Moving in response to an unseen visual stimulus.

OBJECTIVE: Whether consciousness has a causal role in voluntary movements is not clear. Backward masking blocks a stimulus from becoming conscious, but it can trigger movement in a reaction time paradigm. We hypothesize that if backward masking is used in a choice reaction time paradigm, when the visible stimulus (S2) differs from the masked stimulus (S1), the movement will often differ from conscious intent. We did such a study employing electroencephalography (EEG) to explore the brain activity associated with this effect. METHODS: Twenty healthy adults participated in a choice reaction time task with a backwardly masked stimulus and EEG. They moved right or left hand in response to the direction of an arrow. S2 was congruent or incongruent with S1. When incongruent, responses were frequently concordant with S1, with faster reaction time than when responding to S2 and thought to be a mistake. RESULTS: We show that it is possible to trigger movements from the unperceived stimuli indicating consciousness is not causal since the movement was not in accord with intent. EEG showed information flow from occipital cortex to motor cortex. CONCLUSIONS: Occipital activity was the same despite response, but the parietal and frontal EEG differed. When responding to S1, the motor cortex responded as soon as information arrived, and when responding to S2, the motor cortex responded with a delay allowing for other brain processing prior to movement initiation. While the exact time of conscious recognition of S2 is not clear, when there is a response to S1, the frontal cortex signals an "error", but this is apparently too late to veto the movement. SIGNIFICANCE: While consciousness does not initiate the movement, it monitors the concordance of intent and result.

Physiological and introspective antecedents of tics and movements in adults with tic disorders.

OBJECTIVE: To investigate the subjective phenomenon and the neural underpinnings of tics compared with voluntary movements in patients with tic disorders. METHODS: We recorded electroencephalographic and electromyographic data while subjects completed a Libet clock paradigm. Patients and healthy volunteers reported the times of W (willing to move) and M (movement occurrence) while performing voluntary movements. This was repeated only for the patients for the tics. RESULTS: In the patients, W and M times preceding voluntary movements and tics did not significantly differ from voluntary movements of healthy volunteers. The Bereitschaftspotentials in the patients were similar to healthy volunteers. Tics were only assessable for 7 patients due to artifacts. Two subjects did not show Bereitschaftspotentials, and they reported the lowest levels of tic voluntariness. 5 subjects did not show beta band event-related desynchronization before tics. CONCLUSIONS: For patients, the sense of volition for tics is similar to that of their voluntary movements which is similar to normal. Patients showed dissociations between the Bereitschaftspotential and beta desynchronization for tics, with 5/7 showing normal Bereitschaftspotentials and 2/7 showing desynchronization. The absence of desynchronization may suggest attempts to suppress tics. SIGNIFICANCE: This physiology shows a difference for most tics compared with normal movements.