Reduced drift rate: a biomarker of impaired information processing in functional movement disorders

Abstract

Functional neurological disorder is a common and phenomenologically diverse condition. Resultant disability is caused by both the dominant clinical presentation, e.g. paralysis or tremor and additional symptomatology such as cognitive symptoms. Recently the similarity of neuropsychiatric profiles across a range of functional syndromes has been highlighted. This is suggestive of a common underlying mechanism with a theoretical deficit of information processing proposed. Identification of an experimental biomarker for such deficits could offer novel assessment and therapeutic strategies. In this study, we took the temporal discrimination threshold as a paradigm that can be used to model sensory processing in functional movement disorders. Our hypothesis was that we would be able to delineate markers of slowed information processing in this paradigm removed from the phenomenological presentation with a movement disorder. We recorded both response accuracy and reaction time in a two-choice temporal resolution/discrimination task in 36 patients with functional movement disorders and 36 control subjects. A psychometric function was fitted to accuracy data for each individual revealing both abnormally high threshold values (P = 0.0053) and shallow psychometric slopes in patients (P = 0.0015). Patients with functional movement disorders also had significantly slower response times (P = 0.0065). We then used a well-established model for decision-making (the drift diffusion model) that uses both response accuracy and reaction time data to estimate mechanistic physiological dimensions of decision-making and sensory processing. This revealed pathologically reduced drift rate in the patient group, a parameter that quantifies the quality and rate of information accumulation within this sensory task (P = 0.002). We discuss how the deficits we observed in patients with functional movement disorders are likely to stem from abnormal allocation of attention that impairs the quality of sensory information available. Within a predictive coding framework sensory information could be down-weighted in favour of predictions encoded by the prior. Our results therefore offer a parsimonious account for a range of experimental and clinical findings. Reduced drift rate is a potential experimental marker for a generalized deficit in information processing across functional disorders that allows diverse symptomatology to be quantified under a common disease framework.

Functional motor disorders associated with other neurological diseases: Beyond the boundaries of “organic” neurology

“…In clinical practice, the overlap between FMDs and other neurological disorders is underrecognized, as shown by the results of multivariate regression analysis demonstrating that time to reach a diagnosis of FMDs in patients with neurological diseases was significantly longer. These data highlight not only the diagnostic challenge when dissecting functional symptoms from other neurological diseases, but also how in modern neurology there is still a dichotomy between “organic” and “functional” disorders, despite strong evidence of shared neurophysiological abnormalities [26–28] or distinctive psychophysical [29,30] and neuroimaging attributes [31] of FMDs. The demonstration of biological abnormalities represents a strong argument against the term “organic”, which historically has been used to label conditions characterized by structural or other pathological changes, but it has also been adopted for neurological diseases such as migraine or genetic epilepsy or dystonia, in which no structural change is found on brain magnetic resonance imaging [32], despite being determined by network abnormalities within the central nervous system.…”

A framework for understanding the pathophysiology of functional neurological disorder

“…The predictive coding framework is one computational approach to predictive processing which uses the application of Bayesian mathematical principles to develop models of causal inference. Using computational modeling of behavior on a timed-decision task, 50 individuals with motor FND showed deficits in decision-making and sensory processing. From a predictive coding perspective, the authors interpreted this as evidence that predictions were overemphasized in FND relative to the sensory information.…”

Evaluation of Individualized Multi‐Disciplinary Inpatient Treatment for Functional Movement Disorders

“…However, the duration of interventions and therapeutic settings varied considerably between these studies and there are still knowledge gaps with respect to prognostic factors and long-term outcome of different therapeutic strategies. Against this background and because there is some evidence that FMDs reflect maladaptive integration of psychological and physical functions 12,13 one of the open questions is whether patients would benefit from a combination of psychological and physical treatment in a multi-disciplinary team as has recently been suggested. [14][15][16][17][18][19] To this end, we here report results of a rater-blinded retrospective evaluation of a multi-disciplinary inpatient intervention including neurologists, psychologists, and physiotherapists in a group of 31 patients with FMD from a large center for medical treatment and rehabilitation for patients with Parkinson's disease and other movement disorders.…”

“…However, the duration of interventions and therapeutic settings varied considerably between these studies and there are still knowledge gaps with respect to prognostic factors and long‐term outcome of different therapeutic strategies. Against this background and because there is some evidence that FMDs reflect maladaptive integration of psychological and physical functions 12 , 13 one of the open questions is whether patients would benefit from a combination of psychological and physical treatment in a multi‐disciplinary team as has recently been suggested. 14 , 15 , 16 , 17 , 18 , 19 …”