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Title: The neuropsychology of decision-making
Author: Deakin, J. B.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2005
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This thesis is concerned with emotional decision-making and impulsivity. In a series of experiments, the processes involved in emotional decision-making were investigated. Techniques used included both established, neuropsychologically validated tasks known to be dependent on frontal lobe function, and an entirely novel paradigm. A probabilistic decision-making and gambling task has been widely used to delineate the differences between pathological groups and normal volunteers, and attempts to distinguish between different components of impulsivity. In a large cohort of healthy volunteers, older volunteers took longer to make poorer decisions but risked less when gambling. This pattern of changes is unlike the patterns demonstrated by any of the pathological groups previously reported. A factor analysis of the results suggest that the variability across healthy volunteers on the four measures of impulsivity provided by the task can be explained by two orthogonal components, only one of these is influenced by age. Delay aversion is an unusual phenomenon in adult humans. A new task was designed so that optimal choice and delay-averse behaviour could be distinguished. In previous delay aversion tasks, volunteers can either pick the best option and be delay tolerant or the less optimal choice and be delay-averse. I predicted that one would be able to detect smaller variations in delay aversion when there was less of a cost to the delay-averse choice. Normal adults chose optimally and did not demonstrate delay aversion. When age-groups were compared it was seen that older adults did not choose optimally, but in fact choose the sub-optimal and most delay-tolerant option. A group of individuals known to have a decision-making deficit are those with frontal variant frontotemporal dementia. It was hypothesised that this was due to a lack of availability of 5-HT which could be corrected by paroxetine. A double-blind randomised controlled trial was used to detect improved performance on this task (and other tasks sensitive to tryptophan depletion) as well as any change in symptoms using a course of paroxetine (an SSRI). Paroxetine did not improve symptoms and in fact impaired test performance. Surprisingly, long term administration of paroxetine produced a pattern of effects on the test battery which was almost indistinguishable from that of tryptophan depletion. Diazepam is known to have receptors distributed throughout the prefrontal cortex but its effects on tests of orbitofrontal function have never before been examined. In contrast to the well-known sedative effects of diazepam, I demonstrate disinhibitory effects on two speeded reaction time tasks. It is shown that diazepam can impair performance on reaction time tasks both by impairing sensitivity and by increasing the bias to respond. Furthermore diazepam impaired performance on tests of frontal lobe function including both tests of planning and decision-making that depend predominantly on dorsolateral and orbitofrontal regions of the PFC respectively. In summary, a variety of factors have been found that impinge on decision-making. It is demonstrated that older adults are less able to make an optimal choice both when deciding between rewards of different probabilities and when deciding between rewards which come after a different delay. Volunteers who take diazepam are more likely to pick the larger reward associated with the less optimal choice than normal volunteers and demonstrate impulsive behaviour in two speeded reaction time tests. Paroxetine had no effect on decision-making in an impaired group of volunteers with frontotemporal dementia but had a paradoxical effect of impairing performance in tasks that are impaired by damage to orbitofrontal cortex and by tryptophan depletion. Possible neuronal substrates for these effects are discussed, as are explanations for the surprising effects of paroxetine and future directions in therapeutics. Finally a tentative neurochemical and anatomical model is detailed that would account for the effects described here.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available