A Bayesian account of ‘hysteria’, 2012, Edwards et al

A Bayesian account of ‘hysteria’
Mark J. Edwards, Rick A. Adams, Harriet Brown, Isabel Pareés and Karl J. Friston

This article provides a neurobiological account of symptoms that have been called ‘hysterical’, ‘psychogenic’ or ‘medically unexplained’, which we will call functional motor and sensory symptoms.

We use a neurobiologically informed model of hierarchical Bayesian inference in the brain to explain functional motor and sensory symptoms in terms of perception and action arising from inference based on prior beliefs and sensory information. This explanation exploits the key balance between prior beliefs and sensory evidence that is mediated by (body focused) attention, symptom expectations, physical and emotional experiences and beliefs about illness. Crucially, this furnishes an explanation at three different levels: (i) underlying neuromodulatory (synaptic) mechanisms; (ii) cognitive and experiential processes (attention and attribution of agency); and (iii) formal computations that underlie perceptual inference (representation of uncertainty or precision).

Our explanation involves primary and secondary failures of inference; the primary failure is the (autonomous) emergence of a percept or belief that is held with undue certainty (precision) following top-down attentional modulation of synaptic gain. This belief can constitute a sensory percept (or its absence) or induce movement (or its absence). The secondary failure of inference is when the ensuing percept (and any somatosensory consequences) is falsely inferred to be a symptom to explain why its content was not predicted by the source of attentional modulation.

This account accommodates several fundamental observations about functional motor and sensory symptoms, including: (i) their induction and maintenance by attention; (ii) their modification by expectation, prior experience and cultural beliefs and (iii) their involuntary and symptomatic nature.

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The main problem with this (Mark Edwards) paper is that the model is purely hypothetical, it still has no plausible empirical basis (10 years later).

Another research group tried to use this 'explanation' for fatigue and I discussed some of the problems with the 'Bayesian' hypothesis a few years ago (namely the model has no theoretical application to fatigue or pain sensations at all):
https://www.s4me.info/threads/towar...-greenhouse-tucknott-et-al.16228/#post-280690

Notions of prediction models only apply to functions of the brain that actually require detailed predictions - the brain does not need to make detailed predictions about pain or fatigue because there is no need for real-time control over those signals, so there is no complex Bayesian model of prediction along the lines of what the authors describe for those sensory inputs. Instead, spinal reflexes from pain receptors cover the rapid responses to nociception type afferent stimulus which covers the immediate latency gap, before slower behavioural responses can be chosen and put into action. Secondly, the often hypothesised problem in chronic pain conditions is increased sensitivity to afferent inputs, which is the opposite to what is predicted by Edwards' FND model.

The brain does however make detailed predictions about motor function/control (including things like object tracking in the visual cortex), because our nerves aren't fast enough to provide real-time control. So the brain instead has a detailed proprioceptive model of the body which it then compares (with latency) with the afferent feedback to make small adjustments to the model/expectations over time. It is this sort of model of motor control that could in principle be theoretically modelled using a Bayesian approach.

The main problem is that there is no empirical basis for their model. They are proposing that in functional motor disorders, the brain decides to over focus on precision (trying to predict noise) and then increasing the feedback at higher hierarchical levels of processing so that the sensory feedback is decreased and the system is effectively marking it's own homework. First of all, this is the opposite to how Bayesian models are normally constructed - when prediction errors occur, the system increases lower level feedback and tries to reduce the precision of the predictions.

But the primary characteristic of "functional" motor disorders is that there is more than one motor control programme, each of which involves different functional brain connectivity. The discussion of attentional mechanisms in their model doesn't account for this switching at all, it instead suggests a sudden shift in the single motor programme, which has no empirical basis - our models of proprioception and motor control are developed over a lifetime, they aren't suddenly re-written in a few moments. It also doesn't explain why visual feedback is also suddenly ignored when the patient suffers from the motor disorder symptom. Whereas empirically (for these disorders), attentional process do seem to switch to an entirely different motor control programme.

The more I think about these disorders, the more that I think some of these "functional" motor disorders may be a neurodevelopmental syndrome (slightly akin to autism), the key mistake being that the brain has developed and maintained more than one motor control programme in parallel over time.

 

I appreciate you summarizing and explaining the main problems with this unproven theory. Thank you @Snow Leopard.

 

From Stone's writings they seem to believe that they have finally put hysteria on a scientific basis, hence the continual assertion that it is an opt in disease nowadays rather than what you get when you can't find anything else. The fact this is delusional I leave aside for now.

Snow Leopard said

There must be many neurodevelopmental problems to a greater or lesser degree. If they are not bad enough to prevent the passing on of genes they will not be selected out so a lot of the differences between people in general life as well as medicine.

Having said that, the majority of people with an FND diagnosis get it because they go to a doctor about a new condition and there is no reason to believe they do not have a normal type of disease, just one that we can't test for yet.

Hysteria was a nineteenth century mistake and has no place in modern medicine.