Hello Old Friend the use of frequency specific speech phonemes in cortical and behavioural testing of infants

Transcription

1 Hello Old Friend the use of frequency specific speech phonemes in cortical and behavioural testing of infants Andrea Kelly 1,3 Denice Bos 2 Suzanne Purdy 3 Michael Sanders 3 Daniel Kim 1 1. Auckland District Health Board 2. Dilworth Audiology 3. University of Auckland

2 Overall Aim The roll out of universal newborn hearing screening has emphasised the need for accurate testing of infants and young children with hearing loss with and without amplification Create a set of stimuli that could be used for both behavioural and objective testing depending upon the clinical needs

3 Infant Testing Infants with developmental delay, and even infants without any disability may be difficult to test behaviourally with the usual frequency specific stimuli such as narrow band noises and warble tones. It has long been established that one of the most effective stimuli for VRA is speech

4 Type of Stimuli Early VRA research established the optimum criteria for maximising response reliability and efficiency that is still in use today Thompson and Thompson, 1972, found that the best auditory stimuli were speech, filtered speech narrow band noises and lastly pure tones. RESPONSE OF INFANTS AND YOUNG CHILDREN AS A FUNCTION OF AUDITORY STIMULI AND TEST METHODS. MARIE THOMPSON and GARY THOMPSON Journal of Speech and Hearing Research,

5 7 12 months months Thompson and Thompson, 1972

6 Thompson and Thompson, 1972

7 Results of this study supported previous research (Eisenberg, 1965; Hoversten and Moncur, 1969; Ling et al., 1970), which indicated that the auditory responses of infants varied as a function of the test stimulus. These studies also showed that for newborn infants and babies up to about one year old, pure tones consistently produced fewer responses than speech or other complex stimuli, regardless of the intensity level at which the stimuli were compared.

8 Ling 6 Sounds In 1976, Dr Daniel Ling identified a number of speech sounds with concentrations of energy that spanned the entire range of speech frequencies. Ling proposed the development of spoken language in pre-lingually hearing-impaired individuals depends on the perception and discrimination of a broad range of speech sounds.

9 Ling Sounds

10 The Ling sound test is typically used as a quick test to verify audiometric threshold assessment and/or the fitting of hearing aids or cochlear implants (Agung et al, 2005). It is also used to assess the ability of the listener to detect and discriminate between six different speech sounds that largely encompass the speech spectrum (Agung et al, 2006).

11 Cortical evoked potentials to speech stimuli in infants and adults Numerous studies have shown the successful use of phonemes to evoke auditory cortical potentials from the 1980s Novak et al, 1989 Sharma et al, 1997 Agung et al, 2006 Cone and Whitaker, 2013 Carter et al, 2013

12 Agung et al, 2006 Used short and long Ling Sounds to show how the cortical responses varied in amplitude and latency. Possible to see frequency specific differences in the grand mean responses Grand average waveforms for the 500 msec stimuli showing the shift in latency across vowels and between consonants with low- and high-frequency emphasis

13 Agung et al, found cortical auditory evoked responses could be elicited by speech sounds that encompassed the entire speech frequency range, with shorter duration sounds producing CAEPs with larger amplitudes and shorter latencies than longer duration sounds. Speech sounds that were dominated by highfrequency energy produced CAEPs that were significantly different in terms of N1 and P2 amplitude when compared to sounds dominated by lower-frequency energy.

14 While CAEPs may be used to objectively measure differences in neural encoding and perception of spectrally different speech sounds, the differences between CAEPs are not sufficient to use as a measure of discrimination for each of the Ling seven sounds (Agung et al, 2006).

15 Cone and Whitaker 2013 Used 50ms Ling sounds and /o/ 50 ms duration was chosen: 1. the brevity of the speech sound tokens was representative of the duration of consonants and vowels of running speech. 2. brief tokens limit the duration of the large electrical artifact produced by a cochlear implant on CAEP recordings

16 Cone and Whittaker 2013 Tested 36 normal hearing infants (average age 8 months) Behavioural thresholds to speech phonemes Cortical evoked potential thresholds to speech phonemes in same infants Concluded that tonal or speech stimuli can be used in the awake, passively alert infant and they could be used in the verification of hearing aid or cochlear implant benefit

17 Cone and Whitaker, 2013 Individual subject waveforms to separate speech phonemes tracked down to threshold

18 Research question Could the effectiveness of speech phonemes as demonstrated in the previous studies be enhanced by further manipulation of the stimuli to create more frequency specific results that can be used clinically in the diagnosis and habilitation of hearing impaired infants?

19 Stimuli chosen Stimuli based on Ling sounds Used new recordings with a NZ speaker Stimuli chosen were based on their ability to span the important frequencies of speech with a high frequency emphasis. Importance of the high frequencies due to changing cochlear implant candidacy criteria essential to check if adequate amplification is being obtained and also due to need to assess claims of hearing aid manufacturers that they can amplify beyond 4kHz

20 Bos, 2013 Aims: To develop three frequency-specific speech tokens suitable for assessing auditory discrimination with aided CAEPs, and assess the effects of hearing aid processing on the speech tokens with realistic real world hearing aid settings The effects of hearing aid processing on the speech tokens were investigated using two hearing aids representative of the options available to New Zealand infants. Realistic settings were chosen since hearing aid validation using aided CAEPs would ideally occur with the hearing aids programmed with the infant s everyday settings, so as to obtain as much information as possible about the benefit the infant receives from the fitting.

21 Stimuli Speech tokens centred on 1000 and 4000 Hz provide information regarding access to the lower and higher ends of the frequency range important for speech, respectively. A speech token centred on 6000 Hz provides information regarding access to the higher frequency speech sounds (fricatives and affricates) that hearing impaired children have difficulty learning to produce (Moeller, Hoover, Putman, Arbataitis, Bohnenkamp, Peterson, Wood, et al., 2007).

22 Method the phonemes a/, /ʃ/ and /s/ were digitally manipulated to create three 100ms speech tokens representing 1000, 4000 and 6000 Hz, respectively. The effects of hearing aid processing on the speech tokens were assessed using two different hearing aids, and the speech tokens were subjectively assessed by six adults. CAEPs were measured in nine normally hearing infants in response to the speech tokens, and three narrow band noise (NBN) stimuli with centre frequencies corresponding to the speech tokens, as a reference for frequency specificity. Stimuli were presented in the sound field for clinical relevance.

23 Each phoneme was truncated to a spectrally representative 200 ms segment and was then time-compressed with a ratio of 200 to a final duration of 100 ms, Both truncation and time-compression were used in an effort to preserve as much spectral information of the original phoneme as possible. Shaped with linear ramping of 20 ms duration with a 60 ms plateau. Band pass filtered using a second-order Butterworth filter, the filters were one third of an octave in bandwidth, with the centre frequency of each filter corresponding to the frequency each phoneme aimed to represent.

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27 Spectrum of the narrowband noises 0 and speech tokens db FS (Full Scale) /a/ 6000 Hz NBN 4000 Hz NBN 1000 Hz NBN /sh/ /s/ Frequency (Hz)

28 Comparison of spectra from another study using filtered speech tokens Frequency spectra of the three HEARLab stimuli: /m/, /g/ and /t/. K. Munro et al., 2011, Ear and Hearing, 32(6),

29 Hearing Aid Characteristics Both hearing aids (Oticon Safari and Phonak Nios) were programmed for a flat 50 db HL SNHL using the manufacturer s software The hearing aids were programmed to the DSL v5.0a paediatric prescription, and both hearing aids were set to 100% prescribed gain. All other features (e.g. noise reduction, feedback management) were left in the default prescribed settings selected by the manufacturer, with the exception of the frequency lowering feature of the Phonak hearing aid, which was disabled.

30 Method In all experimental conditions, the spectral and temporal characteristics and levels of each speech token was measured at the tympanic membrane using a probe microphone system. In the unaided condition, no hearing aid was worn and the ear canal remained unoccluded. Measurements were conducted in the right ear. In both aided conditions, the volunteer wore the hearing aid on the right ear. Each hearing aid tone hook was connected to standard behindthe-ear (BTE) tubing which was coupled to an insert earphone.

31 Under both aided conditions, the energy peaks of each speech token sample correspond to the centre frequencies of the original electronic speech tokens, with no unwanted frequency distortions to the speech tokens being created.

32 Aided samples were generally comparable to the unaided sample in the temporal domain. However, there was a difference between the aided and unaided samples at the onset of each speech token, showing the hearing aid processing had affected the rise times of the speech tokens.

33 Compared to the unaided conditions, processing by both hearing aids introduced an overshoot to all samples. Consequently, rise times of all aided samples were reduced compared the unaided samples.

34 Conclusions about the hearing aid processing The three speech tokens retained sufficient speech-like characteristics to be treated as speech and thus amplified by both the Phonak and Oticon hearing aids. Despite the preservation of speech-like characteristics during hearing aid processing, an overshoot was produced for all speech tokens following hearing aid processing, resulting in reduction of rise times for all speech tokens. Changes in rise times differed between the Phonak and Oticon hearing aids and between speech tokens.

35 Conclusions of Part 1 The speech tokens were sufficiently speechlike to be processed as speech by both hearing aids Enabled the possibility of using the stimuli for both objective and behavioural testing of aided hearing

36 Cortical evoked potential testing was undertaken in 9 normal hearing infants Example of an infant CAEP waveform, and three measures of infant CAEP morphology: P1 latency, N250 latency and P1-N250 amplitude.

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40 Amplification of Speech Tokens Spectral analysis of the speech tokens in the unaided and aided conditions showed that both hearing aids faithfully transduced the speech tokens and did not alter their spectra. An increase in gain was seen between the unaided condition and both aided conditions, for all speech tokens, indicating that both hearing aids amplified the speech tokens.

41 Cortical results Repeatable CAEPs were evoked in all participants tested with the speech tokens. CAEP waveforms for all speech tokens and NBN stimuli were variable among participants. there were no statistically significant effects of stimulus frequency or type on P1 and N250 latencies and P1-N250 amplitudes and no significant interactions between stimulus frequency or type

42 Conclusions of Bos Features of modern digital hearing aids such as noise reduction and feedback suppression algorithms are designed to recognise nonspeech stimuli (such as noise or feedback signals) and attenuate them to improve listening comfort (Dillon, 2001). Consequently, acceptance of the speech tokens as speech by hearing aids is an important consideration if the speech tokens are to be used for aided CAEPs. Since the speech tokens were amplified, it appears that they retained sufficient speech-like characteristics to be accepted as speech by the particular hearing aids used in this study.

43 Conclusions continued CAEPs were reliably present in response to all speech tokens and NBN stimuli in all participants who were tested with these stimuli, consistent with previous evidence that CAEPs are reliably present in normally hearing, awake infants (Kurtzberg et al., 1984; Kushnerenko et al., 2002; Novak et al., 1989; Pasman, Rotteveel, Maassen, & Visco, 1999; Purdy et al., 2010; Wunderlich et al., 2006).

44 Clinical use of stimuli in the audiological testing of hearing impaired children Cortical recordings in developmentally delayed children where it has not been possible to obtain behavioural responses to sound Visual response audiometry testing in both aided and unaided conditions

45 7yr old female, /s/ Aided Unaided

46 I yr old male, would not do VRA /sh/ /ah/ Thresholds from ABR

47 ANSD baby identified at birth with a normal hearing twin Left click Right click

48 Mother not concerned about lack of language for first 3 years 3 years later, first reasonably reliable audiogram

49 Unaided corticals ah sh ss

50 Aided testing light to medium hearing aid use

51 4k NBN corticals - sound recover

52 VRA to speech tokens Calibration issues with the speech tokens, speech or tonal? Behavioral audiograms from 9 normal hearing adults to NBN, warble tones and speech tokens

53 Calibration of speech tokens warble NBN SP 1k k k

54 Aided testing 4 year old boy Mild CHARGE syndrome Difficult to test Did not like NBN for VRA, responsive to speech tokens

55 10yr old Down Syndrome child not able to do aided speech testing

56 Concluding thoughts Cortical evoked potentials may not be sufficiently sensitive to very small changes in hearing aid characteristics to guide very fine-tuning, on the basis of current evidence (Carter et al, 2013) However are effective at demonstrating audibility of sounds has been achieved, often this is sufficient to answer the clinical question But they are still dependent upon subject cooperation cannot be obtained every time, some infants or children wont tolerate electrode application or are too restless or sleepy

57 Unanswered questions about the effect of the shortened rise times caused by the hearing aid processing does this result in a clinically significant concern? looks unlikely at this stage

58 VRA to frequency specific speech phonemes increases the stimuli available to audiologists to test young infants and developmentally delayed children who may be disinterested in NBN or warble tone stimuli Functional gain testing is possible with modern hearing aids with automatic signal processing features enabled, which is representative of everyday usage

59 Functional gain has a role as a quick validation of hearing aid settings but does not replace the accuracy that can be obtained with prescription formula approach and Real Ear Measures as a validation of the required amplification.