Brett A. Martin

Thresholds for auditory brain stem responses to tones in notched noise from infants and young children with normal hearing or sensorineural hearing loss.

Objective To assess the accuracy of threshold estimates determined using the auditory brain stem responses (ABRs) to brief tones presented in notched noise in a group of infants and young children with normal hearing or sensorineural hearing loss (SNHL). Design The thresholds for ABRs to brief duration 500, 2000, and 4000 Hz tones presented in notched-noise masking were evaluated in infants and young children with normal hearing (N = 34) or SNHL (N = 54). Tone-evoked ABR thresholds were compared with behavioral thresholds obtained at follow-up audiologic assessments, for a total of 220 comparisons. Results ABR thresholds for the infants with bilateral normal hearing were 23.6,12.9, and 12.6 dB nHL for 500, 2000 and 4000 Hz, respectively. Most (92 to 100%) infants with normal hearing showed ABRs to 30 dB nHL tones. Across all subjects (i.e., those with normal hearing and those with impaired hearing), high (20.94) correlations were found between the ABR and behavioral thresholds. The mean differences between ABR (dB nHL) and behavioral (dB HL) thresholds across all subjects were 8.6, -0.4, and -4.3 dB for 500, 2000, and 4000 Hz, respectively. Overall, 98% of the ABR thresholds were within 30 dB of the behavioral thresholds, 93% were within 20 dB, and 80% were within 15 dB. Conclusions These threshold results for the ABR to brief tones in notched noise obtained for infants and young children are similar to those obtained in similar studies of adults. The technique may be used clinically with reasonable accuracy to estimate pure-tone behavioral thresholds in infants and young children who are referred for diagnostic threshold ABR testing.

Speech evoked potentials: from the laboratory to the clinic.

Speech-evoked auditory event-related potentials (ERPs) provide insight into the neural mechanisms underlying speech processing. For this reason, ERPs are of great value to hearing scientists and audiologists. This article will provide an overview of ERPs frequently used to examine the processing of speech and other sound stimuli. These ERPs include the P1–N1–P2 complex, acoustic change complex, mismatch negativity, and P3 responses. In addition, we focus on the application of these speech-evoked potentials for the assessment of (1) the effects of hearing loss on the neural encoding of speech allowing for behavioral detection and discrimination; (2) improvements in the neural processing of speech with amplification (hearing aids, cochlear implants); and (3) the impact of auditory training on the neural processing of speech. Studies in these three areas are reviewed and implications for audiologists are discussed.

Cortical, auditory, evoked potentials in response to changes of spectrum and amplitude

The acoustic change complex (ACC) is a scalp-recorded negative-positive voltage swing elicited by a change during an otherwise steady-state sound. The ACC was obtained from eight adults in response to changes of amplitude and/or spectral envelope at the temporal center of a three-formant synthetic vowel lasting 800 ms. In the absence of spectral change, the group mean waveforms showed a clear ACC to amplitude increments of 2 dB or more and decrements of 3 dB or more. In the presence of a change of second formant frequency (from perceived /u/ to perceived /i/), amplitude increments increased the magnitude of the ACC but amplitude decrements had little or no effect. The fact that the just detectable amplitude change is close to the psychoacoustic limits of the auditory system augurs well for the clinical application of the ACC. The failure to find a condition under which the spectrally elicited ACC is diminished by a small change of amplitude supports the conclusion that the observed ACC to a change of spectral envelope reflects some aspect of cortical frequency coding. Taken together, these findings support the potential value of the ACC as an objective index of auditory discrimination capacity.

Cortical, auditory, event-related potentials in response to periodic and aperiodic stimuli with the same spectral envelope.

OBJECTIVE 1) To determine whether the N1-P2 acoustic change complex is elicited by a change of periodicity in the middle of an ongoing stimulus, in the absence of changes of spectral envelope or rms intensity. 2) To compare the N1-P2 acoustic change complex with the mismatch negativity elicited by the same stimuli in terms of amplitude and signal to noise ratio. DESIGN The signals used in this study were a tonal complex and a band of noise having the same spectral envelope and rms intensity. For elicitation of the acoustic change complex, the signals were concatenated to produce two stimuli that changed in the middle (noise-tone, tone-noise). Two control stimuli were created by concatenating two copies of the noise and two copies of the tone (noise-only, tone-only). The stimuli were presented using an onset-to-onset interstimulus interval of 3 sec. For elicitation of the mismatch negativity, the tonal complex and noise band stimuli were presented using an oddball paradigm (deviant probability = 0.14) with an onset-to-onset interstimulus interval of 600 msec. The stimuli were presented via headphones at 80 dB SPL to 10 adults with normal hearing. Subjects watched a silent video during testing. RESULTS The responses to the noise-only and tone-only stimuli showed a clear N1-P2 complex to the onset of stimulation followed by a sustained potential that continued until the offset of stimulation. The noise-tone and tone-noise stimuli elicited an additional N1-P2 acoustic change complex in response to the change in periodicity occurring in the middle. The acoustic change complex was larger for the tone-noise stimulus than for the noise-tone stimulus. A clear mismatch negativity was elicited by both the noise band and tonal complex stimuli. In contrast to the acoustic change complex, there was no significant difference in amplitude across the two stimuli. The acoustic change complex was a more sensitive index of peripheral discrimination capacity than the mismatch negativity, primarily because its average amplitude was 2.5 times as large. CONCLUSIONS These findings indicate that both the acoustic change complex and the mismatch negativity are sensitive indexes of the neural processing of changes in periodicity, though the acoustic change complex has an advantage in terms of amplitude. The results support the possible utility of the acoustic change complex as a clinical tool in the assessment of peripheral speech perception capacity.

Cortical evoked response To acoustic change within a syllable

Objective: To investigate whether the evoked potential to a complex naturally produced speech syllable could be decomposed to reflect the contributions of the acoustic events contained in the constituent phonemes. Design: Auditory cortical evoked potentials N1 and P2 were obtained in eight adults with normal hearing. Three naturally produced speech stimuli were used: 1) the syllable [sei]; 2) the sibilant [s], extracted from the syllable; 3) the vowel [ei] extracted from the syllable. The isolated sibilant and vowel preserved the same time relationships to the sampling window as they did in the complete syllable. Evoked potentials were collected at Fz, Cz, Pz, A1, and A2, referenced to the nose. Results: In the group mean waveforms, clear responses were observed to both the sibilant and the isolated vowel. Although the response to the [s] was weaker than that to [ei], both had N1 and P2 components with latencies, in relation to sound onset, appropriate to cortical onset potentials. The vowel onset response was preserved in the response to the complete syllable though with reduced amplitude. This pattern was observable in six of the eight waveforms from individual subjects. Conclusions: It seems likely that the response to [ei] within the complete syllable reflects changes of cortical activation caused by amplitude or spectral change at the transition from consonant to vowel. The change from aperiodic to periodic stimulation may also produce changes in cortical activation that contribute to the observed response. Whatever the mechanism, the important conclusion is that the auditory cortical evoked potential to complex, time‐varying speech waveforms can reflect features of the underlying acoustic patterns. Such potentials may have value in the evaluation of speech perception capacity in young hearing‐impaired children.

The effects of decreased audibility produced by high-pass noise masking on cortical event-related potentials to speech sounds /ba/ and /da/

This study investigated the effects of decreased audibility produced by high-pass noise masking on cortical event-related potentials (ERPs) N1, N2, and P3 to the speech sounds /ba/and/da/presented at 65 and 80 dB SPL. Normal-hearing subjects pressed a button in response to the deviant sound in an oddball paradigm. Broadband masking noise was presented at an intensity sufficient to completely mask the response to the 65-dB SPL speech sounds, and subsequently high-pass filtered at 4000, 2000, 1000, 500, and 250 Hz. With high-pass masking noise, pure-tone behavioral thresholds increased by an average of 38 dB at the high-pass cutoff and by 50 dB one octave above the cutoff frequency. Results show that as the cutoff frequency of the high-pass masker was lowered, ERP latencies to speech sounds increased and amplitudes decreased. The cutoff frequency where these changes first occurred and the rate of the change differed for N1 compared to N2, P3, and the behavioral measures. N1 showed gradual changes as the masker cutoff frequency was lowered. N2, P3, and behavioral measures showed marked changes below a masker cutoff of 2000 Hz. These results indicate that the decreased audibility resulting from the noise masking affects the various ERP components in a differential manner. N1 is related to the presence of audible stimulus energy, being present whether audible stimuli are discriminable or not. In contrast, N2 and P3 were absent when the stimuli were audible but not discriminable (i.e., when the second formant transitions were masked), reflecting stimulus discrimination. These data have implications regarding the effects of decreased audibility on cortical processing of speech sounds and for the study of cortical ERPs in populations with hearing impairment.

The effects of broadband noise masking on cortical event-related potentials to speech sounds /ba/ and /da/

Objective: To systematically investigate in normal‐hearing listeners the effects of decreased audibility produced by broadband noise masking on the cortical event‐related potentials (ERPs) N1, N2, and P3 to the speech sounds /ba/ and/da/. Design: Ten normal‐hearing adult listeners actively (button‐press response) discriminated the speech sounds /ba/ and /da/ presented in quiet (no masking) or with broadband masking noise (BBN), using an ERP oddball paradigm. The BBN was presented at 50, 60, and 70 dB SPL when speech sounds were presented at 65 dB ppe SPL and at 60, 70 and, 80 dB SPL when speech sounds were presented at 80 dB ppe SPL. Results: On average, the 50, 60, 70, and 80 dB SPL BBN maskers produced behavioral threshold elevations of 18, 25, 35, and 48 dB (average for 250 to 4000 Hz), respectively. The BBN maskers produced significant decreases (relative to quiet condition) in ERP amplitudes and behavioral discriminability. These decreases did not occur, however, until the noise masker intensity (in dB SPL) was equal to or greater than the speech stimulus intensity (in dB ppe SPL), that is, until speech to noise ratios (SNRs) were ≤0 dB. N1 remained present even after N2, P3, and behavioral discriminability were absent. In contrast to amplitudes, ERP and behavioral latencies showed significant decreases at higher (better) SNRs. Significant latency increases occurred when the noise maskers were within 10 to 20 dB of the stimuli (i.e., SNR ≤ 20 dB). The effects of masking were greater for responses to /da/ compared with /ba/. Latency increases occurred with less masking for N1 than for P3 or behavioral reaction time, with N2 falling in between. Conclusions: These results indicate that decreased audibility as a result of masking affects the various ERP peaks in a differential manner and that latencies are more sensitive indicators of these masking effects than are amplitudes.

Test-retest reliability of cortical evoked potentials using naturally produced speech sounds

Objective To determine if naturally produced speech stimuli evoke distinct neural response patterns that can be reliably recorded in individuals. Design Auditory cortical evoked potentials were obtained from seven normal-hearing young adults in response to four naturally produced speech tokens (/bi/, /pi/, /&U0283;i/, and /si/). Stimuli were tokens from the standardized UCLA version of the Nonsense Syllable Test (NST) (Dubno & Schaefer, 1992). Using a repeated measures design, subjects were tested and then retested within an 8-day period. Results Auditory cortical evoked potentials elicited by naturally produced speech sounds were reliably recorded in individuals. Also, naturally produced speech tokens, representing different acoustic cues, evoked distinct neural response patterns. Conclusions 1) Cortical evoked potentials elicited by naturally produced speech sounds can be reliably recorded in individuals. 2) Naturally produced speech tokens, representing different acoustic cues, evoke distinct neural response patterns. 3) Given the reliability of the response, this work has potential application to the study of neural processing of speech in individuals with communication disorders as well as changes over time after various types of auditory rehabilitation.

Maturation of mismatch negativity: A scalp current density analysis

Objective Auditory evoked potentials provide the opportunity to better understand the central processing of auditory stimuli, which is the basis of speech and language perception. The purpose of this study was to examine maturational changes in the topography of one of these auditory evoked potentials, the mismatch negativity (MMN), using scalp current density (SCD) analysis. Design Subjects were children ages 4 to 11 yr (N = 53), and adults (N = 12). Stimuli were 85 dB peSPL 1000 Hz standard tones and 1200 Hz deviant tones (deviant probability = 0.15). Auditory evoked potentials were recorded using surface electrodes placed at 32 locations on the head while subjects ignored the stimuli by watching a silent video. Results Significant maturational changes in topography of MMN were seen over frontal and left lateral sites. Conclusions Differences in MMN for the children compared to adults indicate that the MMN generators or their orientation, and thus the neural processes underlying discrimination of simple tones, are not yet mature by 11 yr of age.

Effects of low-pass noise masking on auditory event-related potentials to speech.

Objective: This study investigated the effects of decreased audibility in low-frequency spectral regions, produced by low-pass noise masking, on cortical event-related potentials (ERPs) to the speech sounds /ba/ and /da/. Design: The speech sounds were presented to normal-hearing adults (N = 10) at 65- and 80-dB peak-to-peak equivalent SPL while they were engaged in an active condition (pressing a button to deviant sounds) and a passive condition (ignoring the stimuli and reading a book). Broadband masking noise was simultaneously presented at an intensity sufficient to mask the response to the 65-dB speech sounds and subsequently low-pass filtered. The conditions were quiet (no masking), low-pass noise cutoff frequencies of 250, 500, 1000, 2000, and 4000 Hz, and broadband noise. Results: As the cutoff frequency of the low-pass noise masker was raised, ERP latencies increased and amplitudes decreased. The low-pass noise affected N1 differently than the other ERP or behavioral measures, particularly for responses to 80-dB speech stimuli. N1 showed a smaller decrease in amplitude and a smaller increase in latency compared with the other measures. Further, the cutoff frequency where changes first occurred was different for N1. For 80-dB stimuli, N1 amplitudes showed significant changes when the low-pass noise masker cutoff was raised to 4000 Hz. In contrast, d′, MMN, N2, and P3 amplitudes did not change significantly until the low-pass noise masker was raised to 2000 Hz. N1 latencies showed significant changes when the low-pass noise masker was raised to 1000 Hz, whereas RT, MMN, N2, and P3 latencies did not change significantly until the low-pass noise masker was raised to 2000 Hz. No significant differences in response amplitudes were seen across the hemispheres (electrode sites C3M versus C4M) in quiet, or in masking noise. Conclusions: These results indicate that decreased audibility, resulting from the masking, affects N1 in a differential manner compared with MMN, N2, P3, and behavioral measures. N1 indexes the presence of audible stimulus energy, being present when speech sounds are audible, whether or not they are discriminable. MMN indexes stimulus discrimination at a pre-attentive level. It was present only when behavioral measures indicated the ability to differentiate the speech sounds. N2 and P3 also were present only when the speech sounds were behaviorally discriminated. N2 and P3 index stimulus discrimination at a conscious level. These cortical ERP in low-pass noise studies provide insight into the changes in brain processes and behavioral performance that occur when audibility is reduced, such as with low frequency hearing loss.