Effects of sensorineural hearing loss on cortical event-related potential and behavioral measures of speech-sound processing

OBJECTIVE: To investigate systematically the effects of sensorineural hearing loss on cortical event-related potentials (ERPs) N1, MMN, N2 and P3 and their associated behavioral measures (d' sensitivity and reaction time) to the speech sounds /ba/ and /da/ presented at 65 and 80 dB ppe SPL. DESIGN: Cortical ERPs were recorded to /ba/ and /da/ speech stimuli presented at 65 and 80 dB ppe SPL from 20 normal-hearing adults and 20 adults who are hearing impaired. The degree of sensorineural impairments at 1000 to 2000 Hz ranged from mild losses (defined as 25 to 49 dB HL) to severe/profound losses (75 to 120 dB HL). The speech stimuli were presented in an oddball paradigm and the cortical ERPs were recorded in both active and passive listening conditions for each stimulus intensity. RESULTS: Both ERP amplitudes and behavioral discrimination (d') scores were lower for listeners with sensorineural hearing loss than for those with normal hearing. However, these differences in response strength were evident only for those listeners whose average hearing loss at 1000 to 2000 Hz exceeded 60 dB HL for the lower intensity stimuli and exceeded 75 dB HL for the higher intensity stimuli. In contrast, prolongations in the ERP and behavioral latencies, relative to responses from normal-hearing subjects, began with even mild (25 to 49 dB HL) threshold elevations. The amplitude and latency response changes that occurred with sensorineural hearing loss were significantly greater for the later ERP peaks (N2/P3) and behavioral discrimination measures (d' and RT) in comparison with earlier (N1, MMN) responses. CONCLUSIONS: The results indicate that latency measures are more sensitive indicators of the early effects of decreased audibility than are response strength (amplitude, d' or percent correct) measures. Sensorineural hearing loss has a greater impact on higher level or "nonsensory" cortical processing in comparison with lower level or "sensory" cortical processing. Possible physiologic mechanisms within the cortex that may be responsible for these response changes are presented. Lastly, the possible clinical significance of these ERP and behavioral findings is discussed.     

Speech-recognition performance after long-term hearing aid use

Larson et al (2000) reported the findings of a multicenter, NIDCDNA clinical trial that compared hearing aid performance for three output limiting circuits in 360 adults with symmetrical sensorineural hearing loss. The current study was undertaken to examine long-term hearing aid benefit in this same group of participants following five to six years of hearing aid use. The speech-recognition portion of the follow-up study enrolled 108 participants from the original study, 85% of whom were current hearing aid users and 15% of whom had not worn hearing aids during the past month (nonusers). Recognition performance in sound field on the NU-6 (quiet at 62 dB SPL) and the CST (quiet at 74 dB SPL and with -3 and 3 dB signal-to-babble ratios [S/B] at 62 and 74 dB SPL) was measured unaided and aided whenever possible. Speech-recognition abilities decreased significantly since the original study. Speech-recognition decrements were observed regardless of the speech materials (NU-6 and CST), test condition (quiet and noise), S/B (-3 and 3 dB), or stimulus level (62 and 74 dB SPL). Despite decreases in speech recognition, hearing aid benefit remained largely unchanged since the original study; aided performance exceeded unaided performance regardless of presentation level or noise condition. As in the original study, the relations among stimulus level, S/B, and speech-recognition performance were complex.     

The use of cortical auditory evoked potentials to evaluate neural encoding of speech sounds in adults

There has been considerable recent interest in the use of cortical auditory evoked potentials (CAEPs) as an electrophysiological measure of human speech encoding in individuals with normal as well as impaired auditory systems. The development of such electrophysiological measures such as CAEPs is important because they can be used to evaluate the benefits of hearing aids and cochlear implants in infants, young children, and adults that cannot cooperate for behavioral speech discrimination testing. The current study determined whether CAEPs produced by seven different speech sounds, which together cover a broad range of frequencies across the speech spectrum, could be differentiated from each other based on response latency and amplitude measures. CAEPs were recorded from ten adults with normal hearing in response to speech stimuli presented at a conversational level (65 dB SPL) via a loudspeaker. Cortical responses were reliably elicited by each of the speech sounds in all participants. CAEPs produced by speech sounds dominated by high-frequency energy were significantly different in amplitude from CAEPs produced by sounds dominated by lower-frequency energy. Significant effects of stimulus duration were also observed, with shorter duration stimuli producing larger amplitudes and earlier latencies than longer duration stimuli. This research demonstrates that CAEPs can be reliably evoked by sounds that encompass the entire speech frequency range. Further, CAEP latencies and amplitudes may provide an objective indication that spectrally different speech sounds are encoded differently at the cortical level.     

The relationship between cortical auditory evoked potential (CAEP) detection and estimated audibility in infants with sensorineural hearing loss

Abstract Objective: To determine the effectiveness of objective statistical detection in CAEP testing to evaluate audibility in young infants with sensorineural hearing loss. Design: CAEP recordings to speech-based stimuli were made at three presentation levels (55, 65, or 75 dB SPL) when a group of hearing-impaired infants were either aided or unaided. Later-obtained behavioral audiograms were used as the gold standard against which to evaluate the accuracy of the automatic detection of the presence/absence of CAEP responses. Study sample: Participants were 18 infants with confirmed sensorineural hearing loss. Results: Higher sensation levels led to a greater number of present CAEP responses being detected. More CAEP waveforms were detected in the aided condition than in the unaided condition. Conclusion: Our results suggest that the presence/absence of CAEP responses defined by the automatic statistical criterion was effective in showing whether increased sensation levels provided by amplification were sufficient to reach the cortex. This was clearly apparent from the significant increase in cortical detections when comparing unaided with aided testing.     

Audibility and speech perception of children using wide dynamic range compression hearing AIDS

PURPOSE: This study examined the relation of audibility for frequency-specific sounds and the Speech Intelligibility Index (SII) to speech perception abilities of children with sensorineural hearing loss using digital signal-processing hearing aids with wide dynamic range compression. METHOD: Twenty-six children age 5-15 years with pure-tone averages (0.5, 1.0, and 2.0 kHz) from 60-98 dB HL participated. Three subgroups were created based on the compression characteristics of each hearing aid. Minimum audibility was determined using aided thresholds for frequency-modulated tones and the SII calculated at 55 and 70 dB SPL using the simulated real-ear output of the hearing aid. The Lexical Neighborhood Test (LNT; K. I. Kirk, D. B. Pisoni, & M. J. Osberger, 1995) was presented at 50 and 70 dB SPL. RESULTS: LNT scores at 70 dB SPL were significantly higher than at 50 dB SPL. Average aided thresholds at 0.5, 1.0, and 2.0 kHz were negatively correlated with LNT scores at 50 dB SPL, and SIIs at 55 and 70 dB SPL were positively correlated with LNT scores at 50 and 70 dB SPL. CONCLUSIONS: Results support using aided thresholds and speech test scores at soft to loud levels as part of the amplification fitting process.     

The effects of decreased audibility produced by high-pass noise masking on N1 and the mismatch negativity to speech sounds /ba/and/da.

This study investigated the effects of decreased audibility produced by high-pass noise masking on the cortical event-related potentials (ERPs) N1 and mismatch negativity (MMN) to the speech sounds /ba/ and /da/, presented at 65 dB SPL. ERPs were recorded while normal listeners (N = 11) ignored the stimuli and read a book. Broadband masking noise was simultaneously presented at an intensity sufficient to mask the response to the speech sounds, and subsequently high-pass filtered. The conditions were QUIET (no noise); high-pass cutoff frequencies of 4000, 2000, 1000, 500, and 250 Hz; and broadband noise. Behavioral measures of discrimination of the speech sounds (d' and reaction time) were obtained separately from the ERPs for each listener and condition. As the cutoff frequency of the high-pass masker was lowered, ERP latencies increased and amplitudes decreased. The cutoff frequency where changes first occurred differed for N1 and MMN. N1 showed small systematic changes across frequency beginning with the 4000-Hz high-pass noise. MMN and behavioral measures showed large changes that occurred at approximately 1000 Hz. These results indicate that decreased audibility, resulting from the masking, affects N1 and the MMN in a differential manner. N1 reflects the presence of audible stimulus energy, being present in all conditions where stimuli were audible, whether or not they were discriminable. The MMN is present only for those conditions where stimuli were behaviorally discriminable. These studies of cortical ERPs in high-pass noise studies provide insight into the changes in brain processes and behavioral performance that occur when audibility is reduced, as in hearing loss.     

Magnitude estimation of loudness. III: Performance of selected hearing aid users

Aided and unaided loudness functions for narrow-band noise stimuli were obtained from hearing-impaired listeners with a magnitude estimation procedure. A comparison of aided loudness functions with those obtained from normal-hearing subjects suggests that the hearing aids did not restore normal loudness relations among the spectral components of speech stimuli. Instead, aided loudness functions tend to reflect an interaction between the abnormal loudness growth that frequently characterizes sensorineural hearing loss and the saturation characteristics of the hearing aid.     

Speech recognition performance of patients with sensorineural hearing loss under unaided and aided conditions using linear and compression hearing AIDS

OBJECTIVES: This study compared speech recognition performance on the Northwestern University Auditory Test No. 6 (NU-6) and the Connected Speech Test (CST) for three hearing aid circuits (peak clipping [PC], compression limiting [CL], and wide dynamic range compression [WDRC]) in adults with symmetrical sensorineural hearing loss. The study also questioned whether or not hearing aid benefit for the three circuits was dependent upon the speech level and the signal-to-babble ratio (S/B) and upon the degree and slope of hearing loss. DESIGN: Unaided speech recognition performance for NU-6 and CST materials presented from a loudspeaker at 0 degrees was measured during Visit 1, and both unaided and aided performance was measured at 3-mo intervals during Visits 2 to 4. The NU-6 was presented in quiet at a conversational speech level of 62 dB SPL. The CST was presented in 10 listening conditions-three S/B (-3, 0, and 3 dB) at each of three speech levels (soft speech at 52 dB SPL, conversational speech at 62 dB SPL, and loud speech at 74 dB SPL) and in quiet at 74 dB SPL. Uncorrelated multi-talker babble was presented from two loudspeakers at 45 degrees on each side of the main speaker. Hearing aid benefit was examined for 360 subjects divided into four groups of hearing loss, pure tone average <40 dB HL and slope <10 dB/octave or >10 dB/octave and hearing loss >40 dB HL for the two slope categories. RESULTS: Hearing aid benefit (aided minus unaided performance) measured on the NU-6 in quiet exceeded 31 rau for all three circuits. Although small statistical advantages were found for the WDRC, the differences were approximately 2% and are not considered clinically relevant. Unaided CST performance showed a complex relationship between presentation level and signal-to-babble ratio that was further confounded by the degree of hearing loss. For the two mild hearing loss groups and for each of the three nominal signal-to-babble ratios, CST performance decreased by 20 rau for the -3 dB S/B to 6 rau for the 3 dB S/B as speech level increased from 52 to 74 dB SPL. In contrast, unaided performance increased by 32 to 13 rau with signal level for all signal-to-babble ratios for the two >40 dB hearing loss groups. Overall, aided CST performance exceeded unaided performance for all 10 conditions. As expected, hearing aid benefit was greatest (27 rau) for soft speech and smallest for loud speech (6 rau). Differences among the hearing aid circuits were small with only one significant difference; the WDRC at 62/0 was poorer by 3 rau than the other two circuits. When the CST data were analyzed as a function of hearing loss, five pair-wise comparisons were significant. In contrast to the unaided performance, aided performance for all hearing loss groups decreased as presentation level increased, even though the signal-to-babble ratio was constant. CONCLUSIONS: All three hearing aids circuits provided benefit over the unaided condition in both quiet and noise. The greatest benefit was measured for soft speech in the more severe hearing loss groups. Although only small differences were measured among the three hearing aid circuits, significant differences favored the PC and CL circuits over the WDRC in the mild hearing loss groups and favored the WDRC over the PC in the more severe, sloping hearing loss group. An interesting interaction between speech level, signal-to-babble ratio, degree of hearing loss, and amplification was found. For a constant signal-to-babble ratio, recognition performance decreased as speech level increased from 52 to 74 dB SPL. The effect was most marked in the milder hearing loss groups and in the aided conditions, and occurred at even the lowest speech levels.     

Auditory steady-state responses and word recognition scores in normal-hearing and hearing-impaired adults

OBJECTIVE: The number of steady-state responses evoked by the independent amplitude and frequency modulation (IAFM) of tones has been related to the ability to discriminate speech sounds as measured by word recognition scores (WRS). In the present study IAFM stimulus parameters were adjusted to resemble the acoustic properties of everyday speech to see how well responses to these speech-modeled stimuli were related to WRS. DESIGN: We separately measured WRS and IAFM responses at a stimulus intensity of 70 dB SPL in three groups of subjects: young normal-hearing, elderly normal-hearing, and elderly hearing-impaired. We used two series of IAFM stimuli, one with modulation frequencies near 40 Hz and the other with modulation frequencies near 80 Hz. The IAFM stimuli, consisting of four carrier frequencies each independently modulated in frequency and amplitude, could evoke up to eight separate responses in one ear. We recorded IAFM responses and WRS measurements in quiet and in the presence of speech-masking noise at 67 dB SPL or 70 dB SPL. We then evaluated the hearing-impaired subjects with and without their hearing aids to see whether an improvement in WRS would be reflected in an increased number of responses to the IAFM stimulus. RESULTS: The correlations between WRS and the number of IAFM responses recognized as significantly different from the background were between 0.70 and 0.81 for the 40 Hz stimuli, between 0.73 and 0.82 for the 80 Hz stimuli, and between 0.76 and 0.85 for the combined assessment of 40 and 80 Hz responses. Response amplitudes at 80 Hz were smaller in the hearing-impaired than in the normal-hearing subjects. Response amplitudes for the 40 Hz stimuli varied with the state of arousal and this effect made it impossible to compare amplitudes across the different groups. Hearing aids increased both the WRS and the number of significant IAFM responses at 40 Hz and 80 Hz. Masking decreased the WRS and the number of significant responses. CONCLUSIONS: IAFM responses are significantly correlated with WRS and may provide an objective tool for examining the brain's ability to process the auditory information needed to perceive speech.     

Improvements in speech intelligibility in quiet and in noise produced by two-channel compression hearing aids

Eight subjects suffering from bilateral sensorineural hearing losses with recruitment were fitted binaurally with two-channel compression hearing aids, worn behind the ear. After they had worn the aids for some time, measures of speech intelligibility were compared for two conditions: listening unaided, and listening aided. the dynamic range for speech, defined as the difference in level between the speech reception threshold in quiet and the highest comfortable level for speech, was substantially increased in the aided condition for seven of the eight subjects (the exception was a subject with almost normal low-frequency hearing). Speech reception thresholds were also measured in two levels of background noise ('babble'), 60 and 75 dB SPL. Seven of the eight subjects showed a reduced speech reception threshold (i.e. an improvement) in the aided condition for at least one of the two noise levels, although the size of the improvement differed considerably from one subject to another. The subjects were also given a battery of psycho-acoustical tests in an attempt to better characterise their hearing loss, and to gain more insight into individual differences. Results of measurements of frequency selectivity, frequency discrimination, temporal acuity and temporal masking are described and related to the measures of speech intelligibility.     

Acoustical changes of loudly spoken speech and their effects on speech recognition in hearing-impaired listeners.

The level of speech is usually increased in conversations with unaided hearing-impaired listeners. However, the speaker may talk at conversational levels to aided hearing-impaired persons. In this case, the level of speech is electronically increased by the hearing aid. In the present study, the acoustical changes of loudly spoken speech and their effects on speech recognition were investigated in 20 patients with sensorineural hearing loss. Eight test words of the German Speech Intelligibility Test ('Freiburger Sprachtest') were recorded at original levels of 60 and 75 dB SPL by a male speaker. Both recordings were presented to hearing-impaired subjects at a playback level of 75 dB SPL. Thus, the level of the 60 dB SPL recording was increased electronically by 15 dB. For the 75 dB SPL recording, playback and recording levels were identical. The average whole-word score was 49% for the 60 dB SPL recording and 39% for the 75 dB SPL recording. This difference was statistically significant (0.002 less than P less than 0.005). The results of the speech recognition ability tests could be explained by the acoustical changes of the loudly spoken speech. In the 75 dB SPL recording, the levels of voiceless fricatives, nasals and plosives were significantly lower than in the 60 dB SPL recording. Spectrally, the fundamental frequency was increased and the configuration of the first formant was altered in the 75 dB SPL recording. The significance of the findings for clinical speech audiometry and hearing aid evaluation is discussed.     

The relationship between cortical auditory evoked potential (CAEP) detection and estimated audibility in infants with sensorineural hearing loss

Abstract

Objective: To determine the effectiveness of objective statistical detection in CAEP testing to evaluate audibility in young infants with sensorineural hearing loss. Design: CAEP recordings to speech-based stimuli were made at three presentation levels (55, 65, or 75 dB SPL) when a group of hearing-impaired infants were either aided or unaided. Later-obtained behavioral audiograms were used as the gold standard against which to evaluate the accuracy of the automatic detection of the presence/absence of CAEP responses. Study sample: Participants were 18 infants with confirmed sensorineural hearing loss. Results: Higher sensation levels led to a greater number of present CAEP responses being detected. More CAEP waveforms were detected in the aided condition than in the unaided condition. Conclusion: Our results suggest that the presence/absence of CAEP responses defined by the automatic statistical criterion was effective in showing whether increased sensation levels provided by amplification were sufficient to reach the cortex. This was clearly apparent from the significant increase in cortical detections when comparing unaided with aided testing.     

Reliability and validity of judgments of sound quality in elderly hearing aid wearers

OBJECTIVE: The aim of the present study was to evaluate a particular sound-quality rating procedure, referred to here as the Judgments of Sound Quality (JSQ) test, for potential use as an outcome measure with elderly hearing aid wearers. The validity and reliability of the JSQ as an outcome measure were determined for unaided and aided listening conditions. DESIGN: A repeated-measures design was employed with two primary independent variables, each with two values: 1) aided condition (aided versus unaided listening); and 2) trial (test versus retest). Sixteen elderly, hearing-impaired hearing aid users participated in the study. The participants provided ratings on eight dimensions of sound quality (softness, brightness, clarity, fullness, nearness, loudness, spaciousness, and total impression) under four stimulus conditions (speech at 65 dB SPL with a +8 dB signal-to-noise ratio, speech at 65 dB SPL in quiet, music at 90 dB SPL, and music at 75 dB SPL). Test and retest ratings were obtained in both unaided and aided conditions. RESULTS: No significant differences were found between median test and retest scores on the JSQ. The median ratings for each sound-quality dimension were found to have moderate test-retest reliability, with test-retest correlations (r) ranging from 0.20 to 0.73 (median r value = 0.58) for the unaided listening condition, and from 0.23 to 0.85 (median r value = 0.51) for the aided listening condition. Test validity was established through significant differences in JSQ ratings for various stimulus pairs (e.g., speech in quiet versus speech in noise). In addition, significant differences were observed between unaided and aided ratings for the dimensions of clarity, nearness, loudness, and total impression with aided JSQ ratings approaching normative "ideal" values established previously. CONCLUSIONS: The JSQ appears to be a potentially useful measure of hearing aid outcome, especially when using group data to document the benefits of amplification. Additional efforts should be directed at improving the reliability of the JSQ, however, before application to hearing aid wearers on an individual basis.     

Evaluation of electroacoustic test signals I: comparison with amplified speech

OBJECTIVE: To evaluate the ability of clinical test signals to match the aided levels of real speech, across a range of hearing aid circuit types and strengths. DESIGN: Hearing aids (N = 41) were set to DSL targets for moderate, severe, and profound hearing losses. These hearing aids were tested with three test signals (Fonix Pure Tones, Fonix Composite Noise, and Audioscan Swept), as well as with running speech. The difference between the aided test signal and the aided speech was calculated. RESULTS: Accuracy of matches between aided test signals and aided speech levels depended on circuit type, signal type, and test level. CONCLUSIONS: Clinical test signals can more accurately match the aided levels of speech for all types of hearing aids if they are 1) speech-weighted and 2) temporally modulated. Matches were more accurate at low to moderate test levels (i.e., 50 to 70 dB SPL), and less accurate at high test levels (i.e., 85 dB SPL).     

Hearing performance in noise of cochlear implant patients versus severely-profoundly hearing-impaired patients with hearing aids

It is possible for most post-lingually deaf patients to attain significant open speech recognition following cochlear implantation. In contrast, many severely-profoundly sensorineural hearing-impaired patients receive no benefit from their hearing aids, especially in situations with background noise. The aim of the study was to evaluate the speech recognition ability in quiet and in noise of post-lingually deaf adults implanted with Combi 40/40+ cochlear implants versus severely-profoundly sensorineural hearing-impaired patients fitted with hearing aids. For this purpose, we tested two groups of patients: one that had received cochlear implants (n=22) and a group of subjects with severe-profound sensorineural hearing impairment, fitted with hearing aids (n = 15). All of the patients were tested using the Hochmaier, Schultz, and Moser Discrimination Test in quiet and noise. The results of the study demonstrate that most of our cochlear implant patients received a substantial benefit from their implant, achieving scores of 70 to 100 per cent (mean, 90 per cent) for the numbers test and 10 to 72 per cent (mean, 43 per cent) for the monosyllable test 1 year after implantation. Even in situations with background noise, scores of 1 to 99 per cent (mean, 45.65 per cent) for a signal to noise ratio (SNR) of +15 dB 1 year following the implantation improved to 7 to 95 per cent (mean, 50.7 per cent) at 2 years and 8 to 99 per cent (mean, 60 per cent) at 3 years after implantation. These results are significantly (p<0.04) superior to the hearing aid patients' scores of 1 to 64.2 per cent (mean, 26.7 per cent) for a SNR of 15 dB. The results of the present study may have clinical implications in regard to selection of candidates for cochlear implantation.     

Representation of amplified speech at cortical level in good and poor hearing aid performers

IntroductionHearing aid users reject their own hearing aid because of annoyance with background noise. The reason for dissatisfaction is located anywhere from the hearing aid microphone to the integrity of neurons along the auditory pathway. In this preview, the output of hearing aid was recorded at the level of ear canal and at auditory cortex in good and poor hearing aid users, who were classified using acceptable noise level.ObjectiveTo study the representation of amplified speech in good and poor hearing aid performers.MethodsA total of 60 participants (age ranged 15–65 years) with moderate bilateral sensorineural hearing impairment grouped into good (n = 35) and poor (n = 25) hearing aid performers. Gap detection test and aided SNR 50 were administered. In addition, ear canal acoustic measures and cortical auditory evoked potentials were recorded in unaided and aided conditions at 65 dB SPL.ResultsHearing aid minimally alters temporal contrast of speech reflected in envelope difference index. Although having similar temporal impairment, acoustic characteristics of amplified speech sounds and SNR 50 scores from both groups, the aided cortical auditory evoked potentials surprisingly showed significant earlier latencies and higher amplitudes in good performers than poor performers. In addition, good and poor performers classified based on annoyance level was predicted by latencies of 2N1 and 2P2 components of acoustic change complex. Further, a follow-up revealed hearing aid use has relation with acceptance towards noise.ConclusionParticipants who are willing to accept noise from those who are not willing to accept noise have subtle physiological changes evident at the auditory cortex, which supports the hearing aid usage.     

Effects of hearing aid amplification and stimulus intensity on cortical auditory evoked potentials

Hearing aid amplification can be used as a model for studying the effects of auditory stimulation on the central auditory system (CAS). We examined the effects of stimulus presentation level on the physiological detection of sound in unaided and aided conditions. P1, N1, P2, and N2 cortical evoked potentials were recorded in sound field from 13 normal-hearing young adults in response to a 1000-Hz tone presented at seven stimulus intensity levels. As expected, peak amplitudes increased and peak latencies decreased with increasing intensity for unaided and aided conditions. However, there was no significant effect of amplification on latencies or amplitudes. Taken together, these results demonstrate that 20 dB of hearing aid gain affects neural responses differently than 20 dB of stimulus intensity change. Hearing aid signal processing is discussed as a possible contributor to these results. This study demonstrates (1) the importance of controlling for stimulus intensity when evoking responses in aided conditions, and (2) the need to better understand the interaction between the hearing aid and the CAS.     

The post-auricular muscle response: an objective electrophysiological method for evaluating hearing sensitivity

Post-auricular muscle responses (PAMRs) were recorded in sixteen adults with normal hearing and twenty adults with sensorineural hearing loss. Click stimuli were presented at 20 to 80 dB nHL via insert earphones. Only one ear was tested in hearing-impaired subjects, but normal-hearing subjects were tested monaurally and binaurally. PAMR amplitudes declined and latencies increased with decreasing click intensity. Both binaural stimulation and eye turn enhanced the PAMR. In hearing-impaired subjects, PAMR thresholds were correlated with audiometric thresholds for the eyes-turned condition. All normal-hearing subjects had PAMR when recording conditions were optimized and half had responses for the least optimal condition (20 dB nHL, monaural, eyes front). With eyes turned and monaural clicks at 35 dB nHL, the level widely used for infant hearing screening, most normal-hearing adults had a PAMR. Thus the PAMR is a robust response that may be a useful adjunct to ABR for objective hearing assessment.     

A comparison of two-channel and single-channel compression hearing aids

Eight subjects with bilateral sensorineural hearing losses took part in a trial comparing listening unaided with listening binaurally through two types of hearing aid, aid A and aid B. Both aids incorporated slow-acting automatic gain control (AGC) operating on the whole speech signal. However, aid A also incorporated two-channel syllabic compression. The two aids were chosen to be as similar as possible in other respects, and both were worn behind the ear. Subjects were tested in a counter-balanced order, and had at least 2 weeks of everyday experience with each aid before testing took place. Performance was evaluated in three ways: by measuring speech intelligibility in quiet for sentences at three peak sound levels, 55, 70 and 85 dB SPL; by measuring the level of speech required for 50% intelligibility (called the SRT) of sentences in two levels of speech-shaped noise, 60 and 75 dB SPL; and by administering questionnaires about experience with the aids in everyday life. Both aid A and aid B improved the intelligibility of speech in quiet relative to unaided listening, particularly at the lowest sound level. However, aid A gave lower (i.e., superior) SRTs in speech-shaped noise than aid B or unaided listening. The questionnaires also indicated that aid A gave better performance in noisy situations. The results strongly suggest that two-channel syllabic compression, combined with slow-acting AGC operating on the whole speech signal, can give superior results to slow-acting AGC alone, particularly in noisy situations.     

Cortical encoding of speech acoustics: Effects of noise and amplification

Objective: To investigate speech stimuli and background-noise-dependent changes in cortical auditory-evoked potentials (CAEPs) in unaided and aided conditions, and determine amplification effects on CAEPs. Design: CAEPs to naturally produced syllables in quiet and in multi-talker babble were recorded, with and without a hearing aid in the right ear. At least 300 artifact-free trials for each participant were required to measure latencies and amplitudes of CAEPs. Acoustic characteristics of the hearing-aid-transduced stimuli were measured using in-the-canal probe microphone measurements to determine unaided versus aided SNR and to compare stimulus acoustic characteristics to CAEP findings. Study sample: Ten participants with normal hearing, aged 19 to 35 years. Results: CAEP latencies and amplitudes showed significant effects of speech contrast, background noise, and amplification. N1 and P2 components varied differently across conditions. In general, cortical processing in noise was influenced by SNR and the spectrum of the speech stimuli. Hearing-aid-induced spectral and temporal changes to the speech stimuli affected P1-N1-P2 components. Amplification produced complex effects on latencies and amplitudes across speech stimuli and CAEP components, and for quiet versus noise conditions. Conclusion: CAEP components reflect spectral and temporal characteristics of speech stimuli and acoustic changes induced by background noise and amplification.