Interaural time effects on the frequency-following response

Frequency-following responses (FFRs) were recorded to evaluate differences between monaural and binaural waveforms and waveforms evoked by stimuli with interaural time disparities. Eight normal-hearing adult females served as subjects. The stimuli were monaural and binaural 450-Hz tonebursts at 65 and 60 dB SL and interaural time differences of 0 and 660 microseconds, respectively. Normalized amplitudes and periodicities of FFR waveforms within and between subjects were compared. The results showed asymmetric FFR to the various stimuli used in this study. Binaural FFR waveforms were greater than monaural but smaller than summed monaural FFRs. Binaural FFR amplitudes evoked by a zero time difference were greater than amplitudes evoked by a 660-microseconds difference. Additionally, tight phase-locked periodicities were evoked in the FFR monaurally and binaurally. The averaged FFR periodicity to all stimulus conditions from all subjects was 2.29 msec, differing only 6.8 microseconds from the period of the 450-Hz stimulus. In contrast, monaural and binaural neurons in the lower brain stem typically exhibit much less synchroneities to low-frequency tones than the FFR. These data provide evidence that the FFR is not simply a sum of neuronal action potentials. The findings suggest instead the presence of brainstem neuronal networks. Such putative neuronal ensembles apparently maintain a closer correspondence to the period of a low-frequency sound, whether monaural or binaural, than the discharge patterns of single neurons.