Sound-field auditory steady-state response (sASSR) is a useful tool to measure a person's thresholds and speech perception abilities even when wearing hearing prosthetic devices (J Am Acad Audiol. 2002 Apr;13(4):205). However, most auditory evoked responses to cochlear implant (CI) stimulation are accompanied by artifacts. In this study, we demonstrated the presence of such artifacts by (1) comparing the sASSR between CI recipients and normal-hearing controls and (2) recording the sASSR on a human cadaver with a CI.
This IRB-approved study involved a normal-hearing (NH) group of 15 adults and a CI group of eight adults, including six with unilateral implants and two with bilateral implants (Cochlear Nucleus). The study also involved one human cadaver (an 80-year-old woman) implanted with a Cochlear N24(CA) in the right ear and tested eight hours post-mortem. Core temperature was maintained at 84.6oF. Neural response telemetry was attempted but not observed, demonstrating the absence of a compound action potential.
Electrophysiological sASSR thresholds and behavioral thresholds to the ASSR stimulus were measured from NH and CI participants using the IHS SmartEP-ASSR system in a soundproof booth. We used four carrier frequencies (500, 1,000, 2,000, and 4,000 Hz) that were amplitude-modulated at 79, 87, 95, and 103 Hz, respectively, and presented them using a speaker. Starting at 50 dB SPL, the modified Hughson-Westlake procedure was used to obtain sASSR and behavioral thresholds (J Am Acad Audiol. 1998 Oct;9(5):315). The IHS SmartEP-ASSR recording was done using a speaker set at 70 to 20 dB SPL in 10 dB steps. In CI cases, ASSR recordings were also performed with the processor on and off.
RESULTS AND CONCLUSION
Across all carrier and modulation frequencies, the average ASSR threshold was 25 dB SPL for the NH and 20 dB SPL for the CI group [F (3,46) = 14.391, p<0.001)]. For the NH group, the average behavioral threshold was 4 dB SPL, much lower than its corresponding ASSR threshold as shown in previous studies (J Am Acad Audiol. 1998). In contrast, the average behavioral threshold was 21 dB SPL for the CI group, similar to its ASSR threshold, suggesting a stimulation artifact. The artifact was also suggested by the CI users’ subjective reports during the test that they did not hear anything, although a “significant” ASSR was recorded. The artifact was confirmed by the strong presence of the “ASSR” in the human cadaver that disappeared when the processor was turned off. Although ASSR has become increasingly important in the objective measurement of hearing, its application to CI requires extreme caution.