The Impact of Natural Aging on Perceptual Decision Making

Abstract

The transformation of incoming sensory information into an appropriate motor response is known as perceptual decision making, and constitutes a key computation within the human brain. Mathematical models have proved extremely useful in the description and explanation of perceptual decision making, with some of the model-based predictions also being borne out at the neural level. Recent advancements in electrophysiology have made it possible to parse the individual levels of perceptual decision making with a high degree of temporal precision (O? Connell, Dockree, & Kelly, 2012). An overarching aim of this thesis was to utilise insights from previously existing model-based investigations of perceptual decision making to generate hypotheses to be tested within healthy older populations. Across all experimental chapters, the behavioural and EEG data of older adults were compared to younger adults in order to analyse the ways in which perceptual decision making changes as a result of aging. Experiment 1 consisted of an investigation into the speed-accuracy trade-off, i.e. the inverse relationship between speed and accuracy which characterises the decisions we make. In this experiment, it was found that, when changing across speed and accuracy conditions, older adults displayed similar accuracy adjustments but comparatively less speeding of response times than the younger group. However, older adults had significantly stronger sensory representations of the evidence, indicating the potential effect of task engagement and attention on performance. Older adults were found to have less flexible motor preparation as measured by beta-band activity, and no Speed/Accuracy effects were found at the abstract evidence accumulation level in either age group. A final key finding from this chapter was the lack of functional impairment observed in older adults (as indexed by a points reward system), despite these alterations at the neural level. Therefore, this experiment suggests that older adults may favour a more conservative form of responding, if this can be achieved with no loss of reward. Experiment 2 contains some key insights for research in perceptual decision making generally, as well as for research on strategic adaptations in older adults specifically. This experiment addressed a major unanswered question in the literature: is leakage present in human perceptual decision making? A neural signature of leak was isolated in the human brain with the use of a novel task paradigm involving the insertion of gaps within periods of coherent motion evidence. A characteristic dip in the abstract evidence accumulation signal in the younger group indicated that leak may be a major strategic adjustment within continuous monitoring contexts. The older group did not exhibit this pattern, and it is therefore argued that older adults differ from younger adults in terms of their use of leak as a strategic decision adaptation. Experiment 3 involved an examination of the impact of aging on prior-based perceptual decision making. The main aim of this research was to investigate the efficiency with which older adults can utilise cues to inform and enhance their decision making. Interestingly, it was found that older adults did not display any deficits in terms of the use of priors in decision making when compared to younger adults. This indicates that adaptation based on prior information within perceptual decision making may be retained with age. Prior cues were found to primarily have an impact at the level of motor preparation, whereby more beta-band activity was observed within cued conditions than neutral conditions. This neural signal did differ significantly as a function of age, but older adults did not show impairment at the behavioural level, despite differing neural effects. Overall, it is concluded that natural aging impacts perceptual decision making across a variety of neural levels, but that these effects do not necessarily entail functional deficits at the behavioural level.