Cognition in aging and age-related disease - Kensinger - 2009 - Article

In recent years, it has become clear that significant cognitive decline is not an inevitable consequence of aging. Some people, in fact most people, do not show significant decline as get older. In fact, there are three different effects aging can have on cognitive functioning. First, for most people, aging is associated with relatively little cognitive decline. This is called healthy aging or successful aging. Common cognitive declines for people with healthy aging are: problems with paying attention to relevant information and ignoring irrelevant information; word-finding difficulties, and; problems with remembering the context in which information was learned. Second, for some people, memory declines significantly with age, yet it does not prevent the performance of daily activities. This type is also called mild cognitive impairment. Third, for others, aging is associated with severe cognitive deficits, such that these deficits hinder daily functioning and impede the ability to live independently. This type of cognition in aging is also called dementia. In this chapter, the cognitive changes are discussed that are associated with healthy aging as well as the theories that are being developed to explain these cognitive changes. Further, these age-related declines are contrasted with the ones that are common in mild cognitive impairment and dementia (specifically: Alzheimer's disease). .

There are two main sets of theories that explain the cognitive changes in aging. First, the domain-general theories state that there is one central or core shared ability underlying all of the tasks on which older adults are impaired. Three such core abilities have been proposed to explain the cognitive declines: (1) sensory deficits; (2) inhibition, and; (3) speed of processing. According to the first, the cognitive declines may be attributed to changes in sensation (i.e., deficits in vision and hearing). According to the second, cognitive declines are related to older adults' inability to ignore information in the environment, while focusing on goal-relevant information. Hence, it suggests that the cognitive declines can be explained by the growing inhibitory deficits of older adults. Third, older adults have a slower speed of processing than younger adults, which may explain not only the slower responses at the motor level, but also the slower responses at the cognitive level.

The domain-general theories can be contrasted with the domain-specific theories of cognitive aging. According to these domain-specific theories, age-related cognitive decline cannot be explained by a single shared ability, but rather by changes that have a larger impact on one area of cognition than on another. For instance, it appears that older adults often have difficulties retrieving the appropriate name for a person, place, or thing. One may say "it is on the tip of my tongue", implying that a person has access to a word's meaning, but does not remember the phonological features of the word. Some researchers have suggested that these word-finding difficulties arise from the fact that, with age, the links connecting one unit to another in the memory system become weaker. Hence, more links have to be "active" to recall the correct name for an object of person. Similarly, although older adults with healthy aging processes are generally quite good at remembering people or items, they seem to encounter difficulties remembering the contextual details of an event. Where did the event take place? Research suggests two broad types of domain-specific memorial deficits underlying this decreased ability to remember contextual information. First, older adults have difficulties with initiating affecting encoding strategies that would promote memory for the associative details of an experience. In short, older adults appear to express deficits at encoding. Second, older adults seem to have difficulties either forming a long-lasting "link" between an item and its context, or in retrieving that link representation.

In the above sections, we discussed some cognitive deficits that are common in healthy aging. There are, however, still many cognitive functions preserved. Examples are: crystallized intelligence (i.e., the ability to retrieve and use information that has been acquired throughout a lifetime) and emotional regulation.

A lot of studies have been conducted examining the neural changes with healthy aging, using structural MRI, functional MRI, or PET. The vast majority of these studies have shown that the largest changes in structure and function occur in the prefrontal cortex and the medial temporal lobe. There is, for instance, evidence of atrophy in both the gray and white matter in the prefrontal cortex. Other brain regions, such as the cortical and subcortical regions, remain relatively preserved across the lifespan.

As mentioned above, people with mild cognitive impairment (MCI) show more than "healthy" cognitive declines, yet this does not hinder their daily functioning. A diagnosis of MCI requires subjective memory complaints, and impairment in one area of cognition with scores at least 1.5 standard deviations below age-scaled norms, but with deficits not severe enough to interfere with daily living and functioning (which would result in a diagnosis of dementia). Note, however, that there are many commonalities between MCI and Alzheimer's disease in the neuropathological and genetic features. Both people with MCI and Alzheimer's disease have significant structural and functional changes in the medial temporal lobe, as well as alterations in the concentration of amyloid-beta protein.

Dementia can have many different causes, although Alzheimer's disease is the most common one, accounting for two thirds of all patients with dementia. As Alzheimer's disease, which was first described by Alois Alzheimer in 1907, can only be confirmed by autopsy, its clinical diagnosis must be an exclusionary one. That is, there must be memory impairment plus decline in at least one other area of cognition (language, motor function, attention, executive function, personality, or object recognition). The deficits have a gradual onset and progress continually over time and irreversibly. The most notable deficit for patients with Alzheimer's disease is the inability to remember information encountered in the recent past (episodic memory). In fact, episodic memory is the best way of distinguishing people with Alzheimer's disease from healthy older adults. Although the episodic memory is heavily affected by AD, the semantic memory (general world knowledge) is relatively spared with mild AD. Yet, this may get worse, as the disease progresses. With regard to the brain regions, we see the following with AD: in the beginning, the medial-temporal lobe regions are most affected, while later on plaques and tangles are apparent throughout various regions in the brain.