Learning and Motivation

Learning and Motivation 41 (2010) 273 286 Contents lists available at ScienceDirect Learning and Motivation journal homepage: www.elsevier.com/locate/l&m What is learned when concept learning fails? A theory of restricted-domain relational learning Anthony A. Wright, Mark T. Lickteig University of Texas Health Science Center at Houston, United States article info abstract Keywords: Abstract-concept learning Relational learning Item-specific learning Matching-to-sample Same different Monkeys Pigeons Two matching-to-sample (MTS) and four same/different (S/D) experiments employed tests to distinguish between item-specific learning and relational learning. One MTS experiment showed item-specific learning when concept learning failed (i.e., no novelstimulus transfer). Another MTS experiment showed item-specific learning when pigeons novel-stimulus transfer decreased because they chose familiar training comparisons instead of matching novel comparisons. In 8-item and 3-item S/D tasks, pigeons and monkeys were accurate with unfamiliar training-stimulus pairings, stimulus inversions, and distorted stimuli, suggesting relational learning within a domain restricted to the training stimuli (i.e., no novelstimulus transfer). In 32-item S/D tasks, pigeons with previous 8-item training showed less transfer than those without prior training, suggesting a carryover of restricted-domain relational learning. Pigeons shifted from 1024-item to 8-item S/D tasks showed reinstatement of restricted-domain relational learning. These findings are important in specifying which types of learning occur in these tasks, showing that subjects failing novel-stimulus transfer are not required to switch from item-specific to relational learning as a training set is expanded, and demonstrating that concept learning failure is not proof of item-specific learning. 2010 Elsevier Inc. All rights reserved. Corresponding author at: Department of Neurobiology and Anatomy, University of Texas Medical School at Houston, P.O. Box 20708, Houston, TX 77225, United States. E-mail address: anthony.a.wright@uth.tmc.edu (A.A. Wright). 0023-9690/$ see front matter 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.lmot.2010.08.004

274 A.A. Wright, M.T. Lickteig / Learning and Motivation 41 (2010) 273 286 The focus of this article is what animals learn when a task can be learned in different ways. Different ways of learning (i.e., learning strategies 1 ) can affect whether relationships among stimuli will be learned and transfer will occur. An example from Stewart Hulse s work was whether starlings would learn the interval relationships among random notes or the absolute properties (e.g., pitch) of notes making up a fixed sequence (Page, Hulse, & Cynx, 1989). Another related example would be whether pigeons would learn relationships among ordered lists of pictures or just the associative chain between one stimulus and the next response (Terrace, 1993, 2005). In both examples, the birds showed tendencies for learning absolute stimulus-response associations (i.e., item-specific learning) rather than relationships among stimuli making up the fixed sequences. Instead of fixed sequences of stimuli, we have explored the issue of relational vs. item-specific learning in more simplified settings containing three or two simultaneously presented stimuli in matching-to-sample (MTS) and same/different (S/D) tasks, respectively. In MTS, pigeons choose between a comparison stimulus that matches the sample and one that does not. In S/D, pigeons choose between a stimulus that may be identical to the sample and a default stimulus (white rectangle in our studies) signifying that the two stimuli are different. As it turns out, animals can learn these tasks either item-specifically or relationally. They can learn the associative response chain between stimulus pairs or the configural pattern produced by the unique combinations of the stimuli; these types of learning will be referred to as item-specific learning. Alternatively, the animals can learn the relationship between pairs of stimuli in S/D or the relationships between the each comparison and the sample in MTS; this type of learning will be referred to as relational learning. It is not possible to determine which type of learning (i.e., which learning strategy) has occurred without tests of novel stimuli and tests where the training stimuli are manipulated. The main test for determining how a task was learned, indeed the only test for the most part, has been transfer to novel stimuli. Transfer to novel stimuli is very strong evidence that subjects have learned the relationship among stimuli and the abstract concept. But what does it mean if subjects fail to transfer to novel stimuli? What have they learned in that case? Failures of novel-item transfer (i.e., absence of abstract-concept learning) have for a half century been taken as evidence for item-specific learning, despite little or no direct evidence (e.g., Carter & Werner, 1978; Lashley, 1938; Premack, 1978; Premack, 1983a; Premack, 1983b; Spence, 1952; Zentall & Hogan, 1974). Even hierarchical theories of learning ability and intelligence have been based on this null hypothesis (e.g., D Amato, Salmon, & Colombo, 1985; Herrnstein, 1990; Mackintosh, 1994; Macphail, 1996; Thomas, 1980, 1996; Thompson, 1995; Tomasello & Call, 1997). In the first section of this article, some tests that my collaborators and I have conducted for item-specific and relational learning in MTS experiments with pigeons are discussed. In the second section, tests for item-specific and relational learning in S/D experiments with pigeons and monkeys are discussed. Matching-to-sample Item-specific learning when concept learning fails The MTS task has been used to study conditional discriminations and concept learning in humans and non-human species for over a half century. In the MTS experiments discussed here, three cartoon stimuli (Duck, Apple, and Grapes) were used as training stimuli, and the different trial configurations are shown in Fig. 1 (Wright, 1997). The sample stimulus is the center cartoon and comparison (choice) stimuli are located on the sides in the shape of a triangle. Stimuli were presented from the chamber floor with the computer monitor pointed up. For most groups, the sample appeared first and the pigeon pecked the sample a number to times depending upon the group. After the requisite number of sample responses, the comparison stimuli appeared, and the pigeon made a single choice response to one of 1 The term strategy refers to what is learned or how the problem is solved, when the type of learning can lead to different solutions. There is no intended implication that subjects willfully choose a strategy. Although there may be individual and species predispositions regarding these learning strategies, the particular training conditions will undoubtedly have an impact on the learning strategy or type of learning actually employed.

Fig. 1. Matching-to-sample displays with the center stimulus being the sample and the two stimuli lower and to the side of the sample being the two comparison stimuli. The training displays are shown in the top two rows and each row is a counterbalanced set of 6 displays. One set was used for training and the other for testing following acquisition. Examples of novel-stimulus test displays are shown in the bottom row. A.A. Wright, M.T. Lickteig / Learning and Motivation 41 (2010) 273 286 275

276 A.A. Wright, M.T. Lickteig / Learning and Motivation 41 (2010) 273 286 Fig. 2. Baseline and transfer performance for groups of pigeons trained with different sample response requirements. Training displays and testing displays (counterbalanced within groups) are shown in the top 2 rows of stimulus displays in Fig. 1. Examples of novel-stimulus displays are shown in the bottom row in Fig. 1. Error bars are standard errors of the mean. the comparison stimuli. A choice response of the matching comparison stimulus was reinforced with an average of five wheat seeds placed on top of the correct comparison stimulus; stimuli remained in view for 8 s while the pigeon ate the seeds. Incorrect choices were unrewarded. During training, incorrect choices were followed by an 8-s timeout, and the trial was repeated (correction procedure). The number of sample responses varied between groups (0, 1, 10, or 20). For the 0-response group, all three stimuli appeared together and all that was required of the subject was a single peck to one or the other comparison stimulus. There were 84 trials per daily session separated by 15-s intertrial intervals. Four pigeons were trained and tested in each group. Fig. 1 shows two sets of 6 training displays. In each set, each cartoon appeared twice as a sample stimulus and once on the left and once on the right as both a correct and an incorrect comparison stimulus. One set was used for training and the other for testing (counterbalanced within each group). The rationale for split-set training was to provide a test for item-specific learning (e.g., Carter & Eckerman, 1975; Carter & Werner, 1978; Premack, 1978; Zentall & Hogan, 1974). For example, if the pigeons had learned the associative (if...then) chain: If sample Duck, then choose comparison Duck, then that same learning strategy should function equally well with the test set producing accurate performance (equivalent to baseline). The same logic would hold for the other two training stimuli. The pigeons were trained until they were 80% correct and >70% correct for 3 sessions with each of the 6 training displays. They were then tested on daily sessions with one trial each of the 6 test displays imbedded within the 84 training trials for 10 sessions. Following the test with the untrained displays, they were tested in a similar manner with novel stimuli (6 novel transfer trials per day). If they had learned by the associative-chaining strategy, then they should show full transfer to untrained displays and no transfer to novel-stimulus displays. None of the groups showed the predicted pattern of results for the if then, associative-chaining strategy as shown in Fig. 2. For that prediction to be correct, the middle (slant lines) bar for testing displays should have been equivalent to baseline training (unfilled bar) and the right (crossed) bar for novel-stimulus transfer should have been at chance performance. Instead, what happened was that the 0- and 1-response groups showed little or no transfer on both the novel-stimulus and the untrainedset tests. By contrast, the 20-response group showed full transfer (e.g., equivalent to baseline) on both tests. The 10-response group showed intermediate transfer on both tests, which is interesting because it shows that these different types of learning (i.e., learning strategies) are not all-or-none. Full transfer (equivalent to baseline) by the 20-response group shows that this group had fully learned the matching concept and did so with only half the number of possible training displays.

A.A. Wright, M.T. Lickteig / Learning and Motivation 41 (2010) 273 286 277 Learning the matching concept demonstrates that this group had learned the MTS task relationally. The lack of transfer on either test by the 0- and 1-response groups suggests that these groups had learned the configural pattern of each training display. For example, when they saw the first display in Fig. 1 (duck duck apple) they learned to make their choice response to left side of the display where the comparison duck was located. Because each training pattern was unique, they could not transfer their learned performance to the untrained patterns of the testing set. The relational learning and full transfer by the 20-response group suggests that the domain over which this group could apply their relational learning encompassed a large set of hundreds of different cartoons of colored objects, animals, and black and white line drawings. By contrast, the 0- and 1-response groups learned the task item-specifically by learning the configural pattern (or gestalt) of each training display, and their performance domain encompassed only those six specific training displays. What the 10-response group learned is less clear; they may have learned some displays item-specifically and others relationally or applied relational learning sporadically. Since there were no obvious similarities between cartoons on correct transfer trials and the training stimuli, any simple generalization process seems unlikely. Such a conclusion is in keeping with a similar conclusion regarding novel-stimulus transfer and relational learning in S/D tasks we have conducted with pigeons and monkeys (Wright & Katz, 2006, 2007). Item-specific learning (choose-familiar) in MTS A related MTS experiment was conducted with another 20-response group in the same apparatus, with the same stimuli and training procedures, but with a newer monitor and a higher refresh rate. 2 These subjects showed an intermediate level of novel-stimulus transfer more like the previous 10-response group, but provided an opportunity to test the prediction that The pigeon s poor record on generalized match-to-sample (on transferring to novel items) [may have been] due more to experimental artifacts than to limitations in capacity...transfer tests must not place familiar [comparisons] in competition with novel [comparisons]... (Premack, 1983b, p. 129). Premack s hypothesis that subjects would choose the familiar (training) comparison over the matching novel comparison, would be a special case of item-specific responding. In that case, absolute factors of the familiar stimuli associated with reinforcement would override the matching relationship between the novel comparison and the (matching) novel sample. An intermediate level of transfer was particularly important for a fair test of Premack s hypothesis so that relational learning would not completely overwhelm any choose-familiar tendencies if they existed. Results from the familiar vs. novel test for the three pigeons with intermediate novel-stimulus transfer are shown in Fig. 3. When both comparison stimuli were novel (novel novel test), these pigeons showed modest transfer (69%). By contrast, when the non-matching comparison on transfer trials was one of the training stimuli (novel familiar test), there was no transfer. These results provide the first evidence for the prediction made by Premack (1983b) that pigeons would show a tendency to choose a familiar stimulus over one that matches the sample stimulus. This tendency represents one of the most basic forms of item-specific responding: Choose the stimulus that has been most frequently associated with reinforcement. Conclusions from the two MTS experiments discussed would be that when pigeons do not transfer to novel stimuli in the MTS task, there is little evidence that they have learned the MTS task relationally. Indeed, when pigeons showed no transfer to novel stimuli in the first experiment (response groups 0 & 1), they did not transfer to (untrained) combinations of the same three stimuli despite having seen these individual stimuli hundreds of times in specific training combinations. The pigeons of these groups learned the MTS task item-specifically by learning the configural pattern of each training display. Pigeons in the second experiment showed a special case of item-specific learning by choosing the stimulus with a greater history of reinforcement, thereby diminishing their previously demonstrated relational learning of choosing a novel comparison stimulus that matched a novel sample. 2 Unpublished master s thesis project by Mark T. Lickteig.

278 A.A. Wright, M.T. Lickteig / Learning and Motivation 41 (2010) 273 286 Fig. 3. Baseline and transfer performance for two pigeons showing partial novel-stimulus transfer (novel novel) and a lack of novel-stimulus transfer (familiar novel) when the non-matching comparison stimulus was one of the training stimuli. Same/different Four experiments are discussed in this section. To anticipate the conclusions, the results from these experiments suggest that pigeons and monkeys relationally learn the same/different (S/D) task, even when they show no transfer to novel stimuli (i.e., no abstract-concept learning). This tendency toward relational learning in our S/D tasks contrasts with the item-specific learning by pigeons in MTS tasks of the previous section. Pigeons and monkeys learn 8-item S/D tasks relationally In these experiments, we trained groups of four experimentally naïve pigeons and monkeys in S/D tasks with 8 training pictures (travel slides). The rationale for these experiments was that other pigeons and monkeys had learned this same S/D task with 8 items but had shown no novel-stimulus transfer (Katz & Wright, 2006; Katz, Wright, & Bachevalier, 2002). If they could not transfer to novel stimuli and therefore had not learned the abstract concept, then what did they learn? Since these subjects eventually all learned the S/D task relationally as the training set was expanded, these tests were conducted to determine whether subjects would initially learn the S/D task item-specifically and later transition to relational learning or initially learn the task relationally even though they showed no novel-stimulus transfer (Wright & Katz, 2009). The training procedures were similar in all four S/D experiments, with some differences in numbers of initial training pairs. Pigeons and rhesus monkeys were all trained with the same items, the same displays, the same choice responses, the same visual-angles of the displays, the same performance criteria, and the same test items. All chambers had the same video monitors and touch screens mounted flush in stimulus panels. Also mounted on the stimulus panel was a juice tube and a pellet cup (rhesus), or a grain hopper (pigeons). The monkeys (but not pigeons) had Plexiglas templates with stimulus cutouts to direct responses. The actual display size for pigeons was smaller than for monkeys to produce similar visual-angles when the pigeons were pecking stimuli compared to the monkeys touching stimuli. Responses were shaped by successive approximations or autoshaped (1 7 sessions) to white rectangles (successively presented) in the different response area and in the lower picture position. S/D training began with presentation of the upper picture. Following a single touch/peck, the lower picture and the white rectangle were simultaneously presented along with the upper picture. If the two

A.A. Wright, M.T. Lickteig / Learning and Motivation 41 (2010) 273 286 279 pictures were the same, a touch/peck to the lower picture was correct and was rewarded. If the two pictures were different, a touch/peck to the white rectangle was correct and was rewarded. After the choice response, the display was extinguished. A correct choice resulted in a 0.5-s tone and reinforcement: Banana pellet or Tang for monkeys and mixed grain for pigeons. Amount of drink or grain varied on an individual subject basis. Incorrect choices were not reinforced. Choices were followed by a 15-s ITI. Starting on the fifth training session, incorrect choices were followed by correction procedure: A darkened 15-s timeout, the ITI, and repeat of the incorrect trial. Results presented here were based only on first-trial performance. Responses to the upper picture were increased to 10 for monkeys and 20 for pigeons over sessions 1 15. Sessions contained 96 pseudorandom trials (48 same and 48 different), and stimulus pairs were counterbalanced to the degree possible. When performance reached a criterion of 80% correct or better for three consecutive sessions, the correction procedure was removed and then the criterion was reestablished. There were 40 training pairs (8 same, 32 different) and 24 testing pairs (untrained set). The 8 training pictures appeared equally often in top and bottom positions on both same and different trials during training, so individual stimuli could not cue the correct choice response. For this reason, none of the same pairs of stimuli could be reserved for testing following acquisition because those pictures would have been associated ONLY with different trials and could have cued different responses. The 8 training pictures also appeared equally often in top and bottom positions on testing pairs. Untrained-set testing Following acquisition, transfer to the 24 testing pairs (untrained set) was tested for 20 consecutive sessions. Each test session contained 96 trials. There were 84 baseline/training trials (48 same/36 different) and 12 transfer trials. Test pairs were counterbalanced over two test sessions and were peudorandomly placed in test sessions following trial 7. Correct responses on transfer trials were reinforced to avoid subjects learning that the test pairs signaled no-reinforcement/extinction. Novel-stimulus testing Following untrained-set testing, novel-stimulus testing was conducted for six consecutive sessions. Each testing session contained 100 trials (90 baseline and 10 transfer: 5 same and 5 different transfer trials). (See Wright & Katz, 2006, Fig. 4, for novel test pairs used.) Correct performance was reinforced as on baseline trials. Stimulus-inversion testing Transfer was tested to training pairs with the stimulus pictures turned upside down (but maintaining their original top/bottom positions) for five consecutive sessions. Each test session contained 96 trials (86 baseline and 10 transfer trials: 5 same and 5 different transfer trials) with random selection without replacement. Correct responses on transfer trials were reinforced. Results Fig. 4 shows that monkeys and pigeons transferred accurately on the untrained-set test and on the stimulus-inversion test. They also showed similar accurate performances on first presentations of stimulus pairs from these two tests. The results of these tests are contrary to what would be expected if the pigeons and monkeys had learned the training-stimulus pairs, item-specifically. If monkeys and pigeons had learned these training pairs by item-specific associations, then they should have been at chance performance (50% correct) on both tests. But instead, monkeys and pigeons were 87% and 81% correct on the untrained-set test and 73% and 78% on the stimulus-inversion test, respectively. These transfer performances are substantial (by any measure) and suggest that both species were learning something other than item-specific associations. Specifically, these results suggest that pigeons and monkeys were learning this 8-item S/D task relationally. Results from the novel-stimulus test offer additional evidence about their apparent relational learning. None of the subjects transferred accurately to novel stimuli. Thus, whatever they learned did not translate to accurate novel-stimulus transfer. The lack of transfer to novel stimuli shows that these

280 A.A. Wright, M.T. Lickteig / Learning and Motivation 41 (2010) 273 286 Fig. 4. Baseline and transfer test results for rhesus monkeys and pigeons. The untrained-set test tested transfer to the stimulus pairs not used in training. The novel-stimulus test tested transfer to 30 same and 30 different trials composed using 90 novel pictures. The stimulus-inversion test tested transfer to training stimulus pairs with the pictures of the training pairs inverted. See Wright & Katz, 2009 for color reproductions of stimulus pairs used in training and pairs reserved for the untrained-set test. The dotted line is chance performance. Error bars are standard errors of the mean. subjects were unable to expand their relational learning beyond the set of the eight training stimuli a case for restricted-domain relational learning. Pigeons learn a 3-item S/D task relationally In this experiment, pigeons were trained with 3 stimuli (Elmore, Wright, Rivera, & Katz, 2009). Three training stimuli is the minimum number of stimuli in the S/D task for counterbalancing roles during training, while at the same time being able to reserve some stimulus pairs for later testing. The rationale for using the minimum number of training stimuli was to maximize chances that these pigeons would learn the S/D task item-specifically and then tests similar to the previous 8-item S/D experiment could help reveal what was controlling their item-specific behavior. Three pigeons were trained with 3 same pairs and 3 different pairs.

A.A. Wright, M.T. Lickteig / Learning and Motivation 41 (2010) 273 286 281 Fig. 5. Examples of training displays showing the three training stimuli and the white rectangle which was the response area when the two pictures were different. The lower three pictures are shape distortions of the apples, flower, and cat used in the shape test. The stimuli, displays, and stimulus pairs are shown in Fig. 5. Training procedures were similar to those of the previous 8-item S/D experiment. Pigeons pecked the upper picture 20 times, followed by the lower picture and the white rectangle with the upper picture remaining so that all 3 stimuli would be visible simultaneously. Training sessions were 100 trials (50 same, 50 different) with the same acquisition criterion of 80% correct for three sessions. Transfer testing was conducted with novel stimuli (10 trials for 6 sessions) and with different pairs not used in training (9 test trials for 8 sessions). Fig. 6 shows results from the two pigeons that learned this S/D relationally. Like the pigeons from the 8-item S/D task, these pigeons did not show significant novel-stimulus transfer. Performance with the untrained different pairs, although near chance initially, rapidly improved over the course of the 8 test sessions. Their accuracy over the last three test sessions is shown in Fig. 6. Following the untrainedpairs test, these pigeons were tested with inversions of some training-stimulus pairs. The logic for this test was that if the pigeons had learned the correct same and different responses to the individual training pairs (i.e., item-specific associations), then distorting the stimuli by inverting them should disrupt this performance. Nine stimulus-inversion test trials were substituted into each of six 96-trial sessions consisting of an original same or different training configuration (i.e., the top and bottom positions were not changed), but with both the sample and the probe turned upside down. The total number of same and different test trials was balanced over the 6 daily sessions with a total of 27 same and 27 different test trials. These two pigeons showed virtually complete transfer to inversions of the training stimuli, as shown in the third set of histograms in Fig. 6, with transfer being stable at that high level throughout transfer. They were then tested with shape distortions of the training stimuli. The logic was similar to the stimulus-inversion test; distortions should interfere with item-specific learning, but not relational learning. Four stimulus-inversion test trials were substituted into each of six 96-trial sessions consisting of an original same or different training configuration but with shapes distorted using the twirl feature in Jasc Paint Shop Pro 7.04. As shown in Fig. 5 these distortions made pronounced changes to the shapes of the stimuli. Fig. 6 (right-hand histograms) shows that these two pigeons transferred completely (i.e., equivalent to baseline) to these distortions and did so throughout testing. Color was more important to the maintenance of relational processing than shape. When the apple s color was made purple, the flower green, and the cat pink, then transfer was at chance performance.

282 A.A. Wright, M.T. Lickteig / Learning and Motivation 41 (2010) 273 286 Fig. 6. Novel-stimulus transfer, untrained-stimulus pair transfer, and stimulus-inversion transfer (unfilled bars) for 2 pigeons showing restricted-domain relational learning. Filled bars are baseline performance during transfer testing, respectively. Error bars are standard errors of the mean. Nevertheless, more subtle color changes (yellow-green apples, orange flower, gray cat) had only a small detrimental effect on transfer, showing that there was some range of color changes that would support transfer performance. Furthermore, preserving original colors irrespective of shape did not maintain the original high level of transfer when the three pictures were divided into 20 rectangles and randomly rearranged (Elmore et al., 2009). Taken together, these tests showing maintained transfer performance with untrained pairs, stimulus inversions, and shape distortions are contrary to what would be expected of item-specific learning. If pigeons had learned the task item-specifically one would have expected little or no transfer to untrained pairs, and less transfer to stimulus inversions and shape distortions than shown in Fig. 6. It may be important to emphasize that these good transfer performances were largely unexpected. We had expected item-specific learning in this S/D experiment with only 3 training stimuli and only 6 stimulus pairs to be learned. To show that the results could have come out differently, a third pigeon showed no transfer (52%) to untrained-stimulus pairs, less transfer to inversions (79%), and less transfer to shape distortions (70%), in addition to no significant novel-stimulus transfer. This pigeon s results were more in keeping with what we had expected from item-specific learning in this task. In conclusion, the excellent transfer performances by the two pigeons shown in Fig. 6 provide evidence that these pigeons appear to have learned this 3-item S/D task relationally. They apparently related the bottom item to the top item and based their choice decision on whether the two items were the same or different. Despite this apparent relational learning, they showed virtually no transfer to novel stimuli. Therefore, the conclusion is that these pigeons could perform relational learning within the confines of the domain circumscribed by these three stimuli a case of restricted-domain relational learning. Restricted-domain relational learning affects subsequent learning The previously discussed S/D experiments showed restricted-domain relational learning by pigeons and monkeys in 8-item and 3-item S/D tasks. The stimuli contained a wide range of shapes, colors, and features. Therefore, it is somewhat puzzling that the domain would be restricted to just those particular training stimuli. We have also shown that considerable additional training with these 8 items does not alter restriction of this relational-learning domain. Pigeons given repeated cycles (7)

A.A. Wright, M.T. Lickteig / Learning and Motivation 41 (2010) 273 286 283 Fig. 7. Mean baseline and transfer performance for the 32-item group on the right compared to mean baseline and transfer performance by the 8-item group on the left following learning with the 8-item and 32-item training sets. Dotted line is chance performance. Error bars are standard errors of the mean. of training with the same 8 items (matched to sessions by others with expanding training sets), followed by novel-stimulus testing, showed no transfer and hence no change in their restricted domain (Katz & Wright, 2006). Such ardent (stable) restricted-domain learning might have consequences for subsequent learning. To test this possibility, we trained a group of pigeons initially with a 32-item set and compared their transfer performance to a group trained initially with the 8-item set and then subsequently expanded to the same 32-item set (Nakamura, Wright, Katz, Bodily, & Sturz, 2009). Both groups were trained with all possible stimulus pairings (i.e., no pairs reserved for later testing). The baseline and transfer performances from these two groups of pigeons are shown in Fig. 7. Transfer to novel stimuli was substantially better (14%) for the group trained initially with 32 items, showing that prior training carried over to learning of the 32-item set by the 8-item group. Better transfer by the 32-item group shows that this group s relational learning more easily spread to novelitem pairs and this better transfer also apparently affected their acquisition of the task. Remarkably, the 32-item group learned the S/D task of 1024 item pairs (32 32) as rapidly as the 8-item group learned with only 64 item pairs. Transfer of relational learning (i.e., concept learning) during acquisition is the only way such a result could occur. Learning relationally with a larger training set (i.e., more exemplars of the S/D rule) results in a larger domain over which this rule can be applied than the domain created by expanding the training set from 8 to 32 items. An expanded domain can become restricted The S/D experiments discussed show that relational learning can occur in the absence of novelstimulus transfer and suggest restricted-domain relational learning. But as the training set increases in size, so too does the domain of relational learning. The domain broadens at an ever increasing rate as the set size is expanded and eventually encompasses the universe of such pictures, resulting in full abstract-concept learning. One issue is whether a fully learned S/D abstract concept would remain intact despite retraining on the original 8-item set. Alternatively, the original 8-item training set might

284 A.A. Wright, M.T. Lickteig / Learning and Motivation 41 (2010) 273 286 Fig. 8. Mean performance on baseline and novel-stimulus transfer trials for the six pigeons during their initial 8-item transfer test, 1024 transfer test, and 8-item retest. The dashed line is chance performance. Error bars are standard errors of the mean. be an occasion setter and (partially) reinstate previous restricted-domain relational learning. To test these possibilities, we returned 6 pigeons to their original 8-item set, trained them for a minimum of 7 daily sessions, tested them for novel-stimulus transfer and repeated this training-testing cycle four times to test stability (Katz, Wright, & Sturz, 2010). The procedures were the same as those described in the previous S/D experiment (including training with all possible stimulus combinations) with 8 items. Fig. 8 shows the results of primary interest to the present discussion. When these subjects returned to 8-item training, their transfer performance decreased relative to what it had been with a 1024- item training set (and with training sets of 256 and 512 items). Moreover, their baseline performance increased by about as much as their transfer performance decreased. Although transfer performance was clearly better than it had been when they were initially trained on the 8-item set, it was nevertheless significantly less than it had been just prior when they were trained with the large 1024-item set. The results suggest a partial reinstatement of restricted-domain relational learning. Conclusions The experiments discussed in this article show that some MTS and S/D tasks can be learned by either item-specific or relational learning. Traditionally, novel-stimulus transfer has been the test for relational learning because abstract-concept learning is based upon relational learning. But in the S/D studies discussed in this article, other types of tests suggested that relational learning has occurred despite novel-stimulus transfer failure. Monkeys and pigeons transferred accurately to untrained pairings of 8 training pictures and to pairs made up of pairs of inverted training pictures. If they had learned the same and different responses to each of the 40 training pairs, then performance should have been considerably below baseline performance on both of these tests. But performance was frequently not below baseline. The subjects learned relationships between picture pairs, but those relationships were limited to the 8-item training set as shown by a lack of novel-stimulus transfer. In support of this conclusion about relational learning, follow up experiments with these same subjects with training set-size expansions showed gradual increases in novel-stimulus transfer for each subject indicating no shift in learning strategy. If learning had originally been item specific and later shifted to relational

A.A. Wright, M.T. Lickteig / Learning and Motivation 41 (2010) 273 286 285 learning, then there should have been an abrupt change in transfer at the point of shift to relational learning. Other research discussed showed carryover effects of restricted-domain relational learning from 8-item to 32-item training sets and a partial reinstatement of a restriction on the relational-learning domain when the training set was contracted from 1024 items to 8 items. The rich multidimensional nature of the travel-slide pictures provided advantages for exploring the scope of this restricted relational learning. Nevertheless, the multidimensional nature of the travel-slide pictures does pose a problem for specifying the dimensions of the domain created by the training pictures. To some, the domain might seem clearer if geometric colored shapes had been used. But domain is in the perception of the learners monkeys and pigeons in this case. If simple colored circles had been used (or colored geometrical shapes), then the tests conducted here (novel-stimulus transfer, abstract-concept learning, and dimensional analyses) would have been problematical or impossible. More importantly, the learning results from such experiments would likely have no bearing on learning with the multidimensional travel-slide pictures of these experiments. Control by two dimensions was conducted in one of the studies with three training pictures. Pigeons showed better transfer performance over a range of object-shape changes than object-color changes. Restricted-domain relational learning is not limited to animals. Even adult humans apparently show it in restricted-domain relational learning. For example, humans learning a relationship in one domain (e.g., 3 different-sized cups to add or subtract water to obtain a target volume) have trouble seeing how a similar relationship would apply in another domain (e.g., 3 scale weights to add or subtract coal). Comparison groups with the same amount of training but spread across several domains (e.g., length, area, and volume) transferred accurately to the scale-weight domain (Chen & Mo, 2004). A related finding may be children s relational shift that has been found to be domain specific but becomes more abstract as they age and learn more relationships (Gentner, 1988, 2003; Gentner & Rattermann, 1991; Rattermann & Gentner, 1998). This domain-specific relational shift seems to share aspects with restricted-domain relational learning shown in this article for animals. How the domain for animal relational learning is restricted and how it changes may be unclear, but a similar issue is encountered in human learning. For example, there is considerable agreement that...the relational shift reflects a qualitative change in children s mental representations...can affect his or her reasoning...[but] is nonetheless incomplete in that it does not provide an account of how this change takes place (Doumas, Hummel, & Sandhofer, 2008, p. 21). In any case, with monkeys and pigeons in our S/D task the domain expands as the training set increases (i.e., novel-stimulus transfer increases). The domain is unlikely to expand in small steps along stimulus dimensions in the manner of stimulus generalization. Novel-stimulus transfer and the restricted-domain relational learning on which it is based, appear to expand and encompass more diverse stimuli than would be realizable by any simple generalization process (Wright & Katz, 2007). Indeed, we believe that this is the defining characteristic that makes concept learning or rule learning unique the range of accurate application grows more rapidly than anything that might be expected from simple generalization. Acknowledgments This research and preparation of this manuscript were supported by NIMH grants R01 MH-35202 and R01 MH 072616 to AAW. The author thanks Caitlin Elmore, Jackie Rivera, Kenny Leising and Jeffrey Katz for help with figures and the manuscript. References Carter, D. E., & Eckerman, D. A. (1975). Symbolic matching by pigeons: Rate of learning complex discriminations predicted from sample discriminations. Science, 187, 662 664. Carter, D. E., & Werner, T. J. (1978). Complex learning and information processing by pigeons: A critical analysis. Journal of the Experimental Analysis of Behavior, 29, 565 601. Chen, Z., & Mo, L. (2004). Schema induction in problem solving: A multidimensional analysis. Journal of Experimental Psychology: Learning, Memory, and Cognition, 30, 583 600.

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