The brain and the internal state of the body

embodiment, interoception, visceral perception

Hugo Critchley et al. (2004) state that (pg.189) “the internal state of the body is conveyed through a dedicated lamina-1 spinothalamocortical pathway that converges with vagal afferents”. These afferent nerves are noteworthy for the smallness of their diameter in comparison with the larger afferents that apparently deal with proprioception, the perception of the body in space. Physiologist Bud Craig has written that the difference in size of these nerves signify “a simple physiological distinction between the inside and the outside of the body.’’ (Craig, 2002, p. 657)

The heart connects to the brain through the autonomic or visceral nervous system, traditionally thought to operate mostly without conscious control (breathing being a notable exception).  The sympathetic projections of the autonomic system can increase heart rate in stressful situations, while the parasympathetic fibers slow the heart down when appropriate. There are also two sets of nerves connecting heart and brain:  spinal nerves,with the dorsal root containing afferent sensory projections, and the ventral root for efferent motor fibers. In addition to spinal nerves, the vagus nerve supplies parasympathetic fibers including mostly (85%) afferent fibers, while the rest are brain-to-viscera efferent (brain to motor) fibers that project from the medulla oblongata in the brainstem, and which, if working properly, can rapidly increase or decrease heart rate as needed via innervation of the cardiac muscle. The afferent fibers projecting from viscera to brainstem do so viscerotopically, preserving information about spatial extension or location of the viscera in the body that is subsequently processed (many researchers would say “represented”) in the brain.

Evolutionarily/phylogenetically ancient structures such as the nucleus of the solitary tract and the pons transform the incoming afferent “signal” from viscera such as the heart, and eventually pass it along to the “gateway to the cortex” or thalamus. From there thalamo-cortical fibers project to regions such as insula, cingulate gyrus, and somatosensory and orbitofrontal cortices, regions implicated in interoceptive activity and cognitive processes handling internal body information.   These regions have been investigated for processes corresponding to cardiac activity:

insula

Insula, or the Island of Reil
somatosensory cortex with S1 and S2 colored

somatosensory cortex with S1 and S2 colored

fMRI of orbitofrontal cortex

fMRI of orbitofrontal cortex

Some researchers (notably (Olga Pollatos and Rainer Schandry, 2004), (Gray et al., 2007) have identified a heartbeat evoked potential that can be detected with EEG measurements: an increase in amplitude of neuroactivity or neuronal firing, detectable after averaging many instances together and subtracting “background” activity as noise,  that seems to occur after a heartbeat. It is intriguing, and possibly of great significance for models of body-knowledge and interoceptive information access, that higher amplitude in the heartbeat evoked potential correlates well with better heartbeat perception in the Pollatos and Schandry research.

Data coming from fMRI and other imaging studies (magnetic electroencephalography, or MEG, and cerebral blood flow, or CBF) should shed additional light on the relative activity levels of cortical and subcortical structures that enable conscious perception of heartbeats, as well as unconscious central nervous system response to cardiac processes.

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Modularism vs. globalism in cognitive neuroscience: implications for a science of body-knowledge

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Models of how people are able to access physiological state information should take into account a long-running divide in cognitive neuroscience about to what extent explanations, models, and purported mechanisms privilege local, reductionistic, and/or modular theories, as opposed to global and holistic theories that emphasize connectedness with and interdependence of particular systems to the entire brain. The debate is described by the dynamicist Walter Freeman (Freeman and Holmes, 2005) :

“In one view, cortex is a collection of modules like a piano keyboard, each with its structure, signal, and contribution to behavior. In the other view, the neocortex is a continuous sheet of neuropil in each cerebral hemisphere, which embeds specialized architectures that were induced by axon tips arriving from extracortical sources during embryological development. Localizationists analogize the neocortex to a cocktail party with standing speakers; each module gives a signal that, when activated like a voice in a room, by volume conduction occupies the whole head and overlaps other signals… Globalists analogize neocortex to a planetary surface, the storms of which are generated by intrinsic dynamics and modified by the structural features of the surface”

The issue of “module activation” vs. “global pattern dynamics” should be kept in mind while reviewing the evidence for specific regions as crucial to biological models of sensation or perception. Nonetheless, for researchers investigating the neurophysiological basis of access to interoceptive information or body-knowledge focus on a number of cortical areas of interest, particularly somatosensory cortex, orbitofrontal cortex, insular cortex, and cingular cortex/cingulate gyrus. The somatosensory cortex or (S1) is conceived as containing “maps” of body surface areas. A standard interpretation would explain the perception of touch, temperature, and pain as occurring through sensory nerves, which are joined into the spinal cord, and which eventually route through the thalamus, and then the cortical region known as the postcentral gyrus.

One standard refinement to the traditional model gives the label “primary somatosensory cortex” only to the area shown in red, Brodmann area 3 (Kaas, 1983). In any event, primary somatosensory cortex/S1 is conventionally modeled as having four complete maps of the body surface. Arguably, the biological/anatomical grounding of this concept allows one to state that the somatosensory cortex/SI contains “multiple representations of the sensory surface of the body,” without running the risk of invoking representationalist epistemologies, with their polymorphous and “mentalistic” significations. Over time, a picture has emerged of sensation occurring on the outer surface of the body, and then activating S1: neurons in these regions are firing (generating electrical discharges and secreting “signaling” molecules across synapses) at a higher amplitude. Any model that accounts for how perception and awareness of the body is possible will likely need to reference the role of somatosensory cortex.

Another cortical region implicated in interoception or internal perception is that part of the frontal lobes known as the orbitofrontal cortex, which can be defined as that part of the prefrontal cortex that receives certain key afferent projections from the thalamus (the so-called “gateway to the cortex), which receives afferent projections from the body, including the visceral organs. In theory, enhanced activation of physiological state (such as heart rate increase) should be reflected in increased activation of orbitofrontal cortices.

Studies of the role of cortex in processing internal body state often emphasize the role of the (formerly) obscure structure known as the insula, a cortical structure which is nonetheless tucked away underneath the visible cortical layers. The anterior portion of the insula is especially implicated in interoception and internal body-state “information gain”.

Yet another specialized brain area becomes more active in those psychophysiological processes involving internal body state dynamics: a collection of white-matter fibers known as the cingulate gyrus of the cortex.

Again, it should be stressed that neuroscientists may debate the extent to which any one region’s activity should be privileged against global overall processes. Certainly, S1, orbitofrontal cortices, anterior insula and anterior cingulate gyrus are only one of a series of regions that play a part in allowing visceral perception, interoception, or a gain in information about the inside of the body. Emphasizing the contribution of such discreet areas carries forward the “modularist” tradition, while other models will stress more of a global or holistic system of interactions, which is a classic debate in psychology and neurology (Gardner, 1985). Arguably, the pre-understanding of how much processing is done by local “modules” as opposed to collective and global activities influences the very means of data collection (Freeman and Holmes, 2005).