Effective masking levels for bone-conducted amplitude- and frequency-modulated tones in adults with normal hearing: A behavioural study

Objectives: (1) to estimate the amount of masking needed to eliminate perceptual responses to 1000- and 4000-Hz bone-conducted mixed amplitude-(AM) and frequency-modulated (FM) tonal stimuli in adults with normal hearing, and (2) to compare these findings to recently reported effective masking levels (EMLs) for bone-conducted 80-Hz ASSRs. Design: Stimuli were bone-conducted single sinusoidal tones with carrier frequencies of 1000 and 4000 Hz (Mixed modulation (MM): 100% AM & 25% FM at 85-101 Hz) presented to the temporal bone at 15-45 dB HL for 1000 Hz and 25-35 dB HL for 4000 Hz. Air-conducted 1- and 4-Hz narrow-band noise maskers were presented to both ears simultaneously using ER-3A insert earphones. EMLs for each of the stimuli were determined behaviourally. Study sample: Seventeen adults (mean age: 27.6 years) with normal hearing participated. Results: Overall, EMLs were 10-17 dB higher for perceptual responses compared to ASSRs for 1000 and 4000 Hz. Linear regression analyses revealed that behavioural and ASSR EMLs were not significantly correlated for most of the stimuli presented except for 1000-Hz presented at 45 dB HL (r=.64, p=.013). Conclusions: EMLs are frequency-and testing method-dependent for bone-conducted MM tonal stimuli for normal-hearing adults.