Gestalt similarity principle, difference thresholds and pattern discriminability J

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1 Gestalt similarity principle, difference thresholds and pattern discriminability J ALVIN G. GOLDSTEIN2 UNIVERSITY OF MISSOURI, COLUMBIA Perceptual group.ing within a visual array has been studied as a function of the difference limen between elements composing the array. Two experiments are reported in which the array elements differed in size or in shape. Although there is a relationship between perceptual emergenc e and amount of difference between elements of the display as expected, there was clear evidence that above threshold differences between elements could not be used to predict the emergence of a pattern hidden in the visual array. In 1957 Goldstein proposed an experimentandoffered some preliminary data relevant to the Gestalt psychology problem of perceptual organization of visual stimuli. The present report offers further data from two experiments which confirm the earlier, tentative conclusion that perceptual emergence of a pattern from its background is not a simple linear function of the amount of physical disparity between the elements composing the pattern and the elements composing the background to the pattern. If the Gestalt principles of perceptual organization are considered as hypotheses, and thus are open to empirical test, several difficulties arise when an attempt is made to devise a suitable experiment. For example, the lack of experimental investigations of Gestalt principles has been attributed to the fact that they are qualitative notions (Koffka, 1935, p, 166; Osgood, 1953, pp ), and therefore presumably are not amenable to empirical investigation. Hochberg and McAlister (1953) point out that the principles, although of heuristic value are formulated in subjective and qualitative terms making empirical study difficult. These seem weak arguments because there are methods for studying qualitative and subjective attributes of stimuli, and, just as relevantly, the vast majority of "theoretical" statements in psychology are not couched in quantitative terms, a situation which in no way seems to reduce the amount of empirical research these statements spawn. On the other hand, some progress has been made in applying empirical methods to the study of some Gestalt principles(attneave,1955; Brunswick & Kamiya, 1953; Bower, 1965, 1967; Hochberg &. McAlister, 1953; Hochberg &. Silverstein, 1956; Knight, 1937; Rush, 1937). The concept "similarity," which is basic to the research reported here, has received attention from several investigators but not necessarily in relation to its significance for Gestalt psychology. In the broadest sense, implications of the concept of similarity are fundamental to every area of psychology and to science in general (Attneave, 1950, 1951; Noble, 1957). Moreoever, the Gestalt principles-more commonly called "laws"-are not precisely stated. Granting this imprecision, certain implications still appear to follow from a combination of the written principle plus the meaning inherent in the numerous demonstrations offered by the early advocates of Gestalt psychology. The implications relevant to the "law of similarity" will be developed during the discussion which follows. The principle of similarity has been stated as ".. a tendency for like parts to band together. ' I (Wertheimer in Ellis, 1950, pp, 71-81). Atoneextreme, this means that every object in our visual world is an object because its internal elements "band together" and emerge from a visual background to form a higher level perceptual unit. It seems plausible to assert that in order for an object to be perceived as a unit distinct from its surroundings, the elements of the object must be different psychologically from the elements of the surroundings. Thus, in the law of similarity there is the clear implication that "like parts" will band together and emerge as a higher order visual form as a consequence of some discrepancy between object-elements and surroundelements. Demonstrations of the Gestalt similarity principle are replete with examples of this implication. The question which develops from this reasoning is: How much psychological difference between the object-elements and surround-elements are necessary for perceptual emergence of the object to take place?3 METHOD The measure of the dependent variable was obtained from the responses to a visual display which closely resembled (but was not identical with) the formal aspects of the Ishihara color vision test. It will be recalled, that in each stimulus display of that test, numerous elements (circles) are distributed in an apparently random manner. In reality, each display is composed of two kinds of elements, those making up the "hidden" figures (e.g., the number "2") and those making up the fabric of the background (visual "noise" or interference elements). The figure elements are, of course, a noticeably different hue than the background "noise" elements. Keeping this example in mind, then, in the present experiment, S was presented with a display similar to the one shown in Fig. 1 (top). This array is constructed from two Perception & Psychophysics, Vol. 2 (8) Copyright Psychonomic Press, Goleta, Calif. 377

2 Fig. 1. Complete pattern-recognition stimulus (Top). Embedded in this array is the "hidden" pattern (middle), composed of elements which differ markedly (most extreme ratio in Fig. 2) in size from the elements of the background (bottom). sets of elements; one set is arranged in the nonrandom, "hidden" pattern shown in the middle of Fig. l. This pattern is embedded in a background of visual "noise" elements (Fig. I, bottom) which are randomly distributed. In Experiment 1, the size (diameter) of the pattern-elements was the independent variable systematically manipulated while the background elements remained unchanged. In Experiment 2, using these same background-elements, the shape of the individual pattern elements was systematically varied by reducing the ratio of their vertical to horizontal axes, thereby producing figures progressively more ellipsoid. 4 The most critical feature of the method now becomes relevant: the psychological distance between pattern-elements and background elements was determined for each S by means of a psychological discrimination procedure (pairedcomparisons) in which S was confronted with only two stimuli-one pattern element and one background element-and had to decide whether the stimuli were "same" or "different." Thus, the purpose of the experiment was to measure pattern discriminability as a function of the amount of psychological distance between the elements of the pattern and the elements of the background. Stimuli and Apparatus All stimuli were projected slides, white figures on a dark background.5 Two classes of stimuli were ~sed, corresponding to the two tasks which are described in the next section. In the comparison trials each stimulus slide was composed of two elements; in the pattern discrimination trials the stimulus(fig.1, top) was a matrix composed of 78 background elements and 17 pattern elements (approximate ratio 4.5:1). Projected image size of the individual elements in both classes of stimuli averaged 26 mm diam; separation between contours averaged 5 mm (range 3 to 10 mm in pattern discrimination stimuli). Overall size of the projected matrix was approximately 33 em sq. A standard, 300 W 2 x 2 slide projector with an electrically operated shutter mounted in front of the lens was used to project the stimuli upon a milk glass screen. Connected in series with the shutter was Ss' response key, an interval timer, and an electric clock. Closing Ss' key activated the interval timer which controlled the 2-sec foreperiod between the "ready" signal (given by E) and the presentation of the stimulus. S was instructed to close his key on hearing the "ready" signal. Following the foreperiod, the timer simultaneously opened the shutter and started the clock. Both shutter and clock now remained in this mode until S opened his key, which stopped the clock and closed the shutter thereby ending the trial. Thus, S controlled the length of the time the stimulus was visible to him and E measured this duration from the clock. Distance from S to the rear projection screen was 183 em. The center 378 Perception & Psychophysics, 1967, Vol. 2 (8)

3 1.00 Ecmpcrison Trials ~_.----_.>~...- Comparison Threshold ~ PatternJ Discrimination Trial.. --P:~e~:t Discrimination Threshold Exper RATIO Of PAnERN ElEMENT TO BACKGROUNO ELEMENT Fig. 2. Proportion of correct responses in comparison and pattern recognition conditions. Each point based on 40 responses. of the screen was approximately level with Ss' line of sight. Illumination in the room was low, but no attempt was made to keep the room dark. Procedure In this section a description of the two tasks common to both experiments will be given in the order in which they always appeared. Comparison Trials. The psychological disparity between the elements of the pattern and the elements of the background was measured by the technique of paired comparisons using two stimuli and two categories of response, "same" or "different." If S judged the stimuli on a trial to be "different," he had to specify, in Experiment 1. which was the larger stimulus, or, in Experiment 2, which was the more elliptical ("egg-shaped"), By varying the physical disparity between the two comparison stimuli in the ratios shown on the abscissa of Figs. 2 and 3, ascending and descending trials were presented so that every S judged each comparison on four trials, but only the final two judgments were used in the analysis. Reference to the abscissa of Fig. 2 shows that the six sets of comparison stimuli were presented to S (total trials=24) in Experiment1. and as shown in Fig. 3, five sets in Experiment 2. Before the comparison trials, S was instructed to respond accurately and as quickly as possible by releasing his key as soon as he could make his judgment. Response time and verbal report were recorded by E. Pattern Discrimination Trials. As explained earlier, in the pattern discrimination trials, S was under instruction to find the pattern (Fig. 1. middle) concealed within the matrix stimulus. Just prior to the first trial. S was thoroughly familiarized with the shape of the pattern, and instructed that the "point" of the pattern would be oriented to either the left, right, top, or bottom of the projection screen. Using the apparatus described in the preceding section, I S responded by releasing his key (thereby removing the stimulus from the screen and also stopping the clock) when he could locate the pattern. He then reported the orientation of the stimulus pattern. If after 15 sec of searching, S could not locate the pattern, he was asked to guess at the orientation. As there were six sets of comparison stimuli in Experiment I, an equal number of matrices embodying these six size differences were presented to S. Similarly. in Experiment 2. where there were five sets of comparison stimuli, there were also five matrices. In both experiments a matrix stimulus was presented twice in counterbalanced order to each S. Subjects Ten men and 10 women Ss were tested in a repeated measures design in Experiment 1; the same design was used in Experiment 2 with a different group of 10 men and 10 women. RESULTS Number of correct responses in both experiments was not related to the sex of the S, and the data for men and women have been combined and will be treated together. It will be recalled that the goal of each experiment was to obtain data from the paired comparison trials which could be compared to measures obtained in the matrix conditions. Essentially the question was: What is the visibility of the embedded pattern when its elements differ from background elements by psychological differences equivalent to those measured in the comparison task? This question canbestbe answered if the paired comparison and pattern discrimination measures were compatible. Le. on the same scale. One way to do this is to consider both measures as threshold measures. Thus. a difference threshold obtained in the comparison trials could be compared with a pattern threshold in the pattern discrimination I 1.00 t;! so! Comparison Trials... Comporjson Threshold 1 J ~- -- ~ Po"ern DiscriminationJ Triols.01 RATIO OF PATTERN ELEMENT TO BACKGROOND ELEMENT Exper. n Fig. 3. Proportion of correct responses in comparison and pattern recognition conditions. Each point based on 40 responses p~~~;~., Discriminotion Threshold.07 Perception & Psychophysics Vol. 2 (8) 379

4 Table 1. Frequency and RT or Correct Responses in Comparison Trials and Pattern Recognition Trials.in Experiment 1 Ratio of Pollern Element to Background Element Trials Compari san Condition Correct Freq Responses X RT Pallern Recoqnitien Correct Freq Responses X RT Note: Chance level responding in comparison trials was 10 correct; in pattern recognition, 5 correct. trials. Following this decision, the frequency of correct responses in the paired comparison and pattern discrimination conditions were analyzed in accordance with the usual method of determining thresholds. Since, in the paired comparison condition of both experiments S was permitted to make "equal" judgments, all such responses in the analyses were scored as correct or incorrect in proportion to the frequency of responses already in these categories. It should be obvious that the responses in the pattern discrimination conditions can immediately be categorized either as correct or incorrect because S was forced to make a response even if he was guessing. Figure 2 and 3 present the data of the two experiments in the terms of the foregoing discussion. Specifically, chance level responding in the paired comparison condition is now taken as.50 (a response is now either correct or incorrect), and, following the accepted convention,.75 is designated the "comparison threshold." For the curves representing the pattern condition data,.25 indicates chance responding (on each trial there were four alternative orientations of the pattern), and again followingthe convention,.625 is designated as the "pattern threshold." From these graphs the relationship between the comparison and pattern responses can be seen. In Experiment I, although the two stimuli are clearly perceived as different in the comparison task, the fact that the pattern discrimination curve remains below its threshold suggests that the pattern does not perceptually emerge from its background until the disparity between the pattern elements and the backgroundelements is very great. A similar conclusion is also suggested by the results of Experiment 2, but here the pattern never emerges as a clear percept even though the pattern elements are clearly seen as different from the background elements in the comparison trials. The response times, which were recorded on every trial, offer additional support to the conclusions just mentioned (Table 1 and 2). Tabulation and analysis of these data were restricted to RT of correct responses only, primarily because no coherent reason could be discovered for looking at RT for incorrect trials. As expected, mean RT decreased in the paired comparison trials when the discrimination taskbecame easier, as indicated by the increased number of correct responses, a finding which offers little new information. But even though the RT to the pattern discrimination trials also decreased as the patternbackground disparity increased. the relative amount of this decrease was small and the rather long RTs indicates that the pattern never reached the figural Table 2. Frequency and RT or Correct Responses in Comparison Trials and Pattern Recognition Trials in ElqIeriment 2 Trials Comparison Condition Ratio of Pallern Element to Background Element Correct Freq Responses X RT Pallern Recognition Correct Responses Freq. X RT Note: Chance level responding in comparison trials was 10 correct; in pattern recognition, 5 correct Perception & Psycbophysics, 1967, Vol. 2 (8)

5 "emergence" level. Evidently, the Ss had to search for the pattern; it did not appear to them as a figure on a ground. DISCUSSION If we take the principle of similarity at its face value, so to speak, then in this study the patternelements-which were known to be psychologically different from the background elements-should have emerged as a figure, t.e., as a higher order form, held together by their identity to each other and their difference from the more numerous elements composing the background. The facts from both experiments suggest that supraliminal differences between background and pattern elements is not a sufficient condition for figural emergence. Moreover, even when the pattern-background difference was most extreme, (in this study) there was no evidence from the RT data to indicate that the pattern clearly emerged. To the contrary, the long response times, and the verbal reports of the Ss indicate that the pattern was "pieced together" instead of perceptually emerging as a unit. It is encouraging to find evidence in a recent report (Beck, 1966) supporting this conclusion although the data were obtained in an experiment using a similar but not identical method and altogether different stimuli. Beck concluded that [udged similarity of figures (the "elements" of his study) is a poor predictor of perceptual grouping. The most striking aspect of these experiments was the clear difference between pattern and background elements in the comparison condition and the almost impossible task of finding these same stimuli when they were embedded in the matrix array. It appeared to some observers as though an unusual interaction between the elements was responsible for the apparent assimilation of the pattern elements. This was especially compelling in Experiment 1. This observation was later tested in an experiment which showed that apparent visual size is systematically affected by the presence of adjacent stimuli (Goldstein,1961). Although this finding may be used to explain the results of the present experiment, it does not appear to account for Beck's data (1966). Although in the experiments reported here, no attempt was made to evaluate the "potency" of the dimensions as perceptual organizers, this research represents two studies in a series of investigations aimed at deriving the rules by which perceptual grouping occurs. We hoped to answer the question: Is perceptual emergence a function of the psychological distance between pattern and background elements alone, or is it related to the qualitative features of these two sets of elements? The former interpretation would suggest that perceptual grouping will occur when stimulus elements differing in, e.g.,brightness are a certain psychological distance apart, and the size of this psychological gap would be exactly the same for hue, size, etc. The latter interpretation would suggest that some stimulus dimensions were more "efficient" than others in producing perceptual grouping. In other words, it was intriguing to speculate that dimensions such as, say, brightness and size, would not be equally "potent" in their effectiveness in making a pattern perceptually emerge. Thus, to obtain equal pattern visibility, differences between pattern and background elements would have to be psychologically greater for one dimension than for the other. Obviously, one could also speculate that pattern emergence-figure grouping-was a simple function of the number of just noticeable differences between pattern and background elements irrespective of the sensory dimension. The data from the present experiments could not be used to decide which of these speculations are veridical, not only because a limited number of dimensions were employed, but also because only one ratio of background-to-pattern elements was tested. Future experiments can be designed to help in deciding between these two hypotheses. References.Attneave, F. Dimensions of similarity. Amer. J. Psycho!., Attneave, F. Ability to verbalize similarities among concepts and among visual forms. Amer. Psycholoflist, , 270. (Abstract) Attneave, F. Symmetry, information and memory for patterns. Amer. J. Psuchol., 1955, 68, Beck, J. perceptual grouping produced by changes in orientation and shape. Science, 1966, 154, Bower, T. G. R. Phenomenal identity in infants. Psychon. sci., 1965,3, Bower, T. G. R. Phenomenal identity and form perception in an infant. Percept. & Psychophys., 1967, Boynton, R. M., & Bush, W. R. Recognition of forms against a complex background. J. Opt. Soc. Amer , 46, Brunswick, E., & Kamiya, J. Ecological cue-validity of 'proximity' and of other gestalt factors. Amer. J. Psuchol., , Eriksen, C. W. Location of objects in a visual display as a function of the number of dimensions on which the objects differ. J. expo Psycho/., 1952,44, Eriksen, C. W. Object location in a complex perceptual field. J. expo Psychol., , Eriksen, C. W. Partitioning and saturation of visual displays and efficiency of visual search. J. app/. Psycho/., 1955, 39, Goldstein.A. G. Two proposed studies on configuration perception. In J. W. Wulfeck & J, H. Taylor (Eds.), Form discrimination as related to military problems. Washington, D. C.: National Acad. Sciences-National Res. Council (Publication #561). Goldstein, A. G. The relation of extraneous visual stimuli to apparent size. Psycho/. Rec., , Hochberg, J., & McAlister, E. A quantitative approach to figural "goodness". J. expo Psucnot., , Hochberg, J., & Silverstein, A. A quantitative index of stimulussimilarity: proximity vs. differences in brightness. Amer. J. Psychol , Knight, O. D. The role of the figure-ground relation in perceiving and memorizing visual forms. 'ljnpublished doctoral dissertation. Ohio State University, KoUka, K. Principles of gestalt psyc!1.oi0fly. New York: Harcourt. Brace, Perception & psychophysics, 1967, Vol. 2 (8) 381

6 Noble, C. E. Psychology and the logic of similarity. J. gen. Psychol., 1957, 57, Osgood, C. E. Method and theory in experimental psychology. New York: Oxford, Rush, Grace P. Visual grouping in relation to age. Arch. Psycho/., 1937, 31, No Wertheimer, M. Laws of organization in perceptual forms. In W. D. Ellis (Ed.), A source book of gestalt psychology. New York: Humanities, Pp Notes 1. This research was supported in part by the U. S. Air Force under contract No. AF 18(600)1052, monitored by the Operator Laboratory, Air Force Personnel and Training Research Center, Mather AFB, California. 2. I would like to thank Barry Dworkin for his critical reading of this paper. 3. There have been innumerable studies reported where S was faced with the task of visually searching for a figure or object which differed in hue, or shape from others in his perceptual field (e.g. Boynton & Bush, 1956; Ericksen, 1952, 1953). It is difficult to evaluate the relationship between those studies and the present one because of the differences in method, goal, etc. In any event, most of the earlier studies concentrated on the. problem of locating a single item in a background of visual noise. whereas here the relevant issue was the organization of several items into a new, higher level percept. 4, A third experiment was performed where the independent variable was the thickness of the elements' contours. Results were essentially the same as found in the two studies reported here. 5. The prototypes, from which the slides were derived, were labor; iously constructed in the following manner. Original large scale, India ink drawings, carefully measured, were reproduced and printed in reduced size by the photo offset process upon sheets of glue-backed paper. These reprod uctions were used to make the two-figure comparison slides and the multifigure pattern recognition slides. More than one prototype matrix (pattern recognition array) was constructed for each experiment. In each prototype the distances between the elements were arranged so that unequal proximity between elements could not serve as a cue to the location of the pattern. (Accepted for publication April 24, 1967.) 38~ Perception & Psychophysics, 1967, Vol. ~ (8)