ALL INDIA INSTITUTE OF SPEECH AND HEARING,MYSORE-6 National Symposium on Acoustics : Mysore 2014 NEURAL REPRESENTATION OF DIFFERENT SPEECH STIMULI AT CORTEX IN INDIVIDUALS WITH NORMAL HEARING   Presenter Himanshu Kumar Sanju ALL INDIA INSTITUTE OF SPEECH AND HEARING,MYSORE-6

CAEPs reflect maturation of the human brain through Cortical auditory evoked potentials (CAEPs) are - non-invasive measures - reflects cortical maturation. CAEPs reflect maturation of the human brain through - changes in their latency - amplitude - morphology (Eggermont,1989;Sharma,Dorman & Spahr, 2002;Pang & Taylor, 2000)

CAEPs can also be elicited -using speech stimuli Maturation is likely to have impact on speech and oral language skills, which are primarily acquired through auditory modality. CAEPs can also be elicited -using speech stimuli - help in quantifying the central auditory system Kraus, McGee, Micco,Sharma,Carrell, & Nicol(1993). CAEPs are sensitive tool for evaluating the - audibility of speech sounds in children with hearing impairment. Golding, Purdy,Sharma,Dillon(2006); Sharma, Dorman & Spahr, (2002);Carter,Dillon, Seymour, Seeto,Van(2013).

Need for the study There is a dearth of information - to explore speech evoked CAEPs with different speech stimuli in children and adults with normal hearing. - change in neural representation of different speech stimuli

Aim of the study To find out neural representation of different speech stimuli (/m/,/g/ & /t/) in children and adults with normal hearing.

Method

20 Subjects 10 Children (5-7 Years) 10 Adults (17-24 years)

Participants Selection Criteria Hearing sensitivity within normal limits (≤ 15 dB HL) in both ears. Normal middle ear function as indicated by Immittance evaluation. Not having any history of otologic, neurologic problems and illness on the day of testing.

250 to 8000 Hz for air conduction 250 to 4000 Hz for bone conduction Procedure 250 to 8000 Hz for air conduction Pure Tone Audiometry 250 to 4000 Hz for bone conduction

Immittance Evaluation Tympanometry with 226Hz Immittance Evaluation Reflexomatry Ipsilateral and contralateral acoustic reflex thresholds was measured at 500Hz, 1000Hz, 2000Hz and 4000 Hz.

To verify normal hearing sensitivity of the participants & Click evoked ABR To verify normal hearing sensitivity of the participants & to rule out RCP

Speech evoked cortical potential recording HEARlab (version 1.0) evoked potential system

Speech evoked cortical potential Parameters Speech evoked cortical potential Stimulus /m/ (30 ms ) , /g/ (30ms) /t/ (30 ms) Electrode Placement Reference: M1/M2 Active: Cz Ground: Fz Intensity 65 dB SPL Transducer Loudspeaker Transducer Position 0 degree azimuth Ear Binaural Polarity Alternating Filter setting 1-30 Hz Repetition rate 1.1/sec Total no. of sweeps 200 Impedance < 5 kΩ No. of Channels One Analysis Time 500 ms

Explore speech evoked CAEPs with speech stimuli (/m/, /g/, and /t/) which have a spectral emphasis in LF, MF and HF respectively.

Figure: 3rd octave power spectra for speech sounds /m/, /g/ and /t/ with overall levels normalized at 65 dB SPL

/m/ /g/ /t/ A sample waveform of cortical potentials for speech stimuli (/m/, /g/ & /t/) in children with normal hearing (Red color- /m/; Green color- /g/; Blue color- /t/ speech stimuli).

A sample waveform of cortical potentials for speech stimuli (/m/, /g/ & /t/) adults with normal hearing (Red color- /m/; Green color- /g/; Blue color- /t/ speech stimuli).

Result & Discussion

Descriptive Statistics Mann Whitney U test Non-parametric Test Kruskal Wallis test

P1 Latency N1 Latency Error bar for latency measures of wave P1 and N1 in children and adults.

P1 Amplitude N1 Amplitude Error bar for amplitude measures of wave P1 and N1 in children and adults.

Mann-Whitney U test : Between group comparison (children and adults) Latency Significant differences between children and adult for wave P1 (Z = - 6.93, p <0.05) and wave N1 (Z = - 6.42, p<0.05) Amplitude Significant difference between children and adult for wave P1 (Z = - 4.88, p<0.05) and wave N1 (Z = - 4.34, p<0.05).

Kruskal Wallis test: To compare differences across different speech stimuli in children as well as adults Children No significant differences across stimuli (/m/, /g/ and /t/) Latency measures of wave P1 (ᵡ2= 0.485, df=2, p>0.05) and wave N1 (ᵡ2= 2.56, df=2, p>0.05). Amplitude measures of wave P1 (ᵡ2= 0.589, df = 2, p>0.05) and wave N1 (ᵡ2= 0.143, df=2, p>0.05). Adults No significant difference across stimuli for Latency measures of wave P1 (ᵡ2= 0.356, df=2, p>0.05) and wave N1 (ᵡ2= 5.145, df=2, p>0.05). Amplitude measures of wave P1 (ᵡ2= 0.218, df=2, p>0.05) and wave N1 (ᵡ2= 1.577, df=2, p>0.05).

The present study shows robust wave P1 and N1 responses from both children and adults with normal hearing. In adults though later peaks (wave P2 & N2) were notice it was not mentioned due to lack of occurrences of later peaks in children.

Statistically significant differences between children and adults for latency and amplitude measures of wave P1 and N1 at 0.05 levels. Because CAEPs reflect maturational changes of the human brain in terms of the changes in their latency, amplitude and morphology (Eggermont, 1989; Gilley et al., 2005; Fox et al., 2010).

Another major finding of the present study was no statistically significant difference across different speech stimuli in both children as well as in adults. The above finding is in agreement with other reported literature (Dun, Carter & Dillon, 2012; Munro, Purdy, Ahmed, Begum & Dillon, 2011s

A study done by Munro et al A study done by Munro et al. (2011) on adult listener reported that the there is no differences in latency and amplitude measures across different speech stimuli. A study done by Garinis and Cone (2007) also reported no significant difference in CAEP waveform for latency and amplitude measures using /sa/ and /da/ as speech stimuli. However, the above finding is in contrast with Golding et al (2006) study, they reported /t/ speech stimuli evoked significantly larger in amplitude and earlier in latency than for the other two speech stimuli (/m/ & /g/).

To conclude, differentiation of CAEP responses between speech stimuli, was not being observed while measuring cortical potential as synthesized short duration speech stimuli was used and there was greater spread in amplitude and latency in individuals with normal hearing As far field recording of CAEP causes blurred spatial distribution compared with potential at the source which reduces the temporal and spectral difference between stimuli. However, present study finding should be considered with caution and requires validation on large sample size.

Conclusion The present study clearly shows that due to maturational changes there are differences between children and adults cortical potential responses in terms of latency and amplitude measures. Further, the neural representations of different speech stimuli are shown to be alike in children and adults with normal hearing. There is need for alternative objective electrophysiological measure of speech discrimination in individuals. Probably present study outcomes can help researchers to consider as a reference while performing cortical potential measures in clinical populations like individual with hearing impairment.

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Acknowledgement Director of AIISH, Mysore Organizers of NSA 2014 Participants of our study

Discussion