ILLUSIONS OF PERCEPTION--_continued_.
A. _Passive Illusions (a) as determined by the Organism._
In dealing with the illusions which are related to certain peculiarities
in the nervous organism and the laws of sensibility, I shall commence
with those which are connected with certain limits of sensibility.
_Limits of Sensibility._
To begin with, it is known that the sensation does not always answer to
the external stimulus in its degree or intensity. Thus, a certain amount
of stimulation is necessary before any sensation arises. And this will,
of course, be greater when there is little or no attention directed to
the impression, that is to say, no co-operating central reaction. Thus
it happens that slight stimuli go overlooked, and here illusion may have
its starting-point. The most familiar example of such slight errors is
that of movement. When we are looking at objects, our ocular muscles are
apt to execute very slight movements which escape our notice. Hence we
tend, under certain circumstances, to carry over the retinal result of
the movement, that is to say, the impression produced by a shifting of
the parts of the retinal image to new nervous elements, to the object
itself, and so to transform a "subjective" into an "objective" movement.
In a very interesting work on apparent or illusory movements, Professor
Hoppe has fully investigated the facts of such slight movements, and
endeavoured to specify their causes.[17]
Again, even when the stimulus is sufficient to produce a conscious
impression, the degree of the feeling may not represent the degree of
the stimulus. To take a very inconspicuous case, it is found by Fechner
that a given increase of force in the stimulus produces a less amount of
difference in the resulting sensations when the original stimulus is a
powerful one than when it is a feeble one. It follows from this, that
differences in the degree of our sensations do not exactly correspond to
objective differences. For example, we tend to magnify the differences
of light among objects, all of which are feebly illuminated, that is to
say, to see them much more removed from one another in point of
brightness than when they are more strongly illuminated. Helmholtz
relates that, owing to this tendency, he has occasionally caught
himself, on a dark night, entertaining the illusion that the
comparatively bright objects visible in twilight were self-luminous.[18]
Again, there are limits to the conscious separation of sensations which
are received together, and this fact gives rise to illusion. In general,
the number of distinguishable sensations answers to the number of
external causes; but this is not always the case, and here we naturally
fall into the error of mistaking the number of the stimuli. Reference
has already been made to this fact in connection with the question
whether consciousness can be mistaken as to the character of a present
feeling.
The case of confusing two impressions when the sensory fibres involved
are very near one another, has already been alluded to. Both in touch
and in sight we always take two or more points for one when they are
only separated by an interval that falls below the limits of local
discrimination. It seems to follow from this that our perception of the
world as a continuum, made up of points perfectly continuous one with
another may, for what we know, be illusory. Supposing the universe to
consist of atoms separated by very fine intervals, then it is
demonstrable that it would appear to our sensibility as a continuum,
just as it does now.[19]
Two or more simultaneous sensations are indistinguishable from one
another, not only when they have nearly the same local origin, but under
other circumstances. The blending of partial sensations of tone in a
_klang_-sensation, and the coalescence in certain cases of the
impressions received by way of the two retinas, are examples of this. It
is not quite certain what determines this fusion of two simultaneous
feelings. It may be said generally that it is favoured by similarity
between the sensations;[20] by a comparative feebleness of one of the
feelings; by the fact of habitual concomitance, the two sensations
occurring rarely, if ever, in isolation; and by the presence of a mental
disposition to view them as answering to one external object. These
considerations help us to explain the coalescence of the retinal
impressions and its limits, the fusion of partial tones, and so on.[21]
It is plain that this fusion of sensations, whatever its exact
conditions may be, gives rise to error or wrong interpretation of the
sense-impression. Thus, to take the points of two legs of a pair of
compasses for one point is clearly an illusion of perception. Here is
another and less familiar example. Very cold and smooth surfaces, as
those of metal, often appear to be wet. I never feel sure, after wiping
the blades of my skates, that they are perfectly dry, since they always
seem more or less damp to my hand. What is the reason of this? Helmholtz
explains the phenomenon by saying that the feeling we call by the name
of wetness is a compound sensation consisting of one of temperature and
one of touch proper. These sensations occurring together so frequently,
blend into one, and so we infer, according to the general instinctive
tendency already noticed, that there is one specific quality answering
to the feeling. And since the feeling is nearly always produced by
surfaces moistened by cold liquid, we refer it to this circumstance, and
speak of it as a feeling of wetness. Hence, when the particular
conjunction of sensations arises apart from this external circumstance,
we erroneously infer its presence.[22]
The most interesting case of illusion connected with the fusion of
simultaneous sensations, is that of single vision, or the deeply
organized habit of combining the sensations of what are called the
corresponding points of the two retinas. This coalescence of two
sensations is so far erroneous since it makes us overlook the existence
of two distinct external agencies acting on different parts of the
sensitive surface of the body. And this is the more striking in the case
of looking at solid objects, since here it is demonstrable that the
forces acting on the two retinas are not perfectly similar.
Nevertheless, such a coalescence plainly answers to the fact that these
external agencies usually arise in one and the same object, and this
unity of the object is, of course, the all-important thing to be sure
of.
This habit may, however, beget palpable illusion in another way. In
certain exceptional cases the coalescence does not take place, as when I
look at a distant object and hold a pencil just before my eyes.[23] And
in this case the organized tendency to take one visual impression for
one object asserts its force, and I tend to fall into the illusion of
seeing two separate pencils. If I do not wholly lapse into the error, it
is because my experience has made me vaguely aware that double images
under these circumstances answer to one object, and that if there were
really two pencils present I should have four visual impressions.
Once more, it is a law of sensory stimulation that an impression
persists for an appreciable time after the cessation of the action of
the stimulus. This "after sensation" will clearly lead to illusion, in
so far as we tend to think of the stimulus as still at work. It forms,
indeed, as will be seen by-and-by, the simplest and lowest stage of
hallucination. Sometimes this becomes the first stage of a palpable
error. After listening to a child crying for some time the ear easily
deceives itself into supposing that the noise is continued when it has
actually ceased. Again, after taking a bandage from a finger, the
tingling and other sensations due to the pressure sometimes persist for
a good time, in which case they easily give rise to an illusion that the
finger is still bound.
It follows from this fact of the reverberation of the nervous structures
after the removal of a stimulus, that whenever two discontinuous
stimulations follow one another rapidly enough, they will appear
continuous. This fact is a fruitful source of optical illusion. The
appearance of a blending of the stripes of colours on a rotating disc or
top, of the formation of a ring of light by swinging round a piece of
burning wood, and the illusion of the toy known as the thaumatrope, or
wheel of life, all depend on this persistence of retinal impression.
Many of the startling effects of sleight of hand are undoubtedly due in
part to this principle. If two successive actions or sets of
circumstances to which the attention of the spectator is specially
directed follow one another by a very narrow interval of time, they
easily appear continuous, so that there seems absolutely no time for the
introduction of an intermediate step.[24]
There is another limit to sensibility which is in a manner the opposite
to the one just named. It is a law of nervous stimulation that a
continued activity of any structure results in less and less psychic
result, and that when a stimulus is always at work it ceases in time to
have any appreciable effect. The common illustration of this law is
drawn from the region of sound. A constant noise, as of a mill, ceases
to produce any conscious sensation. This fact, it is plain, may easily
become the commencement of an illusion. Not only may we mistake a
measure of noise for perfect silence,[25] we may misconceive the real
nature of external circumstances by overlooking some continuous
impression.
Curious illustrations of this effect are found in optical illusions,
namely, the errors we make respecting the movement of stationary objects
after continued movement of the eyes. When, for example, in a railway
carriage we have for some time been following the (apparent) movement of
objects, as trees, etc., and turn our eyes to an apparently stationary
object, as the carpet of the compartment, this seems to move in the
contrary direction to that of the trees. Helmholtz's explanation of this
illusion is that when we suppose that we are fixing our eye on the
carpet we are really continuing to move it over the surface by reason of
the organic tendency, already spoken of, to go on doing anything that
has been done. But since we are unaware of this prolonged series of
ocular movements, the muscular feelings having become faint, we take the
impression produced by the sliding of the picture over the retina to be
the result of a movement of the object.[26]
Another limit to our sensibility, which needs to be just touched on
here, is known by the name of the specific energy of the nerves. One and
the same nerve-fibre always reacts in a precisely similar way, whatever
the nature of the stimulus. Thus, when the optic nerve is stimulated in
any manner, whether by light, mechanical pressure, or an electric
current, the same effect, a sensation of light, follows.[27] In a usual
way, a given class of nerve-fibre is only stimulated by one kind of
stimulus. Thus, the retina, in ordinary circumstances, is stimulated by
light. Owing to this fact, there has arisen a deeply organized habit of
translating the impression in one particular way. Thus, I instinctively
regard a sensation received by means of the optic nerve as one caused by
light.
Accordingly, whenever circumstances arise in which a like sensation is
produced by another kind of stimulus, we fall into illusion. The
phosphenes, or circles of light which are seen when the hinder part of
the eyeball is pressed, may be said to be illusory in so far as we
speak of them as perceptions of light, thus referring them to the
external physical agency which usually causes them. The same remark
applies to those "subjective sensations," as they are called, which are
known to have as their physical cause subjective stimuli, consisting, in
the case of sight, in varying conditions of the peripheral organ, as
increased blood-pressure. Strictly speaking, such simple feelings as
these appear to be, involve an ingredient of false perception: in saying
that we _perceive_ light at all, we go beyond the pure sensation,
interpreting this wrongly.
Very closely connected with this limitation of our sensibility is
another which refers to the consciousness of the local seat, or origin
of the impression. This has so far its basis in the sensation itself as
it is well known that (within the limits of local discrimination,
referred to above) sensations have a particular "local" colour, which
varies in the case of each of the nervous fibres by the stimulation of
which they arise.[28] But though this much is known through a difference
in the sensibility, nothing more is known. Nothing can certainly be
ascertained by a mere inspection of the sensation as to the distance the
nervous process has travelled, whether from the peripheral termination
of the fibre or from some intermediate point.
In a general way, we refer our sensations to the peripheral endings of
the nerves concerned, according to what physiologists have called "the
law of eccentricity." Thus I am said to feel the pain caused by a
bruise in the foot in the member itself. This applies also to some of
the sensations of the special senses. Thus, impressions of taste are
clearly localized in the corresponding peripheral terminations.
With respect to the sense of smell, and still more to those of hearing
and sight, where the impression is usually caused by an object at a
distance from the peripheral organ, our attention to this external cause
leads us to overlook in part the "bodily seat" of the sensation. Yet
even here we are dimly aware that the sensation is received by way of a
particular part of the sensitive surface, that is to say, by a
particular sense-organ. Thus, though referring an odour to a distant
flower, we perceive that the sensation of odour has its bodily origin in
the nose. And even in the case of hearing and sight, we vaguely refer
the impressions, as such, to the appropriate sense-organ. There is,
indeed, in these cases a double local reference, a faint one to the
peripheral organ which is acted on, and a more distinct one to the
object or the force in the environment which acts on this.
Now, it may be said that the act of localization is in itself distinctly
illusory, since it is known that the sensation first arises in
connection with the excitation of the sensory _centre_, and not of the
peripheral fibre.[29] Yet it must at least be allowed that this
localization of sensation answers to the important fact that, under
usual circumstances, the agency producing the sensation is applied at
this particular point of the organism, the knowledge of which point is
supposed by modern psychologists to have been very slowly learnt by the
individual and the race, through countless experiments with the moving
organ of touch, assisted by the eye.
Similarly, the reference of the impression, in the case of hearing and
sight, to an object in the environment, though, as we have seen, from
one point of view illusory, clearly answers to a fact of our habitual
experience; for in an immense preponderance of cases at least a visual
or auditory impression does arise through the action on the sense-organ
of a force (ether or air waves) proceeding from a distant object.
In some circumstances, however, even this element of practical truth
disappears, and the localization of the impression, both within and
without the organism, becomes altogether illusory. This result is
involved in the illusions, already spoken of, which arise from the
instinctive tendency to refer sensations to the ordinary kind of
stimulus. Thus, when a feeling resulting from a disturbance in the optic
nerve is interpreted as one of external light vaguely felt to be acting
on the eye, or one resulting from some action set up in the auditory
fibre as a sensation of external sound vaguely felt to be entering the
ear, we see that the error of localization is a consequence of the other
error already characterized.
As I have already observed, an excitation of a nerve at any other point
than the peripheral termination, occurs but rarely in normal life. One
familiar instance is the stimulation of the nerve running to the hand
and fingers, by a sharp blow on the elbow over which it passes. As
everybody knows, this gives rise to a sense of pain at the _extremities_
of the nerve. The most common illustration of such errors of
localization is found in subjective sensations, such as the impression
we sometimes have of something creeping over the skin, of a disagreeable
taste in the mouth, of luminous spots floating across the field of
vision, and so on. The exact physiological seat of these is often a
matter of conjecture only; yet it may safely be said that in many
instances the nervous excitation originates at some point considerably
short of its peripheral extremity: in which case there occurs the
illusion of referring the impressions to the peripheral sense-organ, and
to an external force acting on this.
The most striking instances of these errors of localization are found in
abnormal circumstances. It is well known that a man who has lost a leg
refers all sensations arising from a stimulation of the truncated fibres
to his lost foot, and in some cases has even to convince himself of the
non-existence of his lost member by sight or touch. Patients often
describe these experiences in very odd language. "If," says one of Dr.
Weir Mitchell's patients, "I should say I am more sure of the leg which
ain't than the one which air, I guess I should be about correct."[30]
There is good reason for supposing that this source of error plays a
prominent part in the illusions of the insane. Diseased centres may be
accompanied by disordered peripheral structures, and so subjective
sensation may frequently be the starting-point of the wildest illusions.
Thus, a patient's horror of poison may have its first origin in some
subjective gustatory sensation. Similarly, subjective tactual sensations
may give rise to gross illusions, as when a patient "feels" his body
attacked by foul and destructive creatures.
It may be well to remark that this mistaken interpretation of the seat
or origin of subjective sensation is closely related to hallucination.
In so far as the error involves the ascription of the sensation to a
force external to the sense-organ, this part of the mental process must,
when there is no such force present, be viewed as hallucinatory. Thus,
the feeling of something creeping over the skin is an hallucination in
the sense that it implies the idea of an object external to the skin.
Similarly, the projection of an ocular impression due to retinal
disturbance into the external field of vision, may rightly be named an
hallucination. But the case is not always so clear as this. Thus, for
example, when a gustatory sensation is the result of an altered
condition of the saliva, it may be said that the error is as much an
illusion as an hallucination.[31]
In a wide sense, again, all errors connected with those subjective
sensations which arise from a stimulation of the peripheral regions of
the nerve may be called illusions rather than hallucinations. Or, if
they must be called hallucinations, they may be distinguished as
"peripheral" from those "central" hallucinations which arise through an
internal automatic excitation of the sensory centre. It is plain from
this that the region of subjective sensation is an ambiguous region,
where illusion and hallucination mix and become confused. To this point
I shall have occasion to return by-and-by.
I have now probably said enough respecting the illusions that arise
through the fact of there being fixed limits to our sensibility. The
_rationale_ of these illusions is that whenever the limit is reached, we
tend to ignore it and to interpret the impression in the customary way.
_Variations of Sensibility._
We will now pass to a number of illusions which depend on something
variable in the condition of our sensibility, or some more or less
exceptional organic circumstance. These variations may be momentary and
transient or comparatively permanent. The illusion arises in each case
from our ignoring the variation, and treating a given sensation under
all circumstances as answering to one objective cause.
First of all, the variation of organic state may affect our mental
representation of the strength of the stimulus or external cause. Here
the fluctuation may be a temporary or a permanent one. The first case is
illustrated in the familiar example of taking a room to be brighter
than it is when emerging from a dark one. Another striking example is
that of our sense of the temperature of objects, which is known to be
strictly relative to a previous sensation, or more correctly to the
momentary condition of the organ. Yet, though every intelligent person
knows this, the deeply rooted habit of making sensation the measure of
objective quality asserts its sway, and frequently leads us into
illusion. The well-known experiment of first plunging one hand in cold
water, the other in hot, and then dipping them both in tepid, is a
startling example of this organized tendency. For here we are strongly
disposed to accept the palpable contradiction that the same water is at
once warm and cool.
Far more important than these temporary fluctuations of sensibility are
the permanent alterations. Excessive fatigue, want of proper nutrition,
and certain poisons are well known to be causes of such changes. They
appear most commonly under two forms, exalted sensibility, or
hyperæsthesia, and depressed sensibility, or anæsthesia. In these
conditions flagrant errors are made as to the real magnitude of the
causes of the sensations. These variations may occur in normal life to
some extent. In fairly good health we experience at times strange
exaltations of tactual sensibility, so that a very slight stimulus, such
as the contact of the bed-clothes, becomes greatly exaggerated.
In diseased states of the nervous system these variations of sensibility
become much more striking. The patient who has hyperæsthesia fears to
touch a perfectly smooth surface, or he takes a knock at the door to be
a clap of thunder. The hypochondriac may, through an increase of
organic sensibility, translate organic sensations as the effect of some
living creature gnawing at his vitals. Again, states of anæsthesia lead
to odd illusions among the insane. The common supposition that the body
is dead, or made of wood or of glass, is clearly referable in part to
lowered sensibility of the organism.[32]
It is worth adding, perhaps, that these variations in sensibility give
rise not only to sensory but also to motor illusions. To take a homely
instance, the last miles of a long walk seem much longer than the first,
not only because the sense of fatigue leading us to dwell on the
transition of time tends to magnify the apparent duration, but because
the fatigued muscles and connected nerves yield a new set of sensations
which constitute an exaggerated standard of measurement. A number of
optical illusions illustrate the same thing. Our visual sense of
direction is determined in part by the feelings accompanying the action
of the ocular muscles, and so is closely connected with the perception
of movement, which has already been touched on. If an ocular muscle is
partially paralyzed it takes a much greater "effort" to effect a given
extent of movement than when the muscle is sound. Hence any movement
performed by the eye seems exaggerated. Hence, too, in this condition
objects are seen in a wrong direction; for the patient reasons that they
are where they would seem to be if he had executed a wider movement than
he really has. This may easily be proved by asking him to try to seize
the object with, his hand. The effect is exaggerated when complete
paralysis sets in, and no actual movement occurs in obedience to the
impulse from within.[33]
Variations in the condition of the nerve affect not only the degree, but
also the quality of the sensation, and this fact gives rise to a new
kind of illusion. The curious phenomena of colour-contrast illustrate
momentary alterations of sensibility. When, after looking at a green
colour for a time, I turn my eye to a grey surface and see this of the
complementary rose-red hue, the effect is supposed to be due to a
temporary fatigue of the retina in relation to those ingredients of the
total light in the second case which answer to the partial light in the
first (the green rays).[34]
These momentary modifications of sensibility are of no practical
significance, being almost instantly corrected. Other modifications are
more permanent. It was found by Himly that when the retina is
overexcitable every stimulus is raised in the spectrum scale of colours.
Thus, violet becomes red. An exactly opposite effect is observed when
the retina is torpid.[35] Certain poisons are known to affect the
quality of the colour-impression. Thus, santonin, when taken in any
quantity, makes all colourless objects look yellow. Severe pathological
disturbances are known to involve, in addition to hyperæsthesia and
anæsthesia, what, has been called paræsthesia, that is to say, that
condition in which the quality of sensation is greatly changed. Thus,
for example, to one in this state all food appears to have a metallic
taste, and so on.
If we now glance back at the various groups of illusions just
illustrated, we find that they all have this feature in common: they
depend on the general mental law that when we have to do with the
unfrequent, the unimportant, and therefore unattended to, and the
exceptional, we employ the ordinary, the familiar, and the well-known as
our standard. Thus, whether we are dealing with sensations that fall
below the ordinary limits of our mental experience, or with those which
arise in some exceptional state of the organism, we carry the habits
formed in the much wider region of average every-day perception with us.
In a word, illusion in these cases always arises through what may,
figuratively at least, be described as the application of a rule, valid
for the majority of cases, to an exceptional case.
In the varieties of illusion just considered, the circumstance that
gives the peculiarity to the case thus wrongly interpreted has been
referred to the organism. In the illusions to which we now pass, it will
be referred to the environment. At the same time, it is plain that there
is no very sharp distinction between the two classes. Thus, the visual
illusion produced by pressing the eyeball might be regarded not only as
the result of the organic law of the "specific energy" of the nerves,
but, with almost equal appropriateness, as the consequence of an
exceptional state of things in the environment, namely, the pressure of
a body on the retina. As I have already observed, the classification
here adopted is to be viewed simply as a rough expedient for securing
something like a systematic review of the phenomena.