A Review of Aging and Cognitive Decline
The brain is a machine like all of our organs, and aging gradually destroys its function. All of the various forms of outright dementia are caused by processes that take place in all of us: accumulation of metabolic wastes; failure of clearance and maintenance processes; dysfunction of the immune system and consequent neuroinflammation; diminished rate of creation and integration of new neurons; the countless tiny undetected strokes caused by structural failure of small blood vessels; and so on. As this damage accumulates, there is a steady decline in function. Much of this, however, consists of later consequences of fundamental damage. For example the ongoing destruction of brain tissue in small amounts due to tiny strokes is driven to a considerable degree by stiffening of blood vessels and consequent hypertension, which at root is caused by some combination of inflammation, cellular senescence, and calcification and cross-linking in the extracellular matrix of blood vessel walls. If those line items can be addressed, then the later consequences will be prevented, and the decline of cognitive abilities postponed.
Cognition is critical for functional independence as people age, including whether someone can live independently, manage finances, take medications correctly, and drive safely. In addition, intact cognition is vital for humans to communicate effectively, including processing and integrating sensory information and responding appropriately to others. Cognitive abilities often decline with age. It is important to understand what types of changes in cognition are expected as a part of normal aging and what type of changes might suggest the onset of a brain disease. It is imperative to understand the effects of age on cognition because of the rapidly increasing number of adults over the age of 65 and the increasing prevalence of age-associated neurodegenerative dementias. Because many more people are living longer, the number of people with age-associated neurodegenerative dementias also is increasing rapidly. The Alzheimer's Association estimates that 5.2 million people in the United States had a clinical diagnosis of Alzheimer disease (AD) in 2014, and the number of people with a diagnosis of AD is projected to increase to 13.8 million people in 2050, unless effective preventative or treatment strategies are developed. Thus, it is vital to understand how age impacts cognition and what preventative or treatment strategies might preserve cognition into advanced age. Any approaches that could decrease the negative effects of age on cognition or decrease the risk of developing a neurodegenerative dementia would have a tremendous impact on the quality of life of millions of older adults in the United States.
Cognitive abilities can be divided into several specific cognitive domains including attention, memory, executive cognitive function, language, and visuospatial abilities. Each of these domains has measurable declines with age. For each of these domains, a subject must first perceive the stimulus, process the information, and then respond. Both sensory perception and processing speed decline with age, thus impacting test performance in many cognitive domains. For example, auditory acuity begins to decline after age 30, and up to 70% of subjects age 80 have measurable hearing loss. Also, speech discrimination and sound localization decrease in advance age. In addition to these change in sensory perception, there is a clear decline in processing speed in advancing age with older adults performing these activities more slowly than younger adults. This slowing of processing speed causes worse test performance on many types of tasks that involve a timed response. The most noticeable changes in attention that occur with age are declines in performance on complex attentional tasks such as selective or divided attention. Selective attention is the ability to focus on specific information in an environment while at the same time ignoring irrelevant information. Divided attention is the ability to focus on multiple tasks simultaneously, such as walking an obstacle course and answering questions. Normal subject performance declines progressively with age on these more complex attentional tasks. However, simple attention tasks such as digit span are maintained in normal subjects up to age 80.
Executive cognitive function involves decision making, problem solving, planning and sequencing of responses, and multitasking. Each of these areas of executive cognitive function declines with advancing age. Executive cognitive function is particularly important for novel tasks for which a set of habitual responses is not necessarily the most appropriate response and depends critically on the prefrontal cortex. Performance on tests that are novel, complex, or timed steadily declines with advancing age, as does performance on tests that require inhibiting some responses but not others or involve distinguishing between relevant and irrelevant information. In addition, concept formation, abstraction, and mental flexibility decline with age, especially in subjects older than age 70. There are age-related declines in aspects of visuospatial processing and constructional praxis. Visual recognition of objects, shapes, gestures, and conventional signs remains stable into advanced age. However, visuoperceptual judgment and ability to perceive spatial orientation decline with age. A person's ability to copy a simple figure is not affected by age, but ability to copy a complex design declines with age. On standard IQ measures such as block design and object assembly, much of the declines with age are due to time, but when time is factored out, there are still declines in test performance with increasing age.