Effect of broadband noise on the human brain stem auditory evoked response

Effects of Amplification on Neural Phase Locking, Amplitude, and Latency to a Speech Syllable

OBJECTIVE

Older adults often have trouble adjusting to hearing aids when they start wearing them for the first time. Probe microphone measurements verify appropriate levels of amplification up to the tympanic membrane. Little is known, however, about the effects of amplification on auditory-evoked responses to speech stimuli during initial hearing aid use. The present study assesses the effects of amplification on neural encoding of a speech signal in older adults using hearing aids for the first time. It was hypothesized that amplification results in improved stimulus encoding (higher amplitudes, improved phase locking, and earlier latencies), with greater effects for the regions of the signal that are less audible.

DESIGN

Thirty-seven adults, aged 60 to 85 years with mild to severe sensorineural hearing loss and no prior hearing aid use, were bilaterally fit with Widex Dream 440 receiver-in-the-ear hearing aids. Probe microphone measures were used to adjust the gain of the hearing aids and verify the fitting. Unaided and aided frequency-following responses and cortical auditory-evoked potentials to the stimulus /ga/ were recorded in sound field over the course of 2 days for three conditions: 65 dB SPL and 80 dB SPL in quiet, and 80 dB SPL in six-talker babble (+10 signal to noise ratio).

RESULTS

Responses from midbrain were analyzed in the time regions corresponding to the consonant transition (18 to 68 ms) and the steady state vowel (68 to 170 ms). Generally, amplification increased phase locking and amplitude and decreased latency for the region and presentation conditions that had lower stimulus amplitudes-the transition region and 65 dB SPL level. Responses from cortex showed decreased latency for P1, but an unexpected decrease in N1 amplitude. Previous studies have demonstrated an exaggerated cortical representation of speech in older adults compared to younger adults, possibly because of an increase in neural resources necessary to encode the signal. Therefore, a decrease in N1 amplitude with amplification and with increased presentation level may suggest that amplification decreases the neural resources necessary for cortical encoding.

CONCLUSION

Increased phase locking and amplitude and decreased latency in midbrain suggest that amplification may improve neural representation of the speech signal in new hearing aid users. The improvement with amplification was also found in cortex, and, in particular, decreased P1 latencies and lower N1 amplitudes may indicate greater neural efficiency. Further investigations will evaluate changes in subcortical and cortical responses during the first 6 months of hearing aid use.

The effects of click and masker spectrum on the auditory brainstem response of bottlenose dolphins (Tursiops truncatus)

Two experiments were performed that investigated the effects of (1) click level and (2) continuous broadband noise on the binaural auditory brainstem response (ABR) of normal-hearing and hearing-impaired bottlenose dolphins (Tursiops truncatus). In addition to spectrally uncompensated clicks and noise, stimuli were digitally compensated to achieve "white" spectra (flat spectral density level) or "pink" spectra (spectral density level rolling off at -3 dB/octave). For experiment 1, in all spectral conditions, ABR peak latencies increased and peak amplitudes decreased with decreasing click level, but interwave intervals changed little. Latency-intensity function (LIF) slopes ranged from -3 to -11 μs/dB. The LIF slopes of ABR peaks evoked by uncompensated clicks were steeper in dolphins with hearing loss. Click level was held constant during experiment 2, and the effect of bilaterally delivered broadband masking noise on the ABR was investigated. Clicks and noise were filtered to create a pink click/noise condition and a white click/noise condition. With increasing levels of masking noise, peak latencies increased (although only P1-P4 white reached significance), peak amplitudes decreased, and interpeak intervals increased (although not significantly). These effects are compared to results reported for terrestrial mammals, and implications for auditory health assessment and biosonar function are discussed.

Study of suppression effect in the brainstem auditory evoked potential

BACKGROUND

the suppression effect with contralateral white noise observed in the brainstem auditory evoked potential can be influenced by the efferent auditory system.

AIM

to evaluate the suppression effect with contralateral white noise in the Brainstem Auditory Evoked Potential of individuals with normal hearing.

METHODS

25 individuals, ranging in age from 18 to 30 years, of both genders, were submitted to a clinical history questionnaire, inspection of the external auditory canal, conventional audiometry, speech audiometry and acoustic immittance measurements. Only individuals with normal hearing thresholds were selected. The selected individuals underwent brainstem auditory evoked potential testing with and without contralateral white noise.

RESULTS

a significant statistical difference was observed between the situations with and without contralateral white noise, for wave I amplitude and waves III and V latencies. No statistical differences were observed for the interpeak latencies.

CONCLUSION

the present study indicated increased latencies and reduced amplitudes of waves I, III and V with contralateral noise, when comparing the situations with and without noise. These results suggest a possible influence of the efferent auditory system on the response modulation of Brainstem auditory evoked potential when contralateral white noise is used.

Brain stem auditory potentials evoked by clicks in the presence of high-pass filtered noise in dogs

This study evaluates the effects of a high-frequency hearing loss simulated by the high-pass-noise masking method, on the click-evoked brain stem-evoked potentials (BAEP) characteristics in dogs. BAEP were obtained in response to rarefaction and condensation click stimuli from 60 dB normal hearing level (NHL, corresponding to 89 dB sound pressure level) to wave V threshold, using steps of 5 dB in eleven 58 to 80-day-old Beagle puppies. Responses were added, providing an equivalent to alternate polarity clicks, and subtracted, providing the rarefaction-condensation potential (RCDP). The procedure was repeated while constant level, high-pass filtered (HPF) noise was superposed to the click. Cut-off frequencies of the successively used filters were 8, 4, 2 and 1 kHz. For each condition, wave V and RCDP thresholds, and slope of the wave V latency-intensity curve (LIC) were collected. The intensity range at which RCDP could not be recorded (pre-RCDP range) was calculated. Compared with the no noise condition, the pre-RCDP range significantly diminished and the wave V threshold significantly increased when the superposed HPF noise reached the 4 kHz area. Wave V LIC slope became significantly steeper with the 2 kHz HPF noise. In this non-invasive model of high-frequency hearing loss, impaired hearing of frequencies from 8 kHz and above escaped detection through click BAEP study in dogs. Frequencies above 13 kHz were however not specifically addressed in this study.

A comparison of the effects of broadband masking noise on the auditory brainstem response in young and older adults

We examined the effects of ipsilateral-direct, continuous, broadband noise on auditory brainstem response (ABR) wave I and V latencies and amplitudes in young adult versus older adult humans. It was hypothesized that age might influence the effects of masking noise on ABR peak latencies and/or amplitudes, given the frequent complaint of older persons' ability to process speech in background noise. Young adults had hearing thresholds of 20 dB HL or better for the octave frequencies from 250 to 8,000 Hz. A subset of older study participants had thresholds of 20 dB HL or better across frequency, but others had thresholds up to 45 dB HL. All data were collected and analyzed with a Nicolet Bravo. An electrode was placed on the tympanic membrane (as well as on high forehead and contralateral mastoid), and a click level of 115 dB pSPL was used to maximize wave I amplitude. Masker conditions included a no-noise control and noise levels ranging from 20 to 70 dB effective masking, in 10 dB steps. With increasing noise level, both age groups showed minimal changes in wave I latency, but substantial increases in wave V latency and I-V interval. Peak amplitudes decreased with increasing noise level. Mean amplitudes were smaller for the older group, most notably for wave I. Mean peak latencies were greater in the older group, but the I-V interval was similar across age groups, as was the change in peak latencies and I-V interval across noise level. ABR parameters for the older adults with hearing meeting the 20-dB HL criterion at all frequencies (older-better) were compared to those who didn't meet this criterion (older-worse). Mean wave I latency was greater and wave V latency and I-V interval were smaller for the older-worse group at all noise levels. Mean wave I and V amplitudes were similar for the older-better and older-worse groups. In participants with normal or near-normal hearing, ABR changes with increasing age included small latency increases and a substantial reduction in wave I amplitude. The effects of ipsilateral-direct masking noise on the click-evoked ABR are similar for young and older adults.

Auditory evoked brainstem and middle latency responses in Macaca mulatta and humans

Early (ABRs) and middle (MLRs) surface-recorded auditory evoked potentials were compared in eight adult monkeys (Macaca mulatta) and eight adult humans. Responses whose probable generators were the cochlear nucleus and lateral lemniscus were of shorter latency and larger amplitude in monkeys. Relative to humans, ABR response latencies in monkeys were less affected by stimulus intensity, stimulus rate, and masker level. In contrast, monkey amplitudes were relatively more affected by those same stimulus parameters. The most prominent MLR wave was longer in latency and greater in amplitude in humans than the homologous wave in monkeys. The reduction in amplitude of that wave with increasing rate was greater for humans than monkeys. Temporal interactions (the effect of prior stimuli on the response to current stimulation) were investigated from a non-linear systems identification framework using maximum length sequences (MLSs). Both monkey and human auditory systems were second and probably third-order systems at the levels assessed. As the separations between the stimulus pulses decreased, evidence for temporal interactions became more prominent, reached a maximum, and then decreased with further decreases in stimulus pulse separation. At the highest stimulus rates presented, variations in temporal spacing among stimuli had less of an effect on monkey than human evoked responses.

Effects of click polarity on brainstem auditory-evoked potentials in cochlear hearing loss: a working hypothesis

The rarefaction-condensation differential potential (RCDP) obtained by subtracting brainstem auditory-evoked potentials (BAEPs) to C clicks from those to R clicks has been studied in 32 normal subjects and 31 cases of cochlear hearing loss. In normal subjects, no RCDP was recorded along the lower 30-55 dB of the JV latency-intensity function, thus defining the pre-RCDP range. The pre-RCDP range was always abolished in losses unmasking BAEPs from lower ( < 1 kHz) tonotopic regions. When the BAEP originated from higher ( > 1 kHz) tonotopic regions, the pre-RCDP range was either reduced or abolished. These results led to a working hypothesis based on single-unit data and stating a dual dependence of polarity effects on variables distributed along the tonotopic and intensity dimensions, with respective break-points at 1 kHz, and at the junction of the tip and tail of unit frequency tuning curves. The 1 kHz break-point could represent the upper frequency limit for phase locking in man.

Effects of signal-to-noise ratio on the auditory brainstem response to tone bursts in notch noise and broadband noise

This study investigated the effects of signal-to-noise ratio (S/N) on the latency and amplitude of the auditory brainstem response (Wave V) using 1 and 4 kHz tone bursts in notch noise and broadband noise. Normal listeners were presented with 40 dB and 80 dB nHL tone bursts in quiet and in noise at S/Ns of 10, 15, 20, and 25 dB. The latency data suggest that at low intensity levels tone bursts in quiet may be preferred to testing in noise. At moderate and high intensities, however, notch noise or broadband noise is preferred to the quiet condition because of the improved frequency specificity provided by the masking. When testing patients with flat or gradually sloping audiometric configurations at moderate to high intensities with 4 kHz, either notch noise or broadband noise with an S/N of 15 to 25 dB may be used. When presenting 1 kHz tone bursts at high intensities to patients having flat and gradually sloping losses, a notch noise at S/Ns of 10 to 25 dB is preferred to broadband noise. The use of a notch noise at 10 dB S/N may be the stimulus of choice when testing patients with moderately to steeply sloping audiometric configurations with 1 or 4 kHz tone bursts.

Effects of signal-to-noise ratio on the auditory brainstem response to 0.5 and 2 kHz tone bursts in broadband noise and highpass noise or notch noise

This study investigated the effects of signal-to-noise ratio (S/N) on the latency and amplitude of th3 auditory brainstem response (Wave V) using 0.5 and 2 kHz tone bursts in highpass/notch noise and broadband noise. Normal listeners were presented with 40 and 80 dB nHL tone bursts in quiet and in noise at S/Ns of 10, 15, 20, and 25 dB. The latency data suggest that, at moderate and high intensities, highpass/notch noise or broadband noise is preferred to the quiet condition because of the improved frequency specificity provided by the masking. Highpass/notch noise appears preferable to broadband noise when testing at moderate to high levels because the former produced larger Wave V amplitudes to 0.5 and 2 kHz tone bursts at 80 dB nHL. The 80 dB nHL data also suggest that S/Ns of 15-25 dB should be selected when the highpass/notch noise is mixed with moderate to high level 0.5 and 2 kHz tone bursts. In contrast to the 80 dB nHL data, Wave V amplitudes to the 40 dB nHL tone bursts suggest that testing in quiet may be preferred to testing in noise when 0.5 and 2 kHz tone bursts are presented at low levels. This is because of the simplicity of instrumentation and because larger amplitudes were observed in quiet than in noise.

Auditory brainstem responses in autism: brainstem dysfunction or peripheral hearing loss?

The advent of electrophysiological techniques for audiologic and neurologic assessment in the late 60s has generated at least 11 auditory brainstem response (ABR) studies in autism designed to test the integrity of the auditory brainstem pathways. The results reported are contradictory, involving prolongation, shortening, and no abnormalities in central transmission latencies. When sample and methodological factors influencing the ABR are taken into consideration in the interpretation of results, the ABR data available at present can be seen as only suggestive, rather than supportive, of brainstem involvement in autism. Paradoxically, these studies revealed the presence of peripheral hearing impairment in a non-negligible number of autistic individuals. Additional evidence of auditory abnormalities as well as the implications for the clinician are considered.