This articlepresented two studies which compared the thresholds obtained from auditorysteady-state response (ASSR) tests and those obtained from click or tone burst evokedauditory brainstem responses (ABRs). The first study presented a comparisonbetween ASSR and ABR results. 51 Cases were included. Participants who exhibitedprofound hearing loss and an interval of more than 24 months between ABR, ASSR,and behavioral threshold tests were excluded.
The participants ranged from mildhearing to severe-to-profound hearing losses. Also, Subjects had differenttypes of hearing losses. Comparisonswere done between behavioral and evoked potential (ABR and ASSR) thresholds at500, 1000, 2000 and 4000Hz. In behavioral threshold tests, 500, 1000 and 4000Hz were used to establish thresholds. In ABR threshold tests, thresholds weredetermined using 100 f.Lsecclick . In ASSR threshold tests, thresholds weredetermined using pure tones at 500, 1000, 2000, and 4000 Hz that were amplitudeand frequency modulated at 90 Hz.
Results show that both c-ABR and ASSR havestrong correlations with the pure-tone audiogram in infants and children withvarious degrees of hearing loss. The correlation of ABR with pure tonethreshold was higher than ASSR at 1000 and 2000 Hz. Also, there was a highercorrelation between ASSR and behavioral thresholds at 500 Hz compared to thec-ABR. Several factors contributed to these results.
First, clicks evoke a moresynchronous neural response than modulated tones. In contrast, the rise timesof a modulated tone are prolonged relative to the click. Also, threshold for ASSR was defined as thelowest level at which a statistically significant (p < .01) PC result wasobtained. For ABR, threshold was defined as the lowest level based on atime-domain waveform that was visually detected and the observer was notblinded to the subjects' audiometric results, so observer bias might haveaffected the response judgments. The high correlation of c-ABR with the pure toneaudiogram also depends on the nature of pure tone test results. In this sample,most audiograms had flat or only mildly sloping configurations becauseparticipants with profound losses have been eliminated. As a result, theindividual pure-tone thresholds would be highly correlated.
So, ABR andpure-tone threshold correlations were high because only one ABR threshold wascompared with four pure-tone thresholds. For the ASSR, four separatecorrelations have been made, one for each ASSR test frequency and pure-tonethreshold. This data suggest that bothc-ABR and ASSR threshold estimates can be used to predict pure-tone thresholdfor infants and children.
In the second study, a direct comparison ofASSR and tone evoked ABR was done. The PC2 algorithm was used for ASSRdetection and the Fsp algorithm for ABR detection. Participants were ten normalhearing female adults and test was performed unilaterally.
All participants hadnormal pure-tone thresholds. For ABR tests, 500 Hz and 4000 Hz tone bursts wereused. ASSR tests were carried out using 41 Hz and 74 Hz MFs for 500 Hz and 41Hz and 95 Hz MFs for 4000 Hz. For ABR and ASSR tests, stimuli were presented at50, 30, 20, 15, 10, 5, and 0 dB SL. For response detection, two approaches wereused which are the visual detection approach and the automatic detectionapproach. For 500 Hz, the ASSR at 41 Hz and tb-ABR detected by Fsp resulted inthe same threshold.
Visual detection of the 500-Hz tb-ABR resulted in a significantlylower threshold compared to other measures at 500 Hz. The 500 Hz ASSR at 74 Hzresulted in the highest threshold. The visual detection of tb-ABR at 4 kHz alsoresulted in the lowest threshold, but this was not significantly different fromthe threshold obtained for ASSR at 95 Hz. The thresholds obtained from 4-kHz ASSRat 41 Hz and for 4-kHz ABR-Fsp didn’t defer significantly. Over all, results demonstratedthat the tone burst-evoked ABR and the modulated tone-evoked ASSR thresholdswere similar when both were detected with an automatic detection approach andthat thresholds varied with frequency, stimulus rate, and detection method. To conclude, the strength of the firststudy was that participants with a variety of hearing loss types were includedin the study which maximizes the usefulness of the results. Yet, the weaknesseswere that first, participants with profound hearing loss were excluded from thestudy.
Second, the observer reviewed the participants’audiometric status which may lead to tester bias. On the other hand, the strength of the secondstudy was that higher MFs in ASSRs tests were included because they are generatedat the brainstem level, making them comparable to tb-ABR tests. Whilst, theweaknesses were that all participants were females and the tests were performedunilaterally (left ear).