That AA genotypes had a stronger BOLD response to AO in medial prefrontal cortex, but a weaker AO response in the VTA (both relative to ApCO), could seem somewhat at odds— particularly if one interprets the VTA as encoding appetitive stimuli that are also processed in medial frontal cortex. However, a precise interpretation of BOLD responses in the VTA is more complex in light of recent findings. In particular, VTA dopaminergic and other midbrain neurons respond to rewarding and aversive stimuli, and in both excitatory and inhibitory manners (Matsumoto & Hikosaka, 2009; Brischoux et al., 2009; Ungless et al., 2004). It is also possible that local inhibitory responses within the VTA could produce a significant BOLD response, even without excitatory responses from dopamine neurons (Düzel et al., 2009), particularly when considering that the BOLD contrast mechanism does not closely track single or multi-unit transient or phasic spiking (Goense & Logothetis, 2008).
