Error monitoring when no errors are possible: Arbitrary free-choice decisions invoke error monitoring processes.

Some decisions make a difference, but most are arbitrary and inconsequential, like which of several identical new pairs of socks should I wear? Healthy people swiftly make such decisions even with no rational reasons to rely on. In fact, arbitrary decisions have been suggested as demonstrating "free will". However, several clinical populations and some healthy individuals have significant difficulties in making such arbitrary decisions. Here, we investigate the mechanisms involved in arbitrary picking decisions. We show that these decisions, arguably based on a whim, are subject to similar control mechanisms as reasoned decisions. Specifically, error-related negativity (ERN) brain response is elicited in the EEG following change of intention, without an external definition of error, and motor activity in the non-responding hand resembles actual errors both by its muscle EMG temporal dynamics and by the lateralized readiness potential (LRP) pattern. This provides new directions in understanding decision-making and its deficits.