Valuation of opportunity costs by laboratory rats
Peter Shizgal, Center for Studies in Behavioral Neurobiology, Concordia University

My research team and I study the neural, behavioral, and computational mechanisms underlying the evaluation, selection, and pursuit of goal objects by laboratory rats. Direct electrical stimulation of particular brain sites produces a powerfully rewarding effect that leads the rats to work hard in order to procure additional stimulation. The rewarding effect arises from an observable volley of nerve impulse propagating along the axons of identifiable neurons. Thus, this preparation provides an entry point for tracing brain reward circuitry and for studying its interaction with modulators of reward, such as abused drugs and hormonal signals that regulate energy balance. Unlike natural rewards, “consumption” of the electrical reward does not lead to satiety or habituation, making it possible to study reward-seeking behavior over long periods of time under stable, highly controlled conditions.

Although game-theoretic ideas and paradigms have been applied successfully in research on brain reward circuitry in non-human primates, research on brain stimulation reward in rodents has been carried out typically outside of a game-theoretic context. The rodent interacts with a computer-controlled “agent” that does not take into account the strategy employed by the experimental subject and implements instead a fixed “schedule of reinforcement,” a set of rules for delivering rewards according to the amount of work performed and/or the distribution of responses over time. That said, an agent that does take account of the strategies employed by the other agents with which it interacts should benefit from forming an accurate predictive model of the algorithms used by these agents to evaluate benefits, costs and risks. If so, insights into the core processes involved in the evaluation, selection, and pursuit of goals may be germane to the concerns of game-theory researchers.

I will describe experiments on the evaluation of opportunity costs by rats working for rewarding brain stimulation. By allocating time spent working for the electrical reward, the rat must forgo benefits that would accrue from competing behaviors, such as grooming, resting and exploring. We have estimated the psychophysical function that translates the objective opportunity cost into its subjective equivalent. This function has interesting properties. When opportunity costs are very low (e.g., working for only a small fraction of a second triggers delivery of a reward), the subjective cost is invariant. Although the rat may be capable of detecting the difference between 0.1 s and 0.2 s, it does not appear to increment its subjective cost function over such intervals. This makes sense in that there may be no beneficial activities that the rat could substitute for work over such short intervals. As the objective opportunity cost rises, the subjective cost begins to rise as well, and beyond values of ~4 s, we estimate the subjective cost to rise at the same rate as the objective cost. This make sense as well given that a steeper rise in subjective costs would cause the rat to forgo very large expensive rewards that are profitable to pursue, and a more gradual rise would cause the rat to chase smaller, expensive rewards that are not worth the trouble. The form of this “subjective opportunity-cost function” is quite different from the form predicted by hyperbolic temporal discounting. I will propose that time spent working for reward is evaluated differently than time spent waiting for reward after a work requirement has been satisfied. I will also discuss the implications of these results for theories that hold psychological time to be either a scalar or logarithmic function of objective time.