Researchers studied the brain of mice that exhibit hereditary hearing loss, similar to age related hearing loss in humans.
The scientists analysed the density of neurotransmitter receptors in the brain that are crucial for memory formation.
They also researched the extent to which information storage in the brain’s most important memory organ, the hippocampus, was affected.
Adaptability of the brain suffers
Memory is enabled by a process called synaptic plasticity. In the hippocampus, synaptic plasticity was chronically impaired by progressive hearing loss.
The distribution and density of neurotransmitter receptors in sensory and memory regions of the brain also changed constantly. The stronger the hearing impairment, the poorer were both synaptic plasticity and memory ability.
“Our results provide new insights into the putative cause of the relationship between cognitive decline and age-related hearing loss in humans,” said Denise Manahan-Vaughan.
“We believe that the constant changes in neurotransmitter receptor expression caused by progressive hearing loss create shifting sands at the level of sensory information processing that prevent the hippocampus from working effectively”, she adds.
Funding: The study was funded by the Collaborative Research Center (SFB) 874 of the German Research Foundation. SFB 874 “Integration and Representation of Sensory Processes” has existed at RUB since 2010.
The researchers investigate how sensory signals generate neuronal maps, resulting in complex behaviour and memory formation. Daniela Beckmann and Mikro Feldmann have also completed the medical students’ program of the SFB 874 and the International Graduate School of Neuroscience.
It is commonly believed that tinnitus patients may have difficulties with attention span and memory. Many studies have reported that poor cognitive performance was associated with tinnitus. However, unlike hearing loss, which has been reported to be an independent risk factor for dementia, the link between tinnitus and cognitive impairment remains unclear .
Mild cognitive impairment (MCI) refers to an intermediate cognitive state in elderly individuals, somewhere between the state of those aging normally and those with dementia. Patients with MCI are known to have a higher risk of developing dementia.
In a cross-sectional study, Lee et al.  attempted to determine whether there was a relationship between tinnitus and MCI, and established that higher tinnitus handicap inventory (THI) scores, reflecting tinnitus severity, were associated with lower scores on a Korean version of the Montreal Cognitive Assessment.
Similarly, mean hearing levels and tinnitus severity were significant predictors of MCI. The authors  concluded that the cognitive function of aged tinnitus patients should be assessed as part of the initial work-up for tinnitus.
However, the study of Lee et al.  and its conclusions had some limitations. First, the hearing threshold was worse in the MCI group, and the mean hearing threshold showed a stronger relationship with MCI-as manifested by a higher odds ratio- than the THI score. Even worse, multicollinearity tests of mean hearing levels and tinnitus severity should have been performed, but were not.
In addition, the absence of more objective tools weakens the conclusions that can be drawn from their study. Lastly, they did not compare changes in cognitive function before and after tinnitus treatment.
In a similar Chinese study, baseline characteristics (e.g., age, sex, educational level, and hearing threshold) did not show significant relationships with tinnitus severity . In line with the findings of Lee et al.,  that study also found that patients with severe tinnitus (THI ≥38) exhibited distinct cognitive deficits, as evaluated using the Cognitive Abilities Screening Instrument. Moreover, tinnitus severity showed a strong correlation with the extent of cognitive impairment.
They provided objective data with P300 event-related potentials, which reflect cognitive processing. Although the amplitudes of N2 and N3 were not significantly different, the latency values, which are associated with stimulus evaluation, revealed a clear difference according to the severity of tinnitus.
In contrast, an electroencephalographic study reported that cognitive changes in tinnitus patients were associated with changes in hippocampal activity, as well as activity in the anterior cingulate and insula .
The hippocampal area plays an important role in the persistence of tinnitus by updating auditory memory, and it is also related to cognitive dysfunction in patients with tinnitus. Interestingly, hearing loss was not correlated with any of the tested variables in their study. They assumed that this was because they did not test supra-clinical frequencies above 8 kHz.
Tinnitus is inextricable from auditory deafferentation. Therefore, a confounding effect of hearing loss on cognitive function in tinnitus patients is inevitable. A prospective, longitudinal, randomized controlled trial could be the most effective solution for this issue.
It is true that some generators of tinnitus in the brain may share common resources with attention and memory generators in the brain, such as the prefrontal cortex, limbic system, and hippocampus [5,6]. However, it is too early to draw a firm conclusion that tinnitus can cause cognitive dysfunction.
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