In exploration of the environment, intrinsic and extrinsic factors such as the probability and the reward value associated with stimuli can modulate behaviour. Response times are decreased to stimuli that are more rewarding or more likely to appear, and increased to those less rewarding or less likely. It is unclear how these effects interact and how similar the effects are across response modalities. In this thesis these effects were investigated first separately and then concurrently, and across both saccadic and manual responses. The effect of probability was found to be sensitive to the temporal features of the paradigm; an effect on manual responses was seen with longer inter-stimulus intervals (ISI), compared to the saccadic effect with shorter ISIs. The effect of reward was stronger in manual responses than saccadic responses – possibly a result of slower dopaminergic activation within the reward system. The temporal dynamics are postulated to be the reason there is no evidence for a correlation between saccadic and manual responses across the experiments. When manipulations of reward and probability were combined, the probability modulations dominated the effect on responses. It is suggested that this is due to the nature of probabilistic information being an intrinsic feature of the environment that is not affected by the individual observer utility or internal state. Furthermore, no effect of reward was found in healthy older adults with the same paradigm. No correlation between reward and probability effects across participants was found, suggesting that these factors affect different accumulators within a decision-making model framework. The paradigm developed in this thesis provides a feasible way in which to study reward and probability effects in Parkinson’s patients with deep-brain stimulators to the subthalamic nucleus.
|Date of Award||10 May 2016|
- The University of Bristol
|Supervisor||Iain D Gilchrist (Supervisor)|