Electrophysiological Correlates of Loudness using Auditory Steady State Responses

Abstract

The present study investigated the usefulness of Auditory Steady State Response (ASSR) in predicting the loudness growth function in normal hearing children. Objective loudness growth function can be used as a supplementary measure in hearing aid fitting especially in the pediatric populations who have difficulty in reporting such measures of loudness growth. To assess the same, the amplitude intensity function of the multiple steady state response (77 Hz-105 Hz) was compared with two standardized subjective loudness growth procedures : the Cross Modality Matching (CMM) and the Cox contour loudness test in twenty normal hearing children at 500 Hz, 1 kHz, 2 kHz and 4 kHz. Results showed that the two standardized methods were able to adequately predict the loudness growth function and had similar exponents of 0.49 and 0.43. No significant difference in the perception of loudness across the frequencies was noticed using the subjective loudness procedures. In ASSR recordings, the amplitude measures were highly variable. On fitting a non linear power function to the amplitude measure and the perceived loudness measure obtained from the two subjective loudness growth tests separately, a overall modest correlation was observed at all frequencies . Correlation • coefficients between ASSR amplitude growth and Cross Modality Matching were, r = 0.35, 0.58, 0.35 and O.54 at 500 Hz, 1 kHz, 2 kHz and 4 kHz respectively . Similarly, with the Cox contour test, correlation coefficient' were, r = 0.37, 0.58, 0.30 and 0.29 at 500 Hz, 1 kHz, 2 kHz and 4 kHz respectively. Additionally, exponents obtained from each frequency were not similar. The ASSR amplitude growth in the present study only proved to be a modest predictor of the loudness growth function. Variability in the ASSR amplitude, structures involved in both the tasks, for example ASSR being generated from brain stem as opposed to the whole auditory system being involved in both subjective loudness tests, probably could lead to a relatively modest correlation. Also, different parameters used in assessing these two tests should be taken into consideration. The findings of the present study suggest that ASSR should be used with caution in predicting the loudness growth function and such a relationship of the ASSR and loudness growth function in the hearing impaired population must be established before using the tool for clinical purposes.