Investigation of Auditory Nuclei Generating Binaural Masking Level Difference: a Frequency Following Response Study

Abstract

Binaural masking level difference (BMLD) refers to the improved signal detection in presence of noise when either signal or noise go out of phase in one ear with respect to the other. BMLD is considered to be result of the binaural interaction occurring at the level of brainstem. Jenkins and Masterton (1982) suggested MSO neurons to be the probable site of BMLD generation in human. Animal studies in single cell chinchilla and guinea pig have shown lower brainstem to be the site of generation of BMLD (Palmer, Jiang, and McAlpine, 1999). However, these results cannot be implicated to humans directly. Few electrophysiologic studies had aimed to locate site of origin of BMLD in adults. But this issue always remained a matter of discrepancy across researcher. Kevanishivili and Lagidze (1987) used auditory brainstem responses (ABR), middle latency responses (MLR) and slow cortical potentials (SCPs) to estimate site of BMLD origin, they concluded it to be generating from auditory cortical areas. Wong and Stapels (2004) suggested the same using auditory steady state potential (ASSR) as investigating tools. Wilson and Krishnan (2005) recorded BMLD from the auditory brainstem using brainstem evoked potentials (FFRs) but their findings could not be validated as they did not measure and correlated behavioral BMLDs to FFR BMLDs. Need for study: This study was undertaken to investigate FFR BMLD at the brainstem level and correlate the results with that of behavioral BMLD measure to validate the finding of FFR. Results of this study will provide a critical insight on the site of origin of BMLD.Aim and Objective: To systematically measure Binaural masking level difference behaviourally and electrophysiologically using Frequency Following Response in all participants. Methods: Participants: 20 adults (10 males and 10 females) within the age range of 23-30 years participated in this study. All subjects had normal hearing (<20dB HL) thresholds at octave frequencies from 250 to 8000Hz.Procedures and instrumentation: FFRs were recorded using 500Hztone burst stimulus at 70 dB HL in the So and S π conditions. A 40 msec duration white noise was generated using Adobe Audition. Both the stimulus and noise were routed independently to each ear through two independent programmable attenuators. The six different experimental conditions in which FFR were recorded were So, S π , SoNo, SoN and S π No.All neural responses were transformed from the time domain to frequency domain using fast fourier's transformation (FFT) for different conditions. BMLD amplitude for FFR is obtained from differences between SoNo and SoNπ and SoNo and Sπ Noconditions.Behavioural BMLD threshold were measured using OB 922 diagnostic audiometerfor the same conditions as for FFR BMLD. Result and Discussion: FFRs were present in all the individuals for all the BMLD conditions in both frequency and time domain. There was no significant difference between the So and Sπ conditions. Significant differences were obtained between SoNo and SoNπ and SoNo and Sπ Noconditions. Further, no correlation was evident for behavioural BMLD thresholds and FFR BMLD amplitudes. Conclusion: The results of this study revealed that lower auditory brainstem do preserve phase locked neural activity relevant to BMLDs.But no correlation of FFR BMLDs with behavioral FFR uncertainthe role of auditory brainstem in the generation of BMLDs.