Auditory cortical responses in humans with congenital unilateral conductive hearing loss

We recorded auditory evoked magnetic fields from 6 patients with congenital unilateral conductive hearing disorder with a 122-channel whole-head neuromagnetometer. The stimuli were 50-ms 1-kHz tones delivered to the better ear at interstimulus intervals (ISIs) of 2 and 8 s at two different intensities (50 and 70 dB HL). As in normal-hearing subjects, the amplitudes of N100m, the 100-ms response, were larger in 5 patients and the latencies were shorter in 3 patients over the hemisphere contralateral to stimulation. However, in one patient N100m peaked already at 61 ms over the contralateral hemisphere and amplitudes were larger over the ipsilateral hemisphere, possibly reflecting reorganization of the auditory pathways. In 3 patients the latencies were shorter over the ipsilateral hemisphere. The effects of ISI and intensity were similar over both hemispheres and did not differ from those in controls. It seems that congenital unilateral conductive hearing loss does not necessarily lead to any gross disturbances in the human auditory cortex.