A Computational Model of the Functional Role of the Ventral-Striatal D2 Receptor in the Expression of Previously Acquired Behaviors

Abstract

The functional role of dopamine has attracted a great deal of interest ever since it was empirically discovered that dopamine-blocking drugs could be used to treat psychosis. Specifically, the D2 receptor and its expression in the ventral striatum have emerged as pivotal in our understanding of the complex role of the neuromodulator in schizophrenia, reward, and motivation. Our departure from the ubiquitous temporal difference (TD) model of dopamine neuron firing allows us to account for a range of experimental evidence suggesting that ventral striatal dopamine D2 receptor manipulation selectively modulates motivated behavior for distal versus proximal outcomes. Whether an internal model or the TD approach (or a mixture) is better suited to a comprehensive exposition of tonic and phasic dopamine will have important implications for our understanding of reward, motivation, schizophrenia, and impulsivity. We also use the model to help unite some of the leading cognitive hypotheses of dopamine function under a computational umbrella. We have used the model ourselves to stimulate and focus new rounds of experimental research.

A temporal difference account of avoidance learning

“…First, we suggest that a temporaldifference learning model of the CAR can capture its key qualitative, empirical psychological findings. Second, we show that the pharmaco-behavioural findings imply that dopamine is unlikely to be involved in the learning of aversive associations to stimuli, a conclusion that is in contrast to the main theories so far suggested for its role in the CAR, including those based on TD ideas (notably Smith et al 2005Smith et al , 2006Smith et al , 2007. Third, we show that our model helps transcend the difficulties of the classical, qualitative psychological accounts of aversive learning even without appealing to special, additional features of the mechanisms involved.…”

“…However, perhaps the most significant challenge to existing dopaminergic (Smith et al 2005;Smith et al 2006Smith et al , 2007 and indeed some non-dopaminergic (Schmajuk and Zanutto 1997) models of the CAR is from a condition that turns out to resemble escape from fear. In this, the animals are trained with a blockade of dopamine D2 receptors, and thus fail to acquire the AR.…”

“…In more recent years, interest in the CAR resumed as both theoretical (e.g. Smith et al 2005Smith et al , 2006Smith et al , 2007 and psychopharmacological (e.g. Wadenberg and Hicks 1999;Samaha et al 2007) progress was made.…”

“…The learner is said to use a 'forward model' (Wolpert and Miall 1996), or, in a conditioning context, goal-directed actions (Dickinson and Balleine 2001;Daw et al 2005). Two of Smith and co-workers' influential models (Smith et al 2005(Smith et al , 2006 are perhaps most correctly seen in expectancy terms. Dopamine plays two roles in these particular models.…”

Anhedonia in schizophrenia

“…However, evidence of anhedonia preceding frank illness [9], and its elevation in first-degree relatives [7], suggests a primary genetic-biological component, which, of course, may interact synergistically with psychosocial etiologies. The following discussion focuses on dopamine, given its central role in mechanisms of both reward and psychosis, and recent findings that expand and modify traditional theories [32,42].…”