Injury to the frontal lobes or interruptions of subcortical connections with the frontal lobe impair the functions of the frontal lobes, and some of these deficits of frontal lobe function are called executive deficits. Several studies suggest that frontal lobe dysfunction is often associated with aging. For example, Mittenberg, Seidenberg, O'Leary, and DiGiulio (1989) compared older and younger individuals on tests that assessed frontal, parietal, and temporal lobe functions. These investigators found that it was frontal lobe function that best correlated with age.
Some of the most common executive deficits associated with frontal lobe dysfunction are forms of perseverative behavior. One of the tests that patients with frontal lobe perform poorly is the Wisconsin Card Sorting Test. In this test there are a series of cards that have different geometric designs, and these geometric designs have different colors and a different number of designs on each card. The participants are asked to sort the cards into stacks, but the examiner does not explicitly provide the means (shape, color, or number) by which they are to be sorted. Thus, participants might put all the cards that have the same design in the same pile, independent of their colors or numbers. After several successful sorts, however, the examiner indicates that this sort is no longer correct and the participants normally change their sorting strategy, now sorting by color or number. After several correct sorts, the examiner again indicates that this sort is no longer correct, and the person being tested needs to again change their sorting strategy. The patients with frontal injuries after developing a sorting strategy such as sorting by form continue sorting by this strategy even when the examiner indicates that this strategy is no longer the correct one. This form of perseveration where a person does not develop new strategies is called being "stuck in set." Creative innovation requires that a creative person attempt a new strategy, and if this strategy is unsuccessful or only partially successful he or she needs to attempt another strategy. If a creative person gets stuck in set, then his or her creative career will come to an end. Ridderinkhof, Span, and van der Molen (2002) tested a population of older individuals and found that they are more likely to get stuck in set than are younger people. Hence, the problem the older individuals were having on this task was not related to their inability to induce rules of sorting but rather rigidity, or their reluctance to shift once they found a sorting strategy.
The reason why older people have a propensity to get stuck in set or be mentally rigid is unclear, but there at least two possibilities. As I mentioned earlier, neurons communicate by giving off chemicals called neurotransmitters. One of the major neurotransmitters that appears to decrease with aging is dopamine. For example, Volkow et al. (2000) who with functional imaging (PET) found that with aging there was a decrease of dopamine and a decrease of frontal lobe activation. The cells that give off dopamine are found in the midbrain and from the midbrain travel to both the basal ganglia and the cerebral cortex (see Figure 9.1). Patients with Parkinson's disease have a reduced level of dopamine, and they also have evidence for frontal lobe dysfunction (Green et al., 2002). For example, patients with Parkinson's disease often perform poorly on tests such as the Wisconsin Card Sorting Test, frequently getting stuck in set, and thus the reduced creativity seen with aging might be related to cognitive rigidity and this cognitive rigidity might be related to the loss of dopaminergic neurons associated with aging.
I also mentioned earlier that with aging there is a loss of subcortical white matter. Patients who have diseases that injure their white matter, such as multiple sclerosis or multiple small strokes of the white matter, frequently show evidence of frontal lobe dysfunction and do poorly on tests such as the Wisconsin Card Sorting Test because they get stuck in set.
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