Pupil dilations induced by barely conscious reward goal-priming.
The topic of unconscious influences on behavior has long been explored as a way of understanding human performance and the neurobiological correlates of intention, motivation and action. Previous research using transcranial magnetic stimulation has demonstrated that barely visible priming of an action concept, when combined with reward in the form of a consciously perceived positive stimulus, can alter the state of the motor system and enhance the maximal voluntary force level. One possible explanation is that positive stimulus-induced reward signals are processed by the dopaminergic system in the basal ganglia, motivating individuals to increase the effort they invest in particular behaviors, or to recruit the resources necessary for maintaining those behaviors. If so, given that the dopaminergic system has functionally and anatomically close connections with the noradrenergic system, we hypothesize that the state of the noradrenergic system may be enhanced by the same process. In accord with this hypothesis, we observed that barely visible goal priming with reward caused pupil dilation, suggesting that activity in the noradrenergic system increased. Importantly, this enhancement was accompanied by an unconscious increase in handgrip force. This is the first objective evidence that the pupil-linked neuromodulatory system is related to implicit learning of the link between physical exertion and reward, probably in the noradrenergic system, resulting in more forceful voluntary motor action in the absence of conscious awareness.