Feeling free: External influences on endogenous behaviour

When we are presented with two equally appealing options, how does the brain break the symmetry between them and make a choice? Recent research has proposed that when no clear information can guide decisions, we use irrelevant noise to tip the scale in favour of one alternative and decide how to act. In the present study, we investigated this issue exploring how human decisions were influenced by noise in a visual signal that cued instructed or free choice. Participants were presented with random-dot kinematograms, moving unidirectionally either upwards or downwards (in instructed trials) or both upwards and downwards simultaneously (free-choice trials). By varying the coherence of dot motion, we were able to test how moment-to-moment fluctuations in motion energy could influence action selection processes. We also measured participants' awareness of such influence. Our results revealed three novel findings: Participants' choices tended to follow fluctuations in dot motion, showing that sensory noise biased free selection between actions, irrespective of the clarity of the free cue. However, participants appeared to remain unaware of that influence, because subjective ratings of freedom did not correlate with the degree of sensory biasing. In one exception to this general rule, we found that, when participants resisted the bias and made a choice opposite to the one suggested by the stimulus, they reported strong subjective sense of having chosen independently of the stimulation. This result suggests that inhibitory control is tightly linked to the sense of freedom of choice.