Drew Leder is a physician and philosopher. His 1990 book The Absent Body is a tour-de-force!
“According to a scheme employed in physiology, the body’s sensory powers can be divided into three categories. Interoception refers to all sensations of the viscera, that is, the internal organs of the body. It is usually distinguished both from exteroception, our five senses open to the external world, and proprioception, our sense of balance, position, and muscular tension, provided by receptors in muscles, join is, tendons, a ltd the inner ear. In this section I will describe three essential features that structure the interoceptive field; I will term these features respectively qualitative reduction, spatial ambiguity, and spatiotemporal discontinuity.
Ordinarily that which enters the interoceptive field is simultaneously lost to the exteroceptive. Before swallowing the apple I can see, touch, smell, and taste it in all its crimson-tart vividness. Once swallowed, these possibilities are swallowed up as well I can occasion ally; and perhaps unpleasantly, bear or smell evidence of my digestive activity. As in the above example, I can even catch a taste via esophageal reflux. Yet aside from such intermittent and muted evidences, the incorporation of an object into the visceral space involves its withdrawal from exteroceptive experience.
The perceptual field into which the object is received is limited compared to that of the surface body. Interoception does not share the multidimensionality of exteroception, the latter utilizes five sense-modalities which, though tightly interwoven in everyday praxis, have radically divergent spatiotemporal and qualitative properties. Interoception is not devoid of an expressive range and utilizes, physiologists tell us, a variety of sense-receptor types, including mechanoreceptors, nociceptors, and even some thermoreceptors. Yet these are experienced as modulating a single dimension of perception, i.e., “inner sensation” rather than opening onto distinct perceptual worlds.
Furthermore, the qualitative range of this dimension is reduced even when compared to any single exteroceptive mode. Touch, the most analogous form of surface perception, includes within it a huge variety of sensory statements. My acutely articulate skin yields a panoply of tickles, itches, pains, sensations of light and deep pressure, warmth and cold, slow and fast vibrations. The interoceptive vocabulary is not as well developed. In the above example, the stomach and intestines yield a feeling of fullness and cramping. The esophagus burns with an acid reflux. Yet this comes close to exhausting the ordinary sensory experiences of this region. In physiological terms, the viscera have a greatly decreased number and variety of sensory receptors compared to the Surface body, as well as a limited repertoire of motor responses. Experientially, one notices a certain crudeness and generality to most of the messages received. This is a common problem for diagnosing physicians.
An experience of “tightness” in the chest could signal any of a number of cardiac, respiratory, muscular, or even alimentary difficulties, given the imprecision of interoception.
The limited interoceptive vocabulary largely centers around sensations that are affectively charged. Through my outer-directed senses I can survey the exteroceptive field without immediate emotional response. The separation between the perceiver and the perceived makes possible a dispassionate scan. By contrast, visceral sensations grip me from within, often exerting an emotional insistence. As the example suggests, it is the discomforting or painful sensations that speak up most clearly: the crampy stomach, the heartburn, the insistent need for defecation. Like the infant who cries in displeasure but lapses into contentment silently, the viscera seem most able and most articulate in relation to dysfunction (see chapter 3 below). The biological/existential significance of this is clear. It is at times of dysfunction that an insistent and aversive call is needed to compel reparatory action.
While my interoceptive vocabulary is thus most developed in relation to pain, it is limited even here when compared to the body surface. My skin is susceptible to the most exquisite and differentiated tortures if it is cut, burned, pricked, tickled, stretched, struck, pinched. The inner organs exhibit comparatively restricted modes of discomfort. A particular viscus often has its stereotyped ways of responding to almost any noxious stimuli; stomach cramps can result from stress, infection, and food poisoning alike. Moreover, the same general sort of pain, often described as a diffuse aching or burning, is shared in common by many different viscera.
Interoception is reduced compared to surface perception not only in its qualitative range but in its spatial precision. Vision, audition, and touch allow me to locate stimuli to a fine degree. My fingers can tell apart pinpricks separated by only one to two millimeters. While other regions of the surface are less discriminating, I usually have little difficulty in locating cutaneous sensations. By way of contrast, visceral sensations are often vaguely situated with indistinct borders. In my example, I experience midsection fullness and cramps, but there is no clear place where they begin or end, and no precise center.
Pain can suddenly localize when the sensitive membranes lining the visceral cavities become involved. But the inner organs themselves are in many instances simply incapable of registering localized events. Surgeons, for example, have found that they can cut the intestines in two without a conscious patient experiencing significant pain. Like other viscera, the intestines primarily report generalized stimulations involving substantial portions of the organ.
The spatial ambiguity of the visceral depths is accentuated by the phenomenon of referred pain. A process taking place in one organ tan exponentially radiate to adjacent body areas or express itself in a distant location. Hence the pain of a heart attack may originate in the chest area but quickly spread down the left arm. This reflects embryological origins; sensation is referred to that level of the body the viscus occupied in the developing fetus before it descended, dragging nerves along, to its mature position. Thus, I may experience the pain where the organ used to be, not simply where it is now. An almost magical transfer of experience is effected along both spatial and temporal dimensions, weaving the inner body into an ambiguous space.
Moreover, there are physical/phenomenal transfers between any vital organ and the body as a whole that further prohibit strict localization of visceral experience. Ricoeur refers to “this strange mixture of the local and the non-local” that is encountered in phenomena such as pain, hunger, thirst, and all vital needs,’ As the example indicates, hunger is experienced not just in abdominal ache but as heaviness in the limbs, a yearning in the mouth. The visceral organs sustain my body as a whole through processes of digestion, circulation, respiration, and excretion. Hence, when I manifest a visceral-based contentment or dysfunction, this is manifested everywhere and nowhere. A twinge in the finger is clearly located there. But hunger is a complex nexus of heaviness, exhaustion, conative urges, and discomforting sensations that, while gathering into nodes of crystallization, ambiguously inhabits the entirety of the corporeal field.