Binaural interaction in the auditory brainstem in response to homo-phasic and antiphasic stimuli

Binaural level difference (BLD) refers to the lower thresholds obtained when low-frequency signals are presented out-of-phase binaurally, than those obtained for signals in phase. This study was designed to determine if the BLD phenomenon is reflected in the auditory brainstem response (ABR), that is, if ABRs elicited with anti-phasic stimuli yield shorter latencies and/or larger amplitudes when compared to those obtained with homo-phasic stimuli. Ten subjects with normal hearing, participated in the study. Clicks (0.1 ms) were presented at a rate of 5.6/sec, with two in-phase (binaural rarefaction and binaural condensation) and two out-of-phase (Left ear condensation-right ear rarefaction and right ear condensation-left ear rarefaction) stimulus configurations. The stimulus level was 47 dB SL, this level was considered low enough to control for the effects of the acoustic cross-talk and the middle ear reflex. Brain electrical activity was measured across silver electrodes affixed to the forehead and the left earlobe, the nasion served as ground. This electrode montage was used in order to obtain clear earlier waveforms. The earlier of the latencies obtained with homo-phasic stimulation were compared with the earlier of the latencies obtained with anti-phasic stimulation. Repeated-measures ANOVAs, performed on the peaks (P) and troughs (N) of the ABR components, revealed significantly $(p \u3c 0.05)$ earlier latencies for the anti-phasic condition for components IIN, IIIP and VIN. No significant differences were evident for amplitudes. The appearance of BLD in components IIN, IIIP, and VIN of the ABR, in the form of earlier latencies, probably reflects binaural interactions occurring in the cochlear nucleus (efferent pathways), superior olivary complex and the inferior colliculus. The results indicate that the BLD phenomenon is initiated at the brainstem level. Although the BLD is known to be optimal at low frequencies, the current investigation suggests that BLD can be measured in ABR with low-level broadband clicks. Such a measure may have clinical applicability as a \u27sensitized\u27 measure for retrocochlear pathologies