Physiology & Behavior, Voi. 16, pp. 5 2 9 - 5 3 6 . Pergamon Press and Brain Research Publ., 1976. Printed in the U.S.A.
Effects of Hyperstriatal Lesions on Within-Day Serial Reversal Performance in Pigeons EUAN M. MACPHAIL
Laboratory of Experimental Psychology, University of Sussex, Falmer, Brighton BN1 9QG, England (Received 7 April 1975) MACPHAIL, E. M. Effects of hyperstriatal lesions on within-day serial reversal performance in pigeons. PHYSIOL. BEHAV. 16(5) 529-536, 1976. - In Experiment 1, 12 pigeons performed each day for 17 days 3 serial reversals of a simultaneous color discrimination; 6 birds were then given hyperstriatal lesions, and 6 birds were sham operated. Analysis of the effects of lesions on postoperative reversal performance supported 2 hypotheses: first, that the hyperstriatum contains a mechanism involved in the generation of inhibition, and second, that minimal inhibition is generated (in intact birds) in the first of each day's reversals. Experiment 2 investigated the effects of hyperstriatal lesions on serial position reversal; results again supported the notion that the hyperstriatum is involved in response inhibition. Contrasts between the effects obtained in the 2 experiments suggested that inhibition plays a more central role in position, as opposed to color, reversals. Hyperstriatal lesions
Reversallearning
Inhibition
THIS paper reports 2 experiments which investigated the effects of hyperstriatal lesions in pigeons on within-day reversals of a simultaneous discrimination. The particular l e s i o n and behavioral situation were chosen because theoretical accounts for each of them have introduced the notion of inhibition in such a way that certain lesion effects may be predicted. Should these effects be obtained, then further evidence will have been provided, not only for the interpretation of the lesion, but also for the theoretical analysis of reversal learning. This study, in other words, hopes to use the outcome of a physiological investigation to contribute to behavioral theory. The avian hyperstriatal complex consists of three major components, the hyperstriatum ventrale, the hyperstriatum dorsale, and the hyperstriatum accessorium, the latter two structures forming the so-called Wulst [1]. Lesions within this complex disrupt both acquisition and retention of difficult successive discriminations [8, 13, 17], and impair reversals of simultaneous discriminations [ 8,15], but generally have no effect on the acquisition or retention of simultaneous discriminations ([5, 8, 151; Macphail [10] reports an exception to this generalisation). Macphail [8] noted the parallel between these effects and those obtained by certain limbic lesions in mammals and proposed that there was in the avian hyperstriatum a mechanism for response inhibition; this hypothesis is of particular relevance to this study.
In a recent review, Macphail [11] described 3 further hypotheses concerning avian hyperstriatal function. One of these, that hyperstriatal birds suffer from a perceptual deficit, [ 13] was rejected on the grounds that it could not accommodate the finding of selective impairment of reversal, as opposed to acquisition, of simultaneous discriminations. A second hypothesis is that hyperstriatal birds are less likely than normals to shift their mode of response as a consequence of nonreinforcement, a notion derived from Olton's [ 12] account of hippocampal function in the rat; as evidence for this hypothesis, it has been shown that hyperstriatal pigeons are more likely than normals to halt responding altogether in the course of reversal learning [101, a fact that clearly poses problems for the response suppression account. The third hypothesis is that hyperstriatal birds suffer an impairment in selective attention that causes them great difficulty in paying attention to the relevant stimulus in the initial stages of reversals [5,14]; consonant with this view is Stettner's [ 14] report that hyperstriatal quail performing reversals show little more perseverative responding to the former positive stimulus than do normals, but greatly increased position habits. The experiments to be reported here bear on all the hypotheses discussed, and so may help rule out one or more of them. The relevant behavioral situation was described by Macphail [7], in a study of pigeons that performed each day for 17 days 3 reversals of a simultaneous (red-green)
1This research was supported by a grant from the U. K. Medical Research Council; computing facilities were made available by a grant from the U. K. Science Research Council. I am grateful to June Atherton for assistance in preparation of the figures, to Colin Atherton for photographic assistance, and to N. J. Mackintosh and G. Hall for their critical comments on an earlier draft of this manuscript. 529
530
MACPHAIL
discrimination. Birds learned the second of each day's reversal series (R2) more efficiently, in terms of trials to criterion, than they did either the first (R1) or the third (R3). This was unexpected, since, due to overnight forgetting, there was no preference for either stimulus at the start of R l s whereas there was, of course, a preference (for the wrong stimulus) at the start of both R2 and R3s. Analysis of the data showed that the superiority of R2 performance was due primarily to the fact that having made 4 or more correct responses in succession, birds were more likely to complete the criterion run (10 correct responses) without further errors in R2s than they were in Rls. It was argued that in R l s (where few initial errors occurred) no inhibition was generated to the negative stimulus ( S - ) ; thus, after a run of correct choices, birds were faced with a choice between a stimulus (S+) that was gradually accumulating approach strength, and another stimulus ( S - ) that was relatively neutral. In R2s, however, the initial errors should generate inhibition to S - , so that, following a run of correct trials, the stimulus pitted against S+ would not be neutral, but inhibitory. The analysis of R3 performance is complicated, since, after the initial errors, animals will, according to the hypothesis, have generated inhibition to both stimuli; as the superiority of R2s over R3s seemed to be due primarily to fewer position habits, it was proposed that an irrelevant dimension had gained control as a consequence of both colors having become inhibitory. The interpretation advanced so far for this complex situation is, of course, tentative; it is of interest to pursue it, however, as an understanding of the role of inhibition in simultaneous discriminations and their reversals is important, particularly for the behavioral analysis of those lesions that selectively disrupt reversal as opposed to acquisition performance. EXPERIMENT 1 Experiment 1 investigates the effects of hyperstriatal lesions on performance in the red-green serial reversal problem; birds were given extensive pretraining on the reversals prior to operation, since pilot experiments showed that it was very difficult to train naive birds with lesions so that they could complete 3 reversals within a reasonable number of trials in one day. Three aspects of postoperative performance are of interest. First, the effects of lesions on perseverative responding to S - : both the response shift and response inhibition hypotheses predict that these scores should be increased by the lesions, whereas the attention shift model predicts minimal lesion effects. Second, effects on incomplete run scores (sequences of S+ choices preceding the criterion run): the response inhibition hypothesis predicts a much greater effect of lesions on R2 incomplete run scores as compared to R1 scores, as the lower R2 scores in normals are supposedly due to generation of inhibition in R2s; neither of the other 2 hypotheses appears to make any clear prediction concerning incomplete run scores. Third, effects of lesions on position habits: the selective attention hypothesis obviously predicts an increase in these scores following lesions. However, although an absence of any effect on position habits would weaken the selective attention account, an increase would not provide convincing support for the hypothesis if it was accompanied by increases in perseverative errors; this is because if both perseverative errors and position habits increase, it may well
be argued that the increase in the latter is a consequence of the increase in the former. For example, Hall ([4], p 941) has hypothesized that " . . . a rat takes up a position habit when faced with a problem that it cannot solve (e.g. a difficult problem like reversal)." This analysis makes it clear that almost any account that predicts increased perseverative errors may accommodate increased position habits. One conclusion to be drawn is that the situation is not entirely suitable for assessment of the selective attention hypothesis: a fairer test might be an extradimensional shift task, or an examination of generalisation gradients following discrimination training with multiple relevant cues. METHOD
Animals The birds used were 12 adult pigeons (Columba livia) that were maintained at 80% of their ad l i b weights throughout the experiment; ad lib weights ranged from 4 2 0 - 5 2 0 g. The birds were randomly allotted to either the Control Group (N = 6) or the Experimental Group (N = 6) before any training had been given.
Surgery and Histology Experimental Group animals received bilateral lesions in anterior hyperstriatal regions. During the operation, animals were held in a modified stereotaxic instrument while anesthetized with sodium pentobarbital (30 mg]kg), supplemented where necessary with ether. A 3 mA anodal current was delivered for 20 sec to 4 placements in each hemisphere via a stainless steel insect pin electrode, insulated to within 0.5 mm of its tip. The location of the lesions was determined with the aid of the Karten and Hodos [6] atlas; sites were 1 and 3 mm lateral to the midline, at 10 and 11 mm anterior to the interaural zero, the electrode being lowered 2 mm below the brain surface. Control Group animals received sham operations: these birds underwent the normal operative procedures except that no holes were drilled in the skull, and the electrode was not inserted. After completion of postoperative training, Experimental Group animals were given a lethal dose of sodium pentobarbital, and were perfused intracardially with isotonic saline followed by Heidenhain's Susa. Brains were removed, left in Susa fixative for 4 8 h r , and then, following dehydration, embeddin in paraffin. Serial sections were cut at 10u, and every tenth section was stained with haematoxylin and eosin. Reconstructions of the lesions were made by projecting slides showing damage onto a series of standard pigeon brain diagrams adapted from the Karten and Hodos atlas [6].
Apparatus Animals were tested in a pigeon chamber, 30 × 30 × 3 0 c m , which was placed in a dark sound-attenuating cubicle. The chamber contained 3 response keys, 2 cm in dia.; the center key was mounted over a grain feeder at a height of 20 cm above the floor, the remaining keys being 8 cm to either side of the center key at the same level. The center key could be illuminated from behind by white light; the side keys could be illuminated by either red or green light. A houselight in the roof of the chamber was continu-
HYPERSTRIATUM AND RESPONSE INHIBITION ously illuminated and white noise was also provided throughout. The control of the sequence of events in the chamber and the collection of data were carried out on-line by an Elliott 4130 computer. Programmes were written in the Experimental Control Language developed in this laboratory [ 3].
531 more responses that were not classified in the preceding 2 categories); (d) unclassified (that is, any response occurring in a sequence of less than 4 responses that were not classified in either of the first 2 categories). In addition, any irrelevant response in a sequence of 4 or more to the same side was scored as part of a position habit. RESULTS
Pro c edu re Pretraining. Animals were pretrained to peck all three keys by the use of an auto-shaping procedure [2]. Training. Each trial commenced with the illumination of the center key: a response to this key extinguished it and illuminated the two side keys, one with red and one with green light; the position of the colors was varied according to a Gellermann sequence, so that a given color appeared on the same key for no more than 3 trials in succession. Each peck on a side key produced a clearly audible feedback click from a relay attached to the outside wall of the chamber. A total of 5 responses to one of the two side keys constituted a choice, following which both side keys were extinguished. When the choice was correct, animals were given 4 sec access to grain in the hopper, followed by a 10 sec intertrial interval; incorrect choices were followed by a 14 sec intertrial interval. Acquisition and each reversal of the red-green discrimination continued until the criterion of 10 successive correct choices had been achieved, whereupon the reinforcement value of the colors was reversed; this procedure continued until the three daily runs had been completed, when the animal was removed from the apparatus. Where a bird did not initiate a new trial within 5 min of the beginning of the previous trial, the hopper was operated for 8 sec, independent of the bird's behavior; this technique was usually effective in obtaining renewed responding. A maximum of 3 free deliveries of food per day was allowed, and if a bird stopped responding for 5 min on a fourth occasion, training was terminated for that day; on the following day, S+ for the first discrimination of the day was the same side that had been S+ at the end of the uncompleted reversal of the previous day, and as usual, three criterion runs were completed. Days on which 3 reversals were not completed, or on which the first run of the day was in fact a continuation of a preceding day's reversal were discounted, both for the purpose of calculating number of day's training, and from further analysis. Experimental design. Animals from both groups were first pretrained and then tested for 17 days on which they were given three daily runs to criterion, the correct stimulus color being reversed overnight except where a day's testing had been abandoned through failure to maintain responding in the manner described above. Animals of the Experimental Group then received forebrain lesions, while birds from the Control Group underwent sham operations. Both groups were given a minimum of 7 days recovery period after which they were first re-exposed to pretraining and then required to complete 6 more days of the three reversals-a-day training. Data analysis. Trials were classified in the way used in the original report [ 7] ; each trial was scored as either: (a) a p erseverative error, (all initial errors and any errors occurring in a sequence of 4 or more errors); (b) a member of an incomplete run (correct choices in sequences of 4 or m o r e , excluding the criterion run); ( c ) a n irrelevant response (any response occurring in a sequence of 4 or
Histology The brain of one experimental animal was lost. The other 5 animals all showed bilateral damage in anterior regions of the hyperstriatum accessorium, the hyperstriatum dorsale, and the hyperstriatum ventrale; 4 of the 5 birds showed some invasion of the underlying noestriatum. Previous work [ 10] has shown that extensive lesions of the neostriatum do not affect performance in a serial position discrimination reversal. Figure 1 shows reconstructions of the lesions of the 2 animals having respectively the least and the most damage.
A 9.00
A 10. O0
Al1.00
A12.00
A13.00
]
LARGEST
LESION
[]
SMALLEST
LESION
FIG. 1. Reconstructions of the largest and diagrams adapted from the Karten and Hodos tions: E - Ectostriatum; H A - Hyperstriatum Hyperstriatum dorsale; H V - Hyperstriatum striatum (Experiment 1).
smallest lesions on atlas [6]. Abbreviaaccessorium; H D ventrale; N - Neo-
Reversal Preformance Figure 2 shows mean performance of the 2 groups over the 6 postoperative days, trials being broken down into response categories. The figure clearly shows that the normal superiority of R2s over R l s has been eliminated by
532
MACPHAIL
PERSEVERATIVE
J~ 50
20
unclassified i r r e l e v a n t responding, excluding position ha-bits position habits incomplete runs perseverative errors
x
y, X X X X
L
1
3O
Effects of Hyperstriatal Lesions on Within-Day Serial Reversal Performance in Pigeons EUAN M. MACPHAIL
Laboratory of Experimental Psychology, University of Sussex, Falmer, Brighton BN1 9QG, England (Received 7 April 1975) MACPHAIL, E. M. Effects of hyperstriatal lesions on within-day serial reversal performance in pigeons. PHYSIOL. BEHAV. 16(5) 529-536, 1976. - In Experiment 1, 12 pigeons performed each day for 17 days 3 serial reversals of a simultaneous color discrimination; 6 birds were then given hyperstriatal lesions, and 6 birds were sham operated. Analysis of the effects of lesions on postoperative reversal performance supported 2 hypotheses: first, that the hyperstriatum contains a mechanism involved in the generation of inhibition, and second, that minimal inhibition is generated (in intact birds) in the first of each day's reversals. Experiment 2 investigated the effects of hyperstriatal lesions on serial position reversal; results again supported the notion that the hyperstriatum is involved in response inhibition. Contrasts between the effects obtained in the 2 experiments suggested that inhibition plays a more central role in position, as opposed to color, reversals. Hyperstriatal lesions
Reversallearning
Inhibition
THIS paper reports 2 experiments which investigated the effects of hyperstriatal lesions in pigeons on within-day reversals of a simultaneous discrimination. The particular l e s i o n and behavioral situation were chosen because theoretical accounts for each of them have introduced the notion of inhibition in such a way that certain lesion effects may be predicted. Should these effects be obtained, then further evidence will have been provided, not only for the interpretation of the lesion, but also for the theoretical analysis of reversal learning. This study, in other words, hopes to use the outcome of a physiological investigation to contribute to behavioral theory. The avian hyperstriatal complex consists of three major components, the hyperstriatum ventrale, the hyperstriatum dorsale, and the hyperstriatum accessorium, the latter two structures forming the so-called Wulst [1]. Lesions within this complex disrupt both acquisition and retention of difficult successive discriminations [8, 13, 17], and impair reversals of simultaneous discriminations [ 8,15], but generally have no effect on the acquisition or retention of simultaneous discriminations ([5, 8, 151; Macphail [10] reports an exception to this generalisation). Macphail [8] noted the parallel between these effects and those obtained by certain limbic lesions in mammals and proposed that there was in the avian hyperstriatum a mechanism for response inhibition; this hypothesis is of particular relevance to this study.
In a recent review, Macphail [11] described 3 further hypotheses concerning avian hyperstriatal function. One of these, that hyperstriatal birds suffer from a perceptual deficit, [ 13] was rejected on the grounds that it could not accommodate the finding of selective impairment of reversal, as opposed to acquisition, of simultaneous discriminations. A second hypothesis is that hyperstriatal birds are less likely than normals to shift their mode of response as a consequence of nonreinforcement, a notion derived from Olton's [ 12] account of hippocampal function in the rat; as evidence for this hypothesis, it has been shown that hyperstriatal pigeons are more likely than normals to halt responding altogether in the course of reversal learning [101, a fact that clearly poses problems for the response suppression account. The third hypothesis is that hyperstriatal birds suffer an impairment in selective attention that causes them great difficulty in paying attention to the relevant stimulus in the initial stages of reversals [5,14]; consonant with this view is Stettner's [ 14] report that hyperstriatal quail performing reversals show little more perseverative responding to the former positive stimulus than do normals, but greatly increased position habits. The experiments to be reported here bear on all the hypotheses discussed, and so may help rule out one or more of them. The relevant behavioral situation was described by Macphail [7], in a study of pigeons that performed each day for 17 days 3 reversals of a simultaneous (red-green)
1This research was supported by a grant from the U. K. Medical Research Council; computing facilities were made available by a grant from the U. K. Science Research Council. I am grateful to June Atherton for assistance in preparation of the figures, to Colin Atherton for photographic assistance, and to N. J. Mackintosh and G. Hall for their critical comments on an earlier draft of this manuscript. 529
530
MACPHAIL
discrimination. Birds learned the second of each day's reversal series (R2) more efficiently, in terms of trials to criterion, than they did either the first (R1) or the third (R3). This was unexpected, since, due to overnight forgetting, there was no preference for either stimulus at the start of R l s whereas there was, of course, a preference (for the wrong stimulus) at the start of both R2 and R3s. Analysis of the data showed that the superiority of R2 performance was due primarily to the fact that having made 4 or more correct responses in succession, birds were more likely to complete the criterion run (10 correct responses) without further errors in R2s than they were in Rls. It was argued that in R l s (where few initial errors occurred) no inhibition was generated to the negative stimulus ( S - ) ; thus, after a run of correct choices, birds were faced with a choice between a stimulus (S+) that was gradually accumulating approach strength, and another stimulus ( S - ) that was relatively neutral. In R2s, however, the initial errors should generate inhibition to S - , so that, following a run of correct trials, the stimulus pitted against S+ would not be neutral, but inhibitory. The analysis of R3 performance is complicated, since, after the initial errors, animals will, according to the hypothesis, have generated inhibition to both stimuli; as the superiority of R2s over R3s seemed to be due primarily to fewer position habits, it was proposed that an irrelevant dimension had gained control as a consequence of both colors having become inhibitory. The interpretation advanced so far for this complex situation is, of course, tentative; it is of interest to pursue it, however, as an understanding of the role of inhibition in simultaneous discriminations and their reversals is important, particularly for the behavioral analysis of those lesions that selectively disrupt reversal as opposed to acquisition performance. EXPERIMENT 1 Experiment 1 investigates the effects of hyperstriatal lesions on performance in the red-green serial reversal problem; birds were given extensive pretraining on the reversals prior to operation, since pilot experiments showed that it was very difficult to train naive birds with lesions so that they could complete 3 reversals within a reasonable number of trials in one day. Three aspects of postoperative performance are of interest. First, the effects of lesions on perseverative responding to S - : both the response shift and response inhibition hypotheses predict that these scores should be increased by the lesions, whereas the attention shift model predicts minimal lesion effects. Second, effects on incomplete run scores (sequences of S+ choices preceding the criterion run): the response inhibition hypothesis predicts a much greater effect of lesions on R2 incomplete run scores as compared to R1 scores, as the lower R2 scores in normals are supposedly due to generation of inhibition in R2s; neither of the other 2 hypotheses appears to make any clear prediction concerning incomplete run scores. Third, effects of lesions on position habits: the selective attention hypothesis obviously predicts an increase in these scores following lesions. However, although an absence of any effect on position habits would weaken the selective attention account, an increase would not provide convincing support for the hypothesis if it was accompanied by increases in perseverative errors; this is because if both perseverative errors and position habits increase, it may well
be argued that the increase in the latter is a consequence of the increase in the former. For example, Hall ([4], p 941) has hypothesized that " . . . a rat takes up a position habit when faced with a problem that it cannot solve (e.g. a difficult problem like reversal)." This analysis makes it clear that almost any account that predicts increased perseverative errors may accommodate increased position habits. One conclusion to be drawn is that the situation is not entirely suitable for assessment of the selective attention hypothesis: a fairer test might be an extradimensional shift task, or an examination of generalisation gradients following discrimination training with multiple relevant cues. METHOD
Animals The birds used were 12 adult pigeons (Columba livia) that were maintained at 80% of their ad l i b weights throughout the experiment; ad lib weights ranged from 4 2 0 - 5 2 0 g. The birds were randomly allotted to either the Control Group (N = 6) or the Experimental Group (N = 6) before any training had been given.
Surgery and Histology Experimental Group animals received bilateral lesions in anterior hyperstriatal regions. During the operation, animals were held in a modified stereotaxic instrument while anesthetized with sodium pentobarbital (30 mg]kg), supplemented where necessary with ether. A 3 mA anodal current was delivered for 20 sec to 4 placements in each hemisphere via a stainless steel insect pin electrode, insulated to within 0.5 mm of its tip. The location of the lesions was determined with the aid of the Karten and Hodos [6] atlas; sites were 1 and 3 mm lateral to the midline, at 10 and 11 mm anterior to the interaural zero, the electrode being lowered 2 mm below the brain surface. Control Group animals received sham operations: these birds underwent the normal operative procedures except that no holes were drilled in the skull, and the electrode was not inserted. After completion of postoperative training, Experimental Group animals were given a lethal dose of sodium pentobarbital, and were perfused intracardially with isotonic saline followed by Heidenhain's Susa. Brains were removed, left in Susa fixative for 4 8 h r , and then, following dehydration, embeddin in paraffin. Serial sections were cut at 10u, and every tenth section was stained with haematoxylin and eosin. Reconstructions of the lesions were made by projecting slides showing damage onto a series of standard pigeon brain diagrams adapted from the Karten and Hodos atlas [6].
Apparatus Animals were tested in a pigeon chamber, 30 × 30 × 3 0 c m , which was placed in a dark sound-attenuating cubicle. The chamber contained 3 response keys, 2 cm in dia.; the center key was mounted over a grain feeder at a height of 20 cm above the floor, the remaining keys being 8 cm to either side of the center key at the same level. The center key could be illuminated from behind by white light; the side keys could be illuminated by either red or green light. A houselight in the roof of the chamber was continu-
HYPERSTRIATUM AND RESPONSE INHIBITION ously illuminated and white noise was also provided throughout. The control of the sequence of events in the chamber and the collection of data were carried out on-line by an Elliott 4130 computer. Programmes were written in the Experimental Control Language developed in this laboratory [ 3].
531 more responses that were not classified in the preceding 2 categories); (d) unclassified (that is, any response occurring in a sequence of less than 4 responses that were not classified in either of the first 2 categories). In addition, any irrelevant response in a sequence of 4 or more to the same side was scored as part of a position habit. RESULTS
Pro c edu re Pretraining. Animals were pretrained to peck all three keys by the use of an auto-shaping procedure [2]. Training. Each trial commenced with the illumination of the center key: a response to this key extinguished it and illuminated the two side keys, one with red and one with green light; the position of the colors was varied according to a Gellermann sequence, so that a given color appeared on the same key for no more than 3 trials in succession. Each peck on a side key produced a clearly audible feedback click from a relay attached to the outside wall of the chamber. A total of 5 responses to one of the two side keys constituted a choice, following which both side keys were extinguished. When the choice was correct, animals were given 4 sec access to grain in the hopper, followed by a 10 sec intertrial interval; incorrect choices were followed by a 14 sec intertrial interval. Acquisition and each reversal of the red-green discrimination continued until the criterion of 10 successive correct choices had been achieved, whereupon the reinforcement value of the colors was reversed; this procedure continued until the three daily runs had been completed, when the animal was removed from the apparatus. Where a bird did not initiate a new trial within 5 min of the beginning of the previous trial, the hopper was operated for 8 sec, independent of the bird's behavior; this technique was usually effective in obtaining renewed responding. A maximum of 3 free deliveries of food per day was allowed, and if a bird stopped responding for 5 min on a fourth occasion, training was terminated for that day; on the following day, S+ for the first discrimination of the day was the same side that had been S+ at the end of the uncompleted reversal of the previous day, and as usual, three criterion runs were completed. Days on which 3 reversals were not completed, or on which the first run of the day was in fact a continuation of a preceding day's reversal were discounted, both for the purpose of calculating number of day's training, and from further analysis. Experimental design. Animals from both groups were first pretrained and then tested for 17 days on which they were given three daily runs to criterion, the correct stimulus color being reversed overnight except where a day's testing had been abandoned through failure to maintain responding in the manner described above. Animals of the Experimental Group then received forebrain lesions, while birds from the Control Group underwent sham operations. Both groups were given a minimum of 7 days recovery period after which they were first re-exposed to pretraining and then required to complete 6 more days of the three reversals-a-day training. Data analysis. Trials were classified in the way used in the original report [ 7] ; each trial was scored as either: (a) a p erseverative error, (all initial errors and any errors occurring in a sequence of 4 or more errors); (b) a member of an incomplete run (correct choices in sequences of 4 or m o r e , excluding the criterion run); ( c ) a n irrelevant response (any response occurring in a sequence of 4 or
Histology The brain of one experimental animal was lost. The other 5 animals all showed bilateral damage in anterior regions of the hyperstriatum accessorium, the hyperstriatum dorsale, and the hyperstriatum ventrale; 4 of the 5 birds showed some invasion of the underlying noestriatum. Previous work [ 10] has shown that extensive lesions of the neostriatum do not affect performance in a serial position discrimination reversal. Figure 1 shows reconstructions of the lesions of the 2 animals having respectively the least and the most damage.
A 9.00
A 10. O0
Al1.00
A12.00
A13.00
]
LARGEST
LESION
[]
SMALLEST
LESION
FIG. 1. Reconstructions of the largest and diagrams adapted from the Karten and Hodos tions: E - Ectostriatum; H A - Hyperstriatum Hyperstriatum dorsale; H V - Hyperstriatum striatum (Experiment 1).
smallest lesions on atlas [6]. Abbreviaaccessorium; H D ventrale; N - Neo-
Reversal Preformance Figure 2 shows mean performance of the 2 groups over the 6 postoperative days, trials being broken down into response categories. The figure clearly shows that the normal superiority of R2s over R l s has been eliminated by
532
MACPHAIL
PERSEVERATIVE
J~ 50
20
unclassified i r r e l e v a n t responding, excluding position ha-bits position habits incomplete runs perseverative errors
x
y, X X X X
L
1
3O
