Kensinger (2009). Cognition in aging and age related disease. Summary

The most notable cognitive declines as a result of aging are: difficulty paying attention to relevant information and ignoring irrelevant information, word-finding difficulties, problems remembering the context in which information was learned.

Aging affects a range of cognitive functions, but there is one core deficit, according to domain-general theories of aging. There are three possible core deficits:

1. Sensory deficits
   This theory proposes that cognitive changes with aging may be attributed to changes in sensation. Older adults’ performance on cognitive tasks correlates strongly with sensory ability. In young adults, cognitive impairment can arise when to-be processed stimuli are degraded. It is possible that older adults score worse on cognitive tasks requiring auditory information if their auditory sensations are degraded.
2. Inhibition abilities
   This theory proposes that cognitive deficits may relate to their inability to ignore irrelevant information while focusing on the goal-related information. Older adults have a difficult time inhibiting the strong association present in ‘garden-path-sentences’. As a possible result of a decrease in inhibition abilities, older adults have difficulties with task-switching. Difficulty ignoring irrelevant information may explain the reduced (working) memory capacity, because more irrelevant information is stored.
3. Speed of processing
   This theory proposes that cognitive changes with aging relate to the slower processing time of older adults. The slower processing time shows in motor skills and in cognitive skills. Cognitive performance can suffer because the slowed mental operations cannot be carried out within the necessary time frame. The increased time between mental operations can make it more difficult to access previously processed information. A slower processing speed may lead to a poorer encoding of information and a reduced ability to store information.

It is possible to explain the effects of cognitive aging by changes that have a larger impact on one area of cognition than another, according to the domain-general theories of aging. There are two main domain-general theories of cognitive aging:

1. Word-finding difficulties and transmission deficits
   This theory proposes that word-finding difficulties become more common in older adults, because the links connecting one unit to another within the memory system become weaker with age. More links must be active in order to find a word for something in older adults than in younger adults, according to this theory. Words that have a lot of semantic connections, such as everyday objects, are easier for older adults to use than names, which have fewer connections.
2. Contextual memory and associative binding deficits
   Episodic memory includes both item (object) memory and memory for contextual details. Older adults have a difficulty with remembering contextual details. This theory proposes that an associative binding deficit underlies this difficulty. There are to broad memory deficits that underly this, a difficulty with initiating effective coding strategies and a difficulty forming a long-lasting bond between an item and its context.

There are two cognitive functions that remain stable or improve with age:

1. Crystallized intelligence
   This refers to the ability to retrieve and use information that has been acquired throughout a lifetime. This is the knowledge of facts. Older adults are generally very good at this (e.g: factual knowledge & pointing out spelling mistakes).
2. Emotion regulation
   After the age of 60, the ability to regulate emotions starts to improve. This explains the lower rate of depression in older adults. Older adults seem to focus on positive things in their environment and choose activities based on their potential emotional fulfilment. Older adults are more likely to remember contextual information if the event holds any emotional relevance. The emotional memory benefit may be particularly pronounced for positive memories.

Healthy aging is associated with brain changes, but not all regions are affected equally. There are three main changes in the brain associated with healthy aging:

1. Changes in prefrontal cortex
   There is atrophy of grey and white matter. The grey matter decline reflects reductions in the number of cells. The white matter change reflex axonal abnormalities and may result in a slowed neurotransmission. The white matter changes may explain the cognitive slowing. Older adults also show a different pattern of activation in the prefrontal cortex. They show a more bilateral prefrontal activity and young adults show more unilateral prefrontal activity. This might be a compensatory action. It is possible that older adults recruit more prefrontal cortex regions to perform tasks.
2. Medial temporal-lobe changes
   The hippocampus proper shows age-related changes. It is unclear whether there is cell atrophy or neuronal shrinkage. It is also unclear which regions of the hippocampus are affected most. The hippocampus tends to be underrecruited by older adults and this may explain the poorer memory of older adults, as the hippocampus is important in storing vivid memories.
3. Changes in emotion processing regions
   The amygdala and the orbitofrontal cortex are relatively spared in healthy aging. It shows minimal atrophy. This can explain the consistent ability to regulate emotions.

Advancing age can be associated with more severe impairments in recent memory, although these deficits do not impair their ability to function in daily life. A diagnosis of mild cognitive impairment requires subjective memory complaints, an impairment in one area of cognition but with deficits not severe enough to impair their ability to function in daily life. The vast majority of patients with mild cognitive impairment will eventually also develop dementia. The link between mild cognitive impairment and Alzheimer’s disease is supported by the high conversion rate and the overlapping neuropathological and genetic features

Alzheimer’s disease is the most common cause (2/3) for all cases of dementia. The disease can only be confirmed at an autopsy. The clinical profile requires memory impairment plus decline in one other area of cognition. The deficits must have a gradual onset and they must progress continually and irreversibly. There are several results of Alzheimer’s disease:

1. Episodic memory
   Patients with Alzheimer’s disease are unable to remember information encountered in the recent past. This deficit always exists, regardless of type of encoding and type of stimulus.
2. Semantic memory
   Patients with Alzheimer’s disease also show significant semantic memory deficits arise as the disease progresses.
3. Working memory and executive function
   Patients with Alzheimer’s disease also show deficits in the working memory and deficits in the executive functions. Deficits may result in difficulty with focussing attention flexibly and attend to goal-related information.

Plaques and tangles are often apparent throughout the brain in later stages of Alzheimer’s disease. In the early course of the disease, the medial temporal-lobe regions are the most affected. The hippocampal formation also shows atrophy and a volumetric reduction in the entorhinal cortex. The amygdala is affected early on as well. This explains why patients with Alzheimer’s disease do not show an improved memory for emotional information. The nucleus basalis also shows significant cell loss.

In later stages of the disease, there is increased atrophy throughout the medial temporal-lobe and the cellular abnormalities also become apparent in the frontal lobe and throughout the temporal lobe.

Mild cognitive impairment and Alzheimer’s disease do not have a single cause, but arise due to a combination of many factors, both environmental and genetical.