sensation

sensation A sensation is a subjective experience resulting from the stimulation of a sensory receptor (a specialized nerve cell that is excited by some physical or chemical stimulus). Sensations are presumed to arise as a result of nerve activity in certain areas of the cerebral cortex, which receive incoming signals from the sensory receptors.

Aristotle recognized only five senses — vision, hearing, taste, smell and touch, each associated with a particular sense organ (counting the skin as a sense organ). The term ‘special sense’ is still used for vision, hearing, taste and smell. In reality, there are many more sensory systems, including those that mediate the sensations of pain and temperature, those of the positions of parts of the body (proprioception), and those related to balance (conveyed by the vestibular apparatus). A more fundamental way of classifying sensory receptors is according to the types of physical stimuli to which they respond. It is remarkable that all the majesty of our sensory experiences depends only on sensitivity to light, and to mechanical, chemical, and thermal stimulation.

The experience of our own bodies, somatic sensation (from the Greek ‘soma’ meaning body), conventionally includes sensations arising from nerve endings in skin, muscle, bones, and joints. A different term, visceral sensation, is used to describe awareness (usually discomfort) arising from receptors in the internal organs (viscera). Such sensations are only vaguely localized compared to somatic sensations, and in some instances may be ‘referred’, that is, felt to be coming from a site quite different from where the excitation arises. For instance, the pain of angina, originating from sensory fibres in the muscle of the heart, is typically felt in the left shoulder and upper arm. Some visceral sensations, such as the feeling of a full bladder, are associated with normal functions; others signal abnormality, such as the pain of intestinal colic or a growing tumour.

Sensory experiences have distinct subjective qualities, such as colour, pitch, tickle, bitterness, and floral. These are sometimes referred to as ‘modalities’ or ‘sub-modalities’ of sensation, especially for sensations derived from the skin. Valiant, partially successful, attempts have been made to correlate each such mental property with a particular specialized type of sensory receptor, nerve pathway or region of the brain. In 1823, the British neurologist and anatomist, Sir Charles Bell, first described evidence for what subsequently became known as Müller's Law of ‘Specific Nerve Energies’ (after the German physiologist, Johannes Müller). Bell wrote: ‘every nerve of sense is limited in its experience, and can minister to certain perceptions only’. He and Müller cited examples of false sensations, elicited by inappropriate stimulation of particular nerves. For instance, pressing on the side of the eyeball with a finger, which causes direct, mechanical stimulation of the retina, gives rise to a ‘phosphene’ — a visual sensation in the form of a curious blob in the part of the visual field corresponding to that region of the retina. Certainly, many modalities of sensation can be identified with particular specialization of the sensory receptors involved. Touch, vibration, warmth, and coldness are indubitably associated with particular, highly specialized nerve endings in the skin.

Sensory receptors provide information about the quality of the stimuli that they detect, giving rise to what philosophers call the qualia of conscious perceptual experience. We see colour and brightness, hear pitch and timbre, and taste sweetness and sourness: we can distinguish whether an object touching the skin is sharp or smooth, hot or cold. But stimulus quality is not the only kind of information that sense organs provide. They also transmit data on the intensity, duration, and location of the stimulus. Generally speaking (though there are exceptions), the quality of a sensation is determined by which type of specialized nerve ending or receptor cell is stimulated, but the perceptual intensity (brightness, loudness, etc.) depends on how strongly the receptor is stimulated.

However, there is not perfect congruence between particular individual sensory receptors and the qualia – the units of subjective experience. For example, our retina contains just three types of cone receptor cell which have slightly different but overlapping spectral absorption. The large number of different colours that we can distinguish are somehow derived by the brain from the relative strength of the signals from these three basic detectors.

pain is usually thought of as rather different from other modalities, in that it bears no simple relationship to the physical world. It is, most simply, a perception resulting from a noxious stimulus — one associated with actual or potential tissue damage. But pain may be caused by excessive stimulation of receptors ordinarily involved with other modalities: for example heat on the skin is felt as heat, but excessive heat is felt also as pain.

Although, strictly, sensation refers only to conscious feelings, it is clear that much of the massive, unremitting inflow of information from sensory nerves does not enter consciousness. Even for the special senses, we are aware at any moment only of what we attend to, which is a tiny fraction of the information streaming in. Indeed, much sensory processing, essential for the regulation of the body, is entirely unconscious. The receptors in muscles, joints and tendons, some of which give rise to conscious proprioception, are involved in the constant, unconscious task of regulating posture and guiding movement. The heart, lungs, and major blood vessels have a variety of specialized sensory endings, conveying information to the brain about the pressure and composition of the blood, and about the stretching of the lungs. Within the brain itself, the hypothalamus and parts of the medulla contain sensory nerve cells that are specialized to detect such properties as the acidity of the blood, its temperature and the concentration of glucose, salt and other constituents. These unconscious sensory systems play an essential part in homeostasis — the maintenance of the internal environment of the body, by initiating reflex regulation of breathing, heart rate, blood vessel size, sweating and shivering, as well as in regulating essential behaviour such eating and drinking.

Psychologists and philosophers often draw a distinction between sensation (said to have a raw, unprocessed quality) and perception, the interpreted meaning of sensory activity. This view can be traced to Immanuel Kant's ‘transcendental’ philosophy, namely to the view that knowledge of the world arises from sensory experience, furnished by the mind with such ‘archetypal’ properties as space, time, relation, and causality.

The distinction between sensation and perception is hard to defend on the basis of what we now know about how sensory receptors and their associated brain areas work. At each stage, even at the sensory receptors themselves, the particular, selective characteristics of the nerve cells impose expectation and order on sensory signals. The detection of stimuli and their cognitive interpretation, to provide knowledge of the world, are inextricably linked within our sensory systems. Effortless though our perceptions seem, they involve immense ‘computational’ tasks. More than half of the human cerebral cortex is devoted to analysing sensory signals.

‘Common sense’, meaning native intelligence, derives from the medieval term sensus communis. This described the place in the fluid-filled chambers of the brain at which signals from all the sense organs were supposed to mix together, to provide the ingredients of imagination and rational thought. The near-miraculous process that intervenes between the irritation of the membrane of a sensory receptor and our perception of the world is arguably the most intelligent thing that we do.

Colin Blakemore

See also cerebral cortex; cerebral ventricles; hearing; pain; perception; proprioception; sensory receptors; somatic sensation; taste and smell; vestibular system; visceral sensation; vision