The frequency-following response in subjects with profound unilateral hearing loss.

Frequency-following responses (FFRs) evoked by 500 c/sec tone bursts 14 msec in duration, presented at 50 dB SL were recorded from Cz--A1 and Cz--A2 electrode derivations using eight subjects with normal bilateral hearing, and eight subjects with profound unilateral hearing loss. Monaural stimulation of either ear in normal subjects, and of the unimpaired ear in hearing-loss subjects, evoked larger responses from the ipsilateral electrode derivation than from the contralateral electrode derivation. Stimulation of the impaired ear in hearing-loss subjects evoked no response. Binaural stimulation in normal-hearing subjects evoked Cz--A1 and Cz--A2 responses of equal magnitude, each larger than either the ipsilateral or contralateral monaural response, but each slightly smaller than the sum of the ipsilateral and contralateral monaural responses. Binaural stimulation in hearing-loss subjects evoked responses equivalent to those obtained monaurally. The results provide evidence of binaural interaction in normal-hearing subjects and indicate that FFR arises from at least two separate symmetric neural sources, possibly by iterative activation of brainstem evoked response (BER) generators.