(D) Sensation and Movement (cont.)
Qualitative Continuity in Sensations
It is interesting to note that all possible sensations of colours, of tone, and of temperature constitute as many groups of qualitative continua. By continuum is here meant a series of presentations changing gradually in quality, i.e., so that any two differ less the more they approximate in the series. We may represent this relation among presentations spatially, so long as the differences do not exceed three. In this way our normal colour-sensations have been compared to s sphere, in which (a) the maximum of luminosity is at one pole and the minimum at the other ; (b) the series of colours proper (red to violet and through purple back to red), constituting a closed line, are placed round the equator or in zones parallel to it, according to shade ; and (c) the amount of saturation (or absence of white) for any given zone of illumination increases with distance from the axis. The several musical tones, again, have been compared to an ascending spiral, a given tone and its octaves lying in the same perpendicular. Temperature similarly might be represented as ranging in opposite directions, i.e., though heat or through cold, alike. As we frequently experience a continuous range of intensity of varying amount, so we may experience continuos variations in quality, as in looking at the rainbow, for example. Still it is not to be supposed that colours or notes are necessarily presented a continua: that they are such is matter of after-observation.
The groups of sensations known as touches, smells, tastes, on the other hand, do not constitute continua : bitter tastes, for instance, will not shade off into acid or sweet tastes, except, of course, through a gradual diminution of intensity rendering the one quality subliminal followed by a gradual increase from zero in the intensity of the other. This want of continuity might be explained if there were grounds for regarding these groups as more complex than the rest,in so far tertiary colour or vowel-sounds, say, are complex and comparatively discontinuous. But it might equally well be argued that they are simpler than the rest and, as simple and different, are necessarily disparate, while the continuity of colours or tones is due to a gradual change of components.
Our motor presentations contrast with the sensory by their want of striking qualitative difference. We may divide them into two groups (a) motor presentations proper and (b) auxilio-motor presentation. The former answer to our "feelings of muscular effort" or "feelings of innervation." The latter are those presentations due to the straining of tendons, stretching and flexing of the skin, and the like, by which the healthy man knows that his effort to move and followed by movements, and so knows the position of his body and limbs. It is owing to the absence of these presentations that the anaesthetic patient cannot directly tell whether his effort are effectual or not, nor in what position his limbs have been placed by movements from without. Thus under normal circumstances motor presentations are always accompanied by auxilio-motor ; but in disease and in passive movements they are separated and their distinctness thus made manifest. Originally we may suppose auxilio-motor objects to form one imperfectly differentiated continuum, but now, as with sensations, movements have become a collection of special continua, viz., the groups of movements possible to each limb and certain combination of these.
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