Abstract
We are all deeply saddened by the passing of Dr Mark Hallett, a towering and magnanimous figure in Neurology whose wisdom, measured counsel and kind leadership transformed the study of movement disorders. He was Chief of the Human Motor Control Section, National Institute of Neurological Disorders and Stroke (NINDS), National Institutes of Health (NIH), President of the International Parkinson and Movement Disorder Society, President of International Federation of Clinical Neurophysiology and Vice President of the American Neurological Association (ANA). Whilst he will clearly be remembered for his high-level contributions to science, his influence extended far beyond the laboratory and many of us will have benefitted from his support, wisdom and sage advice as he helped build our international collaborations and strengthened our community of clinicians, academics and students.
Dr. Mark Hallett
Dr. Mark Hallett was one of the leading pioneers of clinical neurophysiology across a range of movement disorders, advancing our understanding of voluntary motor control and the pathophysiology of involuntary movement disorders, including tremor, dystonia and other motoric phenomenology. His research elucidated the physiology underlying free will during movement initiation and self-agency perception following motor execution 1 . His contributions to understanding volitional processes enhanced our knowledge of the dysfunctional brain networks involved in the sense of voluntariness that underlies functional neurological disorders, fundamentally transforming our conceptualization of these conditions 2 . Dr. Hallett preferred the term “functional” rather than “psychogenic” movement disorders, reflecting a more neurobiologically informed conceptual framework that enabled more appropriate treatment strategies that could improve outcomes by reducing stigma for both patients and caregivers.
Understanding the physiological characteristics of movement, as well as the pathophysiology of underpinning movement disorders was the driving force throughout Dr. Hallett's illustrious career. He was one of the first to employ transcranial magnetic stimulation (TMS) to study the pathophysiology of dystonia, revealing deficits in surround inhibition, abnormal sensorimotor integrations, and maladaptive plasticity, which have guided novel therapeutic interventions 3 . Indeed, his leadership in the dystonia community, substantially advanced our understanding of the clinical characteristics and pathophysiological mechanisms underlying diverse dystonia subtypes. His work also elucidated myoclonus subtyping, notably contributing to the recent classification of myoclonus in Axis Ib based on distinctive electrophysiological features 4 . His investigations into brain networks underlying tremor provided crucial insights into the pathophysiology of various tremor sub-types, including Parkinson's disease tremor, essential tremor, and tremor in dystonia 5 . He coined the term “tremor of stability” for Parkinson's tremor, which perfectly summarizes how this type of tremor emerges during motorically static contexts: at rest, during stable posturing (re-emergent tremor), and even after repeated, identical movements 6 . He even envisioned that this observation could link the “oscillation model” of Parkinson's disease (where bradykinesia arises due to beta excess in the basal ganglia) to the “dimmer-switch model” (where Parkinson's tremor is initiated in the basal ganglia and is further amplified in the cerebello-thalamo-cortical circuit), given that beta activity emerges specifically during motorically stable conditions5–7. His work also flagged freezing as network-level disorder involving impaired motor initiation and abnormal cortical–basal ganglia dynamics 8 .
Beyond his outstanding contributions to the neuroscience community and the field of movement disorders, Dr. Hallett was an exemplary mentor, teacher, and true role model for students and fellows. Those who had the privilege of undertaking their fellowships at the National Institutes of Health (NIH), had the privilege of getting to know a scientist of exceptional intellect, precision, remarkable kindness, open-mindedness, humility, and a true sense of humanity. His door was always open for fellows to ask any questions and engage in discussions with valued ideas and creativity. His accessibility, generosity in fostering scientific discussions, and trust profoundly influenced countless trainees and colleagues. One example is the way he combined clinical neurology and neuroscience: he explicitly invited research fellows to join his weekly clinics and contribute to the discussion, thereby using clinical observations to drive scientific questions and vice versa, using neuroscientific models to solve clinical puzzles inspiring a new generation of clinician scientists. Dr. Hallett demonstrated genuine concern for those around him and remembered small details of each fellow. His dedication to trainees extended beyond formal training as he continued to provide support, guidance, and professional opportunities throughout the careers of so many. The “Former Hallett Fellows Get-Togethers” at each International Parkinson's Disease & Movement Disorders Conference were the highlight of the meeting, where new international collaborations between his former fellows were born. He knew like no other that collaboration was the key towards advancing science to improve the quality of lives for patients.
Many of us will fondly recall his calm, diplomatic voice in complex debates and his excellence in synthesizing its main points. He was a unifier who listened before speaking; a mentor who built teams rather than empires. He will be deeply missed by his mentees, his collaborators and the many patients whose lives he touched. In his absence, his humility, intellectual generosity and spirit of unity will remain an inspiration for all who follow his path. We will continue to honor his vision and dedication by contributing to our field and society in his spirit. He will be remembered with the deepest respect and gratitude, we will dearly miss him but Dr. Hallett's legacy and contributions will endure forever.
