Transfer of Serial Reversal Learning in the Pigeon

The Quarterly Journal of Experimental Psychology (1986) 38B, 81-95 Transfer of Serial Reversal Learning in the Pigeon P. J. Durlach and N. J. Mackintosh Department of Experimental Psychology, University of Cambridge, Cambridge, U.K. Pigeons were trained on a series of reversals of a simultaneous visual discrimination and were then shifted to a second series of reversals with different visual discriminanda. Pigeons that were given discrimination reversals with one pair of colours (Group Colour) and then shifted to a second pair of colours made fewer errors with the second pair than the first. In contrast, pigeons that were initially given reversals with a pair of orientations (Group Orientation) and then shifted to colours made as many errors during colour reversals as Group Colour had during initial colour training. When birds in Group Colour were subsequently shifted to orientation discrimination reversals, they performed no better than Group Orientation had during initial orientation training. The present results suggest that positive transfer from one series of discrimination reversals to a second, independent series may be constrained by the nature of the stimulus shift. INTRODUCTION It is well established that, in common with most other vertebrates that have been tested, pigeons trained on a series of reversals of a discrimination problem show a significant improvement over the series, learning later reversals with fewer errors than earlier reversals (Macphail, 1982). The cause of this improvement, however, has been a matter of contention. Perhaps the simplest explanation was that proposed by Gonzalez, Behrend, and Bitterman (1967). They suggested that the reduction in errors occurs because, as proactive interference develops over the course Requests for reprints should be sent to P. J. Durlach, Department of Experimental Psychology, University of Cambridge, Downing Street, Cambridge CB2 3EB, U.K. This work was supported by a U.K. S.E.R.C. grant to N. J. Mackintosh and A. Dickinson, and by the North Atlantic Treaty Organization under a grant awarded in 1983 to P. J. Durlach. 0 1986 The Experimental Psychology Society

82 P. J. Durlach and N. J. Mackintosh of repeated reversals, animals fail to remember which alternative was correct during the previous session; therefore, during later reversals, subjects score at chance, rather than below chance, at the outset of each new reversal. There is good evidence that such proactive interference does occur during the course of a series of reversals (Miller, Hansen, and Thomas, 1972; Staddon and Frank, 1974); however, whether increases in proactive interference alone can account for the improvement in performance has been questioned (Mackintosh, 1969; Williams, 1976). Mackintosh (1969) has suggested that learning to maintain attention towards the relevant cues is another important factor in serial reversal improvement. Unlike proactive interference, this attentional factor can explain why subjects sometimes show an increase in rate of withinproblem learning (e.g., Mackintosh and Little, 1969a). Some evidence pointing to an attentional factor is the finding that pigeons made fewer errors on a series of colour discrimination reversals if trained consistently with colours than if alternated between colour and spatial discrimination reversals (Mackintosh and Little, 1969a). A third factor that has been suggested as important in producing efficient reversal performance is the development of a win-stay/loseshift strategy-i.e., learning to use the outcome of one trial as a cue to predict the outcome of the next (Hull, 1952; Macphail, 1982; Williams, 1976). In his review of serial reversal learning in pigeons, Macphail (1982) adduces three lines of evidence in support of this factor: one-trial reversal learning, more rapid reversal learning with a short than with a long intertrial interval, and positive transfer from serial reversal training with one set of stimuli to a new set of discriminanda. The finding of onetrial reversal learning may be prima facie evidence for a win-stay/ lose-shift strategy; however, the one study in which pigeons were able to attain this level of performance (Gossette and Hood, 1968) used a procedure in which an overt mediating response (standing in front of the correct choice throughout the session) was available. The finding that pigeons learn reversals more rapidly with a short than with a long intertrial interval (Williams, 1971, 1976) is consistent with the claim that efficient performance depends on the short-term memory of the outcome of one trial to predict the correct choice on the next; however, it is equally consistent with the suggestion that proactive interference, not only between sessions but also within sessions, affects performance. Proactive interference from earlier reversals may improve performance by ensuring that subjects respond around chance, rather than below chance, at the start of a new reversal; but it may also interfere with performance if subjects cannot remember which alternative is currently correct from one trial to the next. The longer the intertrial interval, the greater this interference would presumably be.

Serial Reversal 83 Finally, the finding of positive transfer from serial reversal training with one discrimination problem to a new, independent problem, would seem quite difficult to explain without appealing to some underlying response strategy; however, for such a conclusion, it would have to be clear that the positive transfer did not depend on nonspecific factors such as habituation to the apparatus or neutralization of position preferences. This issue of transfer, on which unfortunately there is relatively little evidence, is the main focus of the present experiment. We have traced three studies in which transfer from one set of reversals to another was assessed. Positive transfer would be evidenced by fewer errors during transfer, compared with a comparable period during original training. In two of the studies, however, the main focus was on whether the efficient performance attained after extended training on the first problem was disrupted by the transfer to a new problem. There is little question that it is. Stearns and Bitterman (1965), having trained pigeons on a series of reversals of a simultaneous blue-green discrimination, shifted the birds to a red-yellow discrimination, followed by 10 reversals of this new problem. Not only was there a substantial increase in the number of errors on the transfer problem (compared with the terminal performance on the original problem), but performance in transfer appears to have been rather less efficient than it was over the initial 10 reversals of the blue-green discrimination. Since neither the stimuli nor the exact training procedures were the same for training and transfer, these results cannot be taken as serious evidence of negative transfer; but neither do they encourage the belief that there might be positive transfer. Staddon and Frank (1974) trained pigeons on reversals between a triangle on a red background and a cross on a green background, using a free-operant, successive discrimination (multiple variable interval, extinction schedule) until birds were making more than 90% of their responses to the positive keylight on the first day of each new reversal. When shifted to a new pair of stimuli, blue and yellow keylights, the pigeons showed a marked increase in errors and did not attain their former level of accuracy for a further 20 reversals. The one study explicitly designed to compare performance during transfer to performance during original training with the very same discriminanda was carried out by Miller, Hansen and Thomas (1972). One group of pigeons was trained on a series of reversals with colours and was then shifted to a discrimination between line orientations, and one reversal of this discrimination. A second group of pigeons was given only the discrimination with the orientations and one reversal. Transfer was assessed by comparing, between groups, performance on the orientation discrimination and its reversal. The groups did not differ in the

84 P. J. Durlach and N. J. Mackintosh speed of learning the initial orientation problem, but fewer errors on its reversal were made by the colour-reversal birds than by the birds that had received no prior training. This seems like good evidence of positive transfer; however, two points are worth nothing about this study. The procedure, like that of Staddon and Frank (1974), was a free-operant successive discrimination. Performance on such discriminations may be partly under the control of reinforcement in a way that cannot occur in discrete-trial simultaneous discriminations. In the multiple schedule procedure, recent reinforcement is a valid predictor of whether continuing to peck will be reinforced. If the food reinforcement served as a discriminative stimulus for responding, positive transfer in the pretrained group might be due simply to the maintained presence of this discriminative stimulus in transfer. Secondly, if a bird continues to respond to its former correct stimulus and fails to respond to the new correct stimulus during a reversal with this free-operant procedure, it will essentially be on an extinction schedule and may cease responding. This, in fact, occurred during the first colour reversal in Miller et al. s experiment: 5 of the 24 birds required remedial autoshaping in order to make them resume responding. Although it is implied that no bird required remedial training on the critical test reversal, it seems possible that birds with no prior reversal experience would have been more likely to cease responding than those that had had extensive experience with intermittent reinforcement. This would imply that the poorer reversal performance of the control group might have been due to their failure to make as much contact with the scheduled contingencies of reinforcement; this cannot be determined from the data presented, as Miller et al. provided only a percentage measure and not absolute levels of responding. There is surprisingly little evidence, therefore, as to whether pigeons will show positive transfer from one series of reversals to another. The present experiment was intended to address this issue. One group of birds was given reversals of a simultaneous discrimination with one pair of colours and was then shifted to reversals with a different pair of colours (with the identity of the colours counterbalanced across birds). Fewer errors with the second pair than the first, over a comparable period, would provide some indication of positive transfer. This is not, of course, a particularly conservative comparison, for one can imagine a variety of potential sources of positive transfer that might not be the product of reversal learning per se; however, only if reversal-trained birds performed more accurately on their second than on their first series of reversals would there be any point in further experimental analysis designed to elucidate whether such transfer reflected relatively uninteresting sources such as habituation to the apparatus, or experience

Serial Reversal 85 with intermittent reinforcement, or rather more interesting sources such as the maintenance of attention or the development of a win-stay/ lose-shift strategy. We did, however, make provision for one further comparison. A second group of birds was first trained on a lineorientation discrimination and a series of its reversals before being switched to reversals with the colours. Comparison of the performance of the two groups during transfer allowed us to assess the possibility that pigeons might show more intradimensional than extradimensional transfer of reversal learning (colour to colour versus orientation to colour). As a further test of this, the pigeons trained on the two series of colours were finally transferred to the line-orientation discrimination and its reversals; their performance during this stage was compared to that of the initial performance of the orientation-trained birds. Method Subjects The subjects were 16 wild pigeons. The birds had previously participated in several autoshaping experiments carried out in an apparatus different from the one used in the present experiment. Throughout the experiment they were housed individually, with free access to water and grit, but they were maintained at 80% of their free-freeding weight. Apparatus The apparatus consisted of four 2-key pigeon operant chambers of internal dimensions 39 cm x 26 cm x 36 cm. The ceiling and two walls of each chamber were made of wood; the wall that contained the door was made of smoked Plexiglas, permitting one-way viewing under the appropriate lighting conditions. The fourth wall was a metal panel that could easily be removed from the box. The panels had a 5 cm x 6 cm food magazine located in the centre and 4 cm above the wire-mesh floor. Operation of the magazine provided access to mixed grain, illuminated a 8-W bulb located within the magazine and extinguished the normally-on 8-W houselight, which was located 15 cm above the magazine aperture. The panel also had two response keys, each 2.5 cm in diameter, located 28 cm from the floor and 6 cm from the side walls. Projectors located behind each key enabled the presentation of two keylight stimuli during each session. Three pairs of keylight stimuli were used throughout the experiment: Red/ Yellow, Blue/Green, and Vertical/Horizontal (grids of black lines on a white background). These particular colour pairs were chosen because previous experiments have suggested that, although there is substantial stimulus generalization between red and yellow and between blue and green, there is relatively little across the elements of these pairs (Mackintosh and Little, 1969b; Zentall, Edwards, Moore and Hogan, 1981).

86 P. I. Durlach and N. 1. Mackintosh Procedure Pretraining. Because of their previous history, birds required little magazine training. All birds were given varying amounts of pretraining with the four colour stimuli, to ensure that they would respond at the start of the main training phase. This involved either response-independent autoshaping in which a 5-sec colour presentation was followed immediately by 5-sec access to food from the magazine, or response-dependent autoshaping, in which a response terminated the keylight stimulus and was followed immediately by 5-sec access to food; if no response occurred, the stimulus terminated after 5 sec and was followed by 5-sec access to food. Each bird received equal experience with all four colours and with responding on both keys; however, birds required different amounts of training from one another. Birds were subsequently assigned to one of two groups, distinguished by whether colours or orientations would be used during serial reversal training. Mean percent trials with a response over the last 24 trials of pretraining for Group Colour and Group Orientation were 79.6 and 87.5, respectively. Birds in Group Orientation were given three additional sessions of response-dependent autoshaping with Horizontal and Vertical; each session contained 12 presentations of each orientation on each response key, at an average rate of one per minute. Serial reversal training. During serial reversal training, subjects had to choose between the elements of a stimulus pair (Red/Yellow, Blue/Green, or Horizontal/Vertical). At the beginning of each trial, the two response keys were simultaneously illuminated, each with one element of the pair. A single peck at the correct stimulus turned off the keylights and operated the magazine for 4 sec. An incorrect choice turned off the keylights and resulted in a 4-sec blackout (of the houselight). This was followed by reillumination of the houselight and the correct choice, a peck at which turned off the stimulus and operated the magazine for 4 sec. If a bird failed to make a choice within one minute, it was counted as an error. If, within 10 sec, a bird failed to respond to the singly lit keylight that followed a blackout, the keylight was turned off and the magazine was operated for 4 sec. Thus every trial ended in a pairing of the correct stimulus with food. Location of the correct choice on the right or left key was determined randomly, subject to the constraint that it occur equally often on each side within 50 trials. Trials occurred, on average, every 15 sec, with a range of 5 to 20 sec. The houselight was lit throughout, except during operation of the magazine and during blackouts. The 8 birds in Group Orientation were trained with Horizontal and Vertical; half the birds began with Horizontal correct and half with Vertical correct. Four of the eight birds in Group Colour were trained with the Red/Yellow pair and four with the Blue/green pair. Within each of these subgroups, half of the birds began with one of the colours correct, and the rest began with the alternative colour correct. Birds received training with the same discrimination until they reached a criterion of 9 out of 10 consecutive trials correct. Each session terminated when the bird reached this criterion or after 50 trials. If criterion had not been met, the same contingencies were in force the next day; if criterion had been met, the reinforcement contingencies were reversed until criterion was once again met. Thus, reversal of contingencies was instituted between sessions only. Birds continued to receive this reversal training until they met the 9-out-of- 10

Serial Reversal 87 criterion on three consecutive days, or until they had completed 32 reversals following the first discrimination problem. Transfer. After achieving the 9-out-of-10 criterion three days in a row, or after completing 32 reversals, training continued as before, but with a new pair of stimuli. Birds in Group Colour were switched to a new colour pair (each bird received one of the eight possible correct colour to correct colour transitions). Half of the birds in Group Orientation were switched to Red/Yellow and half switched to Blue/Green. Within each subgroup, half received one colour correct, whereas the rest received the other colour correct on the first problem. Birds were trained on this transfer problem for at least eight reversals following the first discrimination. After completing transfer with the new colour pair (9-out-of-10 on three consecutive days) seven of the eight birds in Group Colour were given similar training with Horizontal and Vertical. Three of the birds were originally given Horizontal correct, and four were given Vertical correct. Reversal training with the orientations was carried out until eight reversals following the first discrimination problem had been completed. Results and Discussion Number of errors per reversal was taken as the primary dependent variable; other measures such as trials per reversal followed essentially the same pattern. Training Groups Colour and Orientation did not differ significantly in the number of errors made during the first discrimination problem. Mean number of errors on the first problem was 13.2 and 22.4 for these groups, respectively [F( 1, 14) = 1.35; p > 0.251; the groups ranged in number of errors from 1 to 49 for Group Colour, and 3 to 42 for Group Orientation. During subsequent reversals, Group Colour made significantly fewer errors than Group Orientation; mean errors per reversal were 38.6 for Group Colour and 54.7 for Group Orientation [F(1, 14)=5.52; p<o.o5]. The attempt to bring groups to the same level of reversal performance by running them to a criterion was unfortunately not achieved, as only one of the eight birds in Group Orientation actually met criterion before reversal 32, as compared with six out of eight birds in Group Colour. Vincent curves showing the distribution of errors made during training are plotted in Figure 1. These data were calculated by dividing each bird s reversals into sixths, interpolating as necessary for fractions, and then dividing errors in each sixth by the bird s total errors (Hilgard, 1938). This conversion to proportions of total errors allowed examinations of the distribution of errors in the two groups, independent of the

88 P. J. Durlach and N. J. Mackintosh Jvl $2 $15 W L L > O W a z &lo a 0 a 0 a 0 COLOURS 1 2 3 4 5 6 SIXTHS OF REVERSALS Figure 1. Mean proportion of total errors per sixth of reversals in original training for Group Colour (open circles) and Group Orientation (filled circles). overall difference in number of errors. Analysis of these data using the method of orthogonal polynomials indicated that both groups improved over reversals, but that the pattern of improvement was different across groups [there was a significant quadratic trend in the group by sixth interaction, F(1, 14)= 12.24; p<o.ol]. In the analysis of trend in the simple effect of sixths, a linear trend only was evident for Group Colour [F( 1, 14) = 8.92; p < 0.011, whereas significant quadratic and cubic trends were found for Group Orientation [Fs( 1, 14) = 9.15 and 5.52; ps < 0.01 and 0.05, respectively]. Thus, although the performance of Group Orientation was inferior to that of Group Colour, both groups did show a reduction in errors over reversals. If, late in training, birds solved reversals more efficiently than they had solved the original discrimination problem, it would provide support for the claim that birds had developed a response or attentional strategy. There was no evidence that this occurred, however. Birds in Group Colour did not perform reliably better on their last reversal of training than on their first discrimination of training [17.1 vs. 13.2 mean errors, respectively; F( 1, 7) = 1.62; p > 0.241. Birds in Group Orientation

Serial Reversal 89 performed marginally worse on their linal reversal with orientations compared with their first orientation problem [41.5 vs. 22.4 mean errors, respectively; F( 1 7) = 3.97; 0.05 <p < 0.11. Superior performance on later reversals than on the initial discrimination may be possible, but it did not occur in the present experiment. Transfer Groups Colour and Orientation did not differ significantly in mean number of errors made on the first discrimination problem with the transfer colour stimuli. Mean number of errors was 15.9 and 17.5 for Groups Colour and Orientation, respectively. This level of performance was comparable to that exhibited by Group Colour on its first discrimination problem in original training (mean of 13.2 errors). Fewer errors on the first discrimination of transfer, as compared with the first discrimination of training, would be expected if performance at the termination of training was superior to performance on the initial discrimination problem of training. As discussed previously, this did not occur. Figure 2 displays the data of principal interest: mean number of errors made during the first 8 reversals of transfer to the new pair of colours for both groups (dashed lines), along with the analogous data from Group Colour during original training (solid lines). The first comparison considered was that of original training versus transfer for Group Colour. A within-subject analysis of variance with factors of test (training vs. transfer) and reversal (1-8) indicated that birds in Group Colour did perform significantly better during the beginning of transfer than the beginning of training [main effect of test F(1, 7)=7.56; p CO.051. There was no significant effect of reversal [F(7, 49)= 1.27; p > 0.251, nor a test by reversal interaction (F< 1). Although leaving ambiguous the source, these data suggest that there is, indeed, positive transfer from serial reversal training with one pair of colours to serial reversal training with a different pair of colours. Comparison of the transfer results of Group Colour and Group Orientation suggests that this positive transfer may have been restricted to the intradimensional shift. A mixed analysis of variance (Group x Reversal) indicated that Group Colour made fewer errors during these transfer reversals than did Group Orientation [F( 1 14) = 5.0; p < 0.051; there was no significant effect of reversals nor a group x reversal interaction (Fs< 1). Thus, birds shifted to new colour stimuli showed better serial reversal performance if they had had prior serial reversal training with other colours than with orientations. These results suggest that in the present task, the degree of transfer across sets of stimuli is greater if

90 P. J. Durlach and N. J. Mackintosh 10G 3----0 GROUP COLOUR: COLOUR TRAINING >--o GROUP COLOUR: COLOUR TRANSFER In 8 E (r W 80 60 W [L m f Z z r 40 *--a,. GROUP ORIENTATION: COLOUR TRANSFER V / R / / 7 \ I 20 1 2 3 L 5 6 1 0 REVERSALS Figure 2. Mean number of errors committed on each colour discrimination reversal over the first eight reversals of training (open circles, solid lines) and transfer (open circles, dashed lines) for Group Colour, and of transfer for Group Orientation (filled circles, dashed lines). the shift is intradimensional than extradimensional. In order to assess whether there was any transfer from reversal training with orientations to reversal training with colours, the performance of Group Orientation in transfer was compared with the performance of Group Colour in original training. As can be seen in Figure 2, if anything, Group Orientation performed somewhat worse during transfer with colours than Group Colour had during original training with colours, suggesting no positive effect; there was no significant effect of group [F(1, 14) = 2.07; p > 0.151, nor of reversals, nor a group by reversal interaction (Fs < 1). In summary, then, birds in Group Colour showed evidence of significant positive transfer from training to transfer, but birds in Group Orientation failed to do so. The fact that Groups Colour and Orientation performed differently in transfer has implications for the possible

Serial Reversal 91 sources of the positive transfer exhibited by birds in Group Colour. The positive transfer of Group Colour cannot be attributed merely to factors such as habituation to the apparatus or to experience with intermittent reinforcement, because these factors were equivalent (or possibly even greater) for Group Orientation. The results presented above suggest that the difference between Groups Colour and Orientation in transfer was due to greater positive transfer with the intradimensional than the extradimensional shift. Caution must be used in drawing this conclusion, however. Birds in Group Orientation failed to reach the same level of performance as birds in Group Colour by the end of the original training phase; therefore, their inferior performance in transfer may have occurred, not because of the nature of the shift, but rather because they had not become very competent at serial reversal. To check further on the possibility that significant positive transfer fails to occur across extradimensional shifts in this paradigm, 7 of the 8 birds in Group Colour were trained to criterion on their colour transfer problem and then shifted to serial reversal with the orientation stimuli (one bird, which failed to finish transfer before the other 7 had finished testing with orientations, was not run; as this bird was the poorest performer in Group Colour, its omission would be expected to improve the performance of Group Colour on transfer to orientations). If the difference between Groups Colour and Orientation on transfer to colours was due to their different levels of performance at the end of original training, then Group Colour (now having reached criterion and having performed well on two colour problems) would be expected to show positive transfer to orientations. In contrast, if the difference between Group Colour and Orientation on transfer to colours was due to the nature of the shift (intradimensional versus extradimensional), then Group Colour would not be expected to show positive transfer when shifted finally to orientations. In order to assess this transfer, the performance of the birds in Group Colour on their first 8 reversals with orientations was compared with the performance of birds in Group Orientation on their first 8 reversals in original training. As could be expected on the basis of the transfer reversal 1-8 data, Group Colour made fewer errors in reaching criterion on their second than their first colour problem [26.3 versus 40.1 mean errors per reversal, respectively; F( 1, 6) = 6.72; p < 0.051. Group Colour made more errors on the first discrimination problem with orientations than Group Orientation had. Mean errors were 40.9 for Group Colour and 22.4 for Group Orientation; this difference was marginally reliable [F( 1, 13) = 3.86; 0.05 < p < 0.11, and might have been due to the fact that Group Colour had not received autoshaping with the orientations

92 P. J. Durlach and N. J. Mackintosh during pretraining. Figure 3 shows the mean number of errors made during the first 8 reversals of transfer to orientations for Group Colour (open circles) along with mean number of errors made during the first 8 reversals of original training for Group Orientation (closed circles). Analysis of these data failed to reveal a significant difference between groups. Although Group Orientation made somewhat more errors than Group Colour (means of 64.6 and 57.7 errors per reversal, respectively), this difference was far from reliable (F< 1). There was a significant main effect of reversal [F(7, 91) = 3.22; p < 0.011 but no Group x Reversal interaction [F(7, 91) = 1.13; p > 0.251. That the performance of Group Orientation in original training and Group Colour in transfer to orientations failed to differ significantly is compatible with the conclusion 1 3---OGROUP COLOUR: ORIENTATION TRANSFER -GROUP ORIENTATION: ORIENTATlON TRAINING 1 2 3 4 5 6 7 0 REVERSALS Figure 3. Mean number of errors committed on each orientation discrimination reversal over the first eight reversals of training for Group Orientation (filled circles) and of transfer for Group Colour (open circles). Vertical bars show the standard error of the mean for each group for each reversal.

Serial Reversal 93 that transfer in this task is detectable across intradimensional but not extradimensional shifts. The present results suggest that positive transfer from one series of discrimination reversals to a second, independent series of reversals does occur in pigeons; however, that transfer may be restricted to cases in which the shift in discriminanda from training to transfer is an intradimensional one. Not only did birds in Group Orientation perform more poorly in transfer than did birds in Group Colour, they did not perform any better in transfer than birds in Group Colour had in original training. Moreover, birds in Group Colour failed to provide any evidence of positive transfer when they were finally shifted to reversals of the orientation discrimination. There are two problems in firmly concluding that intradimensional but not extradimensional transfer occurs in the present task. First, the conclusion rests on accepting null results. Second, it overlooks the problem that reversal learning with orientations was much more difficult than with colours; the failure of Group Colour to show any transfer to orientations can only be taken as meaningful if reversal improvement with these stimuli is, in fact, obtainable. Although the majority of birds in Group Orientation failed to reach criterion in acquision, birds did show an improvement in performance. It is clear that the present results are compatible with several previous findings from other paradigms of superior intradimensional, as compared with extradimensional transfer from one discrimination problem to another, e.g., Isaacs and Duncan (1962) with humans, Shepp and Schrier (1969) with monkeys, Shepp and Eimas (1964) with rats, and Mackintosh and Little (1969b, 1970) with pigeons. The two reservations above aside, the conclusion that intradimensional, but little extradimensional transfer of serial reversal learning occurs, appears not an unreasonable one. What might be the source of this intradimensional transfer exhibited by Group Colour? General factors such as habituation to the apparatus or experience with partial reinforcement cannot alone account for the transfer; birds in Group Orientation also experienced these factors but failed to exhibit transfer. Proactive interference, an important factor influencing improvement over a series of reversals, could underlie positive transfer if the training stimuli and transfer stimuli were highly confusable. Although it seems unlikely that Red/Yellow and Blue/Green are highly confusable pairs, it is likely that they are more confusable than colours and orientations; it is therefore possible that the differential transfer between intra- and extradimensional shifts was due to differences in proactive interference from prior training. If this were the case, the performance difference between intra- and extradimensionally

94 P. J. Durlach and N. J. Mackintosh shifted birds should be observable right from the start of each new reversal. Analysis of performance on the first 10 trials of each reversal (minimum number possible per reversal) during transfer to colours (reversals 1-8) failed to reveal any differences in errors committed by Group Orientation vs. Group Colour. The average number of errors made over the first 10 trials of each reversal during transfer was 5.6 and 5.8 for Groups Colour and Orientation, respectively. There was no effect of reversal [F(7,98) = 1.68; p > 0.11, nor a group x reversal interaction (F< 1). Other measures, such as numbers of errors before the first correct choice, also failed to reveal group differences. It appears, therefore, that the difference in transfer performance between groups was not due to differences in proactive interference from prior training. Attentional factors, which have also been identified as important in within-series improvement, may have played an important role in producing the observed pattern of transfer. Serial reversal training may have strengthened attention to the relevant dimension: colours for Group Colour, and orientations for Group Orientation. This strengthening of attention would have benefited Group Colour in transfer to new colours, but not Group Orientation in transfer to colours, or Group Colour in transfer to orientations. Unlike a strategy-learning account of reversal learning, such an attentional account suggests the possibility that the positive transfer of Group Colour was not necessarily a consequence of serial reversal training, per se; any procedure that resulted in strengthened attention to colours might have produced positive transfer to subsequent reversal performance with colours. The present experiment does not address whether the transfer was a result specifically of reversal training. Finally, it is possible that the development of a win-stay/lose-shift strategy was a source of serial reversal improvement and transfer; however, the failure to observe transfer by Group Colour when finally shifted to orientations would imply that if such a strategy existed, it was dimension-specific; that is, its operation was not independent of the stimuli present at the time it was acquired. It is worth noting that, in contrast, other animals, both primates (Warren, 1966) and nonprimates, including other species of birds (Kamil, Jones, Pietrewicz and Mauldin, 1977) have shown precisely such stimulus-independent transfer. References Gonzalez, R. C., Brehend, E. R. and Bitterman, M. E. (1967). Reversal learning in bird and fish. Science, 158, 519-521. Gcssette, R. L. and Hood, P. (1968). Successive discrimination reversal as a

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