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Cognitive aging studies the changes in cognitive processes (for example, memory, intellectual, speed, learning and language) which occur with increasing age. Age-related cognitive changes are part of normal aging that affects all the adult population. There are three different types of cognitive aging; life-long declines, late-life declines, and life-long stability. Life-long decline is defined as a gradual decline of cognitive processes throughout life span. This affects cognitive executive functions of such as processing speed, working memory, and encoding of information into episodic memory. Late-life decline is defined as cognitive processes that decline at late adulthood or life. Some examples of cognitive processes that decline during late adulthood are short term memory and semantic memory (tip-of-the tongue states). Life-long stability is defined as cognitive processes that do not decline throughout life span, for example, autobiographical memory, implicit memory, automatic processes, and emotional process.

The process of cognitive decline is reflected in the deterioration of the brain as we age. However, the brain can compensate for this deterioration by reorganising itself in the area that has been affected. Studies have found that older adults use different areas of the brain when performing the same task. For example, older adults used both hemispheres of the brain while younger adults used only one hemisphere of the brain.

Research Methods Used in Investigating Cognitive Aging
The most common method used in cognitive aging research is the age-comparative cross sectional study, in which the researcher compares data of two different age groups of participants (e.g. young adults vs. older adults). Another method which is becoming increasingly popular is the longitudinal design. This method allows the researcher to study individual change over a long period of time, which can last several decades.

When does cognitive decline begin?
Salthouse stated two reasons why it is important to study the age at which cognition begins to decline. Firstly, cognitive aging interventions are more effective when they are applied early in the course of cognitive decline. Secondly, the knowledge of when cognitive decline begins may facilitate our understanding of what causes cognitive decline. For example, if cognitive decline occurs in early adulthood, then we can determine that it is unlikely to be caused by pathological cognitive changes (such as dementia) or environmental changes (such as retirement) which are common in later life.

Longitudinal studies investigating the age related decline of cognitive abilities indicate there to be no evidence of age-related differences in cognition until late adulthood (after 60 years old). However, this is not a universally accepted conclusion as some research indicates cognitive decline to occur at middle age (between 40 and 50 years old). Cross-sectional study studies have generally found cognitive decline to begin in early adulthood (between 20 and 30 years old). Interestingly, both longitudinal and cross sectional studies have revealed there to be a much greater magnitude of decline of cognitive abilities in adults over 60 years old.

Methodological limitations with the methods (longitudinal and cross-sectional designs) utilised in the investigation of age at which cognitive decline begins may offer an explanation for the discrepancy in research. Cross sectional studies are confounded by bias due to the influence of cohort effects, which have been found to significantly overestimate the effect of age in cognitive decline. A well known example of cohort differences is the Flynn effect which refers to the generational increase of fluid intelligence. Longitudinal studies are confounded by practice effects which have been found to underestimate the age-related decline of cognitive abilities. Thus it has been suggested only multivariate longitudinal studies, age-comparative experimental interventions, and a combination of the two methods will lead to a full understanding of age related cognitive decline.

Intelligence
Cognitive abilities related to intelligence fall into one of two distinct categories: fluid intelligence or crystallised intelligence. Fluid intelligence has been defined as the ‘ability to reason’ and is independent of previously acquired knowledge (p 6829). These abilities are important for logical thinking and novel problem solving and are assumed to have a neurobiological basis. In contrast, crystallised intelligence is influenced by an individual’s experiences. Learned skills, general knowledge gained through education and verbal knowledge are products of crystallised abilities.

Research into the age-related decline of intelligence indicates that fluid and crystallised intelligence do not decline uniformly; fluid abilities have been shown to decline throughout adulthood, whereas crystallised abilities have been shown to maintain throughout adulthood. Thus, although older adults’ performances on fluid intelligence tasks are much lower than younger adults, tasks which demand knowledge and learned skill are not negatively associated with age. The decline of fluid intelligence with age is also apparent on a neurological level, with evidence showing the age-related changes in fluid intelligence to be related to increased white matter hyperintensity burden. Interestingly, the age-related decline of fluid intelligence has been shown to be related to the age-related slowing of information processing ; indicating speed of processing may be an underlying mechanism of fluid intelligence.

Attention
Attention has been defined as ‘the collection of processes regulating the allocation of humans limited cognitive resources’ (262). Different types of attentional processing rely upon a separate sub-process of attention. Interestingly, the age related decline of these attentional sub processes of attention is not uniform.

Selective Attention
Selective attention is the ability to process relevant environmental information whilst ignoring irrelevant information ; an important function as attention is of limited capacity. As we get older, our ability to selectively attend to information significantly decreases. Research has shown, compared to younger adults, older adult are more distracted by irrelevant information. They are also less able to disregard irrelevant information, as shown by visual search task which involve scanning for a target among a field of distracters. One of the popular tests of selective attention is the Stroop task where the participants have to ignore the incongruent colour name and selectively attend to the colour of the ink. West and Alain found older adults’ had significantly higher reaction times on the Stroop task compared to younger adults; age increases the Stroop effect.

An interesting study conducted by McDowd and Fillion showed older adults to have a decreased ability to ignore irrelevant information at a physiological level. Using skin conductance orienting response as a measure (which increases in the presence of novel items), it was found that younger participants habituated more quickly when asked to ignore a stimulus compared with when they were asked to attend. Older adults however, did not show any difference in orienting in the presence of stimuli they were asked to attend compared with stimuli they were asked to ignore.

Although a lot of research exists indicating an age related decline in selective attention, the conclusion is not universally accepted. . Gorth and Allen argued ‘that older adults are just as effective as young adults at selectively attending to relevant information if information about what is relevant is made available to them’ (p 292). Older adults performance in selective attention tasks have been shown to significantly improve when they are familiar with the target, given more practice and when the targets are made more distinct.

Divided Attention
The ability to perform/ process two or more tasks simultaneously is called divided attention. Research indicates that as we get older, our ability to divide attention is significantly reduced. However, older adults reduction in divided attention is only apparent in more difficult and complex tasks; there are no age differences in performing multiple simple tasks simultaneously. The decreased ability in divided attention has significant implications for older adults in their everyday lives. Verghease, Buschke, Viola, Katz, Hall, Kuslansky, & Lipton showed walking whilst talking (a divided attention task) increased older adult risk of falling. To compensate older adults often focus their attention more on walking then other tasks, thus their ability to do tasks, such as memorising items, whilst walking is significantly reduced. Older adults driving ability have also been shown to be more negatively affected when they are required to divide attention compared to younger drivers. For example, when required to navigate whilst driving, older adults driving ability (measured by steering accuracy) was significantly worse compared to younger drivers.

Importantly, age-related differences shown in divided attention have been found to be reduced with practice . In fact, practice eliminates any divided attention deficit in both young and older participants, although older adults required more practice than younger adults

Sustained Attention
Sustained attention refers to the ability to concentrate on/process task information for a sustained period of time and is most commonly measured using vigilance tasks. Research indicates that although older adults do not show any age differences in ‘vigilance decrement’ (decline in vigilance performance over time) they are less accurate in their detection of targets than younger participants. However, these age differences in vigilance tasks have been attributed to secondary aging deficits rather than sustained attention. Thus, older adults’ ability to sustain attention is comparable to younger adults.

Memory
A decline in memory capabilities is one of the most well recognised cognitive processes to decline with age; most older adults are reported to believe their memory capabilities will inevitably decline. However, research indicates that this decline is not universal, as some types of memory are not vulnerable to decline.

Short-term memory
Most studies regarding short-term memory have indicated only a slight decline with increasing age. For instance, results have shown that older adults ability to perform basic short-term memory span tests (where participants have to repeat what the researcher had said to them) is comparable to younger adults.

Working memory
Working memory gradually declines as human brain ages. This is because of the biological decline of the frontal lobe. A study has been done on older adults using backward span test (where participants have to repeat the items said by the researcher in a reversed order). Results reported that older adults were unable to perform as well as the basic short-term memory span tests. The participants are confused by the forward and reverse order of the item as the items are similar to each other. Furthermore, participants might not have the same capabilities as younger adults to mentally process the items. This point is further elaborated that older adults were unsuccessful in transferring the items to long term memory from working memory to be stored.

Another reason of working memory decline could be that older adults are more susceptible towards irrelevant or unimportant distractions. This is related to the decline inhibition processes.

Semantic memory
There is a slight decline in semantic memory such as vocabulary, knowledge of historical facts , knowledge of concepts , production of category exemplars and retrieving familiar words (see more in Weakened Memory Connections). Even though studies have shown that there is only slight decline, older adults tend to believe that there is a great reduction in their semantic memory. This in turn has affected the psychological state of older adults, for example, feelings of low self-esteem and low self-confidence.

It has been argued that semantic memory does not decline to the same extent as other types of memory, as it is the compensatory base for other areas of memory and cognitive decline. For example, it may compensate for declines in episodic memory, memory for scene information , face recognition , comprehension, memory for textual information and spoken language.

Episodic memory
Older adults tend to remember memories of life events from one or two decades ago. Retention of an event weakens/declines after a long period of time unless the life event is well learned or rehearsed. According to a longitudinal study conducted by Zelinski and Burnight (1997), older adults exhibited gradual decline in the process of recalling lists or tasks, which occurred in the past minutes, hours or days. On the other hand, there was no decline in recognition memory. Such evidence was first shown in a study conducted by Schonfield and Robertson (1966) and has been replicated numerous times with similar results. It has been discussed that there is a higher demand of attention when older adults perform processes of recalling in contrast to recognition.

Besides that, older adults tend to have better remembrance of events which occurred during early adulthood. This is known as the reminiscence bump. Research findings have indicated that older adults are capable of retaining these memories because these events occurred during the peak of cognitive performance. Furthermore, most of lives significant events (e.g. graduation and getting married) tend to take place during early adulthood and thus at the peak of cognitive performance.

Prospective memory
There are two different classifications of prospective memory; time based (which requires the participant to initiate a task at a certain time) and event based (which requires the participant to initiate a task when a particular prompt is present). Researchers have found mixed results in regards to the study of time based and event based prospective memory. One study found older adults performed better in event-based in contrast to time based tasks while another study found older adults performed poorly in both tasks. Other research has suggested that such findings could be associated with the cues presented in each study. When a cue is presented, older adults are better able to remember the prospective tasks. Besides that, background tasks (which are done together with a prospective memory task) could also play a role in the decrement of prospective memory performance in old age. Older adults tend to perform poorly when the background tasks are interesting.

Cognitive Slowing
Cognitive slowing has been theorized to be an underlying pinning of age related decline in a variety of cognitive abilities. A single reason to why slowing occurs has not been determined. It is believed to be a result of either reduced neural connections, greater noise in the nervous system or an increased proportion in the loss of information at each step of processing. Nonetheless research has identified that older adults response latencies (excluding the affects of age-related motor slowing) on a range of cognitive tests, are approximately 1.5 times slower than younger adults

Research has indicated that the speed of reading declines with age, as older adults spend more time reading complex sentences in order to recall the information correctly. In comparison younger adults take longer reading infrequent words and new concepts introduced into a particular text. Research has also suggested that cognitive slowing is associated with language comprehension and recollection. For example, older adults portrayed greater difficulty recalling speech segments when they were presented at a fast rate and when given the option older adults chose to listen to shorter slower speech segments. Cognitive slowing has also been associated with reductions in fluid intelligence and recognizing famous names and faces. Although opponents argue the idea of cognitive slowing is a reductionist approach, a link has formed between cognitive speed and competent performance on everyday living tasks such as reading medication labels, counting out change and looking up numbers in a phone book.

Deficit in Inhibitory Processes
The second theory of cognitive aging proposes that aging weakens inhibitory control, making it difficult for older adults to suppress irrelevant information. To suppress such information one must have the ability to (a) control access to attention’s focus (b) delete irrelevant information from attention and working memory and (c) suppress inappropriate responses. Thus an inefficient inhibition framework makes it more difficult for aging adults to accurately process and carry out certain tasks.

Research has indicated that older adults are easily distracted while reading, for example, by a word printed in a different typeface. As a result the speed of reading is reduced and comprehension and memory of the text declines. The inability to suppress irrelevant information also affects the ability to ignore distracting speech while carrying out a task. Consider a study conducted by Tun and colleagues, which compared the ability of young and old adults to ignore competing speech while listening to word lists. Results supported the inhibition deficit theory as older adults were less able to suppress the competing speech and recalled fewer words. A recent study supported such findings as individual differences in inhibitory control were found to predict interference from a competing talker. Research has suggested that the inhibition deficit also contributes to problems when identifying words. In particular when low frequency words (e.g. brood and wool) and lengthy words (e.g. holiday) are used.

Weakened Memory Connections
The third cognitive theory of aging, developed by Burke, MacKay and colleagues (1991), proposes cognitive decline occurs as a result of weakened memory connections which lead to poorer activation of target information. This theory (also known as the Transmission Deficit Model) states language production relies on the strength of connections within a network that includes semantic (conceptual) and phonological (speech sounds) levels. According to Burke and MacKay, the phonological level is more susceptible to lapses in retrieval as there are no inherent connections between word sounds in comparison to inherent connections between concepts. Therefore when certain words are not used recently or frequently, the phonological connections weaken even further making it difficult to retrieve a word even when the meaning is known. This theory is often supported by the tip-of-the-tongue-experience (TOT).

Frequencies of TOTs increase throughout adulthood as a result of age related weakening between the semantic and phonological links. When older participants were provided a definition of an infrequent word (e.g. umbrella), they were often able to evoke the meaning and/or visualize the object but not retrieve the actual word. Thus supporting the idea of semantic and phonological representations. TOT’s are more frequent when a word is presented as a proper name (e.g. Mr Farmer) compared to an occupation (e.g. farmer). It is argued the semantic connections are stronger for common names (such as occupations) as there is often more than one acceptable name for the item (e.g. farmer, farmhand, rancher). Fortunately it is suggested connections between semantic and phonological levels improve with greater use.

Neurobiological Changes
The disciplines of cognitive psychology and neuroscience have recently come together to investigate the neural bases of cognitive aging. This approach proposes that neurobiological changes are at the root of normal cognitive decline in old age. Deficits in domains such as working memory, attention and executive functions, are similar to neuropsychological profiles of individuals with brain damage to the prefrontal cortex.

Research has indicated that older adults do not activate areas in the frontal cortex to the same extent of younger adults. For example, PET and fMRI studies indicated a reduction of cerebral blood flow when participants purposefully committed information to memory and when participants memorized faces. This reduced activation may be due to age related change in brain size, in particular a reduction in brain volume and enlarged ventricles. Cognitive decline may also be a result of age related reductions in the dopamine neurochemical system, which is believed to regulate the prefrontal cortex. Unfortunately pharmacological agents (e.g. Levodopa), which increase dopamine levels in the brain, have not reduced the rate of cognitive decline associated with aging. It is argued such medications cannot work fully due to fewer dopamine receptors in the prefrontal cortex as a result of the aging process.

Retirement
Rohwedder and Willis explain the relationship between cognitive decline and retirement with their "use-it-or-lose-it" hypothesis, where people are encouraged to do mental exercises in order to maintain their cognitive abilities. Furthermore, the extensive research, which was conducted in the past few years, has practically confirmed the validity of their findings. Rohwedder' and Willis explain the nature of their findings by referring to the “unengaged lifestyle hypothesis”. They have discovered that a lifestyle transition from cognitive stimulating environment to a less stimulating one may create favorable conditions for the cognitive decline of a person. In addition, the level of dependence between cognitive decline and retirement is very much influenced by the employment status of the person. It is concluded that intellectual work is more beneficial than the labor work. For instance, there is evidence that even years after the retirement from an intellectual work, cognitive performance is still preserved.

Health and well-being
A feeling of personal accomplishment is strongly influenced by the person’s health. But, the human body becomes more vulnerable to illnesses with ageing. Furthermore, a decrease on the person’s well-being often leads to impair performance in daily activities, loss of interest, fatigue (medical), loss of confidence and finally to depression. These age-related behavioural changes can lead to impaired cognitive performance as proposed by Deary. According to the “common cause” theory of aging, which links physical health and cognitive performance, there are certain biological processes such as oxidative stress and hormonal changes, which affect cognition. In addition, cardiovascular disease, which is generally an age-associated illness, can have a strong influence on cognitive decline, as it can lead to Ischaemic due to interruption in blood flow, causing permanent or temporary impairment in brain function. Finally, cognitive impairment is also considered to be influenced by obesity in elderly people. Numerous studies have been undertaken in order to prove this relation. For example, a 27-year prospective study of over 10000 men and women, emphasized that people who were obese in midlife had a 74% increased risk of dementia later in life in comparison to overweight people who had only a 35% increased risk.

Environmental conditions
Air pollution – It is well known that air pollutants can severely damage human lungs. But new studies have also identified air pollutants as the main culprit for cardiovascular disease, which may lead to cognitive decline.

Socioeconomic status – The connection between the social environment and cognitive decline in later age is not fully agreeable and consistent. However, lower socio-economic status leads to stress, unhealthy food, poor nutrition and less education, which researchers consider as causes for cognitive decline.

Psychological stress – The connection between stress and cognitive performance is purely biological. Conditions like post-traumatic stress disorder and chronic stress lessened the hippocampus. This pathological changes cause memory and learning problems.

Cognitive interventions
These days we speak greatly about cognitive interventions as a way of improving abilities which slightly decrease with age. The most popular method is memory training, in which adults use different techniques (such as mnemonics, concentration, attention strategies, personal insight and self-monitoring) to improve cognitive performance in dual-shifting, visual search, recognition, recall, spatial perception and memory efficiency. According to Wilkinson, cognitive therapy is a good method for reducing the depression and anxiety, which are common sufferings of elders. In a cognitive-behavior study run by Galleghren, Tohmpson and Steffren, which was conducted with people in the mean age of 62, found 72 percent of the participants showed no sign of depression after the therapy in comparison to people who did not undergo the therapy.

Social interventions
People often isolate themselves after retirement, as they have lost their social environment. For this reason social programs, which aim to integrate isolated people back into society, can be considered to have a huge positive impact on cognitive health. For example, a recently launched program, where older adults meet young adults, revealed the positive effects of keeping alive the connection between generations. Furthermore, internet based social networks like Facebook and Twitter can overcome isolation, as they help people keep in touch all the time all over the world. In addition, such social networks are easy, cheap and do not require physical activity which is suitable in later age, especially for people with disabilities.

Nutritional interventions
The role of dietary fiber in slowing-down the cognitive decline in aging has a big support in scientific circles. Supplements such as B-9 and B-12 as well as Omega-3 fatty acid are found to be essential for the robust operation of the memory and the brain. It is believed that antioxidant vitamins, vegetables and fruits increase the aging neurons activity and help them to work properly. . In contrast, a diet which is reach in sugar and cholesterol often leads to poorer cognitive abilities in elderly people. . Although a lot of researches have been conducted supporting the link between nutrition and their effects in healthy aging, further exploration needs to be done.

Physical activity interventions
Cardiovascular fitness and cardiorespiratory fitness are both beneficial in terms of cognitive improvement in elderly people. A meta-analysis conducted by Colcombe and Kramer, indicated that exercise which included aerobic training and strengths and flexibility training had positive influence on cognitive performance in contrast to practicing only one of such exercises. Cardiorespiratory fitness can influence cognitive performance in old people in various ways :

1.	Reduce the risk of diabetes, cardiovascular disease and hypertension. All of them have a strong link with cognitive decline.

2.	Another positive benefit is its impact of reducing the cerebral blood flow which if high may cause stroke in later age.

3.	Connection between nerve growth and cardiorespiratory fitness has been reported in experiments, conducted on mice. However, further investigation is needed to prove this connection.

Preventions
Unfortunately there is no cure or medicine which can stop cognitive decline. However, research has indentified various prevention strategies that may slow the process down.

Education
Education can be considered as negatively correlated to cognitive decline in old people, due to several reasons. Firstly, educated people are generally most likely to have a healthy lifestyle. Secondly, people, who are engaged with intellectual activity, have better cognitive performances than those who exercise labour work. These suggestions are exemplified by the theory of “use-it or lose-it”. Furthermore, Katzman argues that “education increases brain reserve by increasing synaptic density in neocortical association cortex, leading to a delay of symptoms by 4 to 5 years in those with AD (and probably, other dementing disorders) hence halving the prevalence of dementia.” (p. 17). He supported his theory by pointing out that educated people have synaptic density which delays the symptoms of decline for couple of years. Thus, it may be inferred that education has a vital role in keeping the brain healthy.

Healthy lifestyle
Healthy lifestyle has an impact on later cognition. Confronting unhealthy habits like smoking and drinking reduce the chance of cognitive decline. Eating foods rich in Omega-3 and fruit like blackberry improves cognitive capacity. According to “cognitive reserve” theory, there is a need of occupied lifestyle (physical and mental) not only in late age, but through your whole life. In this way people enrich their “cognitive capacity” leading to delay in the cognitive decline.

SenseCam
In recent years devises which aim to foster brain preservation, have flooded the market. Most of them are not scientifically proven, but some are based on truthful research. One such example is the SenseCam, which originates from research conducted in Cambridge University and Bangor University. The Sense Cam works by collecting memory of day to day activities in a series of photographs. The owner of the SenseCam then studies each photograph in order to draw his/her attention to the various events of the day. One of its main purposes is to help people that suffer from depression and dementia. However, it is also suitable for preventing cognitive decline, as it has a positive impact on human memory and attention.