Susan Small

Associate Professor

Relevant Degree Programs

 

Graduate Student Supervision

Master's Student Supervision (2010-2017)
Cortical auditory evoked potentials to gaps in broadband noise in infants (2016)

Purpose: There are currently no objective measures to evaluate hearing function in young infants with impaired temporal processing (e.g., Auditory Neuropathy Spectrum Disorder (ANSD)). The present study investigated cortical auditory evoked potentials (CAEPs) elicited to gaps in broadband noise to assess temporal processing ability in infants. This method has potential as a clinical tool in these populations. This study was intended as a first step to determine feasibility. Method: Participants were 10 adults and 22 infants with normal hearing. Stimuli were continuous broadband noise with 20, 50 and 100 ms gaps inserted once per second. CAEPs were recorded at Cz referenced to M1. Two replications (minimum of 75 trials each) were included for analysis and judged by three raters for response presence or absence.Results: CAEPs were interpreted as present for the majority of infants and adults to 20-, 50- and 100-ms gaps. The adults had responses in almost all cases, consistent with previous results. In most cases, the morphology of the infant response was consistent with previous results, that is, a single peak at approximately 200 ms. In some cases, however, the infant response was a plateau- shaped peak.Conclusions: These results suggest that it is feasible to record CAEPs in infants to gaps and that infants may have better gap detection abilities than previously thought (20 ms or better). Further research is needed to refine this technique and to extend it to clinical populations for clinical use.

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A comparison of the auditory steady-state and auditory brainstem responses to air- and bone-conducted stimuli in infants with hearing loss (2015)

This study investigates how well the air- (AC) and bone-conduction (BC) auditory steady-state response (ASSR) detects conductive hearing loss compared to the gold standard method, the auditory brainstem response (ABR). Similar studies using infants with sensorineural hearing loss have suggested that the ASSR is an effective method for assessing hearing thresholds and detecting hearing loss in infants. This study compares AC and BC ASSR and ABR thresholds in infants with normal hearing and conductive loss. Twenty-three normal hearing infants and 15 infants with conductive hearing loss (0-6 months) were assessed using the ABR and 80-Hz ASSR elicited to AC and BC stimuli (AM²). Mean thresholds for normal hearing infants were : 500 Hz (i) AC ABR : 25 dB nHL, (ii) BC ABR : 10 dB nHL, (iii) AC ASSR : 30 dB HL , (iv) BC ASSR : 17 dB HL and 2000 Hz (i) AC ABR : 18 dB nHL, (ii) BC ABR : 15 dB nHL, (iii) AC ASSR : 20 dB HL, (iv) BC ASSR : 26 dB HL. For infants with confirmed conductive hearing loss, 500 Hz thresholds air-conduction ABR thresholds increased to approximately 48 dB nHL, while bone-conduction ABR thresholds were approximately 12 dB nHL. Air-conduction ASSR thresholds for infants with conductive hearing loss increased to approximately 37 dB HL and bone-conduction thresholds were approximately 15 dB HL. Overall, mean bone-conduction thresholds were similar between groups, while there was a trend for mean air-conduction thresholds to be higher for infants with conductive hearing loss than infants with normal-hearing for both ABR and ASSR testing methods. Previously suggested “normal levels” in the literature appear to be too high to detect mild conductive hearing loss at 500 Hz. Normal levels of 40 and 30 dB HL is suggested for air- and bone conduction 500 Hz ASSR, respectively, to be more accurate in detecting mild hearing losses. Even with an adjusted “normal level”, it appears to be difficult to use the ASSR to differentiate between normal hearing and mild conductive hearing loss. More research is needed using infants with varying degrees of hearing loss at multiple frequencies.

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Elicitation of the acoustic change complex (ACC) to long-duration speech stimuli in four-month-old infants (2014)

The acoustic change complex (ACC) is a cortical auditory-evoked potential(CAEP) that comprises overlapping slow cortical responses (P1-N1-P2) and occurs inresponse to changes during an ongoing stimulus (Martin, Tremblay, & Korczak, 2008).Research findings suggest that the ACC indicates discrimination at the level of theauditory cortex and provides insight into the brain’s capacity to process acoustic featuresof speech (Kaukoranta, et al., 1987; Ostroff, et al., 1998). Only one study to date hasattempted to record ACCs to speech stimuli in young infants (Small & Werker, 2012).Small and Werker (2012) tested a group of English-learning four-month-old infants withspeech contrasts generated from a synthetic place-of-articulation continuum: a nativedental-dental contrast /dada/, dental-labial contrast /daba/, and a non-native Hindi dentalretroflexcontrast /daDa/. Slow cortical responses resembling adult P1-N1-P2 complexwere recorded for all conditions with significantly prolonged latencies compared withadults. Robust ACCs were elicited in most infants to /daba/ with distinct P1, N1 and P2components, but fewer infants had ACC components in response to /dada/ and /daDa/.The author suggested that the absence of ACCs in /dada/ and /daDa/ conditions might bedue to short stimulus length.The purpose of the present study was to investigate the effect of long-durationspeech stimuli for eliciting ACCs in four-month-old infants. By increasing the stimuluslength from 564 to 816 ms, ACCs were reliably elicited for all stimulus conditions(/dada/, /daba/ and /daDa/) in infants with more distinct cortical components and bettermorphology compared with previous findings (Small & Werker, 2012). The amplitude ofP1 elicited to the acoustic change in /daba/ and /daDa/ was significantly larger comparediiiwith that of P1 for /dada/, indicating that the brain discriminated between the speechtokens. In conclusion, our results support the findings by Small and Werker (2012)showing that adult-like slow cortical responses can be recorded in young infants. Ourresults also suggest that ACCs can be reliably elicited in four-month-old infants givenoptimized stimulus parameters (e.g. longer ISIs and stimulus duration) and providefurther evidence for the use of ACCs as an index of discrimination ability.

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Maturation of skull properties with implications for the fitting and verification of the soft band bone-anchored hearing system for infants and young children (2013)

Soft band bone-anchored hearing systems (BAHS) are optimal for individuals with conductive or mixed hearing losses. Although it is understood that bone-conduction hearing is different between infants and adults, few studies have attempted to explain why these differences exist or how they affect the fitting of a soft band BAHS. The main objectives in this study were: (i) to better understand how properties of the developing skull contribute to the maturation of bone-conduction attenuation and sensitivity, and (ii) to determine how future BAHS fitting and verification protocols should be adjusted for infants and young children. The transcranial attenuation of pure-tone bone-conduction stimuli was measured on infants and young children (age 1 month to 7 years) and adults using sound pressure in the ear canal when the transducer was placed on different positions across the skull. In addition, the mechanical impedance magnitude for the forehead and temporal bone was collected for contact forces of 2, 4, and 5.4 N using an impedance head, a BAHS transducer, and a specially-designed holding device. This study was the first to measure mechanical impedance of the skull, which is an essential component to bone-conduction hearing, on young children and infants.Transcranial attenuation was greatest for young infants, and decreased throughout maturation. Attenuation was also greater from the forehead compared to the contralateral temporal bone for infants and children over 10 months of age. In addition, mechanical impedance was lowest for the youngest infants and increased throughout maturation for low frequencies, but for high frequencies, these infants had the highest impedance on the temporal bone only. The effect of contact force was significant for low frequencies, and the effect of placement was significant for high frequencies. These results suggest that the properties of the developing skull relate to infant-adult differences in transcranial attenuation, and the mechanical impedance of the skin and subcutaneous tissue may explain the infant-adult differences in bone-conduction sensitivity. The results also provide important implications for fitting and verifying output from the BAHS for infants and young children.

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Comparisons of auditory steady-state response and behavioural air- and bone-conduction thresholds in infants and adults with normal hearing (2012)

To improve our understanding of normal responses in infants, the present study compares air-conduction (AC) and bone-conduction (BC) auditory thresholds using both the auditory steady-state response (ASSR) and behavioural testing methods in normal-hearing infants (6-18 months of age) and adults. There are no correction factors available for estimating BC behavioural thresholds; this is a limiting factor to clinical implementation of the ASSR. Additionally, previous studies have reported that ASSR and visual reinforcement audiometry (VRA) thresholds (in dB HL) to air- and bone-conducted stimuli have different frequency-dependent trends and suggest that infants present with an air-bone gap that is not attributable to a conductive pathology; however, this relationship has not been assessed directly. The objectives of the present study are: (i) to compare BC thresholds between methods and provide the initial step towards positing correction factors to predict BC behavioural thresholds and; (ii) to directly compare AC and BC thresholds to provide a more accurate estimation of the maturational ABG. Thresholds were estimated at 500–4000 Hz using AM² stimuli for ASSRs and warbled-tone stimuli for behavioural testing. The results indicated that BC thresholds were, on average, 7–16 dB poorer for ASSR compared to VRA, but varied largely across infants. As expected for the ASSR, frequency-dependent differences in BC sensitivity were found— the 500- and 1000-Hz thresholds were better than the 2000-Hz threshold. For AC ASSR, the 500-Hz thresholds were higher than the other frequencies. There was a tendency for infant and adult ASSR thresholds to differ for BC, but not for AC. Behavioural thresholds for AC and BC were similar between infants and adults and across frequency. The results support the presence of a clinically significant maturational ABG (14 and 17 dB) in the low frequencies for infant ASSRs. The infant behavioural ABG also appeared at 500 Hz, as was posited by Hulecki and Small (2011), but was too small to be practically significant. Clinical consideration of the maturational ABG seems warranted when using ASSRs, but not for VRA. The results also provided preliminary normal levels for AC and BC ASSRs to AM² stimuli.

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Behavioural bone-conduction responses of infants 7-30 months of age to warbled-tone stimuli (2010)

The purpose of the present study is to obtain behavioural bone-conduction thresholds ofinfants 7-15 and 18-30 months of age using a clinical visual reinforcement audiometry (VRA)protocol to determine whether frequency-dependent patterns exist. Assessment of boneconductionhearing is an essential component of an audiological test battery because it providesthe information necessary to differentiate between sensorineural, conductive and mixed hearinglosses. Many studies investigated the maturation of air- and/or bone-conduction thresholds ininfants using objective physiological measures, such as the auditory brainstem response (ABR)and the auditory steady-state response (ASSR), reporting infant-adult differences in air- andbone-conduction hearing sensitivities. Also, frequency-dependent differences have been reportedwhere air-conduction thresholds are better in the high- compared to low-frequencies while boneconductionthresholds are better in the low- compared to high-frequencies. These differencesreveal a low-frequency “maturational” air-bone gap, which reflects a stimulus calibration issuerather than a conductive pathology. Only one published study has reported behavioural boneconductionthresholds for infants (Gravel, 1989). However, maturational changes in boneconductionhearing sensitivity have not been directly investigated using behavioural methods.The present study behaviourally assesses bone-conduction thresholds across frequency (500,1000, 2000 and 4000 Hz) using a clinical VRA protocol for normal-hearing infants 7-15 and 18-30 months of age. It is hypothesized that the frequency-dependent differences identified forinfants using objective techniques will be comparable to the results obtained behaviourally.Therefore, it is expected that all infants tested will have better low- compared to high-frequencythresholds which will become more adult-like with age. Results of this study indicated that,when measured behaviourally, infant’s show frequency-dependent bone-conduction thresholds where their responses at 500 and 1000 Hz are significantly better than those at 2000 and 4000Hz. Compared to previously documented air-conduction thresholds of infants using similar VRAtechniques, there is a difference between air- and bone-conduction thresholds in the lowfrequencies. However, thresholds obtained from the younger group of infants (mean age of I 0.6months) were not significantly different from those obtained from the older group of infants(mean age of 23.0 months) at any frequency, suggesting minimal maturational changes in boneconduction hearing sensitivity between these groups.

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Effective masking levels for bone-conduction auditory steady-state response thresholds in infants (2010)

To obtain ear-specific bone-conduction thresholds, masking of the non-test ear is oftenrequired. Masking is not currently utilized in the pediatric diagnostic test battery, partly becauseeffective 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 ageand adults. Using a two-channel ASSR recording, single 1000- and 4000-Hz bone-conductedAM/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 wouldrequire more and less masking at 1000 and 4000 Hz, respectively. As expected, infants havehigher and lower EMLs at 1000 and 4000 Hz, respectively, compared to adults. When RECDsare accounted for, infants have even higher EMLs at 1000 Hz and similar EMLs at 4000 Hzcompared to adults. This is consistent with the frequency-dependent differences in boneconductionsensitivity for infants. When differences in bone-conduction sensitivity areaccounted for, infants have lower EMLs at both frequencies. When RECDs and boneconductionsensitivity are taken into account, infants have lower EMLs at 1000 Hz and similarEMLs at 4000 Hz. Based on ipsilateral/contralateral asymmetries in masked amplitudes, adultswere 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 dBat 1000 Hz and minimum inter-aural attenuations of greater than 35 dB at 4000 Hz. Similar tobehvaioural investigations, the amplitude findings of this study suggest processing efficiencymay 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) 1000Hz: 48 and 58 dB SPL at 15 and 25 dB HL, respectively, and (ii) 4000 Hz: 40 and 45 dB SPL at25 and 35 dB HL, respectively.

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