Enlarged gamma band response of neuromagnetic auditory evoked fields in a visually impaired subject.

Under acoustic stimulation a phase-locked response in the gamma band (near 40 Hz) in the latency range between 20 and 130 ms is evoked. We report on a considerably visually impaired woman with Grönblad-Strandberg syndrome which involves degeneration at the level of retina, but has no overt central nervous component to the degeneration. The subject exhibited an extraordinarily high power in the phase-locked gamma band response (GBR) which was found to be more than three, and sometimes more than four, standard deviations above the average of a group of 25 subjects with normal vision. Furthermore, the dipoles of her mismatch reaction and M200 were found to be located posteriorly to the dipoles of the M100. Overall, both enlarged GBR and changed cortical representation could be results of cortical plasticity related to visual impairment.