Audiology Practicum Report: Speech Perception Deficits

Speech Perception Deficits in High Frequency Hearing Loss

High frequency hearing loss is said to occur in persons where the sensory hearing cells get damaged or die due to cochlear or eighth nerve lesion. These hair cells perform the vital functions of translating the sound detected in the form of noise in the ear to electrical impulses that the brain is capable of interpreting as recognizable sound. Basal part of cochlea is involved in perceiving the high frequency sound while the apical region is entitled with the perception of high frequency sounds and this is the reason attributed to the initial hearing loss in the high frequency region prior to the lower frequency region. This sort of sloping hearing loss that leads to progressive impairment to sound perception is also referred to as sensorineural hearing loss. The deficits concerning speech perception for poor readers have been attributed to a speech-specific failure in phonological representation evident through temporal order judgment task that resulted due to high frequency hearing loss (Mody, Studdert-Kennedy & Brady, 1997). Moreover studies have been conducted to explore the speech perception deficits for a person with high frequency sloping hearing loss. Studies have shown that people suffering from sensorineural hearing loss are likely to suffer from diminished ability to comprehend speech in complex acoustic learning environment specifically in presence of background noise. Apart from loss of audibility in such situations, a combination of suprathreshold processing deficits is thought to be associated that include altered basilar membrane compression and related changes along with decreased ability to temporal coding. Findings reflected that older persons are more prone to get affected by background noise at low carrier frequencies due to presence of significant correlation between the tone-in-noise detection and random frequency modulation detection thresholds in noise at 500 Hz (Kortlang, Mauermann & Ewert, 2016). Further studies corroborated with the findings revealing that utilization of temporal fine structure in low frequency region further influence the speech recognition because of steep high frequency hearing loss. Supra-threshold deficits in low frequency region were found to be the resultant effect of the high frequency hearing loss. Comparisons among people with normal hearing to those with sensorineural hearing loss revealed significant intra-group variation and difference with respect to filtering conditions in case of speech recognition with similar temporal fine structure (TFS)- speech stimulus bandwidth. Moreover sentence identification in quiet condition was better in contrast to noisy condition. Poorer sentence recognition was evident in the persons with sensorineural hearing loss, within the frequency regions of normal absolute threshold for both quiet and noisy conditions (Li et al., 2015). Another study evaluated the speech understanding of persons with sloping high frequency hearing impairment for the sake of investigating the hearing loss configuration influencing the contribution of speech information across various frequency regions in the concerned population. High frequency speech information was found to be limited by high frequency hearing impairment. Results suggested that the identified deficits in speech understanding were uniform in nature instead of being frequency specific for persons with high frequency thresholds up to 60-80 dB hearing loss (Hornsby & Ricketts, 2006). Thus the speech perception deficits in persons with high frequency sloping hearing loss is found to get affected and vary under diverse conditions depending upon the auditory threshold and background condition, one is exposed to.

Speech Perception in Cochlear Implants

Cochlear implants are considered as essential tools that cater to the specific needs of an otherwise deaf individual thereby aiding in restoring hearing sensation as well as investigate the processing complex patterns of sensory information by the human central nervous system (CNS). The auditory central nervous system (CNS) is responsible for deriving meaningful speech from a plethora of auditory sensory information.  This speech perception in cochlear implants is dependent upon several factors that require closer attention and insight to better understand the notion associated with such phenomenon (Pisoni & Remez, 2008).

1. Change in insertion depth is a major determinant in speech perception in case of cochlear implants. Presentation of spectral information to electrodes determines the central pattern recognition mechanisms. The limited insertion depths of the implanted electrodes in the normal tonotopic locations interfere with the speech perception and better outcomes are presented in case of basally located electrodes. Differing degree of spectral mismatch across patients are common due to differing electrode insertion depths that in turn accounted for variable performance. Effects on post operative speech perception were assessed by means of insertion depth of cochlear implant. Estimation of insertion depth was conducted by means of assessing the angle of the electrode tip along with the intracochlear length of the electrode. Other components of insertion depth comprised of the number of active electrodes and channels utilized by the speech processor. Angle was identified to be the best depth related predictor of post operative speech perception. The predictive variables associated with the cochlear nuclear complex (CNC) words or CNC phenomes stood for duration of deafness in addition to insertion depth thereby depicting the fact that depth of inserting electrode influence post-operative speech perception (Yukawa et al., 2004).
2. The number of channels of simulation is considered as a function to the study of speech recognition. For the sake of good speech recognition in quiet, a consensus has been reached regarding the adequacy of the six channels of spectral information. Distinct improvement in performance is noted as the number of channels is increased from one to six while no further improvement is noticed as the number of channels is increased beyond six. A prospective study corroborated with this finding when compared against acoustic hearing ability. Cochlear implant (CI) listeners exhibiting the highest performance level further showed improved performance as the number of simulating channels increased to seven. On the contrary CI listeners having poorer speech perception did not reveal improved performance as the number of channels rose above four. An important insight of the study helped to highlight on the fact that majority of the CI users are unable to completely utilize the spectral information rendered by the number of electrodes used in their implants (Friesen et al., 2001).
3. The increase in the number of channels is not always suggestive of the better speech perception as various studies have suggested.  Study has shown that in case of all speech measures, performance improved remarkably on application of one to four channels. However an asymptomatic stage was attained on application of seven electrodes. No stark differences were noted for variable number of simulating channels beyond six and performance did not differ between 7, 10 and 20 electrodes (Shannon et al., 2004).

Auditory neuropathy (AN) is recognized as a hearing disorder in which the inner ear receives the sound normally, but the transmission of sound from inner ear to the brain is impaired. Persons suffering from this condition are often reported to encounter normal hearing and in cases they may suffer from hearing loss ranging from mild to severe. The condition is characterized by the presence of normal cochlear function even though the auditory nerve suffers damage or exhibits faulty function (Moore, 1987). An isolated auditory nerve disorder or a generalized peripheral neuropathy may be the contributing factor leading to such condition in which people face difficulty in understanding speech clearly thereby accounting for poor speech perception abilities. The degree of hearing loss may not always accurately predict the speech perception difficulties. A person affected by auditory neuropathy may be able to hear but simultaneously the problem of recognizing spoken words might persist. Sounds are reported to fade in and out in these persons culminating in the generation of the feeling of out of sync (Sininger & Starr, 2001). Studies related to speech perception in individuals affected by auditory neuropathy systematically analyzed the role of noise in the given condition and the utility of cochlear implants to alleviate the problem. Findings revealed that in contrast to the normal hearing, cochlear implant and cochlear impaired controls, the auditory neuropathy affected individuals performed worse in terms of speech recognition in noisy environment. In case of the persons affected by auditory neuropathy, evaluation regarding the intelligibility of clear speech in comparison to the conversational speech was assessed that revealed a clear speech advantage related to intelligibility for all listening conditions as well as for the diverse stimulation modes. Improved temporal features in clear speech coupled with enhanced neural synchrony due to electrical stimulation accounted for the procurement of clear speech advantage in the affected AN patients. Further the study highlighted the feasibility of using cochlear implants in order to improve speech perception in noise (Zeng & Liu, 2006).

The causes behind the speech perception deficits in AN has been explored by many researchers over the years. The reasons attributed may be multifaceted and include the damage caused in the inner hair cells that are regarded as specialized cells located in the inner ear and are responsible for transmitting acoustic information via the neural pathway to the brain. Faulty connections between the inner hair cells ns the nerve responsible for establishing connection between the inner ear and the brain or damage caused to the nerve itself might lead to the pathogenesis of this condition. The amplification function of the inner ear remain intact in AN while the normal synchronous activity in the inner ear gets affected. Impairments are particularly prominent in the temporal processing abilities that lead to poor speech perception abilities in the affected population (Zeng et al., 1999). Sources of other evidences suggested that desynchronized or diminished neural activity or a combination of both leads to development of condition like that of AN. Timing related perceptions including the pitch discrimination at low frequencies, gap detection, temporal integration, temporal modulation detection, signal detection in noise and others are severely impaired. The major contribution is offered by the neural synchrony with regards to the sensory perception that is evident through reduced discharge of the auditory nerve (Zeng et al., 2005).

Speech Perception Deficits in Auditory Neuropathy

Categorical perception refers to the process that enables a person to segregate the various sounds present across a continuous range into discrete categories. In this type of perception, stimuli are better distinguished upon identification of belonging to different categories and are considered as a product of natural consequence. Reflection on the operation of a special speech decoder is offered by the categorical perception alongside occurrence of non-speech signals. Empirical research has shown that the linguistic background do exert an influence on the categorical perception. It has been shown that when beginners learn new language they tend to categorize and assimilate the new sounds in closest approximation to a native sound that in turn may be referred to as the linguistic experience. Both the phonetic as well as the phonological properties of the first language or mother tongue limits the linguistic experience. Categorical and contrastive nature of the phonological attributes along with the gradient and within-category nature of the phonetic attributes together account for the linguistic experience (Moore, 2012). Further researches have brought to the forefront that categorical perception includes perception of the acoustic cues and the manner of articulation. The Perceptual Assimilation Model put forward by Best in the year 1995 helps to understand this influence of linguistic background upon categorical perception that states the perceptual tendency of the humans to assimilate the non-native phonemes in their own phonemic inventory for better understanding. The three probable classifications that has been hypothesized in this respect include the categorized paradigm of some native phoneme in which the applicability may range from excellent to poor, uncategorized vowels or consonants that exist somewhere in between the native phonemes and finally the non- assimilable sound that shares no detectable congruity with any of the native phonemes (Raphael, Borden & Harris, 2007). A recent study strived to throw light upon the effects of linguistic experience and stimulus context in relation to neural organization and categorical perception of speech in the English and Chinese speakers. The findings suggested that tonal contexts impact the encoding with reference to the lexical tones. Multidimensional scaling along with clustering of auditory source responses was utilized in the process. As per the behavioral outcomes, the native Chinese were expressed to show stronger categorical perception through exhibition of more psychometric functions as well as rapid identification of linguistic pitch pattern when compared against the native English speaking control. The neurometric and psychometric identification purposes in case of the Chinese were found to be more closely associated thereby suggesting the vital role of the language experience (Bidelman & Lee, 2015).

References

Bidelman, G. M., & Lee, C. C. (2015). Effects of language experience and stimulus context on the neural organization and categorical perception of speech. Neuroimage, 120, 191-200.

Friesen, L. M., Shannon, R. V., Baskent, D., & Wang, X. (2001). Speech recognition in noise as a function of the number of spectral channels: comparison of acoustic hearing and cochlear implants. The Journal of the Acoustical Society of America, 110(2), 1150-1163.

Hornsby, B. W., & Ricketts, T. A. (2006). The effects of hearing loss on the contribution of high-and low-frequency speech information to speech understanding. II. Sloping hearing loss a. The Journal of the Acoustical Society of America, 119(3), 1752-1763.

Kortlang, S., Mauermann, M., & Ewert, S. D. (2016). Suprathreshold auditory processing deficits in noise: Effects of hearing loss and age. Hearing research, 331, 27-40.

Li, Bei, Limin Hou, Li Xu, Hui Wang, Guang Yang, Shankai Yin, and Yanmei Feng (2015). “Effects of steep high-frequency hearing loss on speech recognition using temporal fine structure in low-frequency region.” Hearing research, 326, 66-74.

Mody, M., Studdert-Kennedy, M., & Brady, S. (1997). Speech perception deficits in poor readers: Auditory processing or phonological coding?. Journal of experimental child psychology, 64(2), 199-231.

Moore, B. C. (1987). Psychophysics of normal and impaired hearing. British medical bulletin, 43(4), 887-908.

Moore, B. C. (2012). An introduction to the psychology of hearing. Brill.

Pisoni, D., & Remez, R. (2008). The handbook of speech perception. John Wiley & Sons.

Raphael, L. J., Borden, G. J., & Harris, K. S. (2007). Speech science primer: Physiology, acoustics, and perception of speech. Lippincott Williams & Wilkins.

Shannon, R. V., Fu, Q. J., Galvin, J., & Friesen, L. (2004). Speech perception with cochlear implants. In Cochlear implants: Auditory prostheses and electric hearing (pp. 334-376). Springer New York.

Sininger, Y., & Starr, A. (2001). Auditory neuropathy: A new perspective on hearing disorders. Cengage Learning.

Yukawa, K., Cohen, L., Blamey, P., Pyman, B., Tungvachirakul, V., & O’Leary, S. (2004). Effects of insertion depth of cochlear implant electrodes upon speech perception. Audiology and Neurotology, 9(3), 163-172.

Zeng, F. G., & Liu, S. (2006). Speech perception in individuals with auditory neuropathy. Journal of Speech, Language, and Hearing Research, 49(2), 367-380.

Zeng, F. G., Kong, Y. Y., Michalewski, H. J., & Starr, A. (2005). Perceptual consequences of disrupted auditory nerve activity. Journal of Neurophysiology, 93(6), 3050-3063.

Zeng, F. G., Oba, S., Garde, S., Sininger, Y., & Starr, A. (1999). Temporal and speech processing deficits in auditory neuropathy. Neuroreport, 10(16), 3429-3435.