Role of phasic activity of the substantia nigra pars compacta dopaminergic neurons during voluntary movement: a case for context.

Despite decades of research the role of the dopaminergic neurons (DANs) of the substantia nigra pars compacta (SNc) in movement and behaviour remains unclear and controversial. The classic model that equates tonic dopaminergic activity with permitting wanted movement and phasic activity with appetitive learning is no longer sufficient with the advent of evidence implicating phasic DAN activity with memory, novelty and saliency encoding, movement, exploration, and motivation. The aim of this review paper is to build a case that the appetitive and movement-related roles of phasic dopaminergic activity can be reconciled by the context in which the phasic activity is taking place. Specifically, we propose that SNc DAN phasic activity acts to integrate moment-by-moment motivational context with ongoing behaviour and thereby adds a motivational gain to contextually appropriate behaviours. We make this case by bringing together the findings of seminal works observing phasic SNc DAN activity at the onset of movement in motivated and unmotivated experimental paradigms, and optogenetic studies probing the causal effect of phasic dopamine (DA). Furthermore, we bring together evidence of phasic SNc DAN activity being responsive to changes in the novelty of an environmental stimulus and that its effect on behaviour may depend on the motivational nature of this stimulus. Finally, we propose possible circuits that might function to integrate memory into the motivational gain attributed to context-specific behaviours.