Visceral Signals Shape Brain Dynamics and Cognition
Authors: Damiano Azzalini, Ignacio Rebollo, Catherine Tallon-Baudry
Journal: Trends in Cognitive Sciences (2019)

Visceral signals automatically contribute to shaping spontaneous large-scale brain dynamics, even when not consciously experienced. To what extent ascending visceral signals can be consciously perceived remains a matter of debate.Neural and behavioral responses to external stimuli can be influenced by temporal contingencies between heartbeats and stimuli.Beyond temporal contingencies, the transient neural response evoked by each heartbeat plays a role in cognitive functions that are usually studied separately, such as emotion, self-related cognition, and also subjective visual perception, calling for new theoretical perspectives.Neural responses to heartbeats may be involved in the generation of the unified viewpoint of consciousness. Most research in cognitive neuroscience explores how external stimuli are processed by the brain. However, the brain also receives input from the internal body. We discuss here how the heart and gastrointestinal (GI) tract intrinsically generate their own electrical activity, thereby continuously sending information to the brain. These ongoing ascending signals actively shape brain dynamics at rest, complementing canonical resting-state networks (RSNs). Cardiac signals also influence the processing of external sensory information and the production of spontaneous, internal cognition. These findings are discussed in relation to interpretative frameworks regarding the functional role of visceral inputs. This active field of research offers a unique opportunity to draw new theories blurring the border between cognition, emotion, and consciousness, as well as between mind and body. Most research in cognitive neuroscience explores how external stimuli are processed by the brain. However, the brain also receives input from the internal body. We discuss here how the heart and gastrointestinal (GI) tract intrinsically generate their own electrical activity, thereby continuously sending information to the brain. These ongoing ascending signals actively shape brain dynamics at rest, complementing canonical resting-state networks (RSNs). Cardiac signals also influence the processing of external sensory information and the production of spontaneous, internal cognition. These findings are discussed in relation to interpretative frameworks regarding the functional role of visceral inputs. This active field of research offers a unique opportunity to draw new theories blurring the border between cognition, emotion, and consciousness, as well as between mind and body. 'The brain is bombarded by information from the environment.' This sentence, in one form or another, acts as an introduction to many textbooks and articles about perception and cognition (e.g., [1.Kandel E.R. et al.Nerve cells, neural circuitry, and behavior.in: Kandel E.R. Principles of Neural Science. 5th edn. McGraw-Hill Professional, 2013: 21-38Google Scholar]). Although the internal environment is sometimes mentioned, the vast majority of the experimental work pertains to the processing of external stimuli. This reflects the dominant paradigm in cognitive neuroscience, where an agent collects information from the external environment via the senses and then reacts to this environment by producing actions (Figure 1A ). By studying the brain, cognitive neuroscience has been highly successful at providing mechanistic explanations of behavior and at revealing the existence of hidden rules and variables, such as for instance the reward prediction error [2.Schultz W. et al.A neural substrate of prediction and reward.Science. 1997; 275: 1593-1599Crossref PubMed Scopus (4664) Google Scholar]. In the following we review extensions of this paradigm that include the interplay between the brain and the internal bodily environment (Figure 1B) to account for the neural implementation of cognition. More specifically, we focus on the heart and the gastrointestinal (GI) tract (see Glossary), and refer the reader to [3.Critchley H.D. Harrison N.A. Visceral influences on brain and behavior.Neuron. 2013; 77: 624-638Abstract Full Text Full Text PDF PubMed Scopus (326) Google Scholar] for a comprehensive review of other types of somatic influences, including notably humoral and immune factors. There are several reasons why cardiac and GI inputs might influence brain dynamics and cognition. The first distinctive feature of these two organs is that they generate their own intrinsic oscillatory electrical activity, even when disconnected from the brain, from the first weeks of gestation until death. We will detail later the intrinsic properties of the heart and the GI tract – but consider an isolated heart in a vat, waiting to be transplanted: it is beating. The heart thus differs from respiration, for instance [4.Tort A.B.L. et al.Respiration-entrained brain rhythms are global but often overlooked.Trends Neurosci. 2018; 41: 186-197Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar]. A respiratory system disconnected from the brain is inert, because breathing commands are generated in the central nervous system (CNS). By contrast, both the heart and the GI tract can be considered as intrinsic oscillators that continuously send information to the brain. Visceral signals can thus be considered as stimuli that influence spontaneous brain dynamics. Second, cardiac and GI signals are characterized by intrinsic time constants in the sub-second (heart) to 20 s range (stomach), compatible with the timescale of cognition. Importantly, these signals reach not only brainstem nuclei but also many subcortical and cortical regions involved in cognitive tasks. Last but not least, monitoring internal visceral variables is a core function of the CNS, and the drive behind many life-sustaining behaviors that are tightly related to high-level sensory and cognitive functions. For instance the basic behavior of searching for food is motivated by internal states such as hunger, and requires visual search, action planning, navigation, and learning and memory. Given all these features, cardiac and GI signals stand out as a continuous source of information that may influence both brain dynamics and high-level cognition. However, experimental research has so far mostly focused on descending influences from brain to heart, analyzing in particular how pain, stress, physical effort, and emotion affect heart rate or heart-rate variability (HRV). In this article we will focus on ascending pathways from viscera to brain, their impact on resting-state brain dynamics, and their influence not only on emotions but also on perception and cognition. The potential role of visceral signals in emotions, perception, and cognition is thought to originate from a common low-level function, homeostatic/allostatic regulation, that would have been harnessed by evolution to develop other, higher-level processes [5.Smith R. et al.The hierarchical basis of neurovisceral integration.Neurosci. Biobehav. Rev. 2017; 75: 274-296Crossref PubMed Scopus (40) Google Scholar]. Homeostasis, a concept identified by Claude Bernard in the 19th century, refers to the automatic adjustment of physiological variables within a given range. 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Other types of allostatic regulation result in adjustments of the cardiovascular system, for example in situations involving novelty, threat, and stress, or physical effort, with potential long-term consequences on health [8.McEwen B.S. Seeman T. Protective and damaging effects of mediators of stress. Elaborating and testing the concepts of allostasis and allostatic load.Ann. N. Y. Acad. Sci. 1999; 896: 30-47Crossref PubMed Google Scholar]. Allostatic regulations have been most often studied from the perspective of a descending control from brain to body, for instance a change in heart rate induced by a preparatory cue, and related to arousal. However, ascending inputs are required to inform the brain about current bodily states, as stated for instance in the recent reformulation of allostatic regulations in the predictive coding framework [9.Seth A.K. Interoceptive inference, emotion, and the embodied self.Trends Cogn. 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Bodily signals, and in particular visceral signals, have long been associated with emotions. We tend to spontaneously associate different emotions with different combinations of bodily regions in a stereotypical manner, as recently shown in a set of elegant behavioral experiments [16.Nummenmaa L. et al.Bodily maps of emotions.Proc. Natl. Acad. Sci. U. S. A. 2014; 111: 646-651Crossref PubMed Scopus (138) Google Scholar, 17.Nummenmaa L. et al.Maps of subjective feelings.Proc. Natl. Acad. Sci. U. S. A. 2018; 115: 9198-9203Crossref PubMed Scopus (1) Google Scholar]. However, the causal role of visceral signals in the generation of emotions is controversial. To briefly summarize a century-old debate, some [18.James W. The Principles of Psychology. Heny Holt, 1890Crossref Google Scholar, 19.Damasio A.R. The somatic marker hypothesis and the possible functions of the prefrontal cortex.Philos. Trans. R. Soc. Lond. Ser. B Biol. 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Another line of proposals refers to the notion of the bodily self, viewed as a multisensory construct (Figure 1D) where tactile, proprioceptive, vestibular, and visceral signals are combined to define the sense of body location in space and the sense of body ownership [9.Seth A.K. Interoceptive inference, emotion, and the embodied self.Trends Cogn. Sci. 2013; 17: 565-573Abstract Full Text Full Text PDF PubMed Scopus (433) Google Scholar, 26.Blanke O. et al.Behavioral, neural, and computational principles of bodily self-consciousness.Neuron. 2015; 88: 145-166Abstract Full Text Full Text PDF PubMed Scopus (169) Google Scholar]. 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Because the temporal coincidence between inputs is a key feature of multisensory integration, this hypothesis predicts that the relative timings of visceral and external signals should be relevant. Our own position is that the neural monitoring of cardiac and GI inputs plays a key role in the generation of first-person perspective by creating a subject-centered frame of reference [28.Park H.D. Tallon-Baudry C. The neural subjective frame: from bodily signals to perceptual consciousness.Philos. Trans. R. Soc. Lond. Ser. B Biol. Sci. 2014; 36920130208Crossref PubMed Scopus (0) Google Scholar, 29.Tallon-Baudry C. et al.The neural monitoring of visceral inputs, rather than attention, accounts for first-person perspective in conscious vision.Cortex. 2018; 102: 139-149Crossref PubMed Scopus (7) Google Scholar]. First-person perspective refers to the unique bodily-centered viewpoint that we have on the environment [30.Blanke O. Metzinger T. 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Because visceral signals are constantly present and distributed across a large number of cortical and subcortical areas, they could provide such an input (Figure 1E) and hence help in binding those maps together into the integrated viewpoint of first-person perspective. In this view, visceral signals would play a role not only in emotions and bodily self but also more generally in conscious perception and cognition (Figure 1F). The different perspectives discussed here share common features, and are related to one another. The first-person perspective account can encompass the proposals related to both emotions and bodily self because both are necessarily experienced from the first-person perspective. In addition, the viewpoint from which we experience the world originates from body location, an important component of the bodily self [30.Blanke O. Metzinger T. Full-body illusions and minimal phenomenal selfhood.Trends Cogn. 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Finally, the different accounts use different levels of descriptions and propose different mechanisms: convergence in the insula, multisensory integration, or generation of a subject-centered frame of reference. Before reviewing experimental evidence on the functional role of visceral inputs, it is worth reviewing the known facts – and the many unknowns – about the generation of visceral signals, and their transduction and transmission up to their central representation. Both the heart and the GI tract are endowed with an intrinsic nervous system [41.Gershon M.D. The enteric nervous system: a second brain.Hosp. Pract. 1999; 34: 31-52Crossref PubMed Google Scholar, 42.Furness J.B. The enteric nervous system and neurogastroenterology.Nat. Rev. Gastroenterol. Hepatol. 2012; 9: 286-294Crossref PubMed Scopus (439) Google Scholar, 43.Armour J.A. et al.Gross and microscopic anatomy of the human intrinsic cardiac nervous system.Anat. 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