Identifying the mechanisms that integrate cardiac interoception with exteroception – CAREX

The last 10 years have uncovered a treasure-trove of experimental evidence that cardiac signals influence exteroceptive (somatosensory, visual) sensory processing, through both cardiac cycle effects (CCEs) and heartbeat evoked responses (HERs). The interplay between exteroception and interoception is sometimes competitive (i.e., attending the interoceptive or the exteroceptive stream), sometimes integrative (i.e., integration of interoception and exteroception contributes to conscious experience by relating external stimulus to the neural representation of the living organism). The rules governing integration vs. competition are not known. Besides, evidence has remained mostly correlational and the nature of the mechanisms involved is largely unknown. Here, we explore the mechanisms of influence of cardiac inputs on exteroceptive signals, in an integrated approach with an interplay of biologically plausible computational modelling and experiments in humans.
In WP1, we will construct a biophysically-based cardio-cortical model, where a cortical mini-column, with ongoing and sensory-evoked activity, is modulated by a dynamic cardiac oscillator (which produces synthetic realistic electrocardiograms). Using this model, we explicitly delineate the mechanisms of this ascending influence. We will validate the model by optimizing the parameters of coupling and the dynamics of the cortical and cardiac modules to fit existing electrocardiogram and intracranial data.
In parallel (WP2), we will obtain further EEG data in female and male participants probing the interaction between cardiac and tactile processing with a critical manipulation of self-relevance, a factor that might govern the balance between cardio-tactile competition vs. cardio-tactile integration (WP2). We posit that self-relevance is a combination of exteroceptive stimulus properties and internal state. Stimulus-induced self-relevance is operationalized here as a tactile stimulus delivered with a near sound (more self-relevant), or with a far sound (less self-relevant), and fluctuations in self-related internal state are indexed by source-localized HERs. We hypothesize that HERs in somatosensory cortex compete with the tactile input and slow down tactile reaction times, while HERs in the self-related default mode network combine with the tactile input according to self-relevance as determined by the near or far sound. WP2 will also shed light on the existence or absence of functional links between CCEs and HERs.
In WP3, we will expand the model developed in WP1 to confront the data from WP2 to test several mechanistic hypotheses on the interactions between intero- and exetero-ceptive stimuli. The aims are i) to determine the conditions under which interactions between exteroceptive inputs and cardiac cycle and/or HERs occur either in a concerted or independent manner in the primary somatosensory cortex, ii) determine whether and how self-relevance can modulate the balance between integration and competition – does self-relevance exert its influence through neuromodulatory mechanisms directly on the somatosensory cortex or is it routed through a complementary default-mode-network module? and iii) use the model to determine the conditions under which the HERs and CCEs either compete or integrate with the sensory information, with different hypotheses on the implementation of self-relevance.
In summary, the project will elaborate neural mechanisms for interoceptive modulation of sensory perception mediated by cardio-cortical coupling. Altogether, this project represents an important step to move the field forward, beyond the accumulation of correlative evidence. It also offers a first step toward a mechanistic understanding of the widely employed but under-specified notion of self-relevance, with potential long-term applications in psychiatry as well as in artificial intelligence and robotics.