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Cognitive aging studies the changes of cognitive processes (for example, memory, intellectual, speed, learning and language) as human ages. 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. Examples of cognitive processes that decline during late adulthood; 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.

Biological Explanations of Cognitive Aging
Cerebral atrophy occurs as human brain ages. The decrement in size and lost of neurons may have contributed the cognitive aging. Even though the brain may have deteriorate as it ages, the brain reorganises itself to compensate the area that has been affected. Studies have found that older adults used different area 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 age-comparative cross sectional study, where the researcher compares the result of two different age groups of participants (e.g. young adults vs. older adults). Another method would be longitudinal study. This method studies a group of participants in a period of time that might lasts for a few decades. The researcher will then be able to collect data of individual changes over a period of time. See more in when does cognitive decline begin

Memory
The most prominent symptom that older adults suffer from cognitive aging would be the decline of memory capabilities. Most older adults generally believe that their memory capabilities have decline. However, studies have found that not all the types of memory declines.

Short-term memory
Most of the studies have found that older adult have a slight decline in short-term memory (Craik et al., 1995 as cited in Hamilton, 2000 ). Studies results have shown that older adults are able to perform basic short-term memory span tests (where participants have to repeat what the researcher had said to them) as good as younger adults (Craik et al., 1995 as cited in Hamilton, 2000 ).

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 declineinhibition processes.

Semantic memory
There is a slight decline in semantic memory such as vocabulary, knowledge of historical facts (Perlmutter et al. 1980 as cited in Kester, Benjamin & Craik, 2002 ), knowledge of concepts (Eustache et al., 1998 as cited in Kester, Benjamin & Craik, 2002 ) and production of category exemplars in exception of retrieval of familiar words (See more in Transmission Deficit). Even though studies have shown that there is only slight decline in semantic memory of older adults, older adults tend to believe that their semantic memory had decline. This has affected the psychological state older adults, for example, feeling of low self-esteem and low self-confidence.

Semantic memory does not decline as much when compare to other type of memories as it could act as compensatory base for other areas of memory and cognitive functions decline (Charness, 2000 as cited in Kester, Benjamin & Craik, 2002 ). For example, it could “compensate for declines in episodic memory (Gillund & Perlmutter, 1993 as cited in Kester, Benjamin & Craik, 2002 p. 548 ), memory for scene information, face recognition and, comprehension and memory for textual information and spoken language” (Wingfield, 2000 as cited in Kester, Benjamin & Craik, 2002 ).

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

Besides that, older adults also tend to have better remembrance of the events happened during early adulthood. This is known as reminiscence bump. Research study indicates that older adults are better able to retain these memories because these events occur during the peak of cognitive performance. Furthermore, most of the significant events, such as, graduation and getting married tend to take place during early adulthood, hence, explains the retention.

Prospective memory
There are two different classification 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 of the study of time based and event based prospective memory. One study has found that older adults performed better in event-based than in time based tasks while another study has found older adults performed poorly in both of the event-based and time-based tasks. Other study postulates that the findings could be associated with cue presented in the study (Zacks, et al., 2000 as cited in Kester, Benjamin & Craik, 2002 ). When cue is presented, older adults are better able to remember the prospective tasks (Zacks, et al., 2000 as cited in Kester, Benjamin & Craik, 2002 ). 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 of older adults. Older adults tend to perform poorly when the background tasks are interesting.