The iPlant is all good and well but endogenous - natural - activation of midbrain dopamine (DA) neurons is just as important. DA neurons, whose function is strengthening of active cell assemblies in the frontal cortex ('attention'), generation of motivation ('desire'), and facilitation of declarative learning in the hippocampus and procedural learning in the striatum, are naturally activated by many nerve bundles, including glutamatergic (Glu) projections from sensory and frontal cortex. Of these two, the frontal cortex is more important, because it receives more DA and expresses more DA receptors, and therefore forms, with these DA neurons, an enormously adaptive and tightly knit circuit that we call 'I'.
In the addictive personality (ADHD humans, SH rats) the ability of the frontal cortex to activate and be activated by midbrain DA neurons is low, making the individual unusually reliant on sensory and other ways of activating midbrain DA neurons. Although a slim circuit and its steep delay of reinforcement gradient ('high impulsivity') may have been adaptive during evolution it can be a profound nuisance in a post-industrial environment that generally rewards long-term thinking and self-discipline. Recieving relatively little dopamine, the frontal cortex of the addictive personality becomes hypersensitive to the transmitter and will gravitate towards environmental and chemical stimulants.