But neither of these provide a robust explanation for the full suite of effects, including the drastic shift of responding to grooming and stereotyped outputs at high doses ( Randrup et al., 1963 Randrup and Munkvad, 1967). Why do animals work harder for food they appear less motivated to consume? It is possible that AMPH somehow affects the perceived cost of effort, or that increased task engagement is a by-product of motoric hyperactivity. Suppression of food-intake by AMPH is also evident in primates ( Foltin, 2001). AMPH-treated rats are less motivated to eat their latency to first consumption is longer, they consume less than usual, and they spend less time eating ( Blundell et al., 1976 Blundell et al., 1979 Leibowitz et al., 1986). Behavioral data, however, suggest that reward valuation is decreased. The most obvious explanation for these effects according to utility theory is that AMPH increases the perceived value of reward. It also increases engagement in learned food-seeking behaviors ( Foltin, 2001 Odum and Shahan, 2004). Moderate doses of d-amphetamine (AMPH) increases the amount of work rodents will exert for reward ( Floresco et al., 2008b Bardgett et al., 2009). For instance, rodents and primates typically choose to exert increased effort if the associated reward is of considerably higher value than that of lower-effort options ( Salamone et al., 1994 Hosokawa et al., 2013). Animals often seek to maximize utility, even if it requires exerting additional effort. Options requiring low effort and yielding a large food reward have high utility to hungry animals, whereas options requiring high effort or yielding little/unwanted food have low utility. Effort, reward, and other factors pertinent for decisions about resource collection are often formalized within the concept of utility ( Phillips et al., 2007 Glimcher et al., 2009). When hungry, however, food rewards usually have sufficient value to motivate task engagement in lieu of these other options. Even well-trained rodents sometimes engage in sleep, grooming, exploration, or other behaviors during laboratory tasks. IntroductionĪnimals can draw from a large repertoire of innate and learned actions to generate behavioral output ( Whishaw and Kolb, 2005). Noise from excessive excitability at high doses overcomes stability enhancement to drive frequent deviation from the script, impairing task execution. We propose that under low-dose AMPH, increased network stability balances moderately increased excitability, which promotes accelerated unfolding of a neural ‘script’ for task execution, despite reduced reward valuation. Low-dose AMPH contracted these trajectories and reduced their variance, whereas high-dose AMPH expanded both. Ensembles of simultaneously recorded neurons generated task-specific trajectories of neural activity encoding past, present, and future events. AMPH decreased signaling of reward, but not effort, in the ACC of freely-moving rats. We investigated neural correlates of this phenomenon in the anterior cingulate cortex (ACC), a brain structure implicated in signaling cost-benefit utility. AMPH typically increases task engagement and the effort animals exert for reward, despite decreasing reward valuation. Psychostimulants such as d-amphetamine (AMPH) often have behavioral effects that appear paradoxical within the framework of optimal choice theory.
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