Reduced drift rate: a biomarker of impaired information processing in functional movement disorders, 2019, Edwards et al

[Andy]

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.

Open access, https://academic.oup.com/brain/advance-article/doi/10.1093/brain/awz387/5684838

The disability associated with functional disorders is often compounded by additional symptoms across body systems that are thought to stem from the same neurobiological vulnerabilities. For example, common neuropsychological symptoms are seen across a range of functional syndromes such as fibromyalgia, chronic fatigue syndrome and functional neurological syndromes with theoretical deficits in attention and slowed information processing proposed (Teodoro et al., 2018). Such findings suggest there should be viable biomarkers; however, standard objective cognitive testing frequently reveals a discordance with subjective symptoms (Teodoro et al., 2018). Identifying quantifiable tools for the assessment of abnormalities in cognitive processing in functional disorders is therefore an unmet research need and if available could have multiple applications in clinical and rehabilitative practice.

 

[Hoopoe]

The experiment performed lacks a sick control group (it does not say they were sick so I assume they were healthy). Comparison with other patients with similar illnesses could invalidate the idea that these findings are specific for functional neurological disorders or that functional neurological disorders have a common cause.

 

[Sean]

Functional neurological disorder is a common and phenomenologically diverse condition.

Just like that.

Move attention from the unresolved is this classification (and its implied causation) correct?, to the definitive instructions on how to deal with it.

Problem solved!

 

[Hoopoe]

And the how to deal with it part is therapy with indoctrination likely to lead to response bias in uncontrolled clinical trials that are then cited as evidence the whole thing is correct.

 

[Mithriel]

Resultant disability is caused by both the dominant clinical presentation, e.g. paralysis or tremor and additional symptomatology such as cognitive symptoms.

My friends with MS talk about the cognitive problems as being one of the worst symptoms (after fatigue!).

Umm, I did not read the paper but how did they measure "the temporal discrimination threshold" if someone has a movement disorder it will take them much longer to react to anything as reaction times are measured using movements.

The fact I automatically consider that they did not take that into account shows how bad I have found their research to be.