The Nature Of Sensation





SENSATION GIVES US OUR WORLD OF QUALITIES.--In actual experience

sensations are never known apart from the objects to which they belong.

This is to say that when we see yellow or red it is always in

connection with some surface, or object; when we taste sour, this

quality belongs to some substance, and so on with all the senses. Yet by

sensation we mean only the simple qualities of objects known in

consciousness as the result of appropriate stimuli applied to

end-organs. We shall later see how by perception these qualities fuse

or combine to form objects, but in the present chapter we shall be

concerned with the qualities only. Sensations are, then, the simplest

and most elementary knowledge we may get from the physical world,--the

red, the blue, the bitter, the cold, the fragrant, and whatever other

qualities may belong to the external world. We shall not for the present

be concerned with the objects or sources from which the qualities may

come.



To quote James on the meaning of sensation: All we can say on this

point is that what we mean by sensations are first things in the way of

consciousness. They are the immediate results upon consciousness of

nerve currents as they enter the brain, and before they have awakened

any suggestions or associations with past experience. But it is obvious

that such immediate sensations can be realized only in the earliest

days of life.



THE ATTRIBUTES OF SENSATION.--Sensations differ from each other in at

least four respects; namely, quality, intensity, extensity, and

duration.



It is a difference in quality that makes us say, This paper is red,

and that, blue; this liquid is sweet, and that, sour. Differences in

quality are therefore fundamental differences in kind. Besides the

quality-differences that exist within the same general field, as of

taste or vision, it is evident that there is a still more fundamental

difference existing between the various fields. One can, for example,

compare red with blue or sweet with sour, and tell which quality he

prefers. But let him try to compare red with sweet, or blue with sour,

and the quality-difference is so profound that there seems to be no

basis for comparison.



Differences in intensity of sensation are familiar to every person who

prefers two lumps of sugar rather than one lump in his coffee; the sweet

is of the same quality in either case, but differs in intensity. In

every field of sensation, the intensity may proceed from the smallest

amount to the greatest amount discernible. In general, the intensity of

the sensation depends on the intensity of the stimulus, though the

condition of the sense-organ as regards fatigue or adaptation to the

stimulus has its effect. It is obvious that a stimulus may be too weak

to produce any sensation; as, for example, a few grains of sugar in a

cup of coffee or a few drops of lemon in a quart of water could not be

detected. It is also true that the intensity of the stimulus may be so

great that an increase in intensity produces no effect on the sensation;

as, for example, the addition of sugar to a solution of saccharine would

not noticeably increase its sweetness. The lowest and highest intensity

points of sensation are called the lower and upper limen, or

threshold, respectively.



By extensity is meant the space-differences of sensations. The touch

of the point of a toothpick on the skin has a different space quality

from the touch of the flat end of a pencil. Low tones seem to have more

volume than high tones. Some pains feel sharp and others dull and

diffuse. The warmth felt from spreading the palms of the hands out to

the fire has a bigness not felt from heating one solitary finger. The

extensity of a sensation depends on the number of nerve endings

stimulated.



The duration of a sensation refers to the time it lasts. This must not

be confused with the duration of the stimulus, which may be either

longer or shorter than the duration of the sensation. Every sensation

must exist for some space of time, long or short, or it would have no

part in consciousness.





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