Auditory Steady-State Response: Comparisons with the Auditory Brainstem
Two studies are
reported in which the threshold estimates from auditory steady-state response
(ASSR) tests are compared to those of click- or toneburst-evoked auditory brainstem
associated with the comparison of the two evoked potentials should be
considered, including frequency specificity, response generators, the effects
of hearing loss and automatic detection algorithms.
Two studies were
undertaken that compared the ASSR with the ABR. First, a retrospective
case review was used to compare ASSR test results in relation to c-ABR. Second,
a direct comparison of ASSR and tb-ABR threshold estimation techniques was
conducted using adult listeners.
Study 1: comparison of
ASSR with ABR results
This study reports a
reanalysis of some data from Rance and colleagues (1995, 1998) to provide some
additional comparisons among audiometric threshold, c-ABR, and ASSRs. Method
were 192 comparisons between behavioral and evoked potential (ABR and ASSR)
thresholds, with 39 at 500 Hz, 78 at 1 kHz, 18 at 2 kHz, and 57 at 4 kHz. Results
The results show
that both c-ABR and ASSR have strong and statistically significant (p < .05) correlations with the pure-tone audiogram in infants and children with various degrees of hearing loss. These data suggest that both c-ABR and ASSR threshold estimates can be used to predict pure-tone threshold for infants and children who have hearing thresholds in the normal to severe-to-profound range. Study 2: direct comparison of ASSR and tone burs~evoked ABR To address the issue of ASSR versus tb-ABR for threshold detection, Kosmider (1997) under took a direct comparison of ASSR and tone- evoked ABR threshold in normal-hearing adults using the PC2 algorithm for ASSR detection and the Fsp algorithm for ABR detection. Method Ten normal-hearing female adults, aged 20 to 35 years, participated in the study. Only one ear was tested for each participant; in six cases, this was the left ear. All participants had normal pure tone thresholds established using standard techniques, with thresholds at or better than 10 dB HL for 250 to 8000 Hz. Tympanometry and acoustic reflex tests were also completed, and the results were within the normal range for all participants. Data Acquisition Parameters ABRs were acquired with the NeuroScan "Scan" system, using a two-channel electrode montage. For ASSR tests, a single recording montage was used. Stimuli For ABR tests, 500- and 4000-Hz tone bursts were used. The ASSR tones were sinusoidally amplitude modulated with a depth of 100 percent and frequency modulated at 10 percent. Response Detection Visual Detection: Threshold was judged to be the lowest stimulus level at which an ABR (wave V) was visually detected in the waveform. Automatic Detection: The Fsp was used as the automatic detection method for ABR. Results The behavioral thresholds for each of the stimuli used in the experiment are summarized in the Table This study showed that tone-ABR and ASSR could both be used to estimate hearing threshold but that the stimulus used (tone burst versus AM + FM tone) and detection method (algorithm versus visual detection) affected the threshold determination. DISCUSSION Calibration and Stimulus Differences Difficulties remain in comparing threshold estimates for tone ABR and ASSR because ofthe difference in stimuli used. Optimizing Fsp When ABR thresholds were estimated from trials meeting the Fsp criterion, the estimated thresholds were significantly higher than those from visual detection. Electrophysiologic techniques can never truly be "objective" if waveforms need to be judged subjectively, and techniques such as Fsp offer a means for making response detection and threshold estimation from ABR objective, with identifiable error rates. Fsp is a robust algorithm and can be improved when more is known about the EEG noise characteristics. ASSR in Adults There, as there is a need to establish ASSR thresholds in normal-hearing listeners with para- metric evaluation of the effects of MF, CF, and subject state, as are only a few studies reporting ASSR thresholds in normal-hearing adults.