Behavioral and neurophysiological signatures of interoceptive enhancements following vagus nerve stimulation.

An accruing body of research has shown that interoception (the sensing of signals from the body's internal milieu) relies on both a direct route (afforded by the vagus nerve) and a secondary route (supported by somatosensory mechanisms). However, no study has causally tested the differential role of these pathways, let alone via direct stimulation. To bridge this gap, we tested whether multidimensional signatures of interoception are modulated by noninvasive vagus nerve stimulation (nVNS). Sixty-three participants were divided into an nVNS and a sham-stimulation group. Before and after stimulation, both groups performed a validated heartbeat detection (HBD) task including a genuinely interoceptive condition (monitoring one's own heartbeat) and a control exteroceptive condition (tracking an aurally presented heartbeat). Electroencephalographic signals were obtained during both conditions to examine modulations of the heartbeat-evoked potential (HEP). Moreover, before and after stimulation, participants were asked to complete a somatosensory heartbeat localization task. Results from the interoceptive condition revealed that, after treatment, only the nVNS group exhibited improved performance and greater HEP modulations. No behavioral differences were found for the exteroceptive control condition, which was nonetheless associated with significant HEP modulations. Finally, no between-group differences were observed regarding the localization of the heartbeat sensations or relevant cardiodynamic variables (heart rate and or heart rate variability). Taken together, these results constitute unprecedented evidence that the vagus nerve plays a direct role in neurovisceral integration during interoception. This finding can constrain mechanistic models of the domain while informing a promising transdiagnostic agenda for interoceptive impairments across neuropsychiatric conditions.

Dynamic neurocognitive changes in interoception after heart transplant.

Heart-brain integration dynamics are critical for interoception (i.e. the sensing of body signals). In this unprecedented longitudinal study, we assessed neurocognitive markers of interoception in patients who underwent orthotopic heart transplants and matched healthy controls. Patients were assessed longitudinally before surgery (T1), a few months later (T2) and a year after (T3). We assessed behavioural (heartbeat detection) and electrophysiological (heartbeat evoked potential) markers of interoception. Heartbeat detection task revealed that pre-surgery (T1) interoception was similar between patients and controls. However, patients were outperformed by controls after heart transplant (T2), but no such differences were observed in the follow-up analysis (T3). Neurophysiologically, although heartbeat evoked potential analyses revealed no differences between groups before the surgery (T1), reduced amplitudes of this event-related potential were found for the patients in the two post-transplant stages (T2, T3). All these significant effects persisted after covariation with different cardiological measures. In sum, this study brings new insights into the adaptive properties of brain-heart pathways.

Fatigue in multiple sclerosis is associated with multimodal interoceptive abnormalities.

BACKGROUND: Fatigue ranks among the most common and disabling symptoms in multiple sclerosis (MS). Recent theoretical works have surmised that this trait might be related to alterations across interoceptive mechanisms. However, this hypothesis has not been empirically evaluated. OBJECTIVES: To determine whether fatigue in MS patients is associated with specific behavioral, structural, and functional disruptions of the interoceptive domain. METHODS: Fatigue levels were established via the Modified Fatigue Impact Scale. Interoception was evaluated through a robust measure indexed by the heartbeat detection task. Structural and functional connectivity properties of key interoceptive hubs were tested by magnetic resonance imaging (MRI) and resting-state functional MRI. Machine learning analyses were employed to perform pairwise classifications. RESULTS: Only patients with fatigue presented with decreased interoceptive accuracy alongside decreased gray matter volume and increased functional connectivity in core interoceptive regions, the insula, and the anterior cingulate cortex. Each of these alterations was positively associated with fatigue. Finally, machine-learning analysis with a combination of the above interoceptive indices (behavioral, structural, and functional) successfully discriminated (area under the curve > 90%) fatigued patients from both non-fatigued and healthy controls. CONCLUSION: This study offers unprecedented evidence suggesting that disruptions of neurocognitive markers subserving interoception may constitute a signature of fatigue in MS.