PP511 - Comparison of New Generation Air- and Bone-Conduction ASSR Thresholds to Behavioral Thresholds

Rational/Purpose 

ASSR is a sustained electrophysiological response phase-locked to the steady component of the stimulus, which can be utilized for objective frequency-specific threshold estimation. First-generation ASSR used amplitude-modulated or mixed-modulated stimulus and research showed a good correlation between ASSR and behavioral thresholds for normal hearing individuals and a better correlation for hearing loss individuals for air conduction stimulation. However, the stimulus and detection paradigm of first-generation ASSR lacked sufficient sensitivity. The new generation ASSR is developed to overcome shortcomings of first-generation ASSR by using narrowband CE-chirps, which generate large amplitude responses, and improved detection paradigm utilizing multiple harmonics spectral and phase analysis. This study investigates the relationship between ASSR and behavioral thresholds in normal-hearing adults for both air- and bone conduction stimulation.  

Methods 

The study employed a nonexperimental correlational design. Pure-tone behavioral and ASSR thresholds for air- and bone-conduction stimulation were obtained and compared from 15 normal-hearing adults (3 males and 12 females). Multiple ASSR techniques in Interacoustics Eclipse systems are utilized. Narrowband CE-chirps with varying repetition rates centered at 500Hz, 1000Hz, 2000Hz, and 4000Hz are presented for AC and 1000Hz, 2000Hz, and 4000Hz for BC. In a two-channel recording montage, the non-inverting electrode was placed at the forehead at the hairline, two inverting electrodes were placed on each ear lobe, and the ground electrode was placed on the forehead underneath the non-inverting electrode. Participants relaxed or slept during the ASSR recording. The modified Michel and Jorgensen (2017) procedure is used for ASSR threshold determination. The ASSR threshold is the lowest level at which there is a response 5dB above and no response 5dB below. A response was negative if residual noise was less than 20nV and significance was less than 99% or if residual noise was ≤20nV and significance was less than 50% after 6 minutes and less than 75% after 9 minutes. 

Results 

Consistent with the previous research, ASSR thresholds and the mean differences between ASSR and behavioral thresholds were lowest for high frequencies than low frequencies for both AC and BC stimulation. The AC ASSR threshold at 500 Hz was significantly higher than thresholds at 1kHz, 2kHz, and 4kHz. There was a significant correlation between air-conduction ASSR and behavioral thresholds solely at 4kHz. Bone conduction ASSR at 4kHz had the lowest mean difference with behavioral thresholds, and 1kHz had the greatest difference. For BC, no significant correlation was obtained between ASSR and behavioral thresholds. 

Conclusion 

Similar to previous research, high-frequency ASSR had the lowest mean difference for AC and BC stimulation. Surprisingly, only 4kHz for AC stimulation significantly correlated with pure-tone behavioral thresholds. These limited correlation findings could be due to the inability to present below 0dBnHL during ASSR testing, whereas participants showed behavioral pure tone thresholds below 0 dB HL. Further research with a larger dataset is needed to explore the relationship between ASSR and pure-tone thresholds among the normal hearing and hearing-impaired populations.