Auditory cortical responses in humans with profound unilateral sensorineural hearing loss from early childhood

We recorded auditory evoked magnetic fields from five patients with profound unilateral sensorineural hearing loss from early childhood, using a 122-channel whole-scalp neuromagnetometer. The stimuli were 50-ms 1-kHz tone bursts delivered to the healthy ear at interstimulus intervals (ISI) of 1, 2, and 4 s. As the normal-hearing controls, four patients had shorter latencies of N100m, the 100-ms response, over the hemisphere contralateral to the stimulation than over the ipsilateral hemisphere. With 1-s ISI, three patients had, instead of N100m, a deflection of opposite polarity at about 100 ms (P100m) after the stimulus onset. A 10-year-old patient had a prominent P100m response, did not produce a clear N100m at any ISI, but had a clear N50m at the 4-s ISI. Four patients had bilateral N200m deflections peaking about 200 ms after the stimulus onset; the fifth patient showed N200m over the right hemisphere. N200m was also observed in the three youngest controls in both hemispheres. The ISI dependence of N100m amplitude and latency was similar in controls and patients. The amplitudes and latencies of N200m did not show any ISI dependence. In patients, the appearance of P100m-N200m deflections of auditory evoked fields, normally present in children, is more pronounced than in controls. The defect apparently delays the development of N100m, possibly by interfering with function of callosal connections.