The relationship between cognition and age-related hearing loss continues to fascinate researchers and professionals in audiology and psychology. Different theories have been proposed in attempts to understand the relationship between sensory functions and cognition with the goal of developing effective interventions to combat the negative effects of aging. These recent studies provide a summary of notable theories and supporting empirical data.
Uchida and colleagues conducted a review of the theories linking hearing loss and cognitive decline (Auris Nasus Larynx. 2018 Aug 31. pii: S0385):
Cognitive Load Theory. Cognitive load is broadly defined as the amount of cognitive or information processing effort required by a person to perform a task. This theory postulates that cognitive load increases in environments with degraded speech or high masking levels because individuals would require significantly greater cognitive efforts to understand speech (primary task) and, therefore, have less cognitive ability to process or inhibit other information (secondary task). It further posits that excessive cognitive overload causes degenerative structural changes in the brain and cognitive decline.
Common Cause Theory. This theory assumes that an age-related common factor causes both cognitive and sensory declines. Possible common factors include, but are not limited to, genetics, stria vascularis degeneration, neural degeneration in the peripheral and/or central nervous systems, cardiovascular/cerebrovascular diseases, oxidative stress, or general physical health. As the common cause(s) affects both sensory and general cognitive functions, intervention to compensate for sensory loss (e.g., amplification for people with hearing loss) would not improve cognitive functions. Sensory loss, however, can be a predictor for cognitive decline and vice versa.
Cascade Theory. This theory proposes that hearing loss triggers a series of negative events that lead to cognitive hearing loss, which makes daily communications difficult and sub-sequently reduces the individuals’ socialization and causes psychosocial and health problems and, finally, cognitive decline. This use-it-or-lose-it theory implies a causal effect between hearing loss and cognitive decline such that a negative influence of brain plasticity (i.e., sensory deficits) reduces neural inputs to the brain and leads to neural atrophy, which eventually results in cognitive decline.
Overdiagnosis or Harbinger Theory. This theory suggests that overdiagnosis is the reason for the association between hearing loss and cognitive function because many cognitive tests are conducted verbally. If the instructions are not audible to individuals with hearing loss, they may get lower test scores and appear to have a cognitive deficit because they do not fully understand the instructions. Alternatively, someone with low cognitive function may not fully understand the instructions for a hearing test, resulting in worse tested hearing thresholds.
SUMMARY OF EMPIRICAL DATA
Uchida and colleagues proceeded to summarize the findings of several randomized controlled trials and studies with a large number of participants. They reported inconsistent results, but did not give conclusions on which theory reasonably explains the relationship between hearing abilities, amplification, and cognitive functions. This integrative summary encapsulates the findings of empirical studies to date:
- The Cognitive Load Theory seems to provide a reasonable explanation for why hearing aids and cochlear implants enhance listeners’ working memory and executive function (Clin Interv Aging. 2018 Oct 13). Specifically, if listeners spend less effort on performing the primary task (e.g., understanding speech), they would have more capacity to remember the information, inhibit non-targeted information, or perform the secondary task quickly (J Speech Lang Hear Res. 2009 Oct;52(5):123). Evidence supporting increased cognitive load causing cognitive decline, however, is lacking. Also, it does not explain why improving hearing abilities does not improve general cognition.
- Findings of studies on hearing abilities and general cognitive function are consistent with the Common Cause Theory, which assumes that an underlying factors(s) affects both sensory and cognitive abilities. As hearing loss does not cause cognitive decline, or vice versa, interventions to compensate hearing loss do not slow down or enhance users’ general cognitive function. Some evidence are discussed below.
- Two seminal studies provided strong support for a specific application of the Cascade Theory (i.e., the use-it-or-lose-it theory). Silman and colleagues examined clinical records of two groups of patients with binaural sensorineural hearing loss and compared their hearing thresholds and speech recognition scores four to five years after the patients were fitted with binaural or monaural hearing aids (J Acoust Soc Am. 1984;76(5):1357). Although there was no change in hearing thresholds for both ears nor in the speech recognition scores of the aided ears, the unaided ears yielded significantly lower speech recognition scores, illustrating the negative effects of auditory deprivation. The speech recognition scores of formerly unaided ears, however, never rebounded to the same level as those of the aided ears four years after the binaural fitting (J Am Acad Audiol. 1990; 1:175). Additionally, studies showing individuals with hearing loss had lower whole brain and gray matter volume compared with those of individuals with normal hearing have been cited as evidence to support the Cascade Theory (Trends Neurosci. 2016:39(7):486). Nevertheless, it is also possible that an underlying cause induced neural atrophy, which caused both sensory and cognitive declines.Additionally, if the Cascade Theory were sound in explaining the relationship between hearing loss and cognition,
- longitudinal studies starting with a hearing aid group and an unaided group should have consistently reported higher general cognitive function for the hearing aid group, which experienced much less auditory deprivation than the other, and
- studies that tested participant performance before and after amplification should have reported a slowing down of general cognitive decline or a rebound on cognition abilities because neuroplasticity would have allowed the reversal of cognitive decline after the sensory deficit is compensated.
- Yet, longitudinal studies have not consistently shown that amplification could slow down general cognitive decline or enhance cognitive functions (Int J Audiol. 2015;54(11):838; J Am Geriatr Soc 2015;63(10):2099).
- The Overdiagnosis Theory can be true in some cases as studies have shown that people with hearing loss are more likely to earn scores indicative of cognitive deficit, and vice versa. This phenomenon is mainly attributed to participants’ difficulties in understanding instructions for hearing or cognitive tests. These findings emphasized the need for clinicians to be mindful of the hearing needs of their patients and make sure they understand test instructions. While it is possible that some individuals are misdiagnosed, it is unlikely that all cases are due to overdiagnosis. A recent study showed that individuals with hearing loss obtained similar cognitive test scores in both unaided and aided conditions (J Am Acad Audiol. 2018;29:648).Further, a growing number of studies showed that the reduction in sensory abilities is not specific to auditory perception. Studies on the visual and vestibular system of older adults also reported comorbid deficits with cognitive declines (Ear Hear. 2016; 37(1):52S; J Gerontol A Biol Sci Med Sci. 2016 Feb;71(2):243). While it is possible that the participants in these studies did not understand the instructions for the visual/vestibular tests, it is equally possible that a common cause(s) negatively affects multiple sensory systems and cognitive functions.
Different theories can explain certain aspects of the relationship between hearing and cognition functions. The general belief about sensory and general cognitive functions is consistent with the Common Cause Theory.
These studies have identified possible underlying causes thresholds and cognitive abilities of C57BL/6J mice at 3, 6, and 15 months old (Mol Med Rep. 2018 Aug;18(2):1726). These mice started to develop hearing loss at 6 months old, and the 15-month-old mice had significantly lower cognitive abilities compared with the 3- and 6-month-olds. The older mice also had significantly lower cell and synapse numbers and elevated matrix metalloproteinase (MMP)-9 protein expression in the auditory cortex and hippocampus. The actions of MMPs affect cell migration, differentiation, growth, inflammatory processes, neovascularization, apoptosis, among others.
Muri and colleagues reported that the use of antibiotics and MMP inhibitors was able to preserve learning, memory, and hearing on pediatric mice with pneumococcal meningitis compared with controls that experienced significant learning, cognitive, and hearing disabilities (J Neuroinflammation. 2018 Aug 21;15(1):233). Post-mortem results indicated that MMP inhibitor significantly reduced apoptosis in the hippocampus and the antibiotics reduced cortical necrosis. Both studies support the possibility of MMP overexpression being one of the bad actors contributing to the reduction of hearing sensitivity and cognitive decline.