Korczak et al (2012) offers a comprehensive tutorial on auditory steady-state response (ASSR) audiometry, previously known as the “40 Hz event related potential.” Their review includes stimulus and recording parameters, as well as roles and limits of ASSR with respect to estimating behavioral auditory thresholds in infants with various types and degrees of hearing loss.
Korczak and colleagues report rate-sensitive differences are likely due to various neural generators in the peripheral and/or central nervous systems. Specifically, slower stimuli presentations (for example 20 Hz or less) may activate responses from the primary auditory cortex, whereas stimulus rates between 20 and 60 Hz may come from the primary auditory cortex, the auditory midbrain and thalamus, and for stimuli presented above 60 Hz, the response may originate in the superior olivary complex, inferior colliculus and cochlear nucleus. With regard to stimuli, they note there are essentially broad-band/non-frequency-specific stimuli and frequency-specific stimuli (clicks, tone-bursts, pure-tones and chirps). The authors address and define the advantages and disadvantages of amplitude modulation (AM) versus frequency modulation (FM) as well as mixed modulation (MM, a combination of FM and AM modulation) presentations, and single frequency versus multi-frequency stimulation techniques.
Korczak et al note that ASSR analysis is objective and is based on statistical methods which predict the presence or absence of a response based on the prescribed statistical degree of accuracy. “Stopping rules” are engaged to terminate a test when a response is detected and when there is no reasonable possibility of detecting a response. Although there are multiple algorithms available, most rely on the signal-to-noise (SNR) of the response to determine if a true response has occurred.
With regard to accuracy of air-conducted ASSRs, the authors note that for adults with mild to severe hearing loss, accuracy was found to be within 5 to 13 dB and for adults with normal hearing the test/re-test reliability is good. For screening purposes with multi-frequency tests for infants and children, Korczak et al recommends using normal screening levels of 50 dB HL at 500 Hz, 45 dB HL at 1000 Hz, 40 dB HL at 2000 Hz and 40 dB HL at 4000 Hz. With regard to bone-conducted (BC) ASSR for adults with normal hearing, the BC-ASSR is a fairly good predictor of behavioral thresholds. For infants, temporal bone or mastoid placement (of the BC oscillator) produces the best (lowest) thresholds. However, the authors note that because there is only limited data available on BC ASSR with infants, when ASSR thresholds are elevated (in infants) it is wise to switch to tone ABR to confirm the type and degree of hearing loss.
For More Information, References, and Recommendations
Beck DL, Speidel DP, Petrak M. (2007) Auditory Steady-State Response - A Beginner's Guide. Hearing Review, November.
Beck DL, Speidel DP, Craig JG. (2009) Developments in Auditory Steady-State Responses (ASSR) Hearing Review, August.
Korczak P, Smart J, Delgado R, Strobel TM, Bradford C. (2012) Tutorial—Auditory Steady-State Responses. Journal of the American Academy of Audiology 23:146–170.