Abstract

Brain stimulation reward (BSR), the effect of electrical stimulation that animals seek to reinitiate, is a useful tool to investigate reward-seeking behaviour and its neural underpinnings. The experiments in this thesis pursue this approach by applying the "reward-mountain" model of performance for BSR. This model provides a framework for describing the computational processes that link the induced neural activity to reward- seeking behaviour. The data to which the model is fit are obtained by measuring operant performance for BSR (time spent pressing a lever) as a function of subjective intensity of the stimulation (controlled by pulse frequency) and opportunity cost (work time required to earn a reward). Determining the stage of neural circuitry responsible for the behavioural impact of physiological manipulations is among the principal goals of this strategy.
At the core of the model is the subject’s computation of “payoff” via the integration of reward intensity and costs. An important initial stage, often overlooked in neuroscientific studies of decision-making, is the transformation of the objective into subjective variables. The formal relationship between these variables (termed psychophysical functions) is often non-linear: what is experienced is not necessarily a direct reflection of the external world. An analysis of these transformations is important for the full understanding of cost-benefit decision-making. A central goal of the
experiments in this thesis is to estimate the psychophysical functions of reward-seeking variables.
Chapter 1 reviews the BSR literature and describes the reward-mountain model. The experiment described in Chapter 2 concerns the valuation of time: the translation of the experimenter-set opportunity cost (the objective price) into the equivalent subjective domain (subjective price). The experiment described in Chapter 3 estimates the frequency-response function of the directly stimulated neurons subserving the rewarding effect. This function translates the experimenter-set pulse frequency (the inducing stimulus) into the firing frequency of the neuron (the induced physiological response). Chapter 4 describes a proof-of-principle study: the ability of the reward-mountain paradigm to detect the effect of a lesion challenge on pursuit of BSR and to link this effect to one or more stages of processing. Chapter 5 concludes with a general discussion.