PRIMATES, 19(1): 231-236, January 1978 231

Subproblem Analysis of Shift Behavior in the Gorilla: A Transition from Independent to Cognitive Behavior*

DONALD ROI~BINS, PHILLIP COMPTON, and STEPHEN HOWARD Emory University

ABSTRACT. Two gorillas (Gorilla gorilla) were trained on a series of two-dimensional prob- lems, each problem involving an extradimentsional shift after original learning, then an in- tradimensional shift, followed by another extradimensional shift. The results indicated that during the early problems the subjects appeared to solve the problems as independent sub- problems, i.e., using non-conceptual behavior. However during the last two or three problems the subjects appeared to shift to a conceptual mode of behavior. These data are interpreted as further evidence of the cognitive capacity of the gorilla.

INTRODUCTION

This report is part of an overall project directed at understanding the cognitive pro- cesses of the great apes (Pongidae), particularly the gorilla (Gorilla gorilla).

One of the classic methods used to differentiate cognitive or conceptual from non- conceptual behavior has been the extradimensional vs reversal shift comparison tech- nique (KENDLER • KENDLER, 1962).

Most recently, MEDIN (1973) and TIGHE (1973) have analyzed the extradimensional shift in terms of the changed and unchanged problem. Specifically, let us assume we have stimuli varying on two dimensions, size and shape. A simultaneous discrimina- tion task is used and subjects are presented with a large triangle and a small square on one trial type and a small triangle and large square on the other trial type. If the rele- vant "concept" or dimension is size and large is positive, then the large triangle of one and the large square of the other trial type are the positive stimuli. During an ex- tradimensional shift the same stimuli are presented, but now shape is relevant, e.g., triangles are positive. Thus, the large triangle of one and the small triangle of the other trial type are the positive stimuli. Note that the large triangle was positive in the training phase so that if subjects learned the problem as two independent subproblems they should not change their choice response on the large triangle-small square trial (the unchanged subproblem) but need only to reverse their previous choice response on the small triangle-large square trial (the changed subproblem). TIGHE (1973) presents a considerable amount of data revealing that lower organisms tend to make little or no errors on the unchanged problem confining almost all of their errors to the changed problem. MEDIN (1973), however, points out that this is a weak assessment of inde- pendent learning in that one can conclude independent learning only if no errors are made on the unchanged problem. TmHE (1973) has also shown that college students

*This research was performed at the Yerkes Regional Primate Center which is supported by Grant RR-00165 from the National Institutes of Health.

232 D. ROBBINS, P. COMPTON, & S. HOWARD

show no differential behavior on the changed in contrast to unchanged subproblem of an extradimensional shift, interpreting this result as evidence of "conceptual" in contrast to independent learning.

In addition, intradimensional shifts have been introduced and faster learning on these shifts in contrast to an extradimensional shift is often taken as evidence of con- ceptual behavior (ZEAMAN & HOUSE, 1963). Referring to the example above, let sub- jects be shifted from a problem that has a large triangle and a small square on one trial and a small triangle and a large square on the other trial type, where "large" is positive, to a medium circle any very small star on one and a very small circle and me- dium star on the other trial type. If, e.g., "very small" is the correct stimulus on each trial we have an intradimensional shift. It should be noted that evidence exists that suggests that rhesus monkeys (Macaca mulatta) can solve intradimensional shifts (JACKSON & PEGRAM, 1970) and that stumptailed monkeys (Macaca arctoides) can solve extradimensional shifts (SCHRIER, 1971).

In the present study two gorillas were trained on a series of problems, each problem involving an extradimensional shift, after original learning, then an intradimensional shift followed by another extradimensional shift. It was hoped, in this manner, we would be able to determine if gorillas were capable of conceptual behavior in this task.

M E T H O D

Subjects. Two gorillas (Gorilla gorilla)were used, one was a nine-year old female (Oko), and the other a 12-year old female (Katumba). Both had extensive experimental his- tories.

Apparatus. A large-animal modified semiautomated Wisconsin General Test Ap- paratus was used (RUMBAUGH, BELL, & GILL, 1972). Basically, the apparatus consisted of two clear Plexiglas panels, arranged in a horizontal array, separated by a metal bar. Across the front of each panel were photoelectric beams, and a response was defined as the first side on which a photoelectric beam was broken. Behind each panel was a metal holder for the stimuli and above the panels was a receptacle for candy. The ap- paratus was mounted on a false door which replaced the door on the subjects' cage after the cagemate was removed. The stimuli were 4• in (10.16• 13.97 cm) cutouts of magazine pictures chosen so that the stimuli were varied but relatively easily discriminable patterns (to the human observer).

Design and Procedure. Each subject was exposed to a series of six problems using a two-choice simultaneous visual discrimination task. Each problem consisted of four phases: (1) original learning; (2) an extradimensional shift; (3) an intradimensional shift; and finally (4) an extradimensional shift. The stimuli were as follows for Phases 1 and 2 (those listed in parentheses were used for the intradimensional shift and extra- dimensional shift that followed in Phases 3 and 4): Problem 1--red and blue triangles and circles (green and yellow stars and squares); 2--horizontal or vertical dark or light grey stripe (45 ~ or 135 ~ very light grey or black stripe); 3 - -one or three capital or small letter "X"(s) (two or four capital or small letter "B"(s); 4 diamond or rectangle with filled-in area in upper left or lower right hand corner (hexagon or heart with

Shift Behavior in the Gorilla 233

filled-in area in area in upper right or lower left hand corner); 5--Ellipse on horizontal or vertical axis with bar above or below (dot above or below horizontal or vertical "figure 8"); 6--Dashed or filled-in dotted rectangle or rectangle within a rectangle (open dots or open large dots of parrallelogram or one within one). For each problem, for Oko, one dimension was randomly selected as being correct to determine the positive stimuli and the other dimension was chosen as the correct one for Katumba. Each phase was continued until the subject made 20 consecutive correct responses. The next phase immediately followed without any signal to the subject. A noncorrec- tion procedure was used. Hershettes (M&M type candy) were used as a reward.

Sessions were conducted five days per week in the afternoon and lasted approxi- mately 15-35 minutes each. At the beginning of each session each subject was present- ed with two multidimensional stimuli, one of which was arbitrarily chosen as correct on the first day of the study. They were given this problem as pretraining and both solved it within one week (10 consecutive correct responses). Subsequently, prior to presenting the experimental problems, they were given five trials on this problem as a warm-up (performance was at least 80 ~ correct throughout). The experimental pro- blems followed. As a result of the brief daily sessions (usually caused by the subjects failure to attend to the apparatus, holidays, scheduling problems, and the like), this study took approximately six months to complete.

A typical trial was as follows: The experimenter turned the light on above the two stimuli; the subject responded by breaking a photocell beam in front of one of the two stimulus pictures; the response terminated the light above the stimuli and reward was delivered if a correct response was made. Since a noncorrection procedure was used, incorrect responses terminated these error trials. The experimenter recorded the sub- jects choice and reward (or nonreward) and set up the stimuli and reward conditions for the next trial during a 30-45 sec intertrial interval. The position of the stimuli was randomly determined within blocks of 10 trials.

RESULTS AND DISCUSSION

Table 1 shows the summary data for all four phases of each problem for both sub- jects. There was a considerable amount of variance in trials to criterion to learn the original task in phase 1. For comparison purposes the ratio of the subproblem with the greater number of errors to total errors was calculated for phase I and reveals little bias in favor of one subproblem over the other since this ratio ranged from .50-.59, where no bias----.50.

The phase 2 extradimensional shift showed little savings in trials to criterion for Oko until problem 5. The phase 4 extradimensional shift was similar until problem 4. The ratio of changed to total errors for these phases ranged from .67-.93, indicating that between 2-13 times as many errors were made on changed in contrast to unchanged errors during these phases. Similarly, during these problems no savings can be seen for the intradimensional shift of phase 3. These data indicate that Oko was solving these problems essentially by treating the subproblems as independent entities. A simi- lar picture emerges for Katumba for problems 1-4. The extradimensional shifts of phases 2 and 4 show unchanged to total error ratios of .73-.97, indicating 2-1/2-27

234 D. ROBBINS, P. COMPTON, t~; S. HOWARD

Table 1. Summary data for the four phases of each problem for both subjects. Phase 1 Phase 2 Phase 3 Phase 4 Original Extra-dimen- Intra-dimen- Extra-dimen- learning sional shift sional shift sional shift

Problem Trials to Trials to Trials to Trials to No. Subject criterion ratio 1) criterion ratio 2~ criterion ratio1) criterion ratio 2) 1 Oko 115 .56 125 .68 125 .53 42 .93

Katumba 65 .54 71 .75 215 .51 105 .97 2 Oko 102 .52 315 .77 317 .57 27 .80

Katumba 49 .52 52 .75 28 .54 37 .88 3 Oko 301 .54 261 .71 409 .56 338 .67

Katumba 129 .57 427 .78 495 .54 200 .93 4 Oko 61 .59 40 .70 23 .52 130 .51

Katumba 311 .50 224 .81 27 .55 379 .73 5 Oko 113 .56 130 .54 39 .55 132 .52

Katumba 67 .52 49 .59 28 .56 44 .55 6 Oko 31 .51 438 .52 47 .57 124 .53

Katumba 277 .54 443 .54 29 .53 245 .54 Means 110.9 .54 284.4 .68 246.3 .54 227.9 .69

1) Represents the ratio of the subproblem with the greater number of errors to total error. 2) Represents the ratio of changed subproblem errors to the total of changed subproblem errors plus unchanged subproblem errors.

times as many changed than unchanged errors. In addition, no savings can be seen for the intradimensional shifts of phase 3. Thus, as for Oko, these data for Katumba also indicate subproblem independence and nonconceptual behavior.

However, problem 4 phase 4, the extradimensional shift phase, for Oko and prob- lems 5 and 6 reveal a marked change in the ratio of changed to total errors. A similar change can be seen for Katumba for problems 5 and 6. For these data the changed to total error ratio ranges f rom .51-.59, indicating tittle bias in terms of less errors on the unchanged in contrast to the changed subproblem. Further, at the same time the in- tradimensional shifts of phase 3 all appear to show less trials to criterion than the im- mediately preceding phase. Taken together these data seem to indicate a shift from subproblem independence and non-conceptual behavior to dependence and a concep- tual or cognitive behavior style. In Figure 1 we have summarized these data in quartiles of learning, for those problems and phases for the extradimensional shift that had high vs. those with close to .50 ratios of changed to total errors. It is clear that during the period when the ratio was relatively high the changed and unchanged learning func- tions were quite unlike. However, when the ratio began to approach .50 these curves became very similar. In Figure 2 is shown the original learning and intradimensional phases and reveals little bias towards one or the other subproblem.

Thus we have demonstrated that the gorilla is capable of demonstrating a cognitive style of behavior in a traditional concept identification study. The cognitive solution style appeared to develop during the course of the study, since the first half of the prob- lems were solved in a non-conceptual manner. This kind of result is reminiscent of the classic "learning-to-learn" phenomenon HARLOW reported years ago (HARLOW, 1949). Whether we can interpret these data as having taught the gorilla to solve problems more efficiently, i. e., conceptually in contrast to independently, is not clear. Subse- quent research must be directed at the longevity of this type of behavior. It is our sus-

Shift Behavior in the Gorilla 235

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picion that the change in behavior may be more or less permanent since some of the phases were stretched out over a period of 20 days or more. However, an even clearer picture will emerge if subjects are presented with a variety of tasks, such as a half- reversal shift (SLAMECI~A, 1968) in which conceptual behavior will retard performance, or simplified versions of the "abstract ion" procedure used by POSNER and others (e.g., POSNER, 1969).

These data are clear in demonstrating that the gorilla is capable of using concep- tual or cognitive information and may suggest procedures in which one could teach organisms, that typically display subproblem independence, a conceptual mode of behavior.

236 D. ROBBINS, P. COMPTON, • S. HOWARD

Acknowledgements. We wish to acknowledge the technical assistance of M. BLUM, C. COCHRAN, and D. MARCUS.

REFERENCES

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RUMaAUan, D. M., C. I. BELL, & T. V. GILL, 1972. Two discrimination test apparatuses for primates. Behavior Research Methods and Instrumentation, 4: 6--10.

SCribER, A. M., 1971. Extradimensional transfer of learning-set formation in stumptailed monkeys. Learning and Motivation, 2: 173-181.

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- - Received July 27, 1976; Accepted October 30, 1976

Authors' Present Addresses: DONALD ROBBINS, Rockefeller University, New York, New York 10021, U.S.A.; PHILLIP COMPTON and STEPHEN HOWARD, Department of Psychology, Emory University, Atlanta, Georgia 30322, U.S.A.