far. Eaccoons apparently respond readily to intensity dif- ference: one observer states that only 4^ difference in the brightness of two grays is necessary to give a basis for the rise of a habit. When tested with white and black (positive) about 90 trials are necessary to eliminate errors. Field observers report that their sense of sight is very deli- cate and much depended upon when they are living in the wild. Porcupines have been similarly tested and the results of such tests show about the same sensitivity to intensity difference that we find in the raccoon. Interesting studies of a similar kind have been made upon the dog. Nearly all of- the investigators using the Pawlow method report that the conditioned reflexes in the dog arising from optical stimulation are dependent upon the intensity of the light. Experiments upon the brightness vision of the dancing mouse show that it has a certain capacity for forming habits based upon intensity difference. In the first place, when confronted with black and w^hite cardboards it ex- hibits an original and positive tendency to go to the black. By training, this tendency can be overcome and the animal can be forced to respond positively either to the black or to the white cardboard at the will of the experimenter. If it is tested with numbers 10 and 20 of the Nendel series of grays, it is found that the habit is formed with exceed- ing difficulty or not at all. Consequently this difference represents, under the conditions of such experimentation, the threshold (D.L.). The number of trials required to eliminate errors in such habits (175 to 42) has been shown to depend upon two factors — the intensity difference be- tween the two grays, and the strength of the electric shock (see p. 204). More careful tests by the use of reflected light show that Weber's law probably holds. When 5, 30, and 80 hefners are chosen as the standard intensities the difference must be approximately yV (i-e., a dancer can ' ' discriminate ' ' between 5 and 4yV hefners, between 20 and 18 hefners, and between 80 and 72 hefners). White, gray, black, and brown mice show a similar readiness to react upon the basis of intensity difference when tested with gray cards, yarns, etc. The white rat and the hooded black- 360 VISION and-white rat require from 500 to 600 trials to perfect the habit of responding positively to the brighter of two stimulus plates when the plates are illuminated respectively by a 2 c.p. and a 16 c.p. tungsten (see p. 206). Some inter- esting experiments have been carried out upon the behavior of bats when forced to fly in a room thickly strung with wires; but since the blind animals miss the wires about as often as the normal ones, it seems evident that the reaction is effected through some cutaneous receptor (p. 424). The bat does respond positively (reflexly) to light at certain seasons; so that there is some evidence that vision plays a role in their general behavior.
Birds. — When chicks are tested with Bradley black and white cardboards with black as the positive stimulus they are found to require a larger number of trials to perfect the habit (310 trials) than where white is used as the positive stimulus (150 trials). This is to be explained on the grounds of the fact that the chick is positive to the more intense of two achromatic light stimuli where the absolute intensity of the lights is not great (Breed). Some recent tests have been made upon the effect on habit formation of varying the strength of the electric shock. The habit consisted of learning to respond positively to the darker of two stimulus plates. While these experiments deal only indirectly with the problem which concerns us here, certain facts come out which bear upon the chick's sensitivity to intensity difference. The three separate problems given the chick were to go towards the darker of two plates when their relative ' ' brightness ' ' stood respectively as follows: These tests were carefully made with an apparatus similar to the one described on p. 78. Neglecting here the effect of varying the intensity of the electric shock, we find that the average number of trials required to learn (1) varied from 44 to 16; to learn (2), 105 to 40; to learn (3), 171 to 53 (Cole).
RESPONSE TO WHITE LIGHT 361 Fish. — We know very little about the fishes' sensitivity to differences in the intensity of white light stimuli. Parker has shown that certain fresh water forms {Am- mocoetes, Aniblyopsis, etc.) are usually negative to light. When their optic nerves are cut the animals still respond as do normal individuals; thus showing that the skin of the fish, like that of the frog, is sensitive to light. The tail is especially sensitive as contrasted with other bodily areas. Strangely enough the skin of marine forms is not sensitive. A number of forms (dogfish, killifish, tomcod), the optic nerves of which had been cut, were tested with very intense arc and sunlight, but no response to the light was forthcom- ing. If Hess is right about the lack of sensitivity to wave- length difference in the fish it follows that white light vision (intensity, form, size) plays a predominating role in its re- sponse to objects. It is probable that the behavior of the fish (in daylight), when placed in currents of water (so- called rheotropism), is in part controlled by optical re- flexes. As is well known, the fish takes a direction of motion against that of the current. It apparently tends to keep the same visual field, — i.e., to swim towards a stationary visual object. Animals with only one eye react as do the normals (Lyon).
Amphibia and reptiles. — Several investigators have shown that both the eye and the skin of the frog is sensi- tive to light. These investigators have concerned them- selves with the mechanics of orientation rather than with the range of sensitivity or with the delicacy of the func- tioning of the visual mechanism. Torrelle shows that the frogs (Bana virescens virescens and B. clamata) are posi- tive to light at room temperature (21° C.) and that when the temperature is raised to 30° C, the rate of positive re- sponse is accelerated. On the other hand, when the tem- perature is lowered to 10° C. the response to light is nega- tive. Riley shows that they (at least Bitfo Americanus Le Conie) are negative even at room temperature when the light is very intense (from projection lantern, about 10,000 candle meter). When the eyes and cerebral hemi- spheres are removed. from the frog {B. pipiens Schrefer) 362 VISION it is found that they are still sensitive to light. Such ani- mals after a time turn towards the source of the light and finally jump towards it. The time of such reactions, though, is greatly increased. It has been shown that this behavior is due to the sensitivity of the skin to light (Parker). Di- rect stimulation of the neural tissue by light will not produce the response. It is interesting to note that frogs possessing only one eye orient to light as do normal frogs. It has further been shown that when definite objects are in the field of vision they are reacted to, whereas the ordi- nary photic responses are inhibited (L. J. Cole). We know very little about the delicacy with which amphibia and reptiles react to intensity difference. Some recent tests have shown that the turtle (Chrysemys marginata) is able to learn the black- white (cardboard) habit. Four animals gave positive results, but one animal showed no improve- ment in 220 trials (Casteel). Vision is very important in the daily adjustments of the tortoise.^* When land and water species of the tortoise are forced to jump from a board into a net of black cloth it is found that the time spent on the board before jumping is shorter in water species than in land species. Total inhibition to jumping, i.e., failure to crawl from the board and fall off in the allotted time (60 minutes), appears at a much less height for the purely land species than for the water-land and water species. It is quite probable that there are tactual and kingesthetic factors here in addition to the visual (Yerkes).
VI. Eesponse to Form and Size In mammals. — Eesponse of monkeys to form and size has so far not been carefully tested. In the ordinary rough ^* All observers agree that one finds the strongest support for the view that moving visual stimuli possess higher stimulating value than stationary ones in the resoonses of amphibia and reptiles. Animals belonging to these groups do not as a rule strike even at food objects unless such ob^^ects are moving. Certain of these animals would unquestionably perish from hunger in a cage filled with plenty unless precautions were taken to dangle the food in front of them until it is struck at.
RESPONSE TO FORM AND SIZE 363 tests the monkey shows the readiness we should expect in picking out a food vessel of given shape from others differ- ing in form. Similar rough tests upon size differences yield the same results. In the size tests the monkey apparently seeks the larger vessels. When tested with designs drawn upon cards monkeys failed to show a high order of dis- crimination. The writer has made some form tests upon the rhesus (spring of 1911). The results were never pub- lished because the work could not be completed. The tests were made in the dark room with the standard method and apparatus. The two monkeys {M. rhesus) tested formed the habit of discriminating between a circle and a square (equal in area) very slowly; not much more rapidly than is found to be the case with some of the lower orders of mammals. In fact " J," the larger of the two monkeys, never became very steady in his response (84^). " B," on the other hand, after perfecting the habit, could be counted upon to run correctly every day (100^ for several days). Substitution of a square which could be inscribed in the standard circle produced no breakdown; nor did marked changes in intensity. In the case of " B," after perfecting the circle-square habit, the hexagon-triangle habit arose very rapidly — in about 40 trials. Nor was the habit disintegrated by interchanging the apex and base of the triangle. IMonkeys are able to learn readily to respond in a given way or to inhibit response to a given visual sig- nal. Thorndike states that their behavior in this respect is truly remarkable. In the light of what we now know about Hans and other highly trained animals, it is just possible that we have here responses based upon a general- ized type of behavior on the part of the experimenter. In rough tests it is found that the raccoon and porcupine easily react to the difference between a circular and a square pasteboard card, and between two cards differing in size. No careful tests with standard methods have been made upon their form and size vision. The dog has been exten- sively investigated by Orbelli, using Pawlow's method. These tests show apparently that the dog is sensitive to form and size, and to differences of shading and form in objects.
364 VISION We cite certain of Orbelli's conclusions as given in the report of Yerkes and Morgulis.
(1) The intensity of a photic stimulus depends not only upon the degree of change in light intensity, but also upon the size of the illuminated or shaded area. These two factors may compensate for one another.
(2) Qualitative (it is to be noted that this does not refer to color) differences in photic stimuli are determined not only by the fluctuations of the intensity of the light, but also by the specific grouping of the illuminated and the dark retinal elements (i.e., by the form of objects).
(a) Early in the formation of a reflex the significance of form is indefinite and the reflex is determined wholly by the appearance of a light or a dark figure.
(6) Later the importance of the definite grouping of light and shadow (form) gradually becomes apparent. This specialization in the reflex is developed slowly.
(c) It is possible, in a short time, to produce a marked dif- ference in the influence of two figures by systematically weakening the reflex to the unfamiliar figure and by rein- forcing the influence of the familiar figure by the use of food.
{d) Rendering diff'erent the influences of a familiar and an unfamiliar figure produces conditions which greatly favor the improvement of discrimination of the familiar from the unfamiliar figure. Under these circumstances un- familiar figures retain their independence to a certain extent.
(e) The different effects produced by various figures depend not upon quantitative differences (intensity of light), but upon qualitative differences, that is, upon the unequal or dissimilar grouping of simultaneously stimulated ret- inal elements.
(3) When a difference in the effects of a familiar and an un- familiar figure has been established, intensification of light, increase in the area of the figure, and repetition of the stimulus do not in- crease the effect produced by the unfamiliar figure.
(4) When a difference in the effects of a familiar and an un- familiar figure has been established, and the unfamiliar figure produces only a slight effect, its influence may be considerably in- creased by tlie introduction of irrelevant stimuli.
(5) Moving objects may act upon the eye of the dog as distinct stimuli. In this case the direction of movement may determine the qualitative difference of stimulation and modify the reaction of the dog.
(a) In the early stages, after a conditioned reflex to move- ment has been established, the direction of the movement is not important and a familiar direction will have the same effect as an unfamiliar direction of movement.
(6) A distinct difference in the influence of movements vary- ing in direction may be produced in a short time (by the RESPONSE TO FORM AND SIZE 365 proces of Avearing out the reflex to one stimulus by repetitions, and of reinforcing the other by giving food in connection with the stimulus). (c) The differences in the effects of different figures, and of movements of various direction, is apparently based upr'n the establishment of a conditioned inhibition. Stimula- tion of some regions of the retina establishes among the various groups of retinal elements the same sort of rela- tion which obtains between the separate receptors of the entire organism.
A long series of tests upon albino rats to test sensitivity to difference in form and size has been made. In the first place, it was found that the introduction of a slowly moving sector in the pathway of one of the light stimuli produced no hastening of the habit. One animal formed the vertical- horizontal line habit and another the 30 mm. — 50 mm. circle habit. In testing the threshold for form it was found that while two rectangles 20 x 30 mm. with their long axes re- spectively horizontal and vertical afforded sufficient dif- ference in stimulating effect for the rise of a habit, forms more widely different (square and circle) did not. Form and size vision in the dancer seems to be not so well de- veloped as in the rat. Tests on the ability of the dancer to form a habit when a star-like figure and a circle were used as stimuli failed to give positive results both at high intensity and at low intensity. They are attracted to some extent by moving objects. The mouse (gray, black, white, brown) has equally poorly developed size and form vision.
In birds. — We may summarize the results of the rough early experiments upon birds somewhat as follows: When tested with wooden boxes differing in form it is found that the English sparrow does not easily react to such differ- ences. On the other hand, both the sparrow and the cow- bird are able to learn different designs (markings on cards). The cowbird is able to distinguish the triangle from all the other forms, the sparrow failing in this test. The pigeon readily learns to discriminate, apparently by vision, the position of a feeding-box placed in a row with others (so-called counting experiments). They show ability to pick out a given form of box, — triangular, square, cylindrical, etc., when all are presented simultaneously 366 VISION (Porter). Chicks will readily learn to leave rice grains untouched and to take wheat grains when the former are glued down to the table. After the chicks have perfected the habit, one can scatter the rice grains loosely among the wheat without their being eaten. If one cuts squares and triangles from green peas and places the squares on the top of a glass plate and the triangles under the plate the chicks readily learn to pick at the squares. When the habit is perfected the triangles and squares may be laid side by side but only the squares will be eaten (Katz and Revesz). The scientific results upon reaction to form in birds are not altogether harmonious. Bingham, who has recently worked over this whole field with chicks as the subjects, comes to the conclusion that while the chick can apparently respond to the difference in form be- tween the circle and the square, and the circle and the triangle, when they are equal in area, yet such responses, after all, are really nothing more than keen perception of size differences. He draws this conclusion from the fact that after the chick has learned the circle-triangle habit with the base of the triangle down, the habit will dis- integrate if the apex is placed down.^^ All observers agree that the chick is sensitive to size difference even when tested by critical methods. In a very carefully controlled ^^ Bingham has raised the whole question as to what is meant by form. " Reactions to optical stimuli which have been inter- preted by observers as indicating form discriminations are probably made on the basis of unequal stimulation of different parts of the retina. If local inequality of excitations on the retina be the basis of these reactions, then the apparent d'serimination of form by the chick is, in reality, a keen perception of size differences." Hunter argues that we should not expect the child to discriminate forms in the abstract in the sense in which form is used by Bing- ham. He states that form discrimination is always " pattern " discrimination. The whole discussion is somewhat academic. It marks an attempt to introduce structuralism into behavior. WTiat we desire to know is how the animal will react in the presence of known and reproducible stimuli when the experimental conditions as regards the control of the animal are standard. Nevertheless it would be a distinct gain to test for " form " discrimination under the conditions which Hunter suggests. His paper should certainly be consulted in planning experiments upon " pattern " and " form " discrimination.
RESPONSE TO FORM AND SIZE 367 experiment upon size it has been shown that a standard circle (6 cm.) and a circle lying between 4.5 and 5 cm. (slightly larger than 4.5 cm. and slightly smaller than 5 cm.) affords a basis for a differential reaction. After the chick has been trained to react positively to the larger of two circles it will continue to react positively to the larger when the absolute size of both has been made proportion- ately larger or smaller.^^ In regard to the chick's use of size, form, and brightness stimuli, Bingham states that dif- ference in size is most potent in determining visual response, and next in order come brightness, general illumination, and then form.
In reptiles and in amphibia. — Experiments on the turtle {Chrysemys marginata) bring out the fact that the animals failed to discriminate between patterns. Two learned to discriminate vertical and horizontal lines and 2 between 2 series of parallel horizontal lines 8 mm. and 2 mm. in width respectively. One turtle learned to discriminate be- tween 2 series of parallel vertical lines 8 mm. and 1 mm. wide, and then between lines 4 mm. and 2 mm. wide, and finally between lines 3 mm. and 2 mm. wide. The average number of trials necessary to form such habits was 183 (Casteel). Habits of avoiding certain food objects can apparently be established in the frog (as in the monkey) in a very few trials. Under the influence of punishment by the electric shock B. clamata learned to avoid earth- worms treated with chemicals in two trials (Schaeffer). Just what such results as these really mean it is impossible to say.
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^® From experiments now in progress at the Nela Physical Lab- oratory (Johnson) it would seem that this observation cannot be confirmed.
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Freiburg, Festschr. d. Univ., 1902, 259. Hunter, W. S., " The Question of Form Perception," Jour. Animal Katz and Revesz, " Experimentell-psychologische Untersuchungen mit Hiihnern," Zeit. f. Psychol, 1908, L, 93. Kinnaman, a. J., " Mental Life of Two Macacus rhesus Monkeys in Captivity," Am. Jour. Psych., 1902, XIII, 98. Lashley, K. S., " Visual Discrimination of Size and Form in the Albino Rat," Jour. Animal Beh., 1912, II, 210. Lyon, E. P., " On Rheotropism in Fish," Am. Jour. Physiol., 1904, Mast, S. 0., Light and the Behavior Organisms. New York, John Wiley & Sons, 1911.
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BIBLIOGRAPHY 369 NicOLAi, G. F., " Die physiologisclie Metliodik zur Erforschung der Tierpsyche," Jour. f. Psychol, und Neurol., 1907, X, 1. Orbelli, L. a., " Reflexes conditionnels du cote de Toeil chez le chien," Archives des Sciences Bih., T. XIV, 1 et 2. Paeker, G. H., " The Skin and Eyes as Receptive Organs in the Re- actions of Frogs to Light," Am. Jour. Physiol., 1902, X, 28.
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Washburn, M. F., and Abbott, Edwina, " Experiments on the Brightness Value of Red for the Light-Adapted Eye of the Rabbit," Jour. Animal Beh., 1912, II, 145. Washburn, M. F., and Bentley, Madison, " The Establishment of an Association Involving Color Discrimination in the Creek Chub Semotilus atromaculatus," Jour. Comp. Neu. and Psych., 1906, Watson, J. B., " Some Experiments Bearing upon Color Vision in Monkeys," Jour. Comp. Neu. and Psych., 1909, XIX, 1. Watson, J. B. and M. I., " A Study of the Responses of Rodents to Monochromatic Light," Jour. Animal Beh., 1913, III, 1. Waugh, K. T., "The Role of Vision in the Mental Life of the Mouse," Jour. Comp. Neu. and Psych., 1910, XX, 549. Yerkes, R. M., The Dancing Mouse. New York, Macmillan, 1907.
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CHAPTER XII AUDITORY AND RELATED FUNCTIONS