2 REVIEW OF THE LITERATURE
2.2 COGNITIVE PERFORMANCE
In this thesis, the term cognitive performance is used to describe
participant’s success in cognitive tests. Hence, lower cognitive performance in this context is not always a marker of abnormal brain function (64) or even cognitive decline due to a normal aging process (7), but may simply mean lower test performance compared to the reference group. MCI, however, is a state of cognitive change in one or more cognitive domains that is not severe enough to be diagnosed as memory disorder but is beyond normal aging and can be objectively tested (65). It can affect memory or other cognitive abilities, such as attention or visuospatial skills, but does not substantially affect the ability to manage in daily life. Having MCI increases the risk of developing dementia, and it can be due to AD, but MCI does not always lead to dementia (65).
Table 3. The most studied cognitive domains, their meanings, and related areas of brain functions summarized from Palta et al. (64).
Cognitive domain Meaning Related brain areas Memory The ability to encode, store, and
retrieve stimuli; usually divided into short-term (working or temporary memory) and
long-term memory
Prefrontal cortex, parietal cortex, cingulate
cortex, thalamus, hippocampus, temporal
lobe Visual-spatial
construction
The ability to handle and arrange objects in space and in
relation to each other
Parietal lobe
Language Expression, fluency, and comprehension of speech, including reading and writing
Left-hemisphere, Broca and Wernicke areas Attention/
Psychomotor speed
The ability and speed of information processing. The ability to concentrate on target
stimuli while ignoring the irrelevant
Prefrontal cortex, parietal cortex, cingulate
cortex, subcortical connections Executive function A broad domain that overlaps
with other domains. The ability to plan, organise, solve problems, set goals, think
abstractively, and switch between cognitive processes
Frontal lobe
Figure 3. Indicative illustration of the related brain areas of the main cognitive domains.
2.2.2Assessment
Cognitive performance can be measured with the use of wide range of cognitive tests, which are broadly reviewed in classic handbooks, such as in Neuropsychological Assessment by Lezak et al. (63). Hence, this short introduction is only focusing on giving an idea of the multiple assessment possibilities.
Cognitive tests are usually multifactorial but are typically categorized by the main cognitive domain and targeted areas of brain function that they are trying to measure (64). Every cognitive domain is complex and
consequently, a single test can only evaluate primary components of each domain (64). Also, many confounding factors, such as education, age, and activity level of the participant may have an impact on the test
performance (64). Examples of some commonly used tests are given in Table 4.
Table 4. Examples of some commonly used cognitive tests summarised from Palta et al (64).
Cognitive domain
Examples of the cognitive tests
Idea of the test Short-term
memory
Wechsler Adult Intelligence Scale (WAIS)
Digit Span Forward or Backward test
Participants are asked to repeat a line of digits in the same order as
they were orally presented, in forward or backward order Long-term
memory
The California Verbal Learning Test
Participants are asked to repeat an orally presented list of words,
immediately and after a 20-minute delay Visual-spatial
construction
Rey-Osterrieth Complex Figure Test
Participants are asked to copy an abstract and complex figure Language The Controlled Oral
Word Association Test
Participants are given a category, such as animals or an initial
letter. During a specified timeframe, they are asked to
produce as many words as possible from the given category Attention/
psychomotor speed
TMT A Participants are asked to connect circled numbers in a numerical
order as fast as they can Executive
function
Stroop Color/Word Interference Test
Participants are asked to state the ink color of color-words. The ink color and the color-word are
not corresponding Global MMSE, MoCA Participants are asked to do
several tasks. The test performance reflects general
cognitive ability rather than function of specific cognitive
domains
MMSE, Mini Mental State Examination; MoCA, Montreal Cognitive Assessment;
TMT A, Trail Making Test, Part A
The purpose of cognitive assessment may be either diagnosis, patient care and planning, treatment planning, remediation, and evaluation, forensic, or research (63). When selecting a cognitive test or a set of tests for research purposes, validity, sensitivity, specificity, and reliability of the tests are things to consider (64). Also, the examiner’s background and familiarity with cognitive testing is one aspect to keep in mind (63).
Eventually, study design, research questions, and study resources influence on the final decisions (64). The prime consideration should be whether the selected tests will effectively measure the study hypotheses (63).
For research purposes, several cognitive test batteries have been developed that include tests of multiple cognitive domains and can be digitally performed, such as the National Institutes of Health (NIH) Toolbox (66), Cambridge Neuropsychologic Test Automated Battery (CANTAB) (67), and COGSTATE (68). However, the use of comprehensive test batteries is limited, and the use of single global cognitive tests, such as the Mini Mental State Examination (MMSE) is common (64).
In future studies, cognitive tests that are available for repeated testing at several time points are needed (64), because only a few of the existing tests take into account the effects of practice well enough (63).
2.2.3The role of diet in cognitive performance
The role of diet in the incidence of dementia was covered in chapter 2.1.4.
It is likely that the same dietary factors may play an at least similar role in maintenance of adequate cognitive performance. This is supported by the meta-analysis by Loughrey et al. (69), which concluded that adherence to the Mediterranean diet may be beneficial for global cognition among healthy older adults. According to the meta-analysis, it is also possible that the Mediterranean diet may be beneficial for the function of some
cognitive domains, such as delayed recall, working memory, processing speed, and reasoning (69).
However, further studies are needed to elucidate if dietary factors
enhancing cognitive performance among healthy people are truly the same factors preventing dementia. As this is not yet wholly clear, in the next
chapters, cognitive test performance and diagnosis outcomes, i.e. all-cause dementia or AD, are discussed separately. In this thesis, measured
cognitive performance in neuropsychological tests and the diagnosis of dementia or AD were analyzed as separare outcomes. These outcomes may represent different levels or stages of the same pathological
continuum, in which diet may have a role. Cognitive variation within the normal range or mild cognitive impairment associated with dietary factors may clarify the significance of food and nutrient intake in cognitive
functioning.
2.3 ANIMAL PROTEIN SOURCES
