Development of filtered speech test in Malayalam: A Comparison between monolinguals (Malayalam) and bilinguals (Malayalam and English)Hearing is a complex process that is often taken for granted. The sounds undergo a series of transformations as sounds strike the eardrum and the acoustic signals are changed into neural signals. These neural signals are then passed from the ear through complicated neural networks to various parts of the brain for additional analysis, and ultimately, recognition or comprehension. The CANS is a highly complex system of neural pathways that processes neural Information received from both ears, and plays an important role in certain aspects of hearing Such as sound localization and extracting auditory signals from background noise.
Normal functioning of the CANS can be affected by a number of developmental and pathological Conditions from various locations in the auditory system. APD refers to limitations in the ongoing transmission, analysis, organization, transformation, elaboration, storage, retrieval, and use of information contained in audible signals” (ASHA, 1992). ASHA formally reconvened on the topic again in 1996, creating Task Force on Central Auditory Processing Consensus Development.
This group identified central auditory process as the auditory system mechanisms and processes responsible for the following behaviors: • Sound localization and lateralization, or ability to know where sound has occurred in space • Auditory discrimination, or ability to distinguish one sound from another • Auditory pattern recognition, or ability to determine similarities and differences in patterns of sounds • Temporal aspects, or abilities to sequence sounds, integrate a sequence of sounds into meaningful combinations, and perceive sounds as separate when they quickly follow one another.• Auditory performance decrements, or ability to perceive speech or other sounds when another signal is present • Auditory performance with degraded acoustic signals, or ability to perceive a signal in which some of the information is missing. Aspects of auditory processing which are impaired in individuals with APD include temporal, spectral and binaural hearing, and also the ability to group and order sounds (Chermak & Musiek, 1997; Bellis, 2002; Moore, 2006). Individuals that are suspected to have APD frequently exhibit misunderstanding of messages, delayed, inconsistent or inappropriate responses when communicating, difficulties with sound localization, difficulty with understanding speech in adverse listening environments, learning difficulties, an inability to ignore irrelevant background noise, and difficulty following complex auditory instructions (ASHA, 2005). Keith summarizes the more commonly reported characteristics of APD in children, these include:? Demonstrates normal-hearing sensitivity but with signi?cant history of prolonged middle ear disease (e.g.
otitis media). ? Responds inconsistently and inappropriately to auditory stimuli or occasions, but, at other times, follows auditory instructions in a normal manner. ? Dif?culty with localizing sounds, including the inability to tell the distance of the sound source, or the inability to differentiate between soft and loud sounds. ? Dif?culty in discriminating between different sounds. ? Dif?culty understanding speech in the presence of background noise. ? Dif?culty with auditory memory, either span or sequence, and poor ability to follow multiple instructions.
? Demonstrates poor listening skills and shows decreased attention for auditory information, distractibility or restlessness while listening in dif?cult listening situations.? Dif?culty understanding rapid speech or individuals with an unfamiliar dialect. Not every child suspected of APD will exhibit all of the characteristics mentioned above. The severity of characteristics, ranging from mild to severe, is unique to each child. There are many adult populations that may also experience CAPD. Changes in structure and function occur throughout the peripheral and central auditory nervous systems as a result of the aging process.
Neural degeneration has been found in the auditory nerve, brainstem, and cortex. Whereas the effect of structural change in the auditory periphery is to attenuate and distort incoming sounds, the major effect of structural change in the central auditory nervous system is the degradation of auditory processing. While CAPD is most commonly identified in individuals with normal hearing, individuals with sensorineural (nerve-related) hearing loss can also experience CAPD.
An individual who does poorly on auditory tests even with amplification may have a central component to their disability. Significant word-discrimination difficulty, when serious pathology has been ruled out, may also signal a processing deficit. Auditory closure is one of the ability of normal listener to utilize intrinsic and extrinsic redundancy to fill in missing or distorted portion of the auditory signal and recognize the whole message.
It plays an important role in the everyday listening activities like background noise, regional dialects, conversation partners who speak with quiet voices etc. Ability of auditory closure rely upon extrinsic and intrinsic redundancy of the signal that allows us to comprehend the overall message and engage in meaningful conversation. Intrinsic CANS redundancy is based on the multiplicity of neural pathways, centers, and decussations; interrelatedness of these pathways, centers, and decussations; and bilateral representation of the auditory system. Aspects of the auditory signal such as the frequency range, sound duration, context in which the message is given, rhythm, and the individual’s familiarity with the semantic, syntactic and phonologic rules of language affect the extrinsic redundancy. Intrinsic redundancy is reduced by CANS lesions.
Extrinsic redundancy of speech can be reduced by various means of degradation such as filtering, time alteration, or noise. (Auditory diagnosis, Silman and Silverman).