A neuroscientific perspective
FND is a disorder characterised by the development of faulty perceptual reasoning about sensorimotor and emotionally valent phenomena (Drane et al., 2021).
Perceptual reasoning refers to the process by which an individual forms beliefs about the causes and effects of events occurring inside and outside the body (Teufel and Fletcher, 2020). As sensorimotor and emotionally valenced phenomena reflect beliefs rather than actual events, it is normal for individuals to consequently interpret them as 'true' (Drane et al., 2021).
In the field of systems computational neuroscience, the idea that it is the brain's predictions that influence how we perceive and interpret the world around us has long been established. Predictive processing theory provides an important framework for understanding neuropsychiatric disorders, where the mechanisms by which prior information influences perception are thought to malfunction, leading to the generation of suboptimal models of predictions about the world (Teufel and Fletcher, 2020).
Bayesian models have been extremely important in advancing our understanding of how the brain works, as they are based on the assumption that perceptual and cognitive reasoning is not simply about current information, but is primarily shaped by predictions based on prior information about the structure and function of the environment around us.
Explanation of terms
Bayesian decision theory
A theory based on the principles of Bayesian probability, describing the optimisation of decisions through the use of probability distributions that quantify the prior probabilities of events or states.
Perceptual and cognitive reasoning
The process by which perceptions and beliefs are formed from a combination of evidence from sense/sensory perceptions and information based on prior experience or knowledge.
Predictive coding
A family of algorithms that capture how the brain performs probabilistic inference by calculating the mismatch between predicted and actual signal magnitude.
Forecasts
Forecasts are generated based on estimates of disregarded or missing information within the forecast processing frame. Information processing may (but need not) be largely forward-looking.
A broader view of the theory of predictive processing is provided by the field of cybernetics, whose name derives from the Greek term for helmsman, and thus combines the idea that a successful agent (i.e. a person capable of autonomy and self-regulation in the context of responding to different situations offered by the environment) must be able to control his environment, not even in the sense of being overly restrictive, but in responding adaptively to the changes that occur in the environment. The concept of predictive coding has had an immeasurable impact on the understanding of neural information processing and the formalisation of the mechanisms behind neuropsychiatric disorders (Taufel and Fletcher, 2020).
In the context of the theory, neurons that transmit predictions about sensory states (prior belief or expectation) communicate with neurons that detect prediction errors, with the aim of developing a generative model that will represent the most optimal explanation for the perceptual information received. When there are many prediction errors, the generative model is updated (Drane et al., 2021), and as the discrepancies decrease, the brain seeks to obtain more evidence for its internal model of the world (Sojka et al., 2018). Thus, perceptual inference could be described as a process during which prediction error is reduced by a change in expectation (Sojka et al., 2018).
In a study by Sadnicka et al., (2020), temporal discrimination was used to study information processing in patients with the motor subtype of functional neurological disorder. They found that FND patients have prolonged and abnormal reaction time profiles, which are driven by increased uncertainty in decision-making. The authors interpreted the results as a consequence of impaired attentional mechanisms, where in FND excessive and body-focused attention competes with other attentional resources, increasing the level of distraction and, consequently, reaction time itself.
The authors also discussed the results in the context of active reasoning, where information travels bilaterally from both the 'bottom-up' (i.e. proprioceptive information or information coming from the sensory organs) and the 'top-down ' (i.e. prior information or information coming from the cortex), and the brain tries to minimise the difference between the two different sources of information. Functional neurological disorder is thought to be characterised by abnormally strong and accurate prior information, which is thought to underlie both the symptoms and the deficit in the sense of self agency over movement and/or overwhelming symptoms. This also explains the reduced quality of sensory information available to the individual during decision-making tasks.
-
Drane, D. L., Fani, N., Hallett, M., Khalsa, S. S., Perez, D. L., and Roberts, N. A. (2021). A framework for understanding the pathophysiology of functional neurological disorder. CNS Spectrums, 26(6), 555-561.
Sadnicka, A., Daum, C., Meppelink, A. M., Manohar, S., and Edwards, M. (2020). Reduced drift rate: a biomarker of impaired information processing in functional movement disorders. Brain, 143(2), 674-683.
Sojka, P., Bareš, M., Kašpárek, T., and Světlák, M. (2018). Processing of emotion in functional neurological disorder. Frontiers in psychiatry, 9, 479.
Teufel, C., and Fletcher, P. C. (2020). Forms of prediction in the nervous system. Nature Reviews Neuroscience, 21(4), 231-242.