Recent research has highlighted the link between progressive presbycusis and the development of dementia.
Many people will have experienced it: when hearing-impaired patients are fitted with hearing aids, they sometimes feel as if they have changed. Symptoms that previously appeared to be depression or (pseudo) dementia will recede, people will again participate in conversations and other events, become interested in things that they used to enjoy, such as music and movies, and sometimes people will really flourish again.
But what happens in the brain when the lack of mental stimulation and impoverishment of acoustic input persists? An association of presbycusis with cognitive decline has been described in various studies, but the extent to which this association is causal has been little researched so far. A new paper highlights how hearing loss could contribute to memory loss.1
The loss of the ability to perceive language in a differentiated manner is the most serious aspect of presbycusis in humans. Those who cannot communicate effectively are cut off from their social environment. Rodents also rely heavily on auditory information for both their social structures and the localization of events in their environment. Earlier studies have already described an adaptive reorganisation of cortical and subcortical structures in the wake of blindness.2
In a new study, scientists from the Ruhr University Bochum, Germany (RUB) examined the density of receptors for neurotransmitters in the brains of mice with a hereditary form of gradually progressive hearing loss that sets in during adulthood. In areas that are central to the formation of memory content, they also analysed how severely the storage of information was impaired.
Learning and memory are made possible by two things: sensory information, which is the substrate, so to speak, and synaptic plasticity, which enables its coding. In the memory center, the hippocampus, this adaptability was chronically reduced by progressive hearing loss. The distribution and density of transmitter-receptors in sensory cortex areas and the hippocampus also changed. This was already evident in early stages of hearing loss. The more severe and persistent the hearing loss, the more synaptic plasticity and memory function were impaired.
The dependence of the hippocampus on sensory information is also illustrated by the fact that the sudden and complete loss of a sensory modality leads to a disruption of its function that lasts for months. How a gradually progressive loss of sensory perception affects the hippocampus was little studied before this work.
Prof. Denise Manahan-Vaughan, Head of RUB's Department of Neurophysiology, summarizes: "We believe that the constant changes in the expression of neurotransmitter receptors caused by progressive hearing loss create a sort of quicksand at the level of processing sensory information that prevents the hippocampus from functioning properly".3
References:
1. Beckmann, D., Feldmann, M., Shchyglo, O. & Manahan-Vaughan, D. Hippocampal Synaptic Plasticity, Spatial Memory, and Neurotransmitter Receptor Expression Are Profoundly Altered by Gradual Loss of Hearing Ability. Cereb Cortex doi:10.1093/cercor/bhaa061.
2. Feldmann, M., Beckmann, D., Eysel, U. T. & Manahan-Vaughan, D. Early Loss of Vision Results in Extensive Reorganization of Plasticity-Related Receptors and Alterations in Hippocampal Function That Extend Through Adulthood. Cereb. Cortex 29, 892–905 (2019).
3. How hearing loss in old age affects the brain. ScienceDaily https://www.sciencedaily.com/releases/2020/04/200423130446.htm.