WHAT IS SENSATION?
"gnoti se auton"
by Michel Cabanac
Department of Physiology, Faculty of Medicine,
Laval University, QUÉBEC, G1K 7P4 CANADA

          INTRODUCTION

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Sensation has always been a matter of interest to philosophers. Plato (4th century BC) held that sensation and opinion are the main screens masking Truth, but the opposite view tended to dominate both before and after him. Heraclitos (5th century BC) taught that knowledge comes to man "through the door of the senses", and Protagoras (5th century BC) that the entire psychic life consists only of sensations. Aristotle (4th century BC), Plato's pupil, returned to the sophists' view that sensation is the gate of the soul. This notion can be traced through history up to nowadays. Hobbles (1651) wrote: "There is no conception in man's mind which hath not at first, totally or in parts, been begotten upon the organs of senses". Condillac (1754), taking the theoretical example of an inert statue, showed that the progressive attribution of the senses would allow the development of a complete mind in the statue. Thus he made it clear that the mind must use the senses to know and understand the world, and that the senses are necessary and sufficient to develop a mind. This notion was also accepted by Kant (1781) for whom, however, the senses were one of the two sources of human knowledge, the other one being understanding. "Sensationalism is the theory that all knowledge originates in sensations; that all cognitions even reflective ideas and ... intuition can be traced back to elementary sensations" (Titchener, 1909).

"All science, whatever the realm of application, has a common origin: the immediate experience of the observing person of the scientist himself" (Spence, 1948). The scientific process then proceeds in the sharing of evidence by two or more persons. <...>

The concept of the senses as portals of the mind has therefore turned to a commonplace statement among modern psychologists (Marks 1981). However Plato's image of sensation as a screen masking truth can be recognized as arising again from two problems: the difference between sensation and perception, and the complexity of attributes.
 

Sensation and Perception

Psychologists of the eighteenth century started to distinguish sensation from perception. For Reid (1785) a sensation occurs when an organ of sense is stimulated, and perception depends upon sensation but also includes a conception of the object perceived and an immediate and irresistible conviction of the object's actual existence. These definitions are still accepted nowadays: Levine and Shefner (1981) define clearly sensation as "the process of detecting a stimulus (or some aspect of it) in the environment", and perception as "the way in which we interpret the information gathered (and processed) by the senses". "In a word, we sense the presence of a stimulus, but we perceive what it is".

However, Schiffman (1982) considers as somewhat obsolete the differentiation between sensation and perception, yet brings a useful light to it: sensation is equated to physiology, and perception to psychology. This explains how two attitudes have developed until now.

For the first group, a stimulus associated with a context acquires a meaning (Titchener, 1909) and, to make perception still more complex, an adequate behavioral response to a stimulus carries a meaning and may modify perception (Tolman 1918).

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An explanation of this refusal to recognize sensation as a real entity might be found in the stimuli used to arouse perception. The psychologists concerned with perception refer almost exclusively to the auditory and visual perceptions (Banks, 1991). <...>

The second attitude accepts the theoretical separating of sensation from perception. Huxley (1954) in his experiment with mescaline on himself, has reported that the drug modified his perception but that his sensations remained intact. Indeed, some psychologists (Geldard, 1972; Marks, 1974; Levine & Shefner, 1981) have no problem with the concept of sensation, perhaps because the stimuli they use, light, sound, temperature, pressure, chemical stimuli, are only slightly context-related, or not at all. Yet the refusal to recognize sensation as an entity, simpler and thus different from perception, pinpoints some weakness in the definition of both sensation and perception and justifies this essay. We shall return briefly to this point after having examined the problem of attributes. "A great deal of confusion would be avoided if psychologists at large recognized the fact that the sensation of experimental psychology is a simple, meaningless (or rather non-meaningful) process definable only by an enumeration of its attributes". (Titchener, 1909).

Attributes

"An attribute of sensation,... , is any aspect or moment or dimension of sensation which fulfills the two conditions of inseparability and independent variability" (Titchener, 1908). It follows from this definition that the attributes are always given when a sensation is given and that the nullifying of any attribute annihilates the sensation.

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As is the case with pleasure, the other attributes also possess attributes of their own: vision has light and color (color has three attributes of its own: hue, brightness, and saturation); audition has pitch, and volumeness as proposed by Titchener (1908). <...>
 

A Simpler Descriptive Model of Sensation

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QUALITY OF SENSATION

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The other trend is the perceptual way to look at sensation with less concern for the sensory organs. This trend qualifies as psychological because it starts with an introspective step. Pain, on top of this approach, can be elicited from all parts of the body without a clearly identified receptor organ. Time (Boring, 1942; Schiffman, 1982), orientation (Schiffman 1982), kinesthesia (Corso, 1967; Schiffman, 1982; Geldard, 1972; Ludel, 1978), perception of space (Schiffman, 1982), organic sensation (visceral, hunger, thirst) (Geldard, 1972) have thus been proposed as sensations in addition to the five senses. However, the common feature of these perceptions is precisely to be vague and to pertain to perception as defined above rather than to sensation sensu stricto.

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An important inference to be drawn from this definition is that sensation is not limited to the five senses. We know, from clinical evidence that pain can be felt from any locus in the body with a small number of exceptions: skin on the center of the cheek and on the olecranon, and the nervous tissue of the brain. Pain is not the sole sensation aroused from inside the body. Mere introspection tells me that, if I close my attention to the outside world and concentrate on my own body, I can feel slight visceral sensation from my limbs, trunk, chest, neck, and head. <...>

The above hypothesis regarding the origin of sensations calls for four remarks.

1) The sensory window open to the outside world is limited to less than the short list <...>
2) In addition to information on the outside world, the brain receives also a vast amount of information about the physiological state of the milieu interieur. <...>

3) The quality of a sensation is determined by anatomy, but the pattern of action potentials within an afferent pathway might also account for this dimension.

4) The identification of a sensation is likely to depend on the existence of a semantic support as a conceptual medium. The short list of tastes, sweet, salt, bitter, and sour has been extended recently by adding one sensation: umami (Yamaguchi, 1987). When no word is available to describe a sensation probably we tend to ignore this sensation.

INTENSITY OF SENSATION

Another advantage of defining sensation as the emergence of a sensitivity into consciousness is to relate sensation to the stimulus in two steps, via physiology: From stimulus to action potential, from action potential to sensation. <...>

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AFFECTIVITY OF SENSATION

Pleasure is the state of mind aroused by a pleasant cause. Titchener (1908) took great efforts to refute the hypothesis that pleasure is a peripheral sensation. This refutation entails that pleasure may reside in any kind of mental experience. <...>

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Sensory Pleasure and Behavior

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Experimental evidence in the narrow field of sensory pleasure thus confirms the Epicurean general principle which states that pleasure and displeasure are linked to the well-being of the organism (Lehman, 1914) and according to which: "when it helps and encourages (the vital movement) it is called pleasure, satisfaction, well-being, which is nothing real but a movement in the heart" (Hobbes,1651).

Pleasure as Seen by Philosophers and Psychologists

The relation of pleasure to behavior was regarded as obvious by the Greek philosophers Aristotle (284-322, BC) and Epicures (241-170, BC, see Conche, 1977): "Life and pleasure, as we can see now, are not separable; for without behavior there is no pleasure, and pleasure improves behavior" (Aristotle).

<...>  One good reason for the rejection of pleasure from the realm of science was the lack of experimental evidence and the fact that the philosophers' conclusions were based on their own introspection only. This rejection may be based on earlier approaches to these phenomena but, "as new techniques are developed, our ideas often have to be revised to encompass the new informations obtained" (Teitelbaum, 1964). <...>
 
 

CONCLUSIONS

Once accepted the cogito ergo sum, the whole scientific knowledge bears on two postulates:

a) a world exists around me, and
b) I can exchange evidence with others.

Both of these postulales need the channel of sensation. Therefore our knowledge of the world, including ourselves is filtered twice. Once by the narrow chemico-physical window of the senses, and once by the biological and cultural format of our brain. The way we see sensation might have, therefore, some repercussion on the way we think.

The first hypothesis proposed here, according to which any afferent fiber is susceptible to arousing a sensation, presents several advantages. First the theory is simple. Then all the various categories of sensations are lumped into one single class whereas classical categorization listed many different sorts of sensations with different attributes. Thus is suggested a fundamental unity of the sensory input to the central nervous system.

The second hypothesis presented here regarding the structure of sensation can be examined from the points of view of both phylogeny and ontogeny. This chapter was devoted to human sensation but we may step back a little in phylogeny and try to guess about the origin of sensation. Medicus (1987) has reflected on the process from a behavioral point of view. Sensation emerged from a purely reflective behavior. A Darwinian approach tells us that sensory messages became conscious when this emergence proved useful to the organisms that first acquired it. To be useful, sensations needed to describe the quality, the intensity, and above all the usefulness of environmental stimuli; therefore, it is likely that sensation was immediately multidimensional, as defined above. Thus sensation gave decisional advantages to the first animals which possessed it, by freeing them from the need for an infinitely complex hardware reflex network in their nervous system.

Finally, if, as we saw in Introduction above, sensation as the gate to the soul is commonplace for psychologists and philosophers, it remains that the structure of sensation has an important consequence. If sensation is the phylogenic and ontogenic origin of the conscious experience, then any conscious event is likely to bear fundamentally the same structure. Indeed, introspection tells me that this is the case, and that any conscious feeling has quality, intensity, affectivity, and duration.