional role of human visual brain areas deprived of visual information. To this end, we recorded brain activity elicited by auditory and somatosensory stimuli in blind human subjects. Activity was recorded in the ‘Attend’ condition, in which subjects detected occasional deviant stimuli presented among repetitive standard stimuli and in the ‘Ignore’ condition, in which subjects did not attend to the stimuli. The results indicate that in the early-blinded subjects, event-related potential (ERP) topography elicited by deviant auditory and somatosensory target stimuli is posterior to that in the sighted (Kujala et al, 1995 Experimental Brain Research 104 519 – 526). This suggests involvement of posterior brain areas in auditory and somatosensory processing in the blind humans. For the auditory modality, activated areas were located with magnetoencephalography (MEG), which indicates involvement of extrastriate occipital areas in detection of auditory targets (Kujala et al, 1995 Experimental Brain Research 103 143 – 146). Visual-cortex plasticity was further studied in subjects who had lost their vision after childhood in order to clarify whether these cross-modal changes are specific to visual deprivation of early onset. In that study, auditory ERP topographies of late-blinded, early-blinded, and sighted subjects were compared. Comparison of posterior topography of ERPs elicited by deviant target stimuli in both early-blinded and late-blinded subjects with that in the sighted subjects suggests visual-cortex involvement in auditory processing even in late-onset blindness (Kujala et al, 1977 Psychophysiology 34 213 – 216). Preliminary MEG recordings in one late-blinded subject provided further support for posterior generators (parietal-occipital) in auditory processing.