Saturday, November 8, 2014

dopaminergic foundations of personality and individual difference

frontiersin |  The dopamine system can be divided into several anatomically defined branches or pathways. The nigrostriatal pathway (projecting from the substantia nigra to the striatum) is involved in motor control, and has long been of interest in the context of Parkinson's Disease and its therapeutic management via dopamine replacement (see Cenci, 2007). It was initially thought that motor control was the primary or even sole function of dopamine (e.g., Koob, 1982). However, this perspective has given way to a reward-processing interpretation of dopamine, focussed primarily on the mesolimbic pathway (projecting from the ventral tegmental area to limbic and forebrain areas including the striatum) (Robbins and Everitt, 1996; Wise, 2004; Schultz, 2007). One early theory helped integrate these diverging perspectives by proposing that the ventral striatum, a target of both nigrostriatal and mesolimbic dopamine, was responsible for converting motivation (i.e., to approach desire goal states) into action (Mogenson et al., 1980).
The reward-processing functions of dopamine have been discussed in terms of motivation by reward, enjoyment of reward, and learning from reward—or “wanting,” “liking” and “learning” (Berridge et al., 2009). Initially it was theorized that dopamine mediated reward “liking”—the hedonic impact of rewarding stimuli (Wise, 1982), and that these pleasure responses sustained reward-directed behavior. This theory enjoyed widespread influence for some time, and explains why dopamine was popularly dubbed “the pleasure chemical,” but has now been abandoned (Wise, 2004). One critique came from the addiction literature, which showed that dopamine-mediated escalation of drug dependence is accompanied by decreased pleasurable responses to those drugs (Robinson and Berridge, 2003). This favors the theory that dopamine mediates motivational “wanting” of reward by conferring stimuli with “incentive salience”—the process through which stimuli become motivationally attractive (Robinson and Berridge, 2003; Berridge et al., 2009). Dopamine is also thought to be responsible for reward learning, with phasic dopamine activity providing the “teacher” signal hypothesized in reinforcement learning models (Schultz et al., 1997; Schultz, 2007). Although reward wanting theories appear compatible with reward learning theories, they have not yet been integrated into a cohesive theoretical framework (see Alcaro et al., 2007).
Dopamine also has a major role in cognitive function and dysfunction. The mesocortical dopamine pathway (projecting from the ventral tegmental area to the dorsolateral prefrontal cortex and the anterior cingulate cortex) is implicated in higher cognitive functions such as working memory and decision-making (Robbins et al., 1996; Arnsten, 1998; Floresco and Magyar, 2006). Although these appear strikingly different to the motivational functions of the mesolimbic dopamine system, mental representations and operations seem likely to facilitate motivated action. That is, the mesocorticolimbic dopamine pathways may jointly coordinate the “anticipation of reward and activation of representations in the PFC needed to achieve it” (Miller and Cohen, 2001, p. 182). The higher cognitive functions of dopamine have implications for creative behavior, which is typically operationalized using tests of cognitive flexibility and divergent thinking. Ashby et al. (1999) suggest that this may explain the apparent impact of induced positive affect on creativity; positive affect is often preceded by reward delivery, which will often stimulate dopamine release. Finally, an enduring theory has posited a central role for dopamine in the cognitive disturbances seen in schizophrenia (e.g., Gray et al., 1991). A later iteration of this theory has related mesocortical dopamine to cognitive deficits (e.g., executive dysfunction) and negative symptoms (e.g., anhedonia), and mesolimbicdopamine to positive symptoms (e.g., hallucinations and delusions) (Lindenmayer et al., 2013).
This brief overview is only intended to orient the reader, illustrate the breadth of processes to which dopamine has been linked, and thereby foreshadow the diversity of topics addressed in this special issue. For more in-depth perspectives on dopamine function the interested reader is encouraged to consult the references cited here.


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