| Summary: | To obtain ear-specific bone-conduction thresholds, masking of the non-test ear is often
required. Masking is not currently utilized in the pediatric diagnostic test battery, partly because
effective masking levels (EMLs) for bone-conducted stimuli in young infants are not known.
The purpose of this study is to determine EMLs for auditory steady-state responses (ASSRs)
elicited by bone-conducted stimuli in a group of normal-hearing infants under six-months of age
and adults. Using a two-channel ASSR recording, single 1000- and 4000-Hz bone-conducted
AM/FM stimuli were masked out with 1 and 4 kHz of narrowband noise presented binaurally.
Taking into consideration maturational differences in real-ear-to-coupler differences (RECDs)
and bone-conduction sensitivity (Small & Stapells, 2008a), it was predicted that infants would
require more and less masking at 1000 and 4000 Hz, respectively. As expected, infants have
higher and lower EMLs at 1000 and 4000 Hz, respectively, compared to adults. When RECDs
are accounted for, infants have even higher EMLs at 1000 Hz and similar EMLs at 4000 Hz
compared to adults. This is consistent with the frequency-dependent differences in boneconduction
sensitivity for infants. When differences in bone-conduction sensitivity are
accounted for, infants have lower EMLs at both frequencies. When RECDs and boneconduction
sensitivity are taken into account, infants have lower EMLs at 1000 Hz and similar
EMLs at 4000 Hz. Based on ipsilateral/contralateral asymmetries in masked amplitudes, adults
were estimated to have inter-aural attenuations of at least 0-5 and 0-10 dB at 1000 and 4000 Hz,
respectively. In contrast, infants were estimated to have inter-aural attenuations of at least 10 dB
at 1000 Hz and minimum inter-aural attenuations of greater than 35 dB at 4000 Hz. Similar to
behvaioural investigations, the amplitude findings of this study suggest processing efficiency
may be immature at 1000 Hz, but not at 4000 Hz. Based on the findings of this study, the following preliminary masking levels for bone-conduction stimuli are recommended: (i) 1000
Hz: 48 and 58 dB SPL at 15 and 25 dB HL, respectively, and (ii) 4000 Hz: 40 and 45 dB SPL at
25 and 35 dB HL, respectively.
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