CONCLUSION A survey of many facts observed in conjuction with the study of the perception of visual motion under discrete successive retinal stimulation indicates various weaknesses in the explanation of such movement as offered by the Gestalttheorie. In order to be adequate the doctrine must provide, we contend, for the following phenomena: (1) simultaneous movements in opposed directions over identical parts of the visual field, (2) visual movement where one member is presented and to it, after a slight delay, another is added, (3) visual movement where one member is removed from the total figure, (4) visual movement under physical sequence but without temporal interval, (5) partial movement where the other object is at rest or before the second object appears, (6) dual movements of various sorts, (7) tri-dimensional movement under precisely the same conditions which give bi-dimensional movement, (8) the significant changes induced in the direction of movement due merely to the change in instruction, and (9) the presence of various intermediate and accessory members. In terms neither of short-circuiting nor of cortical fusions nor of differences in potential, as these are now understood, can the above facts be explained. The resolution of the conflict between observation and theory calls either for a theoretical reinterpretation or for the abandonment of the specific neurological hypotheses underlying the whole of the explanation of visual movement as offered by theorists of the Gestalt. II. THE EFFECT UPON VISUAL MOVEMENT OF COLORED STIMULUS OBJECTS In his study Ueber das Sehen von Bewegung Wertheimer contends that the perception of visual movement may be independent, in a sense, of the quality of the stimulus. Red or white lines drawn upon a dark ground and successively exposed in a Schumann tachistoscope with a certain temporal interval between the objects gave rise, as he reported, to a visual perception of movement from the first to the second object-with no object or color appearing intermediatelyjust as readily as did black lines upon a gray ground. The movement was not of a colored object; it was just movement.1 This apprehendion of 'colorless' movement with colored stimulus objects is very important for the general doctrine of the Gestalt. It may be cited as especially confirmatory of the independent and unitary character of the movement experience. Visual motion per se (i.e., pure phi) has for the Gestalttheorie no qualitative moments. Since such an experience depends physiologically upon a dynamic Struktur, in no sense identical with the perception of the objects in the phenomenal field, it must, by its very nature, be independent of the quality of the stimulus. 2 More recently Dimmick, employing red, green and blue lines drawn upon white ground, red objects on green ground, and blue lines on yellow ground, reports that the apprehension of visual movement conditioned through such successively presented objects was resolvable into a 'grey flash' of brief duration. "Under the descriptive attitude there was always noted a grey patch covering the space between the two members." The phenomenal field of movement then was uncolored; it was filled merely with a gray, presumably cortical in origin. This in spite of the fact that the stimulus objects as well as the background were colored. So Dimmick confirmed, in a manner, Wertheimer's results with respect to the independent character of visual movement under colored stimuli even though he rejected the physiological assumptions and the qualitative descriptions of the latter. From the results of an earlier study we were led seriously to question the primary importance for the visual apprehension of motion of any intermediate grey. We had such a large number of reports in which there was clear movement but no accompanying grey, such as Dimmick described, that we were forced 1 Referring to the character of visual movement with colored stimuli Wertheimer writes, "Sehr deutliche, einheitliche Drehung sinnlich klar zu beschreibung . . . Ganzdrehung klar von a nach b gesehen, von der Mitte ist bezüglich des sonstigen Optischen zu sagen: da war nichts von Streifen, nichts von Rot." Zsch. f. Psychol., 1912, 61, 222. ? F. L. Dimmick, Amer. J. of Psychol., 1920, 31, 330. to believe that its significance had been greatly exaggerated. Frequently our Os reported that the field was partially or wholly filled with a distinctly yellowish light either of a dynamic character-stretching and shooting; or of a more static nature. This filling apparently served to reënforce the impression of movement, since it was referred to as the 'cloud or haze of dust' similar to that raised by a natural object in motion. It could be perceived without movement or movement could be perceived without it, as a brief survey of the reports from our earlier study will show. We felt that additional and valuable information upon the nature of visual movement might appear where the objective conditions were varied by way of a change in the quality of the stimulus objects. By using similarly and dissimilarly colored forms O was provided with a means of discriminating more sharply between the phenomenal objects. At the same time the task of defining the qualitative moments of the phenomenal field of movement would be simplified. Problem. We sought then to determine whether the visual perception of motion was qualitatively independent of the qualitative aspects of the stimulus objects or whethe visual movement varied in quality with a change in the color of the stimulating conditions. That is to say, we sought to 1 determine the hue of the object perceived as moving when the stimulus conditions were variously colored. If movement or the object-perceived-as-moving varies qualitatively from moment to moment with a change in the hue of the stimulus, we may assume that visual movement between two objects is not independent of the perceived figures but that it arises through the immediate integration of the successively apprehended object or objects. We can further assert that, if green lines give clear movement over a greenish field, and red lines give movement over a reddish field, the "existential correlate" of the apprehension of visual motion cannot possibly be said to be a "grey flash" fairly brief in duration. Apparatus and method.—We used the apparatus previously constructed for a study of visual movement. To obtain Amer. J. of Psychol., 1926, 37, 63-116. • Op. cit., p. 76. colored objects we placed in certain of the series (1–16) red and green gelatins over the windows of the tachistoscope as well as over the slitted cards. When the lines were similarly colored we also placed like-colored gelatins across the slits in the box before the eyes of O. Thus O saw two colored lines within or upon a black film or field. In the other series (17-32) we used colored figures drawn upon a colored and colorless ground. The successive objects were separated temporally by three intervals, o, 75 sigma and 150 sigma. The formal instructions were the same as in the previous study. The Os were Dr. P. T. Young (Y) and Dr. E. A. Culler (C). We presented the angular-horizontal and the parallelhorizontal arrangements of the lines in 32 series as follows: I. Objects upon lightless field. Temporal interval: 75 sigma and 150 sigma. Exposure time: 60 sigma. Single exposition of each figure Angular arrangement Ser. 1. Red to red 2. Red to green 3. Green to red 4. Green to green Horizontal arrangement Ser. 5. Red to red 6. Red to green 7. Green to red 8. Green to green Serial exposition; each figure given five times Angular arrangement Ser. 9. Red to red 10. Red to green II. Green to red 12. Green to green Horizontal arrangement Ser. 13. Red to red 14. Red to green 15. Green to red 16. Green to green II. Objects upon colored and colorless fields. No temporal interval. Exposure time: 5 sec. Single exposition of each figure Ser. 17-20; same as Ser. 1-4 Horizontal arrangement Ser. 21-24; same as Ser. 5-8 Serial expositions; each figure given five times Ser. 25-28; same as Ser 9-12 Horizontal arrangement Ser. 29-32; same as Ser. 13-16 Each of the first 16 series was presented to O with two intervals, 75 sigma and 150 sigma. In the method of serial exposition (9-16) the two lines were given five times. The 16 series were then repeated (series 17-32) without interval. The slitted cards were removed at P and black and gray cards with red and green lines, red cards with green lines, and yellow cards with red lines, were placed at H. One object was given for five sec by turning on the weaker of the two lights within the box, and then the other (stronger) was given to wipe out the first. Both were then removed together. Again, both lines were given simultaneously, although only one objectthe brighter-was observable when the stronger light was on, and then the stronger was removed (after 5 sec) leaving the weaker. Under serial exposition the stronger was turned on and off at periods of 1 second. Results; Series 1-16.-Since we are not primarily interested in the spatial extent of the apparent motion under these conditions we shall deal but briefly with that aspect of the experience. Where the lines were similarly colored optimal movement was generally reported. When the members were of dissimilar hue, there was a tendency to report fewer full movements. With unlike hues O reported at times that a line either moved partly across the field; or that it disappeared in motion and reappeared a moment later to complete the movement as a line of a different hue. But even with unlike |