Voluntary or involuntary? A neurophysiologic approach to functional movement disorders

Patients with functional movement disorders (FMD) experience movements as involuntary that share fundamental characteristics with voluntary actions. This apparent paradox raises questions regarding the possible sources of a subjective experience of action. In addition, it poses a yet unresolved diagnostic challenge, namely how to describe or even quantify this experience in a scientifically and clinically useful way. Here, we describe recent experimental approaches that have shed light on the phenomenology of action in FMD. We first outline the sources and content of a subjective experience of action in healthy humans and discuss how this experience may be created in the brain. Turning to FMD, we describe implicit, behavioral measures that have revealed specific abnormalities in the awareness of action in FMD. Based on these abnormalities, we propose a potential, new solution to the paradox of volition in FMD.