Abstract
Goal-directed behavior is a fundamental means by which animals can flexibly solve the challenges posed by variable external and internal conditions. Recently, the processes and brain mechanisms underlying such behavior have been extensively studied from behavioral, neuroscientific and computational perspectives. This research has highlighted the processes underlying goal-directed behavior and associated brain systems including prefrontal cortex, basal ganglia and, in particular therein, the nucleus accumbens (NAcc). This paper focusses on one particular process at the core of goal-directed behavior: how motivational value is assigned to goals on the basis of internal states and environmental stimuli, and how this supports goal selection processes. Various biological and computational accounts have been given of this problem and of related multiple neural and behavior phenomena, but we still lack an integrated hypothesis on the generation and use of value for goal selection. This paper proposes an hypothesis that aims to solve this problem and is based on this key elements: (a) amygdala and hippocampus establish the motivational value of stimuli and goals; (b) prefrontal cortex encodes various types of action outcomes; (c) NAcc integrates different sources of value, representing them in terms of a common currency with the aid of dopamine, and thereby plays a major role in selecting action outcomes within prefrontal cortex. The “goals” pursued by the organism are the outcomes selected by these processes. The hypothesis is developed in the context of a critical review of relevant biological and computational literature which offer it support. The paper shows how the hypothesis has the potential to integrate existing interpretations of motivational value and goal selection.
Highlights
Instrumental learning—the process of acquiring the capacity to select actions based on the utility of their outcomes—is a fundamental means through which animals adapt to changes in their environment
This paper proposes an hypothesis that aims to solve this problem and is based on this key elements: (a) amygdala and hippocampus establish the motivational value of stimuli and goals; (b) prefrontal cortex encodes various types of action outcomes; (c) nucleus accumbens (NAcc) integrates different sources of value, representing them in terms of a common currency with the aid of dopamine, and thereby plays a major role in selecting action outcomes within prefrontal cortex
Our hypothesis makes several key advances in our understanding of goal-directed behavior. It proposes a way for representing value from whatever source in a common currency, namely activity or salience within accumbens, thereby allowing all sources of value to be integrated and processed uniformly
Summary
Instrumental learning—the process of acquiring the capacity to select actions based on the utility of their outcomes—is a fundamental means through which animals adapt to changes in their environment. In a seminal paper, Daw et al (2005) proposed that habitual behavior and its learning can be captured on the basis of model-free reinforcement learning, whereas the functionalities involved in goal-directed behavior can be represented through model-based reinforcement learning Within this framework, a key role has been ascribed to the NAcc in terms of processing of current values and reward predictions (Humphries and Prescott, 2010; Bornstein and Daw, 2011; Penner and Mizumori, 2011; Pennartz et al, 2011; Khamassi and Humphries, 2012). The section continues by explaining the neural basis of the hypothesis In particular it describes the three main components of the hypothesis: (1) the amygdala and hippocampus as the sources of motivational value, (2) the ventral basal ganglia (including NAcc) as the sub-system integrating motivational-value information and selecting goals on this basis, (3) the prefrontal cortex as the main component representing and predicting outcomes, and triggering the execution of actions that lead to these outcomes, based on actionoutcome contingency representations.
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