Engelen, T. | Search bioRxiv

bioRxiv2024

The cardiac, respiratory and gastric rhythms independently modulate corticospinal excitability

Interoception refers to the brains sensing of internal body state and encompasses various bodily systems, notably the cardiac, respiratory and gastric rhythms. Beyond their roles in physiological regulation and emotional states, each of these visceral rhythms has been shown to influence brain activity and cognition, prompting for the development of various interpretative functional frameworks. However, both experimental data and functional hypothesis leave it unclear whether and how each visceral rhythm acts simultaneously and independently on brain activity. Here we address this question by measuring in human participants how the corticospinal excitability of the motor system varies with the phase of each of the three visceral rhythms. We applied single pulse Transcranial Magnetic Stimulation (TMS) over the hand region in primary motor cortex to elicit Motor Evoked Potentials (MEP), whose amplitude reflects corticospinal excitability, and tested whether MEP amplitude depends on the phase of the simultaneously measured cardiac, respiratory and gastric rhythms. All three visceral rhythms were coupled to motor excitability with similar effect sizes at the group level. However, we found no relation between coupling strengths: participants displaying high coupling with one organ did not necessarily display high coupling to the other organs. These results indicate that independent mechanisms could underly the coupling between the cardiac, respiratory and gastric rhythm and motor excitability. We further introduce the concept of individual interoceptive profiles, and show that such interoceptive profiles obtained from objective coupling strength measures were not explained by self-reported awareness of the organ. Altogether our results call for refined specifications of the frameworks offering a functional or clinical interpretation of viscera-brain coupling taking into account both independent mechanisms and individual interoceptive profiles.

neuroscience↗

bioRxiv2018

Faded visual afterimages reappear after TMS over early visual cortex

In the complete absence of small transients in visual inputs (e.g. by experimentally stabilizing an image on the retina, or in everyday life during intent staring), information perceived by the eyes will fade from the perceptual experience. While the mechanisms of visual fading remain poorly understood, one possibility is that higher-level brain regions actively suppress the stable visual signals via targeted inhibitory feedback onto Early Visual Cortex (EVC). Here, we used positive afterimages and multisensory conflict to induce gestaltlike fading of participants own hands. In two separate experiments, participants rated the perceived quality of their hands both before and after Transcranial Magnetic Stimulation (TMS) was applied over EVC. In a first experiment, triple pulse TMS was able to make a faded hand appear less faded after the pulses were applied, compared to placebo pulses. A second experiment demonstrated that this was because triple pulse TMS inoculated the removed hand from fading over time. Interestingly, TMS similarly affected the left and right hand, despite being applied only over right EVC. Together, our results suggest that TMS can lift inhibitory processes in EVC and reverse the effects of visual fading. And it might do so by crossing transcollosal connections, or via multimodal integration sites in which both hands are represented.

neuroscience↗