Veto and vacillation: a neural precursor of the decision to withhold action

The capacity to inhibit a planned action gives human behavior its characteristic flexibility. How this mechanism operates and what factors influence a decision to act or not act remain relatively unexplored. We used EEG readiness potentials (RPs) to examine preparatory activity before each action of an ongoing sequence, in which one action was occasionally omitted. We compared RPs between sequences in which omissions were instructed by a rule (e.g., "omit every fourth action") and sequences in which the participant themselves freely decided which action to omit. RP amplitude was reduced for actions that immediately preceded a voluntary omission but not a rule-based omission. We also used the regular temporal pattern of the action sequences to explore brain processes linked to omitting an action by time-locking EEG averages to the inferred time when an action would have occurred had it not been omitted. When omissions were instructed by a rule, there was a negative-going trend in the EEG, recalling the rising ramp of an RP. No such component was found for voluntary omissions. The results are consistent with a model in which spontaneously fluctuating activity in motor areas of the brain could bias "free" decisions to act or not.