Attenuated heartbeat-evoked potentials in functional neurological disorder

The pathophysiology of functional neurological disorders (FND) has been discussed to include dysfunctions in interoception, the modality about perceiving and processing internal bodily signals. However, findings on abnormal interoception in FND have been inconsistent and mainly limited to measures of accuracy and self-report. Interoceptive neuronal markers have only been investigated in specific symptoms, and interoceptive attentional modulation has been completely overlooked. In a cohort of patients with mixed FND (N = 44) and sex- and age-matched healthy controls (N = 48), we set out to assess first, interoceptive accuracy with an adapted version of the heartbeat counting task; secondly, interoceptive self-report with two different questionnaires (Multidimensional Assessment of Interoceptive Awareness and Interoceptive Accuracy Scale) and thirdly, neuronal trait markers under attention modulation by measuring heartbeat-evoked potentials, the neurophysiological signal related to the heartbeat. We searched for group differences (FND versus controls) across two attentional conditions, by asking participants to either focus on their heartbeat (i.e. interoceptive condition) or an external sound (i.e. exteroceptive control condition). Cardiac covariates (heart rate and heartrate variability or normalized electrocardiac amplitude) were included in the analysis as control. Patients with FND scored lower in both interoceptive self-report questionnaires (P < 0.020), reported higher difficulty concerning the focus towards their heartbeat (P = 0.004), while no significant difference was found in interoceptive accuracy using the heartbeat counting task (P = 0.060). Global field analyses revealed short intervals of a group-by-condition interaction in global field power (285-298 ms) and topographical differences (310-321 ms) confirming that patients with FND have lower overall activity and frontal deactivation during the interoceptive condition. Preselected electrodes for a targeted analysis of the heartbeat-evoked potential based on earlier work revealed a medium effect size attenuation at the frontal-lateralized F8 electrode at 250-595 ms following R-peak for patients with FND (P = 0.028), surviving correction for cardiac covariates. Exploratory analyses further identified an earlier difference at F1 (185-210 ms post-R-peak) in FND patients for interoceptive attention (P = 0.001), also surviving covariate control. While behavioural interoceptive accuracy was marginally preserved, these findings indicate overall altered interoceptive processing in FND, characterized by reduced self-report and difficulty to focus on cardiac signals, along with attenuated neural processes, especially in frontal-lateralized regions that further depend on attentional mechanisms. By identifying objective neural markers of interoceptive dysfunction in FND, this study highlights the involvement of interoception in a multidimensional assessment including the relevance of attention.