The ability to obtain frequency specific information regarding a patient’s hearing sensitivity in an objective manner allows the evaluation of patient populations who cannot be tested through traditional behavioral methods. One method for obtaining this information is the auditory steady state response (ASSR). ASSR permits the testing of multiple carrier frequencies simultaneously and both ears simultaneously, unlike the auditory brainstem response (ABR). ASSR replaces subjective examiner interpretation of the response with statistical analyses not subject to the variability of human observers.
Unlike ABR which has been in use for decades and utilizes relatively consistent stimuli and test protocols, the ASSR has only been in widespread clinical use for the past 6-8 years and consequently does not have the same level of standardization as ABR. ASSR can be elicited by a variety of stimulus types including but not limited to: (1) Sinusoidal amplitude modulated (SAM) tones, (2) Frequency modulated (FM) tones, (3) Mixed modulation (MM) tones, and (4) Toneburst (TB) trains. The ASSR response is found in the frequency domain at the frequency of modulation and is frequently differentiated from unrelated neural activity using an F-statistic to determine if the amplitude of the line spectra at the modulation frequency is statistically different from the surround physiologic noise.
The current study sought to evaluate several common stimuli used in ASSR testing to determine if a more recently introduced stimulus (TB) emerges as a more appropriate stimulus for generating the response. Response detection and collection parameters were standardized so that any differences seen could be attributed to the stimulus. Both behavioral and ASSR thresholds were measured using SAM, MM, and TB stimuli in ten young adults with normal hearing (≤ 15 dB HL from 250-8000 Hz). Comparisons were then made between stimulus types to determine which stimuli could best predict a behavioral response for a pure tone matching the carrier frequency.
The results of the current study indicate that the MM and TB stimuli provide lower ASSR thresholds than do SAM stimuli and that a regression model provides the most accurate estimates of behavioral threshold. The thresholds for an individually presented TB were consistently lower than for a TB at the same frequency that was presented in the multiple simultaneous paradigm (four simultaneous carrier frequencies presented to the ear). However the threshold predictions based on the two measurements were similar so little accuracy in prediction is lost by using multiple simultaneously presented tonebursts. The current study shows that while ASSR can provide reasonable estimates of hearing sensitivity when the mean data are examined for any given individual the accuracy of prediction can vary greatly.