| Summary: | Multiple Auditory Steady-State Responses (ASSRs) will likely be included in the
diagnostic test battery for estimating infant auditory thresholds in the near future;
however, the effects of single versus multiple stimulus presentation in infants has never
been investigated. There are known maturational differences between infants and
adults and these differences may lead to greater interactions between responses to
multiply-presented stimuli in infants. Thus, it is unknown whether or not interactions
between responses to multiple stimuli exist, and if they do, whether or not a single-stimulus
or multiple-stimulus presentation method is more efficient for testing infants. In
the present thesis, two studies were carried out to address this issue. All infants in
Study A participated in three stimulus conditions which differed in the number of stimuli
presented simultaneously. The monotic single (MS) condition consisted of 500, 1000,
2000, and 4000 Hz tones which were presented singly to one ear. The monotic multiple
(MM) condition was composed of four tones (500, 1000, 2000, and 4000 Hz) presented
to one ear simultaneously. The dichotic multiple (DM) condition consisted of eight
tones presented simultaneously to both ears (four tones to each ear). ASSR
amplitudes were obtained from 15 normal infants (mean age: 23.1 weeks) in response
to multiple (MM, DM) and single (MS) air-conduction AM tones [77-1 05 Hz modulation
rates; 60 dBSPL]. Mean single-stimulus amplitudes for 500, 1000, 2000, and 4000 Hz
were 30, 39, 45 and 43 nV, respectively. Presentation of multiple AM tones (i.e., 4
octave-spaced frequencies) to one ear resulted in ASSR amplitudes that were 97%,
87%, 82%, and 70% (for 500, 1000, 2000 and 4000 Hz, respectively) of the
single—stimulus ASSR amplitudes. Results for the dichotic presentation of eight AM
tones show ASSR amplitudes that were 70%, 77%, 67%, and 67% relative to the MS
condition. Although decreases in amplitude occurred using multiple stimuli in infants,
the multiple ASSR remained more efficient than the single-stimulus ASSR (i.e.,
multiple-stimulus amplitudes were greater than single-stimulus amplitudes divided by
M° 5,where M is the number of stimuli). The amplitude reductions seen in the multiple-stimulus
conditions in infants could have origins in the ear canal, middle ear, cochlea
and/or brainstem. Because greater interactions occur in the dichotic multiple-stimulus
condition compared to the monotic multiple-stimulus condition and baseline single-stimulus
condition, brainstem origins of these interactions are likely. In study B, ASSR
thresholds were determined for 500-Hz in the single- and dichotic multiple-stimulus
conditions (14 infants; mean age: 20.2 weeks). Results indicate that ASSR thresholds
for 500 Hz presented in the dichotic-multiple condition were elevated 3 dB compared to
that obtained in the 500-Hz single-stimulus condition. This statistically non-significant
difference is within the range of acceptable test-retest variability, and is thus not of
clinical significance. Results from both studies revealed that the multiple-stimulus
conditions resulted in shorter time-to-criteria, but this did not quite reach significance.
In summary, as with adults, multiple-stimulus presentation in infants is more efficient (in
terms of amplitude and time-to-criteria) than single AM tones. === Medicine, Faculty of === Audiology and Speech Sciences, School of === Graduate
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