Clinical Neurophysiology 124 (2013) 474–480

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Clinical Neurophysiology

journal homepage: www.elsevier .com/locate /c l inph

Stimulus level effects on speech-evoked obligatory cortical auditory evokedpotentials in infants with normal hearing

S.C. Purdy a,b,c,⇑, M. Sharma b, K.J. Munro d, C.L.A. Morgan c

a The University of Auckland, New Zealandb Macquarie University, Hearing CRC, Australiac National Acoustic Laboratories, Sydney, Australiad University of Manchester, United Kingdom

a r t i c l e i n f o

Article history:Accepted 16 September 2012Available online 30 October 2012

Keywords:Cortical auditory evoked potentialInfantSpeechStimulus levelInput–output functionLoudness

1388-2457/$36.00 � 2012 Published by Elsevier Irelahttp://dx.doi.org/10.1016/j.clinph.2012.09.011

⇑ Corresponding author at: School of Psychology (SAuckland, Building 721, Tamaki Innovation Campus,New Zealand. Tel.: +64 9 3737599x82073; fax: +64 9

E-mail address: sc.purdy@auckland.ac.edu (S.C. Pu

h i g h l i g h t s

� This study shows robust cortical auditory evoked potential (CAEP) amplitudes at low stimulus levels ininfants.� The effect of stimulus level on CAEP latencies differs between speech stimuli.� CAEP input–output functions for infants differ from published findings for adults.

a b s t r a c t

Objective: To determine stimulus level effects on speech-evoked cortical auditory evoked potentials(CAEPs) in infants for a low (/m/) and high (/t/) frequency speech sound.Methods: CAEPs were recorded for two natural speech tokens, /m/ and /t/. Participants were 16 infantsaged 3–8 months with no risk factors for hearing impairment, no parental concern regarding hearingor development, and normal tympanograms and otoacoustic emissions. Infants were either tested at lev-els of 30, 50, and 70 dB SPL or at 40, 60, and 80 dB SPL, in counterbalanced order.Results: Input–output functions show different effects of increasing sound level between stimuli. Therewere minimal changes in latency with increase in level for /t/. For /m/, there were approximately 50–60 ms latency increases at soft compared to loud levels. Amplitudes saturated at moderate–high levels(60–80 dB SPL) for both stimuli.Conclusions: Infants’ CAEP input–output functions differ for /t/ versus /m/ and differ from those previ-ously reported for adults for other stimuli. Effects of stimulus and level on CAEPs should be consideredwhen using CAEPs for hearing aid or cochlear implant evaluation in infants.Significance: Speech-evoked CAEPs provide an objective measure of central auditory processing. Possibledifferences in CAEP growth between infants and adults suggest developmental effects on intensity codingby the auditory cortex.

� 2012 Published by Elsevier Ireland Ltd. on behalf of International Federation of ClinicalNeurophysiology.

1. Introduction

Obligatory cortical auditory evoked potentials (CAEPs) can pro-vide source and timing information regarding neural processesthat underlie speech perception (Alain and Tremblay, 2007; Oateset al., 2002; Martin et al., 2008). In addition, CAEPs can be usedclinically to assess auditory system maturation, auditory process-ing ability, and speech audibility in infants with hearing loss(Rapin and Graziani, 1967; Gravel et al., 1989; Sharma et al.,

nd Ltd. on behalf of International F

peech Science), University ofPrivate Bag 92019, Auckland,373 7902.

rdy).

2002; Golding et al., 2007; Martin et al., 2008). CAEPs are robustresponses that can been evoked by clicks, tones, and speechsounds across the life span, in infants (Kurtzberg et al., 1984;Ponton et al., 2002; Purdy et al., 2004), young children (Pontonet al., 1996; Cunningham et al., 2000; Ceponiene et al., 2002;Sharma et al., 2007), adolescents (Ponton et al., 1996; Cunninghamet al., 2000), young adults (Ponton et al., 1996; Cunningham et al.,2000; Wunderlich and Cone-Wesson, 2001; Sharma et al., 2002),and the elderly (Cunningham et al., 2000). Although CAEP peaks,especially P2, are influenced by attention (Coch et al., 2005),obligatory CAEPs are typically recorded using a passive listeningparadigm, with the listener watching a silent subtitled videoor reading. Infants can be tested while they are awake anddistracted by a visually engaging toy or book (Purdy et al.,

ederation of Clinical Neurophysiology.

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2004) or during ‘‘active’’ sleep (Kurtzberg et al., 1984; Novaket al., 1989; Kushnerenko et al., 2001).

Stimulus parameters that influence CAEP characteristics includepresentation rate (Budd et al., 1998; Sharma et al., 2007), stimulusduration (Onishi and Davis, 1968; Agung et al., 2006; Beukes et al.,2009), stimulus level (Madell and Goldstein, 1972; Garinis andCone-Wesson, 2007), and type of speech sound (Kurtzberg, 1989;Sharma and Dorman, 1999; Agung et al., 2006) or tonal stimulusfrequency (Jacobson et al., 1992). Golding et al. (2006) recordedspeech-evoked CAEPs in infants and reported significantly smallerand later P1 latencies for /m/ compared to /t/, across a range ofstimulus durations and inter-stimulus intervals. This is consistentwith a magnetoencephalography study of adult listeners by Gageet al. (1998), that showed larger cortical amplitudes and shorterlatencies for words with initial stop consonants (/b/, /p/, /d/, /t/,/g/, /k/) versus non-stops (/f/, /l/, /m/, /r/, /s/). Gage et al. (1998)suggested that these differences are due to stops having greaterenergy at onset than other sounds. The auditory cortex is domi-nated by neurons with discharge spikes time-locked to the onsetof auditory stimuli (Heil, 1997) and the /t/ stimulus has a much fas-ter onset-to-peak time than /m/ (Golding et al., 2006). Prasher(1980) investigated whether onset effects on CAEP latencies andamplitudes are affected by spectral splatter effects by comparingeffects of rise time for tones and noise, and found that latencies re-duced with stimulus rise time for noise and tonal stimuli, butamplitude increases for short rise times only occurred for tonalstimuli. Thus, both spectral and temporal differences are likely toaccount for the latency and amplitude differences between /m/and /t/ CAEPs reported by Golding et al. (2006).

The effects of stimulus parameters on infant CAEPs have notbeen investigated as extensively as they have in adults (Hyde,1997). A number of studies have shown that, for stimulus ratesof about 1 per second, CAEPs in infants are dominated by the P1peak occurring at about 100–300 ms after stimulus onset, followedby a late negativity at about 350 ms (Kurtzberg et al., 1984; Pontonet al., 1996; Kushnerenko et al., 2001; Sharma et al., 2002; Kinget al., 2008).

The adult CAEP waveform consists of distinct P1, N1, and P2peaks. Up to about 7 years of age the vertex-recorded cortical po-tential often primarily consists of a single positive wave ratherthan a P1–N1–P2 complex. P1 latency shows considerable matura-tional change in the preschool period, reducing from several hun-dred milliseconds in very young infants (e.g. Sharma andDorman, 2006) to approximately 100 ms at 5 years (see reviewby Wunderlich and Cone-Wesson, 2006). CAEPs recorded in adultsusing similar stimulus presentation rates have earlier P1, N1, andP2 peaks occurring at about 50, 100, and 200 ms, respectively.The time range of the adult N1–P2 overlaps with the infant P1occuring between 100 and 300 ms. Ponton et al. identified tangen-tial and radial N1 sources, with differing maturational timecourses, and proposed that P1 is reduced in adults due to phasecancellation of the later parts of the P1 peak by the increasing mag-nitude of N1. Hence, when adult and infant studies are compared itis difficult to separate stimulus influences on P1 and N1 since thesepeaks in the CAEP waveform are likely to have overlapping sources.

Studies reporting effects of stimulus and recording parameterson CAEPs in adults generally focus on N1. Adult findings cannotbe readily extrapolated to infants, however, since P1, N1, and P2have different sources and maturation patterns (Ponton et al.,2002) and thus the effects of stimulus parameters may differ acrossthese peaks. In adults P1 is small; reduced P1 amplitudes are asso-ciated with the emergence of a large N1 in late childhood (Pontonet al., 2002). Thus, intensity effects on P1 in adults may be difficultto observe and have not been widely reported. In general, adult N1and P2 latencies reduce and amplitudes increase as stimuluslevel increases from near-threshold to loud levels, but there are

variations across studies in the shape of the input–output functionfor CAEP latencies and amplitudes (Beagley and Knight, 1967; Pic-ton et al., 1974; Morita et al., 2006; Kaskey et al., 1980; Garinis andCone-Wesson, 2007; Lütkenhöner and Klein, 2007). Differences instimulus type, duration and inter-stimulus interval, and the rangeof presentation levels investigated are likely to account for the var-iation in findings.

To our knowledge CAEP input–output characteristics have notbeen reported for infants. CAEP input–output characteristics areof interest because of the use of CAEPs to evaluate the audibilityof amplified speech in children with hearing impairment (Purdyet al., 2004; Martin et al., 2008). The aim of the current studywas to assess the effects of stimulus level on latencies and ampli-tudes of speech-evoked CAEPs in infants, using two speech sounds(/m/ and /t/) with different spectral and temporal characteristicsthat are known to produce differences in CAEP morphology.

2. Materials and methods

2.1. Participants

Participants were 16 infants (5 girls and 11 boys) aged 2.5–10 months (mean 5.3 months, SD 2.6), with no risk factors forhearing impairment and no parental concerns about hearing.Prior approval for the study was obtained from Ethics ReviewBoard of University of Auckland. All participants had normal1000-Hz tympanograms (Baldwin, 2006) on the day of testingand passed transient evoked otoacoustic emission testing. Infantswere randomly divided into two groups to be tested using levelsof either 30, 50 or 70 dB SPL, or 40, 60 and 80 dB SPL for the twostimuli. This approach was taken since a pilot study suggested itmay not be possible to maintain infant cooperation for long en-ough to test two stimuli and six presentation levels, and partici-pant recruitment was facilitated by having one test session perinfant. Table 1 provides a summary of these two groups. Testingwas terminated when babies could not be distracted anymore.Test sessions lasted 45 min to 2 hours, with testing interruptedwith feeding when necessary. Some infants could not be testedat the three pre-determined levels for the two stimuli, or sometrials did not have enough averages due to excessive stimulusartifact and thus the data could not be included in the statistics.Eleven infants (69%) completed all or 5 out of 6 assigned condi-tions (3 levels, 2 stimuli). For each stimulus and each presenta-tion level, data was obtained from between five and eightparticipants.

2.2. Stimuli

CAEPs were collected in response to two 78 ms natural speechtokens /t/ and /m/. These two sound tokens were chosen with dom-inant energy in the high and low frequencies, respectively (seeFig. 1). The speech tokens were extracted from running speech spo-ken by an Australian male (Golding et al., 2006). The stimuli wereinitial consonants extracted from the speech sample and truncatedto remove the subsequent vowel (/ae/). After this processing the /t/stimulus is perceived as human speech and /m/ has a ‘buzz-like’sound quality. The two stimuli differ spectrally and temporally;/t/ is a high-frequency, aperiodic unvoiced plosive, while /m/ is alower-frequency, periodic voiced nasal sound. The stimuli werepresented at an interstimulus interval of 1125 ms at levels of 30–80 dB SPL, counterbalanced in 10 dB steps. Speech sounds werepresented via a Turbosound IMPACT 50 loudspeaker placed at 0�azimuth (right in front of the infant) at a distance of 1.5 m and aheight of 1.4 m, with the face of the loudspeaker angled down-wards at 45� to reduce variability in the sound at the infant’s ear-canal (Killion and Revit, 1987). This arrangement allowed the

Table 1Summary of the test conditions across participants for the sounds /t/ and /m/. Babies were randomised into group O (odd) or E (even) and were allocated odd or even stimuluslevels depending on which group they were assigned to.

Sounds Group Levels (dB SPL) No. of participants Ratio of males:females Mean age in months ± 1 SD

/t/ O 30 6 6:0 5.2 (3.0)50 8 7:1 5.6 (2.6)70 7 6:1 6.0 (2.4)

E 40 6 4:2 5.3 (2.8)60 6 4:2 5.4 (2.8)80 6 4:2 5.4 (2.8)

/m/ O 30 5 5:0 6.6 (2.7)50 7 6:1 5.6 (2.8)70 6 5:1 6.2 (2.6)

E 40 5 2:3 5.3 (3.3)60 7 2:4 5.6 (3.0)80 6 2:4 5.6 (3.0)

Fig. 1. Time waveforms and frequency spectra of the /m/ and /t/ stimuli.

476 S.C. Purdy et al. / Clinical Neurophysiology 124 (2013) 474–480

person distracting the infant to be located below and to the side ofthe loudspeaker. A flat soundfield frequency response wasachieved by routing the CAEP stimulus through an Australian Mon-itor Synergy SY400 power amplifier and Sabine Graphi-Q GRQ-3102 equalizer that had been adjusted to achieve levels within±2.5 dB from 80 to 10 kHz using a white noise stimulus. Speechstimuli were calibrated in dB SPL (rms) using a half-inch polarizedcondenser free-field microphone connected to a Bruel and Kjaermeasuring amplifier (linear weighting, impulse response) andoscilloscope.

2.3. Evoked response recording

Participants sat on the caregiver’s lap facing a researcher sittingin a small chair below the loudspeaker who distracted the infantusing silent toys and books. Electrodes were attached at C3, Cz,and C4, referenced to the right mastoid with the forehead asground. Only CAEPs measured from the vertex midline (Cz) elec-trode were analyzed, as this location provided the most stable elec-trode attachment and impedance values across the recordingsession. The online artifact rejection rate was set at ±100 lV. Ac-cepted runs had <50% artifact rejection rates. Speech stimuli werepresented via Neuroscan STIM 2 and CAEPs were acquired usingNeuroscan SCAN Acquisition software version 4.3.1 and SynAmps2amplifiers. Online evoked response recording was performingusing a digital bandpass FIR filter of 1–100 Hz and a gain settingof 1000.

2.4. Data analysis

Offline analysis involved converting continuous EEG data toepoch files with a time window of �100 to 600 ms (with baselinecorrection in the pre-stimulus interval). CAEPs were then bandpassfiltered (1–30 Hz low pass filtered, 24 dB/octave) using FIR zerophase shift digital filtering. There were approximately 200 aver-ages for each condition at each level for each participant. P1 laten-cies and amplitudes were measured for all individual averagewaveforms for each stimulus and presentation level. Waveformpeaks were visually identified by three of the researchers. Initialpeak picking was performed by two researchers; a third researcherblinded to the stimulus intensity subsequently reviewed the wave-forms and peaks were identified based on a consensus amongst thethree researchers. P1 was identified as the most robust positivity inthe range of 100–300 ms. If CAEPs were present across C3, C4 andCz electrode positions, then peak amplitudes, latencies and mor-phology were expected to be approximately consistent across elec-trode positions. This was used as a guide only when peak pickingthe response recorded at Cz, as some variations in amplitudesand latencies might occur between central and right/left hemi-sphere electrodes (Paavilainen et al., 1991; Alain and Tremblay,2007). Peak picking was performed at Cz only as electrode contactwas very stable in this midline location but was more variable atC3 and C4. Two-factor repeated-measures (signal [2], level [6])analyses of variance were performed using Statistica 7.0 to deter-mine stimulus and level effects on P1 latencies and amplitudes.

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Planned comparisons were used to investigate differences betweenindividual stimulus levels.

3. Results

Grand average CAEP waveforms recorded in response to /m/and /t/ are shown in Fig. 2. All infants had a P1 response at approx-imately 150–200 ms, followed by a negativity at about 250–300 ms. There is an additional earlier biphasic response (positivityfollowed by a negativity) evident in the /t/ grand average wave-forms for the higher stimulus levels. This biphasic peak evidentin the responses to /t/ at higher stimulus levels is the post auricularresponse (PAMR), occurring at approximately 20–50 ms (Agunget al., 2005; Golding et al., 2006). PAMR is a brainstem responsethat is influenced by the stimulus type, level and eye movement.PAMR is optimally recorded using bilateral, high level, high fre-quency stimuli (Agung et al., 2005).

Overall /m/ produced longer CAEP latencies and smaller ampli-tudes than /t/ (Fig. 3). Because the cohort of children tested witheach stimulus was not identical and the group sizes are small, aconservative nonparametric analysis was performed to seewhether /m/ and /t/ latencies and amplitudes differed significantly.A Kruskal–Wallis ANOVA showed that latencies differed betweenspeech stimuli at 30 dB (p = 0.018), 40 dB (p = 0.006), 50 dB(p = 0.002), 60 dB (p = 0.007) and 80 dB (p = 0.021), but not at70 dB (p = 0.087). P1 amplitudes differed significantly betweenspeech stimuli only at 40 dB (p = 0.029) and 60 dB (p = 0.025). Toimprove statistical power for these comparisons, low (30–40 dB),medium (50–60 dB), and high (70–80 dB) intensity data werepooled and latencies and amplitudes were compared at each level

Fig. 2. Grand average CAEP waveforms (n = 6–8 infants per waveform), showing P1at approximately 150–200 ms across stimulus levels of 30–80 dB SPL, for /t/ (leftpanel) and /m/ (right panel) speech sounds. The early biphasic response evident athigher intensity levels in the /t/ waveforms is post auricular muscle response(PAMR).

between stimuli. As was seen for the comparison of individualintensities, there were significant latency differences betweenstimuli at low (p = 0.004) and medium (p = .005) levels, but notat the highest intensities (p = 0.062). P1 amplitudes for the pooledlevel data did not differ between stimuli for low (p = 0.335), med-ium (p = 0.821), or high (p = .221) intensities.

P1 amplitude varied significantly across levels for /m/ whenstimulus levels were analyzed separately [F(5,20) = 5.22,p = 0.003] and for pooled intensity data [F(2,10) = 9.34, p = 0.005].Results of planned comparisons used to compare amplitudes be-tween stimulus levels are summarized in Table 2. P1 amplitudesat 30 dB SPL were significant smaller than those at 50, 60, 70,and 80 dB SPL. P1 was also significantly smaller at 40 dB SPL thanat 50 and 60 dB SPL. This pattern of /m/ amplitude growth for low–medium stimulus levels (30–60 dB) and saturation at high levels(70–80 dB) was confirmed by the analysis of the pooled data,which showed a significant amplitude difference between lowand medium (p = .006) but not between medium and high(p = 0.536) intensities. Although Fig. 3 shows small, but generallyconsistent increases in P1 amplitudes with stimulus level for /t/,the differences across stimulus level were not statistically signifi-cant when the analysis was performed for separate [F(5,10) =1.85, p = 0.190] intensities, but there was significant amplitudegrowth when intensity levels were pooled [F(2,16) = 3.91,p = 0.042). Amplitudes differed for low versus medium(p = 0.011), but not medium versus high (p = 0.410) intensities.Thus, both stimuli showed a pattern of increasing amplitude withincrease in level from low to medium intensities, with saturation athigher levels.

P1 latencies did not vary significantly with stimulus level for /t/for either separate [F(5,10) = 1.27, p = 0.349] or pooled [F(2,16) =0.33, p = 0.722] analyses. While /t/ latencies were unaffected bystimulus level, /m/ latencies became increasingly shorter as thestimulus became louder. The change in P1 latency with stimuluslevel for /m/ was statistically significant for the pooled level anal-ysis [F(2,18) = 4.89, p = 0.020]. Planned comparisons showed long-er /m/ P1 latencies at 40 dB than at 70 (p = 0.037) or 80 (p = 0.019)dB SPL. Latencies of /m/ responses did not differ when low versusmedium levels were compared (p = 0.465), but were significantlyshorter at high versus medium (p = 0.042) intensities. Thus thegreatest change in /m/ latencies was at high intensities, at the pointwhere the amplitude of the /m/ cortical response is saturating inthese infants.

4. Discussion

The current study was undertaken to determine the effects ofstimulus level on cortical responses to a high and a low frequencyspeech sound in babies. CAEPs were recorded across the range ofstimulus levels for both stimuli. CAEPs show a trend of increasingamplitudes up to 50 dB SPL for /m/ and up to 60 dB SPL for /t/. Thedifferences in amplitude across stimulus level were not significantfor /t/. This may be due to a lack of statistical power as a result ofthe small sample size and amplitude variability across infants.Alternatively, differences in amplitude growth could reflect differ-ences in the nature of the stimuli. This is difficult to answer di-rectly given the variability in response amplitudes and lack ofother test stimuli for comparison. The /t/ stimulus has a relativelyfast stimulus onset and noise-like spectrum, somewhat like a click.Earlier studies using click stimuli have shown CAEP amplitudegrowth with comparable changes in stimulus level. For adult lis-teners, Picton et al. (1974) showed increasing N1 and P2 clickevoked CAEP amplitudes across a wider range of stimulus intensi-ties from 30 to 80 dB sensation level (SL). Similarly, Beagley andKnight (1967) reported little change in amplitude and latency at60–70 dB SL, but below these levels reductions in stimulus level

Fig. 3. Mean P1 amplitudes (top panel) and latencies (bottom panel) for /t/ and /m/ stimuli, across levels from 30 to 80 dB SPL. Error bars indicate ±1 standard error.

Table 2Summary of the outcomes of the statistical planned comparisons (ns = not significant)comparing P1 amplitudes between stimulus levels for /m/. None of the contrasts weresignificant for /t/.

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produced significant changes in amplitude and latency. Other stud-ies using tonal or speech stimuli have found that adult CAEP ampli-tudes saturate at lower intensities, as was seen for the infants inthe current study. Lütkenhöner and Klein (2007) found that, forneuromagnetic recordings of N1 and P2, latencies reduced andamplitudes increased rapidly with increase in tonal stimulus levelfrom 2 to 20 dB SL, with much smaller changes with level between20 and 40 dB SL. Garinis and Cone-Wesson (2007) saw a similarpattern of saturating amplitude growth for consonant–vowelspeech stimuli at levels of 0 to 40 dB SL, but found linear decreasesin latencies with level. Garinis and Cone-Wesson (2007) used aslower stimulus presentation rate than Lütkenhöner and Klein(2007), which may account for the difference in latency results.Morita et al.’s (2006) data is consistent with these studies, showingreducing N1m latencies for tones in quiet from 40 to 50 dB SPL andno change in latency with further increases in stimulus intensity.

For soft presentation levels CAEP amplitudes increased more for/m/ than for /t/. The /m/ stimulus resulted in less variation in laten-cies across levels, and substantial decrease in amplitudes at nearthreshold levels. The /t/ stimulus produced CAEPs with similarlatencies across level and only a small reduction in average ampli-tudes at soft levels. With the increased sample size for the pooleddata, the reduction in amplitude for /t/ when intensity reducedfrom mid to low intensities was significant. The patterns of ampli-tude and latency change across level for the two stimuli were quitedistinct, however. Previous studies have not investigated stimulus

effects on human CAEP input–output functions however, Hart et al.(2003) found differences in the growth of fMRI activation for300 Hz versus 4750 Hz tones for sound levels between 42 and96 dB SPL. For the low frequency tone, the extent of activationwas flat up to 66 dB and then showed a rapid growth which contin-ued up to 96 dB SPL. In contrast, increasing the level of the higherfrequency tones produced a steady growth in the extent of activa-tion across the range of levels studied. Hart et al. (2003) concludedthat recruitment of primary auditory cortical neurons may be dif-ferent at high and low frequencies. The changes in evoked poten-tial amplitudes seen for infants in the current study were verydifferent from this pattern. The findings do support Hart et al.’s(2003) conclusion regarding differences in high versus low fre-quency cortical processing. In addition to possible differences incortical input–output functions between low and high frequencystimuli, the /m/ and /t/ stimuli should produce different patternsof cochlear excitation with increasing stimulus level. The /m/ stim-ulus is a relatively narrowband, low frequency stimulus; increasingstimulus intensity would presumably result in upward spread ofcochlear excitation. The /t/ stimulus has peak energy at approxi-mately 3000 Hz, however, the acoustic spectrum is relatively broadand hence less change in cochlear excitation would be expectedwith increasing stimulus level.

Abdala and Folsom (1995) found that, compared to adults,3 month olds had broader 8000 Hz neural tuning curves derivedusing notched noise masking of the auditory brainstem response.By 6 months of age, low and high frequency tuning was adult-like. In the current study input–output functions for the low fre-quency speech sound /m/ were more adult-like than those ob-served for /t/. For /m/, latencies reduced at high intensities andthere was considerable amplitude growth at low presentationlevels. For the high frequency speech sound /t/, although thepooled analysis showed significant growth in amplitude for /t/from low to mid intensity levels, latencies did not change signif-icantly and the changes in amplitude across intensity were lessthan those seen for /m/. This is consistent with Abdala and Fol-som’s (1995) observation of less mature high frequency neuralprocessing in young infants at the level of the auditorybrainstem.

S.C. Purdy et al. / Clinical Neurophysiology 124 (2013) 474–480 479

Stimulus type influenced P1 morphology, amplitude, and la-tency. The /m/ stimulus resulted in longer P1 latencies and smallerP1 amplitudes than /t/. The results of the current study have impli-cations for aided CAEP testing for clinical evaluation of hearing aidsand cochlear implants; improved stimulus audibility should im-prove CAEP amplitudes and reduce latencies, but only over a rela-tively small range of stimulus levels. Substantial P1 latencyreductions reported in the literature for children receiving cochlearimplants (e.g. Dorman et al., 2007) exceed the range of latencychanges seen in children with normal hearing when stimulus audi-bility improves. This suggests that the changes in P1 seen afterearly cochlear implantation reflect more than just improvedaudibility.

5. Conclusions

CAEP input–output functions for /m/ and /t/ behave differently.Increasing audibility influences speech-evoked CAEP amplitudesand latencies, but only over a limited range of stimulus levels. Ifthe results obtained here are applicable to children with hearingloss, CAEP amplitudes may be robust, even when stimuli are atlow sensation levels, particularly for the high frequency speechsound /t/. Thus, loudness is unlikely to correlate with CAEP ampli-tudes. The presence of CAEPs indicates stimulus audibility, but thecurrent findings for children with normal hearing indicate thatCAEPs may not be useful for determining where speech falls withinthe child’s dynamic range. The American Academy of Audiology(2003) Pediatric Amplification protocol recommends that stimuliused for verification of hearing aid performance should representthe frequency, intensity and temporal aspects of speech and thatspeech perception should be evaluated where possible. Aided CAEPtesting offers the ability to assess speech perception objectively ata young age (Dillon, 2005). The current results indicate that reli-able CAEP responses with characteristic latencies and amplitudescan be recorded to different natural speech stimuli at a range ofintensities in young infants. The CAEP technique and the stimulidescribed here fit well with hearing fitting goals for children ofensuring that hearing aids are not under-amplifying speech andthat they provide beneficial levels of sound across the speech spec-trum (Scollie, 2006).

Acknowledgment

This research was supported in part by a Churchill Fellowshipawarded to the fourth author.

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