It is widely recognized that motivated behaviors have an energetic or activational component, and that animals often must exert effort to overcome the psychological distance that separates them from reinforcing stimuli (Salamone & Correa, 2002; Salamone et al., 2005, 2007, 2010; Robbins & Everitt, 2007). Furthermore, organisms continually make complex effort or time-related evaluations, allocating behavioral resources into goal-directed behaviors based upon assessments of motivational value and response costs. Effort-based decision making in animals is usually assessed by offering choices between reinforcers with high value that can be obtained through instrumental behaviors with high response requirements vs. low value–low cost options (Salamone et al., 2007, 2009). Forebrain circuits involving prefrontal, striatal and pallidal areas are involved in exertion of effort and effort-related choice (Salamone & Correa, 2002; Walton et al., 2006; Floresco & Ghods-Sharifi, 2007; Salamone et al., 2007, 2010; Mingote et al., 2008; Hauber & Sommer, 2009). Manipulations such as accumbens dopamine (DA) depletions, intra-accumbens injections of DA antagonists or adenosine A2A agonists, or excitotoxic lesions of accumbens cell bodies, alter effort-related choice, shifting animals from high value–high cost options to the low value–low cost alternatives (Salamone et al., 1994, 2007, 2010; Hauber & Sommer, 2009; Farrar et al., 2010).
The paper by Day et al. (2011; this issue) is focused upon the role that accumbens neurons play in effort- and delay-related functions. Neuronal activity in the accumbens was monitored using multi-neuron electrophysiological recordings conducted during two cost-based decision tasks. With these distinct procedures, effort-related (i.e. ratio) and delay-related costs were manipulated. In each task, visual cues predicted high value (low effort–immediate) and low value (high effort–delayed) rewards. Trained animals exhibited a preference for high value reinforcers, but they nevertheless overcame high cost requirements to obtain reinforcement. Electrophysiological analyses demonstrated that a subgroup of accumbens neurons exhibited phasic increases in firing rate during cue presentations. In the effort-related decision task (but not the delay-related task) the activity of this population reflected the cost-discounted value of the future response, and on choice trials low-cost-selective neurons responded to the cue more on trials when the rats ultimately chose the low-cost option. Finally, another population of cells exhibited sustained changes in firing rate (excitation or inhibition) while animals completed high effort requirements or waited for delayed reinforcement. The results of the Day et al. (2011) paper provide exciting insights into the neural circuits regulating effort-related decision making, and suggest plausible ways in which manipulations of accumbens cell activity could affect the allocation of instrumental responses in relation to their cost. Moreover, they invite speculation about how the inhibition or excitation of accumbens neurons during completion of the ratio requirements reflects the actions of DA, glutamate or GABA inputs.
The Day et al. (2011) paper, coupled with other recent studies in humans (Botvinick et al., 2009; Croxson et al., 2009; Kurniawan et al., 2010), serves to highlight the emerging importance of research on effort-related functions; these studies could yield insights into the neural mechanisms underlying effort-related dysfunctions such as psychomotor slowing, anergia and fatigue, which are important features of depression and other disorders (Salamone et al., 2007, 2010; Treadway & Zald, 2010).