A number of theories have been put forward to explain the reasons for personality change in dementia. One theory is that perceived personality change is actually the demented person's reasonable reaction to their condition. For example, they may become anxious and depressed at their decrease in cognitive ability, or they may become irritable and angry when unable to remember where they have put things. If this were the case then considerable variability o f change could be expected between demented people depending on their situation and possibly their premorbid personality. In this study large changes in personality were found with most dementia sufferers showing changes in the same direction. The large, global change found here
suggests that the personality changes are unlikely to be solely due to the person's reaction to the disease. Studies which have looked at premorbid personality have found that this has little effect on changes seen in dementia (Petry et al., 1988; Siegler et al., 1991; Chatterjee et al., 1992). Such findings would argue against this theory. The relationship between personality change and cognitive decline found in different studies has often been used to support different theories of personality change. The lack of correlation between personality and cognitive decline found in some studies has been used as evidence to support the theory that personality change may be a direct result of damage to particular neurotransmitter systems or specific areas of the brain, independent of those changes causing cognitive decline. In the current study those clusters covering disagreeable and neurotic traits (Scale 2) and those covering passive traits (Scale 3) showed, a significant but moderate correlation to cognitive decline suggesting that these personality changes may be caused, at least in part, by processes independent of those involved in cognitive change.
Studies which have examined personality change due to brain injury have shown that specific personality changes may occur following damage to particular areas of the brain. Bilateral temporal lobe lesions can result in placidity and hypersexuality, while damage to the nondominant parietal lobes may affect insight and disturb emotional processing (Lilly et al., 1983). Injury to the frontal lobes may result in apathy, a lack of impulse control, irritability and mood changes (Lishman, 1987). Such changes are often seen without accompanying cognitive change suggesting that different areas of the brain or neuro transmitter systems may be involved. As was mentioned previously, Frontal Lobe dementia is characterised specifically by personality changes including either apathy and underactivity or disinhibition and overactivity (Neary, 1990). Involvement of the frontal lobes has been shown in such cases using single photon emission tomography (Neary et al., 1987). Recent studies on neurotransmitter systems suggest that levels of different neurotransmitters can influence personality traits (Mulder, 1992). For example, low levels of a metabolite of serotonin have been linked with violent, disinhibited and impulsive behaviour. Levels of this neurotransmitter have also been associated with behavioural disturbance in DAT (Chui, 1989). Certain neuropsychological tests have been found to be sensitive to frontal lobe pathology. Future studies including such tests would allow correlations between
Another possibility is that the same pathological or neurochemical changes which cause cognitive change are also responsible for personality change. This theory
would be supported by finding a strong correlation between cognitive decline and personality change.
These last two theories are proposing that personality change is a direct result of changes to areas of the brain. Another possibility is that personality change may be a direct result of cognitive decline. The finding of a strong correlation between personality change and cognitive decline may also be used to support this theory. The decrease seen in the Conscientiousness and Openness factors is probably largely a direct result of cognitive decline. Katzman and Karasu (1975) have suggested that there may be a decline in 'moral standards'. They considered that this could occur due to a 'loss of adaptive mechanisms dependent on memory, including the memory of the social consequences of these actions'. Many personality related behaviours are learned, particularly those related to social acceptability. It is possible that loss of memory could result in an increase in disagreeable traits. In the current study, disagreeable traits showed some association with cognitive decline which may support the idea that memory plays a part in socially appropriate behaviours. An explanation for the relationship between passive behaviours and cognitive decline proves more difficult to hypothesise. Perhaps passivity increases as the demented person becomes less able to interpret her or his environment and to know how to respond to it. It is also possible that the neurological changes which cause cognitive decline also affect this aspect of personality.
One final possibility is that personality changes may arise as a combination of the above possible causes: both biological and psychological. Given the complex nature of personality this explanation may be the most likely.
Unlike other reports, this study found that some demented people change in the opposite direction to the majority, becoming less passive or less disagreeable and emotional. While this finding may be attributable to methodological factors, it is more likely to be a real phenomenon. In dementia changes to the brain typically occur in the frontal and temporal lobes, with neuronal degeneration occurring in the limbic areas, and hippocampi and amygdaloid nuclei (Lishman, 1987). However, variants of the typical picture are sometimes seen and this may explain those subjects whose personalities appear to change in a different way to the majority. These individuals may be examples of a sub-type of DAT.