The Animal Mind: A Textbook of Comparative Psychology

On the whole, the weight of evidence is at present in favor of the possession of color vision by fish. § 42. The Problem of Visual Qualities: Reptiles and Amphibia Skin sensitiveness to light has been demonstrated in certain amphibians. The response of the frog to light persists when the animal is blinded, although in the normal animal the eyes are involved in the reaction, since it occurs when the skin is covered and the eyes left intact (405, 538). The skin of salamanders also is sensitive to

light (196). The nature of the ‘dermal light sensation”’ remains a mystery. It can hardly, in frogs, be a painful irritation, since it produces a positive response; and it is not due to heat rays, for it occurs when these are intercepted by passing the light through water. As Parker says, radiant heat and light, “distinct as they seem to our senses, are members of one physical series in that they are both ether vibrations, varying only in wave length” (538). While, then, the nerve endings in human skin are sensitive only to the slower of these vibrations, the heat rays, those in the skin of the frog may respond to the whole series, with what accompanying sensation qualities we cannot say. It is interesting to note that Pearse (566), working with frogs and salamanders, normal and blinded, finds that red light, which stands nearest to heat in vibration frequency, is most effective for the blinded animals, blue light for the normal ones. In the young of frogs and salamanders it has been shown that the skin nerves are the source of dermal reactions to light.

The frog’s eye is sensitive to light rays from all the spectral regions visible to man; the distribution of brightnesses in the spectrum is like that of normal human vision, and the dark-adapted eye shows a shift*of the brightness values to correspond with those of the dark-adapted human eye. One method by which these results were obtained was that of testing the electric effect (action currents) of stimulating frogs’ eyes with light of different colors: the maximum effect for the light-adapted eye was in the yellow green, that for the dark-adapted eye was in the yellow (324 a). Another method was to illuminate food with light of different colors and to observe in what lights it was most readily seized. From the results Hess (305) concludes that amphibian vision is qualitatively like that

of man. Bab&k (11) has studied the effect of different colors on the frog’s breathing; its rate and the movements involved in “throat” and “lung” breathing. The forebrains of the animals had been removed, a proceeding which makes the breathing of the resting animal more regular. He found that each color produced a breathing curve of a certain specific pattern, and concluded that the colors have specific effects on the eye independent of their intensity.

The results with turtles, which are reptiles, correspond to those for amphibians, except that Hess (305) finds the spectrum shortened at the violet end; that is, the turtle does not see beyond the blue. The method used was that of illuminating food with differently colored lights. Hess explains this shortening of the spectrum by the fact that in the turtle eyes, as in those of all birds, a few fishes, and Ornithorhyncus, there are attached to the ends of the cones transparent colored globules like little drops of oil. They are in the turtle mostly red and orange, and would act, Hess thinks, like spectacles of colored glass to cut off the blue and violet rays. The fact that adaptation to darkness apparently occurs in the turtle is of interest because its retina is lacking in rods. The rods, then, cannot be, as they have sometimes been supposed, essential to the process of darkness adaptation.

Many experiments have been made on color discrimination in birds. Most of the older ones were conducted by the method of training the birds to choose between differently colored papers (611), or between compartments illuminated through differently colored glass (647). These experiments made practically no attempt to guard against the possibility that the birds were reacting to differences in the brightness of the lights. Another method used by Katz and Revesz (395) was that of scattering grain on grounds of different colors and noting how often the grain was picked from the several grounds. The positive result of this research was that while fowls with light-adapted eyes pecked equally often at grains on yellow, green, red, and violet grounds, those with dark-adapted eyes never pecked at grain on the red ground. This indicates a process of darkness adaptation like that in the human eye, which sees red as very dark in faint light. In a later investigation the same workers tried scattering red, blue, and green grains of various saturations, mixed with grains stained four different shades of gray. All the grains were stuck fast to the ground except those of a particular color. The fowls showed an ability to discriminate which was about equal to that of the normal human being. It cannot be said, however, that these experiments satisfactorily eliminated the brightness error, since so few shades of gray were used.

The effect of different colored rays on the pupillar reflex of birds was studied by Hess (303). For day birds, he found that the maximal effect was produced by the yellow rays ; for owls, by the yellow-green. That is, the day birds showed the brightness distribution characteristic of the light-adapted human eye with color vision; the night birds the distribution of total color blindness, or darkness adaptation. By his method of observing under what illumination the animals could find food, Hess obtained results leading him to conclude that day birds have a spectrum shortened in the violet end, a fact which he ascribes to the effect of the oil globules in the retina; and that the spectrum for

owls is somewhat longer. Watson (773), on the other hand, working with a more exact apparatus, concludes that the spectrum is visible to the chick and the homing pigeon within the same limits as to man. A study of the electric currents generated by the action of light on the eyes of day and night birds gives evidence confirming the hypothesis that the latter are color-blind: in the day birds, each color gives a characteristic deflection of the galvanometer, not due to its intensity, while no such differences appear for the eyes of night birds (403). Breed (ror, 102), using colored screens through which the light was passed, and offering a choice of passages differently illuminated, obtained evidence of color discrimination in the chick. The preference of the chicks for one color rather than another appeared to depend on the relative brightness of the colors, since it could be reversed when their brightnesses were sufficiently altered. When a blue and red were found between which the chick showed no preference, this was taken as an indication that they looked equally bright to the chick. The bird could, however, be trained to choose one of these two colors; hence the conclusion was reached that it could probably react to a difference in color and not merely to one in brightness. The evidence for color vision in birds has lately been made practically conclusive by the careful experiments of Lashley (413) on the domestic fowl. He used spectral light whose intensity was accurately controlled. The ability of the fowl to distinguish red and green was demonstrated under the following conditions, which ruled out the possibility of discriminating by brightness differences. (1) Each of the lights was alternately reduced to threshold intensity, while the other remained at full intensity.

(2) White light of a constant intensity was substituted for each colored light in turn. (3) Each light was in turn exposed alone, one passage being left dark. Indications of the presence of darkness adaptation in the chick appeared from the facts that light adapted chicks chose red and yellow rather than green, while for dark adapted chicks the preference was reversed. Rouse observed that differently colored lights had on the average different effects in quickening the rate of breathing in the pigeon; the strongest effect being produced by blue, the weakest by red (648).

The earlier experiments on the visual discriminations of mammals, like those with other animals, failed to reckon adequately with the brightness error, the possibility that discriminations between colors are made as a color-blind human being would make them, the colors being seen as different shades of gray (138, 140). The first method, as we have seen, which suggested itself as a means of eliminating this error was that of showing that an animal could, or could not, distinguish a color from the gray which a light-adapted human being would see in its place. Such a gray can be determined by the so-called “flicker method.” If a disk be made of a colored and a gray paper, when it is rotated a little too slowly to give a smooth mixture, the peculiar appearance of “‘flickering” will be observed if the color and the gray are not of equal brightness, but will disappear when a gray equally bright with the color is selected. The determination of this equivalent, however, has really no bearing on the problem of color visionin animals. If they are color-blind, their difficulty would more probably lie in distinguishing

between a color and its brightness equivalent for the colorblind or dark-adapted human eye; and quite possibly the brightness which they see instead of color may be unlike the brightness value of that color to either the light-adapted or the dark-adapted human eye. Kinnaman’s (401) color‘ tests on monkeys, from which he concluded that they possess color vision, employed only the older methods of getting rid of the brightness error: the monkeys, which had learned to identify a vessel covered with a particular colored paper as containing food, were shown to be unequal to the discrimination between gray papers whose brightnesses were to the human eye the same as those of the colors. It was also shown that a colored glass could be picked out many times from among three others covered with gray paper of the same brightness as the color, to human vision. In Cole’s (134) demonstration that raccoons can distinguish colors, the colors used were equated in brightness for the human eye by the flicker method. :

The experiments of Yerkes on the dancing mouse (820) brought into clear relief the danger of trying to eliminate the brightness error by the use of grays equal in brightness with the colors to the human eye. His method consisted in teaching the animals to associate one of two differently illuminated compartments with an electric shock. The intensity of the illuminations could be regulated by varying the distance of the lights from them. When only white lights were used, Weber’s Law was found to hold for the one mouse tested: the animal could distinguish a difference in the brightness of the compartments amounting to about one-tenth of their absolute brightness, within certain limits of absolute brightnesses. Light blue and orange, green and red, violet and red, were discriminated

even when their brightnesses were considerably varied. Yet the probability appeared that these discriminations were based merely on brightness differences, for after a mouse had learned to choose green rather than red, when it was offered a choice between light and darkness, it uniformly preferred the former, although untrained mice showed no such preference. Apparently, then, the green had been previously discriminated simply as the lighter of the two impressions, and to the eye of the mouse, as to that of the color-blind human being, red looks an extremely dark gray.

In some experiments of the writer’s (756) on the rabbit, the method was used of presenting a color with various grays, in successive experimental series, and finding whether or not there existed a gray with which the color was confused. This is the only adequate way of dealing with the brightness error. We found that while the rabbit could be taught, by rewarding it with food for right choices, to distinguish a standard red paper from a number of different gray papers, it invariably failed when a very dark gray, almost black, was presented with the red. Two objections which have been urged against the use of colored papers were met in these experiments. In the first place, it is argued that papers of different colors may differ in surface texture: the possibility that our rabbits reacted to this clew we eliminated by occasionally substituting red and gray velvets for our red and gray papers, a change that did not at all affect the rabbits. Secondly, it has been urged that when colored papers are pasted on cards, they are apt to show wrinkles that might identify them: this we obvi- | ated by pinning on our papers afresh in successive experiments.

pure colors; since, that is, they reflect to the eye light of wave-lengths besides that of their predominant color, the use of pure spectral light is to be preferred. The apparatus by which such light can be used with its intensity accurately controlled is very elaborate, and was devised by Yerkes and Watson (831). Using this apparatus, the Watsons (772) found that rats and rabbits failed to distinguish between red and darkness. A similar indication had been previously obtained by Waugh (775) on the gray mouse; he found that red filters and pigments could be distinguished from gray when the two were equal in brightness to the human eye, but that the discrimination tended to disappear when the red was made lighter, and to improve when it was made darker.

The dog and cat also suffer under the imputation of colorblindness. Tests by Pawlow’s method on the dog failed to indicate that it can react to color differences as such (830). Smith (687), it is true, working with colored papers, argues in favor of the dog’s color vision from the fact that the dogs showed some evidence of learning to distinguish the colors from all the grays used. De Voss and Ganson (184) found that none of the six colored papers they used could be discriminated by cats from all the shades of gray in their series: each color was confused with some particular gray. Even the monkey is suspected of color-blindness: Watson (768) reports as the chief result of his experiments with this animal that red has little or no stimulating power upon it.

. Moopirication of the behavior of animals with reference ~'to the spatial characteristics of the forces acting upon them appears at the very beginning of the scale of animal life, and throughout is quite as important as modification with reference to the kind or quality of such forces. It assumes a number of distinct forms. Some of these suggest to us, interpreting them as we must on the basis of our own experience, no conscious aspect at all; they seem rather mechanical effects upon a passive organism. In other cases, it appears possible that the mental process which we know as space perception, involving the simultaneous awareness of a number of sensations consciously referred to different points in space, may accompany the reaction of an animal with reference to the spatial relations of its environment. And sometimes we can only say that differences in the space characteristics of a stimulus may modify the accompanying sensation in some manner which yet apparently does not involve space perception as we know it. Our task in the following pages will then be to examine the different ways in which animal behavior is adapted to the spatial characteristics of stimuli, and to ask which of these suggest as their conscious accompaniment some form of space perception. A classification of spatially

determined responses that is not, indeed, ideally satisfactory, but may serve our purpose, divides them into five groups : — 1. Reactions adapted to the position of a single stimulus acting at a definite point on the body. 2. Reactions to a continuous stimulus, which involve the assumption of a certain position of the whole body with reference to the stimulus: orienting reactions. . 3. Reactions to a stimulus that moves, i.e., that affects several neighboring points on the body successively.

4. Reactions adapted to the relative position of several stimuli acting simultaneously. 5. Reactions adapted to the distance of an object from the body. Responses to stimulation that are adapted to the point of application of the stimulus are to be found among very simple animals. They may be subdivided into three groups: first, cases where the part of the animal that reacts is the part directly affected by the stimulus; second, cases where the whole animal reacts by a movement in the appropriate direction; and third, cases where a part of the body not directly affected by the stimulus moves toward the point stimulated. j

xz. Amceba furnishes an example of the first class. Its negative reaction occurs by the checking of protoplasmic flow at the point where a strong mechanical stimulus affects the body; its positive reaction by a flowing forward of the protoplasm at the point where a weak stimulus acts, and its food-taking reaction by an enveloping flow on both sides of the point stimulated. This would seem to be the most primitive way of adapting response to the location of a stimulus: the effect is produced just where the force acts, as it might be upon a piece of inanimate matter. In no animal with a nervous system, probably, is the process quite so simple. The bell of the jellyfish contracts at the point where a stimulus, mechanical or photic, is applied ; yet although these responses are made when the nervous system is thrown out of function, they occur more slowly, and in the normal animal the nervous tissue is probably involved, while, of course, a long conduction pathway is traversed when, to use a familiar illustration, the baby pulls. back its hand from the candle flame. 2. Paramecium and other infusoria, planarians, the earthworm, and various other animals give us illustrations Sof movements of the entire body differing according to the point affected by a single stimulus. If the front half of Paramecium be touched, the animal gives the typical avoiding reaction of darting backward and turning to one side; if the hinder end be touched, it moves forward (378, p. 59). On the other hand, it makes no difference in its reactions to stimuli affecting either side of the body; the turning is always to the aboral side even when the stimulus comes from that direction (378, p. 52). If strong mechanical stimulation be applied to the head end of a planarian, there is a response which seems to belong under type (1): the head is turned away from the stimulus. If the hinder region is touched, strong forward crawling movements of the body are produced. The positive reaction in the planarian, turning the head toward the stimulus, also suggests type (1), but in reality it has been shown by Pearl to be a far more complex affair than the mere flow of protoplasm at the stimulated point, and to involve the contraction of several

sets of muscles (561). The earthworm creeps backward if the front half of the body is affected, turns away from a stimulus applied to the side of the anterior end, and creeps forward if the stimulus affects the posterior half of the body (377). In general, a reaction of type (2) rather than type (x) will occur in proportion to the degree in which an organism’s movements are codrdinated and it tends to act as a whole. 3. One of the prettiest examples of the most highly coordinated form of response to a single localized stimulus; namely, movement of some other part of the body toward the point affected, is to be found in the swinging over of the jellyfish’s manubrium toward the spot on the bell touched by food. ‘In the typical feeding reaction,” says Yerkes, “the manubrium bends toward the food. If during such a movement the piece of food be moved to the opposite side of the bell, the manubrium, too, in a few seconds will bend in the opposite direction, that is, again toward the food” (802). The sea urchin responds to mechanical stimulation by moving the spines toward the place stimulated (735). In the higher animals this form of reaction has largely superseded other methods of adapting behavior to a stimulus acting at a definite point. Where grasping appendages exist, the obvious device is to move them toward the point of stimulation in order either to seize or to remove the object. This involves not merely that the effects of the stimulus shall diffuse so as to involve general locomotor movements, but that the effect shall be exerted very definitely upon a particular set of muscles in a particular way. The “‘scratch-reflex”’ of mammals, and the reaction whereby a frog rubs its hind leg on the spot of skin affected by a drop of acid, are further examples.

ment of the reactions described under these three heads? When a stimulus applied at point @ brings about a reaction different from that produced by precisely the same stimulus acting on point }, are the accompanying sensations different, supposing the animal concerned to be conscious? If they are, the difference must be what has been called a difference in local sign. There is certainly no evidence that space perception is concerned. Space perception in our own experience always involves the simultaneous awareness of several stimuli. But where a single stimulus only is operative, the fact that reaction to it is modified by its location cannot mean that the relations of that location to the location of other stimuli are perceived. The truth is that space perception is so constant a factor in our own experience that we cannot imagine how a single sensation can be modified in connection with change of place of the stimulus, where space perception does not exist. A touch at any point on the skin of a human being is referred to a definite point in a constricted space, tactile and visual; it is given its proper place in a complex of sensations. What modification of it would correspond to its location if it stood alone in consciousness, we cannot now conceive. -

§ 47. Class II: Orienting Reactions; Possible Modes of Producing Them Various forces, such as gravity, light, electricity, centrifugal force, currents of water and air, are all influences causing certain organisms to bring their bodies into a definite position. Such reactions, involving the direction of the whole body with reference to a continuous force acting upon it, are known as reactions of orientation. There are various ways in which they might conceivably take place.

(a) They might be due to the “pull” of a force upon the passive body of an animal. In the case of gravity or of a current of wind or water, if one part of the body were heavier or offered ,.more surface to the force, the position assumed could be explained without supposing any activity on the animal’s part. In such a case there would be no reason for thinking of the reaction as conscious. (b) The response might be due to the effect of a force acting unevenly upon the two sides of the body, and thereby unevenly affecting the motor apparatus on the two sides, thus causing the animal to turn until the forces acting upon symmetrical points were balanced. This, although involving activity on the animal’s part, would not, if the force acted directly on the muscles, suggest any conscious accompaniment. If it acted through symmetrically placed sense organs, awareness of the direction from which the force operated might be present.

(c) The orientation might take place by a negative reaction on the animal’s part to a definite stimulus given when the animal was in any other than the final, oriented position. If gravity were the force in question, the stimulus might be the pressure exerted within the body by particles of different density or by the fluid or mineral bodies in a statocyst organ. If the stimulus were light, the organism might be oriented by giving the negative reaction when its head entered a region either brighter or darker than the optimum illumination. In such cases, where the ordinary negative reaction is the only one involved, there is no reason to suppose the occurrence of any conscious accompaniment, other than the possible unpleasantness connected with that reaction.

ized “righting” reaction, given in response either to a stimulus within, say, a statolith organ, or, as in the planarian, to the absence of accustomed contact stimulation on one surface of the body. The reaction in these cases being a specialized one, it is possible that a peculiar sensation quality might be involved. (e) Orientation might take place through a movement occurring when the position of several stimuli perceived simultaneously was disturbed, and tending to restore them to their original position. This is the principle involved, as we shall see, in explaining the rheotropism or current orientation of fishes, and the anemotropism, or orientation to air currents, of insects,as due to an instinct to keep the visual surroundings the same. And this form of orientation alone suggests a true space perception as its conscious accompaniment.

Such being the conceivable ways in which orientation may be brought about, what are the observed facts? They may be considered under the heads of orientation to gravity, to light, and to other forces. To this form of reaction the term ‘“‘geotropism” or “‘geotaxis” has been applied. In various Protozoa negative geotropism, or a tendency to rise against the pull of gravity, has been observed: first by Schwartz in two single-celled organisms frequently classified as plants, Euglena and Chlamydomonas (667); and eight years later by Aderhold, who suggested, without accepting it, the theory that the orientation may be due simply to the greater weight of one end of the organism’s body (2). This view was maintained by Verworn: the action of gravity, he urged, must

be purely passive. It cannot operate as a stimulus to active response on the animal’s part, for a stimulus is always a change in environment, and gravity is a constant force (742). This ignores the fact that the animal’s relations to gravity may change though gravity does not, and also the fact that the continuous action of light is a stimulus. According to Verworn’s theory, the geotropic orientation of a single-celled organism takes place through a series of _ “little falls” whereby the heavier end is directed downward.

Massart opposed this view on the basis of observations which showed that the actual movements of the organisms did not correspond to it, but were the result of active orientation. If response to gravity is passive, then dead - animals should fall through the water in the same position as that assumed by living animals when oriented to gravity. Massart experimented with various Protozoa by killing them and studying their positions in sinking, which he found not always the same as the attitudes assumed in response to gravity (461). There is always the possibility, however, that the methods employed to kill may change the specific gravity of some part of the body. Jensen offered the theory that reaction to gravity may be due to the difference in the water pressure on the two ends of the animal. He asserted that when the air pressure on the water was reduced by exhausting the air above, there was an increase in the geotropism, indicating a relative rather than an absolute sensibility to pressure (382), but Lyon points out that this process may affect the animals in various other ways besides altering the air pressure. Increasing the air pressure, or protecting the surface with oil, has no effect upon geotropism, Lyon finds, and he urges that Jensen’s theory requires enormous sensibility to pressure differences on the organism’s part, as great as that needed by a human

being to note the difference between the air pressure on the head and that on the feet (449). Another suggestion was offered by Davenport (175), namely, that negatively geotropic organisms swim in the direction where the greatest resistance to their progress is offered. This is like one theory put forward to explain rheotropism, or the tendency of animals to swim against currents, and anemotropism, or the ‘‘head against wind” movement of insects ; and as RAdl (622) first and Lyon (448) afterward pointed out, it assumes the fact to be explained, for only if an animal actively opposes a force, will that force exert more pressure at one point of its body than at another. The theory cannot explain why an animal at rest should be oriented. Another argument that tells against it is offered . by experiments showing that animals placed in solutions of the same density as their own bodies, in which, therefore, they have no weight, still display negative geotropism, and that the direction of the response is not reversed when the fluid is made heavier than the animals (449). Lyon’s own theory, accepted by Jennings, is that the stimulus for geotropism is furnished by the action of gravity within the body of the organism, upon substances of different weight which exert varying pressures and take up different positions according to the position of the body (449).