The effects of cognitive functions on audition continue to be at the center of discussion in the hearing community. Cognition is a general term for the conscious mental activities involving thinking, understanding, learning, and remembering. It also includes processes such as knowledge, memory, language, perception, comprehension, concept formation, mental imagery, reasoning, judgment and evaluation, pattern recognition, decision-making, problem solving, computation, and action.
In auditory research, many studies have examined the relationship between various aspects of cognitive functions and auditory performance, for example, the effects of:
- global cognitive functions on speech understanding and hearing aid compression time constants (Foo. J AM Acad Audiolo 2007;18:618o http://bit.ly/2c5K3WR)
- working memory on speech understanding in noise (Humes. J Speech Hear Res 1994;37:465 http://bit.ly/2bLwTPS; Humes. J Acoust Soc Am 2002;112[3 Pt 1]:1112 http://bit.ly/2cpTQYF; Humes. J Speech Lang Hear Res 2007;50:283 http://bit.ly/2cpTGAL) and perceived listening effort spent on understanding speech in noise (Rudner. J Am Acad Audiol 2012;23(8):577 http://bit.ly/2c33eAn);
- hearing aid noise reduction algorithms on cognitive load (Sarampalis. J Speech Lang Hear Res 2009;52:1230 http://bit.ly/2bLyaGH); and
- tinnitus on executive control of attention (Tegg-Quinn. Int J Audiol 2016:1 http://bit.ly/2cjKJ8D).
HEARING LOSS AND COGNITIVE DECLINE
Aging is found to be associated with declines in both cognitive functions and hearing loss (Lin. JAMA Intern Med 2013;25;173:293 http://bit.ly/2beIROu). Hearing loss, in turn, is associated with higher rate of cognitive decline, dementia, depression, and brain atrophy (Lin. Neuroimage 2014;90:84 http://bit.ly/2bOLvMP; Mener. J Am Geriatr Soc 2013;61:1627 http://bit.ly/2beIDGV). Researchers also warn of a bidirectional interaction between hearing aid and cognitive impairment. People with hearing loss tend to exhibit lower cognitive scores than people with either normal hearing or when hearing loss is compensated. People with lower cognitive scores tend to have higher degrees of hearing loss that may be a result of poorer comprehension of test instructions.
Despite the association, there is no evidence supporting a causal relationship between hearing loss and global cognitive function or cognitive decline (Lin. Arch Neurol 2011;63:214 http://bit.ly/2c38IeL; Mulrow. J Speech Hear Res 1992;35:1402 http://bit.ly/2cmiR7X; Allen. Age Ageing 2003;32:189 http://bit.ly/2cq0fTw; Dawes. PLoS One 2015;10 http://bit.ly/2bVgUM7). The general belief is that both cognitive decline and hearing loss are the consequences of a “common cause.” Possible culprits are microvascular conditions or alterations in the nervous system (Fortunato. Acta Otorhinolaryngol Ital 2016;36:155 http://bit.ly/2c1uxIB; Dawes. PLoS One 2015 http://bit.ly/2bVgUM7).
The lack of causal relationship between hearing sensitivity and global cognitive function is further demonstrated in the limited association between amplification and cognition. A cochlear implant study showed that cochlear implantation combined with speech and cognitive rehabilitation training improved the cognitive functions of cochlear implant users (Mosnier. JAMA Otolaryngol Head Neck Surg 2015;141:442 http://bit.ly/2cz7ERV). It is, however, unsure whether the improvement was due to cochlear implantation or cognitive training.
Additionally, some studies reported hearing aids improved specific cognition subscales, such as working memory or cognitive load, if the participants were tested with auditory-dependent tasks (Baltes. Psychol Aging 1997;12:12 http://bit.ly/2c3OPmD). Measures on global cognition functions often showed hearing aid use did not improve cognitive function or slow down the cognitive decline in longitudinal studies.
A CLOSER LOOK AT HEALTH RECORDS
Health records of 4,541 people between the ages of 48 and 92 years living in Beaver Dam, Wisconsin were examined (Dawes. Int J Audiol 2015;54:838 http://bit.ly/2cdzWj9). The health records included those of 666 people with average thresholds at 3 and 4 kHz greater than 40 dB in the better ear but reported no hearing aid use at the time of the study to examine the association between hearing aid use and mental health, social engagement, cognitive function, and physical health over 11 years.
Participants were divided into new hearing aid user group (n=69) and non-user group (n=597) at the baseline. The cognitive function was measured using Mini Mental State Exam (MMSE—global cognitive function measure), trail making, auditory verbal learning, digit-symbol substitution, verbal fluency, and incidence of cognitive impairment. Their hearing handicap, physical health, social engagement, and mental health were also monitored. The tests were then repeated at five and 11 years thereafter.
Results showed that:
- Hearing aid users generally exhibited higher perceived hearing handicap than non-users.
- Hearing aid use had no significant effect on the participants’ cognitive function, physical health, mental health, or social engagements after adjusting for age, gender, and average hearing loss at the baseline, five-year, and 11-year measurements.
- At the 11th year check point, hearing aid users had better scores than non-users in one of the physical health measures, but no significant difference between the hearing aid users and non-users in morbidity rates.
- The conclusions were similar after excluding any non-users who adopted hearing aids (n=91) or any hearing aid users who stopped using hearing aids (n=7).
IMPACT OF HEARING AIDS
Amieva and colleagues examined a total of 3,670 participants aged 65 and older (J Am Geriatr Soc 2015;63:2099 http://bit.ly/2csBRkc). The participants self-reported to have normal hearing (2,394), moderate hearing problems (n=1,139), or major hearing loss (1,139). They were evaluated in their homes at the initial visit and at 1, 3, 5, 8, 10, 13, 15, 17, 20, and 25 years. During the initial visit, a psychologist conducted a neuropsychological evaluation and developed a criteria checklist for dementia. After that, a neurologist either accepted or rejected the diagnosis of those participants who met the criteria. The inclusion of the participant in the study was then confirmed by a panel of neurologists. At each follow-up visit, MMSE and four of eight subscales of Instrumental Activity of Daily Living (telephone use, transportation, medication, and domestic finance) were used to examine participant's global cognitive performance and specific cognitive abilities.
At the baseline, those who have hearing loss are more likely to be less educated males, exhibit more depression symptoms and comorbidities, and show a higher level of dependency than those with normal hearing. Participants with major hearing loss tend to have greater difficulties tracking conversations in background noise, be more dependent on others, and have lower MMSE scores than participants with moderate hearing loss.
To examine the effects of hearing loss and hearing aids on cognitive function, the researchers divided the participants into three groups: participants with normal hearing, non-hearing aid users, and hearing aid users. The results indicate that hearing aid users generally had the highest MMSE scores, and non-hearing aid users had the lowest MMSE scores among the three groups during the 25-year follow-up period.
After adjusting for the age, sex, and educational levels, the authors reported that non-users had a higher cognitive decline rate than those with normal hearing (~1.5 points over the 25-year period) but hearing aid users did not. When all other factors were adjusted, however, there was no difference among the two groups of participants with hearing loss. The authors concluded that hearing aids did not slow down cognitive decline, but they reduced the negative effects of hearing loss on participants’ psychosocial behaviors.