by Vasiliki (Vivian) Iliadou, Aristotle University of Thessaloniki
The importance of the sense of hearing as the main vehicle of information transmission during language acquisition, learning and communication is widely accepted. As hearing extends beyond hearing sensitivity (acuity) measured by an audiogram (Jerger & Musiek, 2000; Iliadou, Chermak, Bamiou, 2015), auditory processing evaluation provides more ecological information on functional everyday hearing. An audiogram measures a person’s ability to recognise very simple sinusoidal tones in an artificially sound-treated environment. This is a simplistic measurement of hearing as individuals depend on hearing complex sounds (i.e. human speech) in an acoustically difficult environment (competing sounds).
The link between Hearing and Cognition is evident in studies of hearing loss. Children with severe sensorineural hearing will typically score low on cognition tests and intelligence measurements, if they are not accommodated accordingly and if their hearing impairment is not treated both effectively and early enough. Early cochlear implantation results in better cognitive and emotional skills leading to the conclusion that early hearing loss-deafness treatment can provide a foundation for a more normalized theory of mind development (Sundqvist, Lyxell, Jönsson, & Heimann 2014). Unilateral hearing loss when left untreated in childhood is recently found to be linked with school failure (Lieu 2013) and decline in language test scores (Fischer & Lieu 2014). The presence of two related deficits associated with unilateral hearing loss: (1) phonological deficit and (2) impaired executive control function may explain the educational difficulties exhibited by children with unilateral hearing loss. Adults with hearing loss may be diagnosed as having Mild Cognitive Impairment (MCI) if their hearing loss is not known, their hearing is not tested prior to the MCI assessment and when provided and fitted with a hearing aid their results on cognition may be within normal limits. Elderly population studies show an independent association between peripheral hearing impairment and reduced neurocognitive performance (Lin, F.R., 2011, Gussekloo, de Craen et al., 2005). This association is linked with accelerated rates of cognitive decline (Lin, F.R., Yaffe, K., et al., 2013) and increased risk of dementia (Gallacher, J., Ilubaera, V., et al., 2012). A recent study (Linn et al 2014) demonstrated accelerated decline in both temporal lobe regional volumes and whole brain volume.
Auditory perception is formulated by hearing sensitivity, as measured by pure tone audiometry, and by auditory processing. Auditory processing includes elements of perceiving speech in noise and temporal resolution, which may be associated with phonological skills and prosody. When referring to auditory perception one important everyday skill is speech perception. This skill is known to be related with cognition (Akeroyd 2008; Gatehouse, Naylor & Elberling 2006, Rönnberg 2003). Specifically the performance on speech perception is linked with working verbal memory measurements (Arlinger, Lunner, Lyxell & Pechora-Fuller, 2009). The impact of hearing loss in speech perception is associated with increased need of cognitive processing capacity (McCoy et al 2005) due to brain resources reallocation for auditory processing. Other possible explanations include shared neuropathology simultaneously influencing both hearing and cognition and social isolation as a result of hearing loss leading to cognitive decline (Barnes, Mendes de Leon et al., 2004; Bennett, Schneider et al.,2006). Temporal resolution refers to the ability to perceive time-related changes in the auditory stimuli. Testing temporal resolution is commonly implemented by the use of gap detection tests. The ability tested is that of the perception of a short period of absence of acoustic energy during a stimulus. Older adults are typically found to have worse gap detection performance with poorer (longer) thresholds than younger adults (Lister, Roberts, & Lister, 2011). Impaired temporal acuity is reported in the elderly with normal hearing or hearing loss (Moore et al. 1992, Snell & Frisina 2000).
The finding of higher incidence (46.7% as opposed to 4.6%) of Central Auditory Processing Disorder (CAPD) in Framingham longitudinal population cohort in subjects that developed Alzheimer’s Disease a decade later in life is intriguing and may have an impact on early diagnosis of AD based on CAP testing. Findings of cognitive deficits or decline in individual’s throughout the age span who are being diagnosed with Auditory Processing Disorder are usually met with scepticism on the diagnostic validity of the auditory processing skills and on the APD nature as a whole. In a few studies it may be attributed to shared neuropathology leading to both APD and Cognition decline or deficit. It is rarely the case that the possibility of APD leading to Cognition decline is taken into consideration. This article points to this possibility in auditory processing testing interpretation. When APD and Cognitive deficits co-exist the option of APD contributing to cognitive decline should not be overlooked considering the data presented in this article.
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