SENSATION
We can best understand the problems of sensation and perception if we
first think of the existence of two great worlds--the world of physical
nature without and the world of mind within. On the one hand is our
material environment, the things we see and hear and touch and taste and
handle; and on the other hand our consciousness, the means by which we
come to know this outer world and adjust ourselves to it. These two
worlds seem in a sense to belong to and require each other. For what
would be the meaning or use of the physical world with no mind to know
or use it; and what would be the use of a mind with nothing to be known
or thought about?
1. HOW WE COME TO KNOW THE EXTERNAL WORLD
There is a marvel about our coming to know the external world which we
shall never be able fully to understand. We have come by this knowledge
so gradually and unconsciously that it now appears to us as commonplace,
and we take for granted many things that it would puzzle us to explain.
KNOWLEDGE THROUGH THE SENSES.--For example, we say, "Of course I see
yonder green tree: it is about ten rods distant." But why "of course"?
Why should objects at a distance from us and with no evident connection
between us and them be known to us at all merely by turning our eyes in
their direction when there is light? Why not rather say with the blind
son of Professor Puiseaux of Paris, who, when asked if he would like to
be restored to sight, answered: "If it were not for curiosity I would
rather have long arms. It seems to me that my hands would teach me
better what is passing in the moon than your eyes or telescopes."
We listen and then say, "Yes, that is a certain bell ringing in the
neighboring village," as if this were the most simple thing in the
world. But why should one piece of metal striking against another a mile
or two away make us aware that there is a bell there at all, let alone
that it is a certain bell whose tone we recognize? Or we pass our
fingers over a piece of cloth and decide, "That is silk." But why,
merely by placing our skin in contact with a bit of material, should we
be able to know its quality, much less that it is cloth and that its
threads were originally spun by an insect? Or we take a sip of liquid
and say, "This milk is sour." But why should we be able by taking the
liquid into the mouth and bringing it into contact with the mucous
membrane to tell that it is milk, and that it possesses the quality
which we call _sour_? Or, once more, we get a whiff of air through the
open window in the springtime and say, "There is a lilac bush in bloom
on the lawn." Yet why, from inhaling air containing particles of lilac,
should we be able to know that there is anything outside, much less that
it is a flower and of a particular variety which we call lilac? Or,
finally, we hold a heated flatiron up near the cheek and say, "This is
too hot! it will burn the cloth." But why by holding this object a foot
away from the face do we know that it is there, let alone knowing its
temperature?
THE UNITY OF SENSORY EXPERIENCE.--Further, our senses come through
experience to have the power of fusing, or combining their knowledge, so
to speak, by which each expresses its knowledge in terms of the others.
Thus we take a glance out of the window and say that the day looks cold,
although we well know that we cannot see _cold_. Or we say that the
melon sounds green, or the bell sounds cracked, although a _crack_ or
_greenness_ cannot be heard. Or we say that the box feels empty,
although _emptiness_ cannot be felt. We have come to associate cold,
originally experienced with days which look like the one we now see,
with this particular appearance, and so we say we see the cold; sounds
like the one coming from the bell we have come to associate with cracked
bells, and that coming from the melon with green melons, until we say
unhesitatingly that the bell sounds cracked and the melon sounds green.
And so with the various senses. Each gleans from the world its own
particular bit of knowledge, but all are finally in a partnership and
what is each one's knowledge belongs to every other one in so far as the
other can use it.
THE SENSORY PROCESSES TO BE EXPLAINED.--The explanation of the ultimate
nature of knowledge, and how we reach it through contact with our
material environment, we will leave to the philosophers. And battles
enough they have over the question, and still others they will have
before the matter is settled. The easier and more important problem for
us is to describe the _processes_ by which the mind comes to know its
environment, and to see how it uses this knowledge in thinking. This
much we shall be able to do, for it is often possible to describe a
process and discover its laws even when we cannot fully explain its
nature and origin. We know the process of digestion and assimilation,
and the laws which govern them, although we do not understand the
ultimate nature and origin of _life_ which makes these possible.
THE QUALITIES OF OBJECTS EXIST IN THE MIND.--Yet even in the relatively
simple description which we have proposed many puzzles confront us, and
one of them appears at the very outset. This is that the qualities which
we usually ascribe to objects really exist in our own minds and not in
the objects at all. Take, for instance, the common qualities of light
and color. The physicist tells us that what we see as light is
occasioned by an incredibly rapid beating of ether waves on the retina
of the eye. All space is filled with this ether; and when it is
light--that is, when some object like the sun or other light-giving body
is present--the ether is set in motion by the vibrating molecules of the
body which is the source of light, its waves strike the retina, a
current is produced and carried to the brain, and we see light. This
means, then, that space, the medium in which we see objects, is not
filled with light (the sensation), but with very rapid waves of ether,
and that the light which we see really occurs in our own minds as the
mental response to the physical stimulus of ether waves. Likewise with
color. Color is produced by ether waves of different lengths and degrees
of rapidity.
Thus ether waves at the rate of 450 billions a second give us the
sensation of red; of 472 billions a second, orange; of 526 billions a
second, yellow; of 589 billions a second, green; of 640 billions a
second, blue; of 722 billions a second, indigo; of 790 billions a
second, violet. What exists outside of us, then, is these ether waves of
different rates, and not the colors (as sensations) themselves. The
beautiful yellow and crimson of a sunset, the variegated colors of a
landscape, the delicate pink in the cheek of a child, the blush of a
rose, the shimmering green of the lake--these reside not in the objects
themselves, but in the consciousness of the one who sees them. The
objects possess but the quality of reflecting back to the eye ether
waves of the particular rate corresponding to the color which we ascribe
to them. Thus "red" objects, and no others, reflect back ether waves of
a rate of 450 billions a second: "white" objects reflect all rates;
"black" objects reflect none.
The case is no different with regard to sound. When we speak of a sound
coming from a bell, what we really mean is that the vibrations of the
bell have set up waves in the air between it and our ear, which have
produced corresponding vibrations in the ear; that a nerve current was
thereby produced; and that a sound was heard. But the sound (i.e.,
sensation) is a mental thing, and exists only in our own consciousness.
What passed between the sounding object and ourselves was waves in the
intervening air, ready to be translated through the machinery of nerves
and brain into the beautiful tones and melodies and harmonies of the
mind. And so with all other sensations.
THE THREE SETS OF FACTORS.--What exists outside of us therefore is a
_stimulus_, some form of physical energy, of a kind suitable to excite
to activity a certain end-organ of taste, or touch, or smell, or sight,
or hearing; what exists within us is the _nervous machinery_ capable of
converting this stimulus into a nerve current which shall produce an
activity in the cortex of the brain; what results is the _mental object_
which we call a _sensation_ of taste, smell, touch, sight, or hearing.
2. 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.
3. SENSORY QUALITIES AND THEIR END-ORGANS
All are familiar with the "five senses" of our elementary physiologies,
sight, hearing, taste, smell, and touch. A more complete study of
sensation reveals nearly three times this number, however. This is to
say that the body is equipped with more than a dozen different kinds of
end-organs, each prepared to receive its own particular type of
stimulus. It must also be understood that some of the end-organs yield
more than one sense. The eye, for example, gives not only visual but
muscular sensations; the ear not only auditory, but tactual; the tongue
not only gustatory, but tactual and cold and warmth sensations.
SIGHT.--Vision is a _distance_ sense; we can see afar off. The stimulus
is _chemical_ in its action; this means that the ether waves, on
striking the retina, cause a chemical change which sets up the nerve
current responsible for the sensation.
The eye, whose general structure is sufficiently described in all
standard physiologies, consists of a visual apparatus designed to bring
the images of objects to a clear focus on the retina at the _fovea_, or
area of clearest vision, near the point of entrance of the optic nerve.
The sensation of sight coming from this retinal image unaided by other
sensations gives us but two qualities, _light and color_. The eye can
distinguish many different grades of light from purest white on through
the various grays to densest black. The range is greater still in color.
We speak of the seven colors of the spectrum, violet, indigo, blue,
green, yellow, orange, and red. But this is not a very serviceable
classification, since the average eye can distinguish about 35,000 color
effects. It is also somewhat bewildering to find that all these colors
seem to be produced from the four fundamental hues, red, green, yellow,
and blue, plus the various tints. These four, combined in varying
proportions and with different degrees of light (i.e., different shades
of gray), yield all the color effects known to the human eye. Herschel
estimates that the workers on the mosaics at Rome must have
distinguished 30,000 different color tones. The _hue_ of a color refers
to its fundamental quality, as red or yellow; the _chroma_, to its
saturation, or the strength of the color; and the _tint,_ to the amount
of brightness (i.e., white) it contains.
HEARING.--Hearing is also a distance sense. The action of its stimulus
is mechanical, which is to say that the vibrations produced in the air
by the sounding body are finally transmitted by the mechanism of the
middle ear to the inner ear. Here the impulse is conveyed through the
liquid of the internal ear to the nerve endings as so many tiny blows,
which produce the nerve current carried to the brain by the auditory
nerve.
The sensation of hearing, like that of sight, gives us two qualities:
namely, _tones_ with their accompanying pitch and timbre, and _noises_.
Tones, or musical sounds, are produced by isochronous or equal-timed
vibrations; thus _C_ of the first octave is produced by 256 vibrations a
second, and if this tone is prolonged the vibration rate will continue
uniformly the same. Noises, on the other hand, are produced by
vibrations which have no uniformity of vibration rate. The ear's
sensibility to pitch extends over about seven octaves. The seven-octave
piano goes down to 27-1/2 vibrations and reaches up to 3,500 vibrations.
Notes of nearly 50,000 vibrations can be heard by an average ear,
however, though these are too painfully shrill to be musical. Taking
into account this upper limit, the range of the ear is about eleven
octaves. The ear, having given us _loudness_ of tones, which depends on
the amplitude of the vibrations, _pitch_, which depends on the rapidity
of the vibrations, and _timbre_, or _quality_, which depends on the
complexity of the vibrations, has no further qualities of sound to
reveal.
TASTE.--The sense of taste is located chiefly in the tongue, over the
surface of which are scattered many minute _taste-bulbs_. These can be
seen as small red specks, most plentifully distributed along the edges
and at the tip of the tongue. The substance tasted must be in
_solution_, and come in contact with the nerve endings. The action of
the stimulus is _chemical_.
The sense of taste recognizes the four qualities of _sour_, _sweet_,
_salt_, and _bitter_. Many of the qualities which we improperly call
tastes are in reality a complex of taste, smell, touch, and temperature.
Smell contributes so largely to the sense of taste that many articles of
food become "tasteless" when we have a catarrh, and many nauseating
doses of medicine can be taken without discomfort if the nose is held.
Probably none of us, if we are careful to exclude all odors by plugging
the nostrils with cotton, can by taste distinguish between scraped
apple, potato, turnip, or beet, or can tell hot milk from tea or coffee
of the same temperature.
SMELL.--In the upper part of the nasal cavity lies a small brownish
patch of mucous membrane. It is here that the olfactory nerve endings
are located. The substance smelled must be volatile, that is, must exist
in gaseous form, and come in direct contact with the nerve endings.
Chemical action results in a nerve current.
The sensations of smell have not been classified so well as those of
taste, and we have no distinct names for them. Neither do we know how
many olfactory qualities the sense of smell is capable of revealing. The
only definite classification of smell qualities is that based on their
pleasantness or the opposite. We also borrow a few terms and speak of
_sweet_ or _fragrant_ odors and _fresh_ or _close_ smells. There is some
evidence when we observe animals, or even primitive men, that the human
race has been evolving greater sensibility to certain odors, while at
the same time there has been a loss of keenness of what we call scent.
VARIOUS SENSATIONS FROM THE SKIN.--The skin, besides being a protective
and excretory organ, affords a lodging-place for the end-organs giving
us our sense of pressure, pain, cold, warmth, tickle, and itch.
_Pressure_ seems to have for its end-organ the _hair-bulbs_ of the skin;
on hairless regions small bulbs called the _corpuscles of Meissner_
serve this purpose. _Pain_ is thought to be mediated by free nerve
endings. _Cold_ depends on end-organs called the _bulbs of Krause_; and
_warmth_ on the _Ruffinian corpuscles_.
Cutaneous or skin sensation may arise from either _mechanical_
stimulation, such as pressure, a blow, or tickling, from _thermal_
stimulation from hot or cold objects, from _electrical_ stimulation, or
from the action of certain _chemicals_, such as acids and the like.
Stimulated mechanically, the skin gives us but two sensation qualities,
_pressure_ and _pain_. Many of the qualities which we commonly ascribe
to the skin sensations are really a complex of cutaneous and muscular
sensations. _Contact_ is light pressure. _Hardness_ and _softness_
depend on the intensity of the pressure. _Roughness_ and _smoothness_
arise from interrupted and continuous pressure, respectively, and
require movement over the rough or smooth surface. _Touch_ depends on
pressure accompanied by the muscular sensations involved in the
movements connected with the act. Pain is clearly a different sensation
from pressure; but any of the cutaneous or muscular sensations may, by
excessive stimulation, be made to pass over into pain. All parts of the
skin are sensitive to pressure and pain; but certain parts, like the
finger tips, and the tip of the tongue, are more highly sensitive than
others. The skin varies also in its sensitivity to _heat_ and _cold_. If
we take a hot or a very cold pencil point and pass it rather lightly and
slowly over the skin, it is easy to discover certain spots from which a
sensation of warmth or of cold flashes out. In this way it is possible
to locate the end-organs of temperature very accurately.
[Illustration: FIG. 17.--Diagram showing distribution of hot and cold
spots on the back of the hand. C, cold spots; H, hot spots.]
THE KINÆSTHETIC SENSES.--The muscles, tendons, and joints also give rise
to perfectly definite sensations, but they have not been named as have
the sensations from most of the other end-organs. _Weight_ is the most
clearly marked of these sensations. It is through the sensations
connected with movements of muscles, tendons, and joints that we come to
judge _form_, _size_, and _distance_.
THE ORGANIC SENSES.--Finally, to the sensations mentioned so far must be
added those which come from the internal organs of the body. From the
alimentary canal we get the sensations of _hunger_, _thirst_, and
_nausea_; from the heart, lungs, and organs of sex come numerous
well-defined but unnamed sensations which play an important part in
making up the feeling-tone of our daily lives.
Thus we see that the senses may be looked upon as the sentries of the
body, standing at the outposts where nature and ourselves meet. They
discover the qualities of the various objects with which we come in
contact and hand them over to the mind in the form of sensations. And
these sensations are the raw material out of which we begin to construct
our material environment. Only as we are equipped with good organs of
sense, especially good eyes and ears, therefore, are we able to enter
fully into the wonderful world about us and receive the stimuli
necessary to our thought and action.
4. PROBLEMS IN OBSERVATION AND INTROSPECTION
1. Observe a schoolroom of children at work with the aim of discovering
any that show defects of vision or hearing. What are the symptoms? What
is the effect of inability to hear or see well upon interest and
attention?
2. Talk with your teacher about testing the eyes and ears of the
children of some school. The simpler tests for vision and hearing are
easily applied, and the expense for material almost nothing. What tests
should be used? Does your school have the test card for vision?
3. Use a rotator or color tops for mixing discs of white and black to
produce different shades of gray. Fix in mind the gray made of half
white and half black; three-fourths white and one-fourth black;
one-fourth-white and three-fourths black.
4. In the same way mix the two complementaries yellow and blue to
produce a gray; mix red and green in the same way. Try various
combinations of the four fundamental colors, and discover how different
colors are produced. Seek for these same colors in nature--sky, leaves,
flowers, etc.
5. Take a large wire nail and push it through a cork so that it can be
handled without touching the metal with the fingers. Now cool it in ice
or very cold water, then dry it and move the point slowly across the
back of the hand. Do you feel occasional thrills of cold as the point
passes over a bulb of Krause? Heat the nail with a match flame or over a
lamp, and perform the same experiment. Do you feel the thrills of heat
from the corpuscles of Ruffini?
6. Try stopping the nostrils with cotton and having someone give you
scraped apple, potato, onion, etc., and see whether, by taste alone, you
can distinguish the difference. Why cannot sulphur be tasted?