The most consistent activation across several studies involving happiness induction is in the basal ganglia (ventral striate and putamen) (Phan et al., 2002). These include recognition of happy faces, pleasant pictures (including attractive faces), recall of happy memories, pleasant sexual arousal and competitive arousal of a successful nature (reviewed in Phan et al., 2002). In one study, transient happiness had no areas of significantly increased activity but was associated with significant and widespread reductions in cortical rCBF, especially in the right prefrontal and bilateral temporal-parietal regions (George et al., 1995).
A number of recent neuroimaging studies have investigated reward mechanisms in humans. These studies have used monetary and nonmonetary rewards during planning and gambling tasks (Elliott et al., 2000; Breiter et al., 2001; Knutson et al., 2001a,b; Delgado et al., 2000), or primary taste rewards (e.g., fruit juice) (O'Doherty et al., 2002). Elliott and colleagues (2000) studied with 15O-water PET the response to non-monetary feedback in planning and guessing tasks and found bilateral activation in the caudate nucleus (dorsal striatum) when feedback was given, as opposed to when it was absent. Delgado et al. (2000) used fMRI in a card game with monetary rewards or punishments. They found increases of activity in dorsal striatum (bilateral caudate nuclei) and left ventral striatum that were more sustained in cases of rewarding rather than punishing outcome. Similarly, Breiter et al. (2001) found that responses to rewarding outcomes increased with monetary value in the nucleus accumbens, sublenticular extended amygdala (SLEA) of the basal forebrain, and hypothalamus.
In their fMRI study, Breiter et al. (2001) found that the prospect of a monetary reward was associated with responses in SLEA and orbital gyrus. In a similar study, Knutson et al. (2001a,b) used fMRI and found that anticipation of increasing rewards elicited ventral striatal (nucleus accumbens) activation, along with increased self-reports of happiness. In contrast, anticipation of increasing punishment did not. Activity in dorsal striate (medial caudate) was present in anticipation of both rewards and punishments.
Using a primary reward (fruit juice) in an fMRI study, O'Doherty et al. (2002) found that expectation of a pleasant taste produced activation in dopaminergic midbrain, amygdala, striatum, and orbitofrontal cortex. Only the latter was activated by reward receipt.
A remarkable PET study by Koepp et al. (1998) examined in vivo dopaminergic activity during a videogame with monetary reward by measuring 11-C raclopride binding to striatal D2 dopamine receptors. They found that binding was significantly reduced in the dorsal and ventral striatum during the video game compared with baseline levels, consistent with increased dopamine release. Importantly, the binding reduction in ventral striatum positively correlated with the performance level during the task. Thus, the anticipation and outcome of a reward (not separable in this study) activate the dorsal and ventral striatum, and this may be mediated by increased firing in dopaminergic mesolimbic neurons, and increased dopamine release. The more recent fMRI data are consistent with such a conclusion (e.g., Knutson et al., 2001a,b).
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