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One versus two discrimination by whitenecked ravens (Corvus cryptoleucus) with non-number dimensions varied
Anim . Behav., 1970, 18, 454-460
ONE VERSUS TWO DISCRIMINATION BY WHITENECKED RAVENS (COR VUS CR YPTOLEUCUS) WITH NON-NUMBER DIMENSIONS VARIED BY LELAND C . SWENSON Department of Psychology, Occidental College, Los Angeles, California 90041
One of the basic components of human counting behaviour is making discriminations on the basis of number . Successful discrimination training with number as the relevant stimulus dimension has been reported for a wide range of mammalian and avian species . However, many of these results can be explained by the subjects' reliance on correlated but non-number cues . These may include pattern cues, area cues, odour cues or cues resulting from unequal motions towards the available alternatives made by the experimenter . Douglas & Whitty (1941), trained various primates to discriminate patterns of five dots from patterns of six dots . The results of later experimenters incorporating more careful controls of density and figural organization indicated that the animals had discriminated by those cues. Kuhn (1953), reported training a rhesus monkey to 95 per cent accuracy on a sixeight problem in 18 718 trials . These results may be due to arrangement (figure) or area cues . Hicks (1956), was able to teach rhesus monkeys to respond to three objects of any shape or arrangement on the cards while ignoring cards bearing two or four objects . Changes in stimuli used resulted in performance decrements but the animals continued to perform more accurately than chance . Even with rigid control of nonnumber cues, recovery was more rapid than original learning . Rensch & Altevogt (1957), trained an elephant to respond to a card having three dots in preference to one marked with four dots . This discrimination did not generalize to, pairs of cards with novel stimulus arrangeents or shapes . Hassmann (1952) reported learning of up to 'fiveness' by squirrels . Baits were placed manually under the stimulus objects and odour was not controlled . Wesley (1959) trained rats to approach cards decorated with two stimuli and avoid those with three . While many non-number cues may not have been controlled, strong negative transfer during a reversal task indicated a high correlation between the cues prepared by the author and those used by the rats . Results with birds were similar to those reported for mammals . Arndt (1939) reported
68 per cent accuracy by pigeons in pecking cards
with two stimuli and not pecking cards with one, three or four stimuli. Since area was not controlled and since area cues were therefore highly correlated with the rewarded choice, the level of accuracy reported seems more consistent with a hypothesis of discrimination by area than discrimination by number . Marold (1939) used grain kernels as stimuli for his parakeets in a one grain versus two grain discrimination task. Terminal performance was only 74 per cent correct suggesting that the discrimination may have been made on the basis of area . Schiemann (1939) reported near 100 per cent accuracy by jackdaws on a three positive, four negative problem . When the dots of the original problem were replaced by mealworms, evidence of number discrimination vanished . Koehler (1951) has been the most successful in training birds to discriminate by number . Extensive control procedures included new irregular plasticine chips for every trial and hidden observers . Following 12 000 trials one raven mastered a six minus, seven rewarded, problem . Parrots and jackdaws learned discriminations of five from six, and pigeons four from five . Logler (1959) trained a parrot to respond differentially in a delayed choice problem dependent upon the number of circles of light presented (one, three and five) in 880 trials . As in a later study of number discrimination in canaries by Pastore (1961), area was not controlled . Zeier (1966), utilized the Skinnerian technique of shaping to train eighty-two pigeons on series of binary decisions . With the use of various guidance techniques (progressive programming, auditory feedback from the keys, and the extra cues of colour and/ or position) three birds managed a programme of eight sequential decisions and nine mastered seven decisions . For most of the pigeons many thousands of trials were required and it was reported that many of the birds appeared to `lock on' to correlated but extraneous cues which appeared to block the acquisition of the desired behaviour . In general, European researches have reported more success in training infrahuman species to discriminate by number than their 454
SWENSON : ONE-TWO DISCRIMINATION BY RAVENS
American counterparts . Their results also tend to suggest levels of avian performance which equal or exceed those of mammals . However with the exception of the work of Koehler and of Zeier, inadequate control of non-number cues precludes any definitive comparisons of either American with European researchers or bird number-discrimination ability with mammalian number-discrimination ability . Wesley (1961), after reviewing reports of number discriminations, suggests that these results may be artifacts, engendered by a naive experimental attitude, and perpetuated by casual technique . Koehler (1956) interprets the difference between his results and less successful attempts as a function of identifying correlated but nonnumber cues and training his subjects to avoid dependence upon any cues but number . He suggests that fixation on extraneous cues which are partly correlated with the reward (area, odour, differential movements by the experimenter, and so on) prevents utilization of the more difficult number cue . For Wesley (1961), careful control means evidence that the final discrimination depends upon the number-cue dimension . It is implied in his position that an organism capable of discrimination by number `works through' a `hierarchy' of available stimulus dimensions until the number discrimination is made . Thus it follows that failure to demonstrate number discrimination after sufficient trials indicates the limits of the subject's capability. For Koehler, extra-number cues are not incidental 'waystations' enroute to discrimination by number, but obstacles which the experimenter must train the subject to avoid . It seems obvious that the failure of an animal to solve a problem in a typical discrimination study may indicate nothing about the possible capabilities the animal may display with a more adequate training procedure . For Koehler the adequate training programme involves reducing the probability of reward to a non-number cue as soon as the animal begins to depend upon that cue . If Koehler's position is essentially correct, then programmes which identify and minimize correlated cues rather than allowing the subject to `work through' his hypothesis may serve in shaping attention to very high difficulty concepts in animals and humans. The present study was designed to study the relationships between the performance of ravens on the simple one versus two number discrimination and the effects of extraneous cue dimen-
455
sions . Following the criticisms of former number-discrimination studies by Wesley (1961), and others, the experimental design was formulated to evaluate and control responses by area, pattern, odour, subtle experimenter movements, and other extraneous cues . The probabilities of the subjects making correct responses by attending to non-number dimensions were determined for those cue dimensions that could not be removed from the experimental setting. Generalization of the concept over novel stimuli was to be tested for those birds who showed clear evidence of responses to the number cues . Following Koehler, it was predicted that the subjects showing the least dependence on extraneous cues should demonstrate the most rapid learning of the concept `one' versus `two' . Methods Subjects Three adult whitenecked ravens (Corvus cryptoleucus) of undetermined sex named Koenig Cathy and Schwartz, served as subjects . The birds were maintained on a diet of canned dog food, Purina rat-chow, eggs from laboratory quail, and occasional table scraps . Apparatus The apparatus was a flat grey painted box, 62 cm wide, 55 cm deep and 37 cm high, with a wood frame and sides and flooring of wire mesh. A masonite board divided the test box into two equal compartments, each with inward swinging doors of white Plexiglas . On each door a metal bracket held the 17 . 4 x 17 . 4 cm milkwhite Plexiglas stimulus cards . A wood frame masked all the edges of the doors . The sides and top of the test box were hidden from the birds by three large masonite shields . Pushing either door inward displaced a switch which activated a mechanism that locked the unpushed door and prevented the birds from making within trial corrections . Baits of 3 g canned dog food were concealed in white cups set in heavy wood blocks near the backs of the two test chambers . The ravens were housed in a cage with four equal-sized compartments, connected by three guillotine doors. One end compartment was provided with an opening to the outside which matched the dimensions of the front of the test box . This opening was closed by a removable door except when testing . During testing the test box was placed against the matching opening and the subject isolated from the other birds by closing
45 6
ANIMAL BEHAVIOUR, 18,
the guillotine door connecting the end compartment with the three other chambers . Pre-training Five days of ten to twenty pre-training trials per day were given to train the subjects to open the swinging doors leading to the two identical test chambers in order to obtain the baits in the cups . On the first trials, the doors were opened by the experimenter and quail eggs placed on the thresholds of both test chambers . On later trials the experimenter opened the doors by decreasing amounts . After acquisition of door opening the quail eggs were replaced by bits of canned dog food . The number of responses made to each chamber were noted for each bird . To control for an initial left or right chamber preference, additional trials were given with the formerly preferred side locked until all birds had obtained baits equally from each chamber. Preceding and during testing all subjects were given free access to water and Purina rat-chow (a non-preferred food) in the cage section fitted with the removable door . Dog food was only available in the test chambers following a correct choice . Experiment 1 Experiment 1 consisted of presenting the birds with a series of circles mounted on the Plexiglas stimulus cards until the subjects met the criterion of 90 per cent correct choices on two successive days or until they had completed 36 consecutive days of testing with 32 trials being given on each day . Stimuli Eight Plexiglas stimulus cards with red Plexi-
3
glas circles of areas 4 to 9 and 12 to 6 cm 2 bonded to them were used (see Fig . 1) . Procedure By rotating the `one' cards (top left corner, top centre, top right corner and so on) and pairing the resulting eight circle orientations with the four possible circle orientations of the `two' cards, thirty-two rotation pairs result . These were assigned numbers . By use of a random number table, thirty-six sets (daily blocks) of the thirty-two rotation pairs were arranged in unique orders . Within blocks, the reinforced `one' card was displayed on the left door sixteen times and on the right door sixteen times, with order determined by Gellerman (1933) series . Both small and large `one' cards were distributed by Gellerman orders within left and right sides, with both sizes appearing equally often on left and right doors. The two circles on the `two' cards were either both small or both large . By pairing both sizes of circles on the `two' cards with both sizes of circles on the `one' cards eight each of the four possible combinations were obtained . Area ratios ranged from 1 to 6 .3 for the combination of a single small circle versus two large circles to 1 .5 to 1 . 0 for the single large circle versus two small circles . Each bird was tested for 16 trials and then allowed to rest (while another bird worked) before finishing the final 16 trials of the daily block. The order of testing was randomized with the restriction that no bird be tested first on two successive days . At the start of each trial the stimulus cards were inserted manually with the experimenter
Fig . 1 . All cards used in experiment 1 . Cards are white Plexiglas with red Plexiglas mounted on them .
SWENSON : ONE-TWO DISCRIMINATION BY RAVENS
hidden behind the masonite shields . The specific pair of cards inserted and their orientation and position were determined by reference to the master schedule . The bait was touched to both bait cups to minimize odour and noise cues . The experimenter then left the testing room and did not return until 30 s after a light flashing outside the testing room signalled that the subject had pushed open one of the two swinging doors bearing the stimulus cards . The 30-s delay allowed time for the bird pushing open the correct (single circle) door to obtain the bait . Results Overall learning. Per cent of correct responses by blocks of days are given for all birds in Fig . 2 . Only Schwartz met the criterion for the `one' from `two' discrimination (90 per cent accuracy on 2 consecutive days) . Koenig gradually reached an average accuracy level of 76 per cent on the last four days of testing . Cathy showed erratic terminal performance with no significant improvement beyond the average accuracy (67 per cent reached on day 20 .) Analysis of the effects of non-number cues . The experimental design provided for the evaluation of the effects of area ratios, position and orientation of the stimuli on the cards . It was hoped that such evaluation would indicate the extent to which all subjects relied upon non-number cues . Both Cathy and Koenig were most accurate when the ratio of the area of the dual circles 83 80 77 74 y 71
S u
6s 6
t 6 U d 69 56
u
5 50
g 47 4
1-4
5-8
9-12
13-16 17-20 21-24 25-28 29-32 33-36
Daily blocks of thirty-two trials
Fig. 2 . Per cent of correct responses for each bird averaged across sets of four daily blocks of thirty-two trials each . Each data point represents 128 trials . •, Schwartz ; p, Koenig ; 0, Cathy .
4 57
to the area of the single circle was 6 . 3 to 1 .0, next in accuracy when the ratio was 2 . 0 to 1 .0 and least accurate when the area of the single circle was larger . This effect of area was significant (P<0 . 01, F=8 .71, df=3264) . The data for Schwartz showed a high accuracy regardless of area (F< 1 . 0, df= 3144) . All birds showed strong initial position preferences in spite of the pretraining equalization procedure . Koenig overcame his right test-chamber preference gradually while Cathy continued to respond mainly to the left door . In the five days preceding his sudden solution, Schwartz made no right side responses . The last five errors were all made to two small circles on the left side, when this card was contrasted with the large single circle on the right (non-preferred) side. For all birds a consistent bias in favour of more correct responses to a `one' card paired with a vertical rotation of the `two' stimuli and less correct responses with a horizontal orientation of the `two' card was observed . All of the birds made more correct responses to some specific card sets of the thirty-two orientation pairs, than to others . Cathy and Schwartz showed similar patterns of `easy' and `difficult' pairs with a 0 . 95 correlation between their respective rank orders of orientation-pair difficulty. The pattern of the relative difficulty of the orientation pairs for Koenig was quite different from that of the other two subjects . These data suggest the operation of a common factor in the performance of Cathy and Schwartz (before solution), which was absent for Koenig . All birds utilized various non-number aspects of the stimulus array . The early failure of Schwartz in using area cues (correlated 0 . 75 with reward) may be related to his ultimate success in solving the problem, while Cathy and Koenig's use of area cues may have interfered with learning a number discrimination . Experiment 2 A series of twenty-four new cards was prepared to test the generality of any `number' concept learned. These consisted of various geometric shapes of red Plexiglas and circles of different areas of black Plexiglas (Fig . 3) . The two birds showing the best performance in experiment 1 were tested. Koenig was given forty trials . Area ratios were computed for all pairs of stimulus cards . An x2 test of deviations from the response pattern predicted by area ratios gave a statistic close to 0. It was concluded that Koenig was
45 8
ANIMAL BEHAVIOUR,
18, 3
Pair numbers 1
2
3
9
4
5
.40k
..
Vk
Card numbers 21
22
23
M
24
A&
Fig. 3 . Stimuli cards used in experiment 2 . Pairs 1 to 5 are red stimuli on white cards . Cards 21 to 24 have black stimuli on white cards.
responding to area and consistently choosing two figures if their combined area was smaller than a single figure . Schwartz was given a total of 330 generalization trials . For the first 120 trials five matched pairs of geometric shapes were used . Accuracy was 91 per cent (seventy-two trials) for the three pairs (squares, rectangles, irregular shapes) in which the area of the single shape was less than the areas of the double shapes . In twenty-four trials Schwartz was 79 per cent correct in discriminating a large tripod from two small tripods (Fig. 3, pair 4) of lesser combined area . For these four pairs of stimuli no errors were made in the first trials . The last pair (no. 5) consisted of a huge triangle of three times the combined areas of the two small triangles . There were only two correct responses in twenty-four trials . In following tests the cards with geometric shapes were mixed with each other . This was followed by the cards having black circles paired first with similar cards, and then with cards having other geometric figures . In all cases accuracy was over 85 per cent except for
single figures of three or more times the combined area of the double figures . By training the subject on pairs with a large (19 .6 cm2) black circle (the largest circles previously used were 12 . 6 cm2), it was possible to break down the aversion to responding to very large stimuli . A huge (34 . 5 cm 2) black triangle (no . 24) presented for the first time was responded to with one error on the third of eight trials . To determine if Schwartz had been using 'boundedness' or extensity cues, cards with two small circles (not shown) very close together were contrasted with large single stimuli (nos . 22 and 24) . In these cases the outer edge to outer edge measure of the two small stimuli was less than the widest extent (distance `bound') of the single stimulus alone . Two errors were made in twenty trials. Because the `one' cards had been used before, it may be argued that individual cards have been learned . To test this, the large triangle from pair 5 (which had been chosen only four times in thirty-eight previous presentations) was paired against a new card
SWENSON : ONE-TWO DISCRIMINATION BY RAVENS
showing two small circles with edges touching (no. 23). The large red triangle was selected on the first trial and in five of the seven following trials . As the large triangle from pair 5 had had a circle removed the possible explanation that Schwart discriminated closed versus open stimulus arrays may also be questioned . Schwartz appeared to have discriminated by number. Accuracy ranged from 90 per cent or better with pairs having `two' cards of area greater or equal to that of the `one' card stimulus to 75 per cent with the `one' card of area greater than that of the stimuli of the `two' cards . The discrimination generalized over shape and colour shifts without training and over extreme area reversals with training . Trials with different shapes on the `two' cards than on the `one' cards were slightly more difficult than trials with similar shapes on each card . Discussion The case for discrimination by number is most defensible for Schwartz, although alternate explanations must be examined . Rossman (1959), working with fish, found that her subjects had responded by array density. On a `one' versus `two' problem either density equals area (which was controlled), or density implies the organism will learn unitary stimulus versus disjunctive stimulus . The latter definition predicts the same results as an assumption of discrimination by number. Wesley (1961) suggests that, in studies where there has been control for odour, unwitting experimenter movement or expression, and other cues, the results may be due to the subjects learning to approach or avoid individual cards . This interpretation seems precluded by Schwartz's all-or-none pattern of solution and by the ease of transfer to new types of cards . A third alternate explanation is discrimination by area `bound' by the outermost edges of the stimuli. By this hypothesis all cards with `two' stimuli `bound' more area than cards with `one' stimulus, as a result of their spatial separation . Cards with lesser outer-edge to outer-edge measurements for the dual stimuli were given in experiment 2, but the subject continued to choose the single stimulus . Following Pastore (1961), any behaviour, which appears to represent discrimination by number and cannot be demonstrated to be the result of observable non-number cues, will be assumed to be based on a `counting' process . This process is not assumed to be the same as that involved in human countings . By the above definition, Schwartz `counted' .
459
After Schwartz had solved the problem in experiment 1, only pairs of cards with a large single stimulus on the right side, and two small circles on the preferred left side were incorrectly discriminated (five out of the last eight such combinations presented in experiment 1) . It appears that the additive effects of two factors favouring an incorrect response were sometimes stronger than the probability of responding on the basis of the newly learned number dimension . Such an interpretation would imply that attention is not restricted to one dimension as required by classical non-continuity theory (Krechevsky 1932) . Koehler (1956) seems to suggest that difficulty in learning of number discriminations is dependent upon the number of confounding dimensions available to the subjects . Since the probability of the subjects paying attention to the number aspect of the stimulus array is initially low, any success while attending to a non-number dimension of the array would make it that much more difficult for the number dimension to be selected . The observation that Schwartz appeared to respond to the fewest non-number dimensions supports Koehler's position . Koenig who was found to have responded primarily on the basis of area cues (predicts reward 75 per cent) and Cathy who showed both area and position (predicts reward 50 per cent) had terminal scores of 76 and 67 per cent respectively . The difficulty of even the most basic numberdiscrimination task for whitenecked ravens should not be taken to represent a limit of ability . Once the subject is trained to attend to number cues, learning of further number discriminations should proceed relatively rapidly . If Koehler is correct, a more efficient design for testing the upper limits of avian numberdiscrimination ability would involve a correction procedure (or shaping) to minimize rewards associated with any consistent pattern of responding to non-number cues . The work of Zeier (1966) suggests both that birds mastering simple `counting' problems can succeed in more difficult tasks and that techniques such as those mentioned above may be beneficial . Another possible direction would be to utilize the fading procedure of Terrance (1963) . This procedure involves training the subject on an easy dimension and then gradually transferring stimulus control to the difficult dimension . The development and experimental comparison of techniques for these types of problems
4 60
ANIMAL BEHAVIOUR, 18, 3
represent some of the most difficult tasks in the education of animals and young humans. Systematic experimentation is required on the interactions of easy cues and abstract dimensions during the learning of these complex dimensions . Summary Three whitenecked ravens (Corvus cryptoleucus) were tested on a one versus two number-discrimination task. One bird showed a sudden jump in performance from chance levels to over 90 per cent accuracy and subsequently showed good transfer to stimuli of new colours and shapes . One bird gradually reached an average accuracy level of about 75 per cent in over 1000 trials . Testing with novel shapes indicated that the previous discrimination could have been predicted by assuming that the bird had responded to area cues . The third subject reached an average accuracy level of 67 per cent and inspection of response patterns indicated that the bird had made its responses on the basis of both position preference and area cues . The obtained results support an hypothesis that information which is partially related to reward may block acquisition of an abstract concept . Suppression of tendencies to respond on the basis of partially correlated information may be a necessary condition for determination of performance limits in animals or humans . Acknowledgments This investigation was supported in part by NSF Grant No . GB 5989, L . J. Stettner, principle investigator . I am indebted to Professor Stettner for his stimulation and assistance . My thanks are also due to Professor F . Wehmer and Professor S . Brent for their advice and criticism . REFERENCES Arndt, W. (1939) . Abschliessende Versuche zur Frage des "Zahl"-Vermogens der H austaube . Z. TierpsychoL, 2, 88-142.
Douglas, J. W. B . & Whitty, C. W. M. (1941) . An investigation of number appreciation in some subhuman primates. J. comp . physiol. Psychol., 31, 129-142. Gellerman, L . W. (1933) . Chance order of alternating stimuli in visual discrimination experiments . J. genet. Psychol., 42, 207-208 . Hassmann, M. (1952). Vom Erlernen unbenannter Anzahlen bei Eichornchen . Z. Tierpsychol., 9, 294-321 . Hicks, L . H. (1956) . An analysis of number concepts formation in the rhesus monkey . J. comp . physiol. Psychol., 49, 212-218. Koehler, O . (1951) . The ability of birds to count . Bull. Anim. Behav., 9, 41-45 . Koehler, O . (1956) . Sprache and unbenanntes denken . In : L'Instinct dans le Comportement des Animeaux et de l.Homme (Ed. by P. P . Grassb), pp . 647-675 . Paris : Masson . Krechevsky, I . (1932). Hypotheses in rats. Psychol. Rev., 39,516-632 . Kuhn, E. (1953). Simultanvergleich geserhener Mengen beim Rhesusaffen. Z. Tierpsychol., 10, 268-296. Logler, P. (1959) . Versuche zur Frage des "Zahl"Vermogens an einem Graupapagei undVergleischsversuche an Menchen . Z. Tierpsychol., 16, 169217. Marold, E. (1939). Versuche an Wellensittichen zur Frages des "Zahl"-Vermogens . Z. Tierpsychol., 3, 170-223 . Pastore, N . (1961). Number sense and "counting ability" in the canary . Z. Tierpsychol., 18, 561-573. Schiemann, K . (1939) . Von Erlernen unbenannter Anzahlen bei Dohlen . Z. Tierpsychol., 16, 605627. Rensch, B . & Altevogt, R . (1957) . The intelligence of elephants . Scient. Am ., February, 421-427. Rossman, A. (1959) . i)ber das `Zhal-Vermogens' der Fische. Z. Tierpsychol., 16, 1-18 . Terrance, H . (1963) . Discrimination learning with and without errors . J. exp . anal. Behav ., 6, 1-27 . Wesley, F . (1959) . Number concept formation in the rat . Z. Tierpsychol., 16, 605-627 . Wesley, F. (1961) . The number concept : A phylogenetic review. Psychol. Bull., 58, 420-428. Zeier, von H . (1966) . Ober seqentielles lernen bei Tauben mit spezieller Berucksichtigung des, "Zahl"Verhaltens . Z. Tierpsychol., 23, 161-189 . (Received 17 January 1969 ; revised 22 December 1969 ; MS. number : A795)
ONE VERSUS TWO DISCRIMINATION BY WHITENECKED RAVENS (COR VUS CR YPTOLEUCUS) WITH NON-NUMBER DIMENSIONS VARIED BY LELAND C . SWENSON Department of Psychology, Occidental College, Los Angeles, California 90041
One of the basic components of human counting behaviour is making discriminations on the basis of number . Successful discrimination training with number as the relevant stimulus dimension has been reported for a wide range of mammalian and avian species . However, many of these results can be explained by the subjects' reliance on correlated but non-number cues . These may include pattern cues, area cues, odour cues or cues resulting from unequal motions towards the available alternatives made by the experimenter . Douglas & Whitty (1941), trained various primates to discriminate patterns of five dots from patterns of six dots . The results of later experimenters incorporating more careful controls of density and figural organization indicated that the animals had discriminated by those cues. Kuhn (1953), reported training a rhesus monkey to 95 per cent accuracy on a sixeight problem in 18 718 trials . These results may be due to arrangement (figure) or area cues . Hicks (1956), was able to teach rhesus monkeys to respond to three objects of any shape or arrangement on the cards while ignoring cards bearing two or four objects . Changes in stimuli used resulted in performance decrements but the animals continued to perform more accurately than chance . Even with rigid control of nonnumber cues, recovery was more rapid than original learning . Rensch & Altevogt (1957), trained an elephant to respond to a card having three dots in preference to one marked with four dots . This discrimination did not generalize to, pairs of cards with novel stimulus arrangeents or shapes . Hassmann (1952) reported learning of up to 'fiveness' by squirrels . Baits were placed manually under the stimulus objects and odour was not controlled . Wesley (1959) trained rats to approach cards decorated with two stimuli and avoid those with three . While many non-number cues may not have been controlled, strong negative transfer during a reversal task indicated a high correlation between the cues prepared by the author and those used by the rats . Results with birds were similar to those reported for mammals . Arndt (1939) reported
68 per cent accuracy by pigeons in pecking cards
with two stimuli and not pecking cards with one, three or four stimuli. Since area was not controlled and since area cues were therefore highly correlated with the rewarded choice, the level of accuracy reported seems more consistent with a hypothesis of discrimination by area than discrimination by number . Marold (1939) used grain kernels as stimuli for his parakeets in a one grain versus two grain discrimination task. Terminal performance was only 74 per cent correct suggesting that the discrimination may have been made on the basis of area . Schiemann (1939) reported near 100 per cent accuracy by jackdaws on a three positive, four negative problem . When the dots of the original problem were replaced by mealworms, evidence of number discrimination vanished . Koehler (1951) has been the most successful in training birds to discriminate by number . Extensive control procedures included new irregular plasticine chips for every trial and hidden observers . Following 12 000 trials one raven mastered a six minus, seven rewarded, problem . Parrots and jackdaws learned discriminations of five from six, and pigeons four from five . Logler (1959) trained a parrot to respond differentially in a delayed choice problem dependent upon the number of circles of light presented (one, three and five) in 880 trials . As in a later study of number discrimination in canaries by Pastore (1961), area was not controlled . Zeier (1966), utilized the Skinnerian technique of shaping to train eighty-two pigeons on series of binary decisions . With the use of various guidance techniques (progressive programming, auditory feedback from the keys, and the extra cues of colour and/ or position) three birds managed a programme of eight sequential decisions and nine mastered seven decisions . For most of the pigeons many thousands of trials were required and it was reported that many of the birds appeared to `lock on' to correlated but extraneous cues which appeared to block the acquisition of the desired behaviour . In general, European researches have reported more success in training infrahuman species to discriminate by number than their 454
SWENSON : ONE-TWO DISCRIMINATION BY RAVENS
American counterparts . Their results also tend to suggest levels of avian performance which equal or exceed those of mammals . However with the exception of the work of Koehler and of Zeier, inadequate control of non-number cues precludes any definitive comparisons of either American with European researchers or bird number-discrimination ability with mammalian number-discrimination ability . Wesley (1961), after reviewing reports of number discriminations, suggests that these results may be artifacts, engendered by a naive experimental attitude, and perpetuated by casual technique . Koehler (1956) interprets the difference between his results and less successful attempts as a function of identifying correlated but nonnumber cues and training his subjects to avoid dependence upon any cues but number . He suggests that fixation on extraneous cues which are partly correlated with the reward (area, odour, differential movements by the experimenter, and so on) prevents utilization of the more difficult number cue . For Wesley (1961), careful control means evidence that the final discrimination depends upon the number-cue dimension . It is implied in his position that an organism capable of discrimination by number `works through' a `hierarchy' of available stimulus dimensions until the number discrimination is made . Thus it follows that failure to demonstrate number discrimination after sufficient trials indicates the limits of the subject's capability. For Koehler, extra-number cues are not incidental 'waystations' enroute to discrimination by number, but obstacles which the experimenter must train the subject to avoid . It seems obvious that the failure of an animal to solve a problem in a typical discrimination study may indicate nothing about the possible capabilities the animal may display with a more adequate training procedure . For Koehler the adequate training programme involves reducing the probability of reward to a non-number cue as soon as the animal begins to depend upon that cue . If Koehler's position is essentially correct, then programmes which identify and minimize correlated cues rather than allowing the subject to `work through' his hypothesis may serve in shaping attention to very high difficulty concepts in animals and humans. The present study was designed to study the relationships between the performance of ravens on the simple one versus two number discrimination and the effects of extraneous cue dimen-
455
sions . Following the criticisms of former number-discrimination studies by Wesley (1961), and others, the experimental design was formulated to evaluate and control responses by area, pattern, odour, subtle experimenter movements, and other extraneous cues . The probabilities of the subjects making correct responses by attending to non-number dimensions were determined for those cue dimensions that could not be removed from the experimental setting. Generalization of the concept over novel stimuli was to be tested for those birds who showed clear evidence of responses to the number cues . Following Koehler, it was predicted that the subjects showing the least dependence on extraneous cues should demonstrate the most rapid learning of the concept `one' versus `two' . Methods Subjects Three adult whitenecked ravens (Corvus cryptoleucus) of undetermined sex named Koenig Cathy and Schwartz, served as subjects . The birds were maintained on a diet of canned dog food, Purina rat-chow, eggs from laboratory quail, and occasional table scraps . Apparatus The apparatus was a flat grey painted box, 62 cm wide, 55 cm deep and 37 cm high, with a wood frame and sides and flooring of wire mesh. A masonite board divided the test box into two equal compartments, each with inward swinging doors of white Plexiglas . On each door a metal bracket held the 17 . 4 x 17 . 4 cm milkwhite Plexiglas stimulus cards . A wood frame masked all the edges of the doors . The sides and top of the test box were hidden from the birds by three large masonite shields . Pushing either door inward displaced a switch which activated a mechanism that locked the unpushed door and prevented the birds from making within trial corrections . Baits of 3 g canned dog food were concealed in white cups set in heavy wood blocks near the backs of the two test chambers . The ravens were housed in a cage with four equal-sized compartments, connected by three guillotine doors. One end compartment was provided with an opening to the outside which matched the dimensions of the front of the test box . This opening was closed by a removable door except when testing . During testing the test box was placed against the matching opening and the subject isolated from the other birds by closing
45 6
ANIMAL BEHAVIOUR, 18,
the guillotine door connecting the end compartment with the three other chambers . Pre-training Five days of ten to twenty pre-training trials per day were given to train the subjects to open the swinging doors leading to the two identical test chambers in order to obtain the baits in the cups . On the first trials, the doors were opened by the experimenter and quail eggs placed on the thresholds of both test chambers . On later trials the experimenter opened the doors by decreasing amounts . After acquisition of door opening the quail eggs were replaced by bits of canned dog food . The number of responses made to each chamber were noted for each bird . To control for an initial left or right chamber preference, additional trials were given with the formerly preferred side locked until all birds had obtained baits equally from each chamber. Preceding and during testing all subjects were given free access to water and Purina rat-chow (a non-preferred food) in the cage section fitted with the removable door . Dog food was only available in the test chambers following a correct choice . Experiment 1 Experiment 1 consisted of presenting the birds with a series of circles mounted on the Plexiglas stimulus cards until the subjects met the criterion of 90 per cent correct choices on two successive days or until they had completed 36 consecutive days of testing with 32 trials being given on each day . Stimuli Eight Plexiglas stimulus cards with red Plexi-
3
glas circles of areas 4 to 9 and 12 to 6 cm 2 bonded to them were used (see Fig . 1) . Procedure By rotating the `one' cards (top left corner, top centre, top right corner and so on) and pairing the resulting eight circle orientations with the four possible circle orientations of the `two' cards, thirty-two rotation pairs result . These were assigned numbers . By use of a random number table, thirty-six sets (daily blocks) of the thirty-two rotation pairs were arranged in unique orders . Within blocks, the reinforced `one' card was displayed on the left door sixteen times and on the right door sixteen times, with order determined by Gellerman (1933) series . Both small and large `one' cards were distributed by Gellerman orders within left and right sides, with both sizes appearing equally often on left and right doors. The two circles on the `two' cards were either both small or both large . By pairing both sizes of circles on the `two' cards with both sizes of circles on the `one' cards eight each of the four possible combinations were obtained . Area ratios ranged from 1 to 6 .3 for the combination of a single small circle versus two large circles to 1 .5 to 1 . 0 for the single large circle versus two small circles . Each bird was tested for 16 trials and then allowed to rest (while another bird worked) before finishing the final 16 trials of the daily block. The order of testing was randomized with the restriction that no bird be tested first on two successive days . At the start of each trial the stimulus cards were inserted manually with the experimenter
Fig . 1 . All cards used in experiment 1 . Cards are white Plexiglas with red Plexiglas mounted on them .
SWENSON : ONE-TWO DISCRIMINATION BY RAVENS
hidden behind the masonite shields . The specific pair of cards inserted and their orientation and position were determined by reference to the master schedule . The bait was touched to both bait cups to minimize odour and noise cues . The experimenter then left the testing room and did not return until 30 s after a light flashing outside the testing room signalled that the subject had pushed open one of the two swinging doors bearing the stimulus cards . The 30-s delay allowed time for the bird pushing open the correct (single circle) door to obtain the bait . Results Overall learning. Per cent of correct responses by blocks of days are given for all birds in Fig . 2 . Only Schwartz met the criterion for the `one' from `two' discrimination (90 per cent accuracy on 2 consecutive days) . Koenig gradually reached an average accuracy level of 76 per cent on the last four days of testing . Cathy showed erratic terminal performance with no significant improvement beyond the average accuracy (67 per cent reached on day 20 .) Analysis of the effects of non-number cues . The experimental design provided for the evaluation of the effects of area ratios, position and orientation of the stimuli on the cards . It was hoped that such evaluation would indicate the extent to which all subjects relied upon non-number cues . Both Cathy and Koenig were most accurate when the ratio of the area of the dual circles 83 80 77 74 y 71
S u
6s 6
t 6 U d 69 56
u
5 50
g 47 4
1-4
5-8
9-12
13-16 17-20 21-24 25-28 29-32 33-36
Daily blocks of thirty-two trials
Fig. 2 . Per cent of correct responses for each bird averaged across sets of four daily blocks of thirty-two trials each . Each data point represents 128 trials . •, Schwartz ; p, Koenig ; 0, Cathy .
4 57
to the area of the single circle was 6 . 3 to 1 .0, next in accuracy when the ratio was 2 . 0 to 1 .0 and least accurate when the area of the single circle was larger . This effect of area was significant (P<0 . 01, F=8 .71, df=3264) . The data for Schwartz showed a high accuracy regardless of area (F< 1 . 0, df= 3144) . All birds showed strong initial position preferences in spite of the pretraining equalization procedure . Koenig overcame his right test-chamber preference gradually while Cathy continued to respond mainly to the left door . In the five days preceding his sudden solution, Schwartz made no right side responses . The last five errors were all made to two small circles on the left side, when this card was contrasted with the large single circle on the right (non-preferred) side. For all birds a consistent bias in favour of more correct responses to a `one' card paired with a vertical rotation of the `two' stimuli and less correct responses with a horizontal orientation of the `two' card was observed . All of the birds made more correct responses to some specific card sets of the thirty-two orientation pairs, than to others . Cathy and Schwartz showed similar patterns of `easy' and `difficult' pairs with a 0 . 95 correlation between their respective rank orders of orientation-pair difficulty. The pattern of the relative difficulty of the orientation pairs for Koenig was quite different from that of the other two subjects . These data suggest the operation of a common factor in the performance of Cathy and Schwartz (before solution), which was absent for Koenig . All birds utilized various non-number aspects of the stimulus array . The early failure of Schwartz in using area cues (correlated 0 . 75 with reward) may be related to his ultimate success in solving the problem, while Cathy and Koenig's use of area cues may have interfered with learning a number discrimination . Experiment 2 A series of twenty-four new cards was prepared to test the generality of any `number' concept learned. These consisted of various geometric shapes of red Plexiglas and circles of different areas of black Plexiglas (Fig . 3) . The two birds showing the best performance in experiment 1 were tested. Koenig was given forty trials . Area ratios were computed for all pairs of stimulus cards . An x2 test of deviations from the response pattern predicted by area ratios gave a statistic close to 0. It was concluded that Koenig was
45 8
ANIMAL BEHAVIOUR,
18, 3
Pair numbers 1
2
3
9
4
5
.40k
..
Vk
Card numbers 21
22
23
M
24
A&
Fig. 3 . Stimuli cards used in experiment 2 . Pairs 1 to 5 are red stimuli on white cards . Cards 21 to 24 have black stimuli on white cards.
responding to area and consistently choosing two figures if their combined area was smaller than a single figure . Schwartz was given a total of 330 generalization trials . For the first 120 trials five matched pairs of geometric shapes were used . Accuracy was 91 per cent (seventy-two trials) for the three pairs (squares, rectangles, irregular shapes) in which the area of the single shape was less than the areas of the double shapes . In twenty-four trials Schwartz was 79 per cent correct in discriminating a large tripod from two small tripods (Fig. 3, pair 4) of lesser combined area . For these four pairs of stimuli no errors were made in the first trials . The last pair (no. 5) consisted of a huge triangle of three times the combined areas of the two small triangles . There were only two correct responses in twenty-four trials . In following tests the cards with geometric shapes were mixed with each other . This was followed by the cards having black circles paired first with similar cards, and then with cards having other geometric figures . In all cases accuracy was over 85 per cent except for
single figures of three or more times the combined area of the double figures . By training the subject on pairs with a large (19 .6 cm2) black circle (the largest circles previously used were 12 . 6 cm2), it was possible to break down the aversion to responding to very large stimuli . A huge (34 . 5 cm 2) black triangle (no . 24) presented for the first time was responded to with one error on the third of eight trials . To determine if Schwartz had been using 'boundedness' or extensity cues, cards with two small circles (not shown) very close together were contrasted with large single stimuli (nos . 22 and 24) . In these cases the outer edge to outer edge measure of the two small stimuli was less than the widest extent (distance `bound') of the single stimulus alone . Two errors were made in twenty trials. Because the `one' cards had been used before, it may be argued that individual cards have been learned . To test this, the large triangle from pair 5 (which had been chosen only four times in thirty-eight previous presentations) was paired against a new card
SWENSON : ONE-TWO DISCRIMINATION BY RAVENS
showing two small circles with edges touching (no. 23). The large red triangle was selected on the first trial and in five of the seven following trials . As the large triangle from pair 5 had had a circle removed the possible explanation that Schwart discriminated closed versus open stimulus arrays may also be questioned . Schwartz appeared to have discriminated by number. Accuracy ranged from 90 per cent or better with pairs having `two' cards of area greater or equal to that of the `one' card stimulus to 75 per cent with the `one' card of area greater than that of the stimuli of the `two' cards . The discrimination generalized over shape and colour shifts without training and over extreme area reversals with training . Trials with different shapes on the `two' cards than on the `one' cards were slightly more difficult than trials with similar shapes on each card . Discussion The case for discrimination by number is most defensible for Schwartz, although alternate explanations must be examined . Rossman (1959), working with fish, found that her subjects had responded by array density. On a `one' versus `two' problem either density equals area (which was controlled), or density implies the organism will learn unitary stimulus versus disjunctive stimulus . The latter definition predicts the same results as an assumption of discrimination by number. Wesley (1961) suggests that, in studies where there has been control for odour, unwitting experimenter movement or expression, and other cues, the results may be due to the subjects learning to approach or avoid individual cards . This interpretation seems precluded by Schwartz's all-or-none pattern of solution and by the ease of transfer to new types of cards . A third alternate explanation is discrimination by area `bound' by the outermost edges of the stimuli. By this hypothesis all cards with `two' stimuli `bound' more area than cards with `one' stimulus, as a result of their spatial separation . Cards with lesser outer-edge to outer-edge measurements for the dual stimuli were given in experiment 2, but the subject continued to choose the single stimulus . Following Pastore (1961), any behaviour, which appears to represent discrimination by number and cannot be demonstrated to be the result of observable non-number cues, will be assumed to be based on a `counting' process . This process is not assumed to be the same as that involved in human countings . By the above definition, Schwartz `counted' .
459
After Schwartz had solved the problem in experiment 1, only pairs of cards with a large single stimulus on the right side, and two small circles on the preferred left side were incorrectly discriminated (five out of the last eight such combinations presented in experiment 1) . It appears that the additive effects of two factors favouring an incorrect response were sometimes stronger than the probability of responding on the basis of the newly learned number dimension . Such an interpretation would imply that attention is not restricted to one dimension as required by classical non-continuity theory (Krechevsky 1932) . Koehler (1956) seems to suggest that difficulty in learning of number discriminations is dependent upon the number of confounding dimensions available to the subjects . Since the probability of the subjects paying attention to the number aspect of the stimulus array is initially low, any success while attending to a non-number dimension of the array would make it that much more difficult for the number dimension to be selected . The observation that Schwartz appeared to respond to the fewest non-number dimensions supports Koehler's position . Koenig who was found to have responded primarily on the basis of area cues (predicts reward 75 per cent) and Cathy who showed both area and position (predicts reward 50 per cent) had terminal scores of 76 and 67 per cent respectively . The difficulty of even the most basic numberdiscrimination task for whitenecked ravens should not be taken to represent a limit of ability . Once the subject is trained to attend to number cues, learning of further number discriminations should proceed relatively rapidly . If Koehler is correct, a more efficient design for testing the upper limits of avian numberdiscrimination ability would involve a correction procedure (or shaping) to minimize rewards associated with any consistent pattern of responding to non-number cues . The work of Zeier (1966) suggests both that birds mastering simple `counting' problems can succeed in more difficult tasks and that techniques such as those mentioned above may be beneficial . Another possible direction would be to utilize the fading procedure of Terrance (1963) . This procedure involves training the subject on an easy dimension and then gradually transferring stimulus control to the difficult dimension . The development and experimental comparison of techniques for these types of problems
4 60
ANIMAL BEHAVIOUR, 18, 3
represent some of the most difficult tasks in the education of animals and young humans. Systematic experimentation is required on the interactions of easy cues and abstract dimensions during the learning of these complex dimensions . Summary Three whitenecked ravens (Corvus cryptoleucus) were tested on a one versus two number-discrimination task. One bird showed a sudden jump in performance from chance levels to over 90 per cent accuracy and subsequently showed good transfer to stimuli of new colours and shapes . One bird gradually reached an average accuracy level of about 75 per cent in over 1000 trials . Testing with novel shapes indicated that the previous discrimination could have been predicted by assuming that the bird had responded to area cues . The third subject reached an average accuracy level of 67 per cent and inspection of response patterns indicated that the bird had made its responses on the basis of both position preference and area cues . The obtained results support an hypothesis that information which is partially related to reward may block acquisition of an abstract concept . Suppression of tendencies to respond on the basis of partially correlated information may be a necessary condition for determination of performance limits in animals or humans . Acknowledgments This investigation was supported in part by NSF Grant No . GB 5989, L . J. Stettner, principle investigator . I am indebted to Professor Stettner for his stimulation and assistance . My thanks are also due to Professor F . Wehmer and Professor S . Brent for their advice and criticism . REFERENCES Arndt, W. (1939) . Abschliessende Versuche zur Frage des "Zahl"-Vermogens der H austaube . Z. TierpsychoL, 2, 88-142.
Douglas, J. W. B . & Whitty, C. W. M. (1941) . An investigation of number appreciation in some subhuman primates. J. comp . physiol. Psychol., 31, 129-142. Gellerman, L . W. (1933) . Chance order of alternating stimuli in visual discrimination experiments . J. genet. Psychol., 42, 207-208 . Hassmann, M. (1952). Vom Erlernen unbenannter Anzahlen bei Eichornchen . Z. Tierpsychol., 9, 294-321 . Hicks, L . H. (1956) . An analysis of number concepts formation in the rhesus monkey . J. comp . physiol. Psychol., 49, 212-218. Koehler, O . (1951) . The ability of birds to count . Bull. Anim. Behav., 9, 41-45 . Koehler, O . (1956) . Sprache and unbenanntes denken . In : L'Instinct dans le Comportement des Animeaux et de l.Homme (Ed. by P. P . Grassb), pp . 647-675 . Paris : Masson . Krechevsky, I . (1932). Hypotheses in rats. Psychol. Rev., 39,516-632 . Kuhn, E. (1953). Simultanvergleich geserhener Mengen beim Rhesusaffen. Z. Tierpsychol., 10, 268-296. Logler, P. (1959) . Versuche zur Frage des "Zahl"Vermogens an einem Graupapagei undVergleischsversuche an Menchen . Z. Tierpsychol., 16, 169217. Marold, E. (1939). Versuche an Wellensittichen zur Frages des "Zahl"-Vermogens . Z. Tierpsychol., 3, 170-223 . Pastore, N . (1961). Number sense and "counting ability" in the canary . Z. Tierpsychol., 18, 561-573. Schiemann, K . (1939) . Von Erlernen unbenannter Anzahlen bei Dohlen . Z. Tierpsychol., 16, 605627. Rensch, B . & Altevogt, R . (1957) . The intelligence of elephants . Scient. Am ., February, 421-427. Rossman, A. (1959) . i)ber das `Zhal-Vermogens' der Fische. Z. Tierpsychol., 16, 1-18 . Terrance, H . (1963) . Discrimination learning with and without errors . J. exp . anal. Behav ., 6, 1-27 . Wesley, F . (1959) . Number concept formation in the rat . Z. Tierpsychol., 16, 605-627 . Wesley, F. (1961) . The number concept : A phylogenetic review. Psychol. Bull., 58, 420-428. Zeier, von H . (1966) . Ober seqentielles lernen bei Tauben mit spezieller Berucksichtigung des, "Zahl"Verhaltens . Z. Tierpsychol., 23, 161-189 . (Received 17 January 1969 ; revised 22 December 1969 ; MS. number : A795)