retention interval - Europe PMC

JOURNAL OF THE EXPERIMENTAL ANALYSIS OF BEHAVIOR

1991, 55,

201-212

NUMBER 2

(MARCH)

DISCRIMINATION TRAINING FACILITATES PIGEONS' PERFORMANCE ON ONE- TRIAL-PER-DAY DELAYED MATCHING OF KEY LOCATION ROBERT J. WILLSON AND DONALD M. WILKIE UNIVERSITY OF BRITISH COLUMBIA

Six pigeons were tested on a one-trial-per-day variant of delayed matching of key location. In one condition, a trial began with the illumination of a pair of quasi-randomly selected pecking keys in a large 10-key test box. Pigeons' pecks to one key (the sample) were reinforced with 8-second access to grain on a variable-interval 30-second schedule, whereas pecks to the other key (the distractor) had no scheduled consequences. In the second condition, the nonreinforced distractor was not presented. In both conditions, subjects were removed from the apparatus after 15 minutes and placed in a holding cage. Subjects were subsequently replaced in the box after a delay (retention interval) of 30 seconds and were reexposed to the illuminated sample and distractor keys for 1 minute. If a pigeon made more pecks to the sample during this interval, the distractor was extinguished and subsequent pecks to the sample were reinforced on the previous schedule for an additional 15 minutes. If, however, a pigeon made more pecks to the distractor, both keys were extinguished and the subject was returned to its home cage. For all subjects, matching-to-sample accuracy was higher in the first condition. In a second experiment, the retention interval was increased to 5, 15, and 30 minutes, and then to 1, 2, 4, 8, 12, and 24 hours. Most subjects remembered the correct key location for up to 4 hours, and in one case, up to 24 hours, demonstrating a spatial-memory proficiency far better than previously reported in this species on delayed matching tasks. The results are discussed in terms of the commonly held distinction between working and reference memory. Key words: delayed matching of key location, spatial memory, working memory, reference memory, proactive interference, key peck, pigeons

Delayed matching of key location is a simple procedure that has been used to examine memory for spatial location in pigeons (Wilkie, 1983a, 1983b, 1984, 1986, 1989; Wilkie & Kennedy, 1987; Wilkie & Summers, 1982; variants of this procedure have been used by Balda & Kamill with Clark's nutcrackers and by Roitblat & Harley, 1988, with rats). Subjects face a square 3 by 3 matrix of pecking keys. Trials begin with the illumination of a randomly selected key, usually for 1 to 2 s. Following a delay (retention interval), the sample key and a randomly selected distractor key are lit. If subjects peck the sample key,

they receive a grain reinforcer; if they peck the distractor key, they do not. Subjects receive approximately 50 trials per daily session, separated by an intertrial interval of about 15 s. In several experiments (e.g., Wilkie & Summers, 1982), the retention interval has been systematically varied. In these experiments, matching accuracy remains high with short delays but decreases to near-chance levels when delays of 8 s or longer are imposed. This sensitivity of pigeons' matching accuracy to delays has been frequently contrasted with the behavior of rats (e.g., Bond, Cook, & Lamb, 1981) and food-caching birds (e.g., Sherry, 1984; Shettleworth, 1983). These species apparently remember spatial location informaThis research was supported by the Natural Sciences tion for much longer periods of time than do and Engineering Research Council of Canada. The results pigeons. However, because the procedures used reported here comprised part of the first author's master's thesis. Peter Graf, Lee Gass, Suzanne MacDonald, and to study spatial memory in these species differ two reviewers made many helpful comments. Shayne Kar- in many ways, it is perhaps premature to argue dal and Kevin Lee assisted with the research. Send cor- that the behavioral differences between foodrespondence regarding this article to either author, De- caching birds and pigeons are the result of partment of Psychology, The University of British adaptive specializations in memory. Also, there Columbia, Vancouver V6T 1Y7, Canada. has been no attempt to vary delayed-matchingI Balda, R. P., & Kamil, A. C. (1986). Natural history, of-location procedures with the goal of imfood-storing ecology and spatial memory in birds. Paper presented at the twenty-eighth annual meeting of the Psy- proving subjects' performance at longer delays. That was the purpose of the present research. chonomic Society, Seattle. 201

202

ROBERT J. WILLSON and DONALD M. WILKIE

EXPERIMENT 1 One factor that has been shown to play a major role in decreasing delayed-matching-tosample accuracy is proactive interference (e.g., Wright, Urcuioli, & Sands, 1986). Wright et al. argued that the short-term memory capabilities of animals have often been grossly underestimated due to proactive interference created by the standard practice of using a small number of samples and repeating these over trials within a session. When trial-unique samples are used, repetition is avoided and performance is enhanced (e.g., Sands & Wright, 1980). Another way of avoiding repetition is to present only one trial per day. This is one of the ways in which we modified the delayed-matching-of-key-location procedure in the present research. We also changed this procedure in five other ways in our attempt to determine whether matching accuracy always decreases rapidly as the delay interval is lengthened. First, because matching accuracy improves with increased sample duration (Wilkie & Summers, 1982), we used an extended presentation of the sample. Second, because matching accuracy improves when responding to the sample is reinforced (Wilkie, 1983c), we reinforced our pigeons' pecking the sample key. The third procedural change was made in light of a finding by Urcuioli and Callender (1989). These investigators gave their pigeons off-baseline training to discriminate between the stimuli (colors and line orientations) that served as samples in matching-to-sample training. Subjects who received such training acquired the matching-to-sample task much more rapidly than did subjects who did not receive such training. Urcuioli and Callender interpreted the facilitation of matching as stemming from enhanced attention to the sample stimuli produced by differential reinforcement. In our first experiment, subjects were tested in two conditions. In one, a single key was lit as the sample and was followed, after a delay, by the sample and a distractor key. In the second, two keys were lit during the initial part of each trial. One of these was the sample; the other was the key that would serve as the distractor during the ensuing choice phase of the trial. During this initial part of the trial, pecks to the sample, but not to the distractor, were reinforced. We reasoned that

differential reinforcement for pecking these two keys might enhance attention to them. The fourth procedural change concerned the way in which memory for the sample was measured. In standard matching-to-sample procedures, subjects choose between the sample and distractor by making a single response, which is scored as correct or incorrect. There is now an accumulating body of evidence that suggests that incorrect choices in memory paradigms are not always due to forgetting (see Brown & Cook, 1986; Dale, 1988; Devenport, 1989; Roitblat & Harley, 1988; Wilkie & Spetch, 1981). Because single incorrect responses may or may not reflect forgetting, we used a procedure that allowed the subject to make several choices on each trial. We recorded the percentage of first responses that were correct, but we also calculated a discrimination ratio of correct to total choices. The final procedural change concerned the spatial layout of the response keys. In previous delayed-matching-of-key-location experiments, the subjects viewed a small matrix of pecking keys in a dimly lit operant chamber. In the present experiment, the sample stimuli were keys in a large transparent 10-sided box. Each pecking key occupied one wall of the chamber; thus, the keys were well separated. The surrounding room and its features were also easily visible from inside the apparatus. Spetch and Edwards (1988) recently demonstrated the importance of global and local landmarks in mediating improved spatial memory performance in pigeons. The present apparatus should enable the subjects to use landmarks to localize the sample stimuli and thus improve their spatial memory performance. METHOD

Subjects Six King pigeons, who had varied experimental histories in standard Skinner boxes, were used. All subjects were maintained at approximately 90% of their free-feeding weights by mixed grain obtained during the experimental sessions and from postsession supplements when necessary. Water, health grit, and crushed oyster shell were available ad libitum in the home cage but not during the experimental sessions. Subjects were tested 6 days per week in the early part of the light

KEY LOCATION MATCHING

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1kdeagn1 Sinner box Grin Dispesr

Wooden base Table

E. Door F. Tvo-vay mirror

6. Cement column

H. Conetor box

Fig. 1. An overhead schematic diagram of the experimental apparatus and testing room used in the present experiment. Numbers indicate the pecking key positions.

wide) on each vertical side and anchored to a large wooden base (95 cm by 90 cm by 30 cm). The angle between adjacent sides was apApparatus proximately 32.7 degrees. Ten of the 11 sides The apparatus was a large 1 1 -sided Skinner had a clear Plexiglas pecking key (2.5 cm in box (Figure 1). The walls of the box were diameter) requiring approximately 0.2 N to clear Plexiglas (20 cm by 30 cm by 7 mm), register a response, centered 15 cm above the reinforced with strips of sheet metal (2.5 cm floor. Each key could be illuminated with red

cycle that was matched to natural sunrise and sunset times.

ROBERT J. WILLSON and DONALD M. WILKIE

204

Table 1 Data from each 10-day block in Experiment 1. Two measures are shown for each subject: Row 1-discrimination ratio, Row 2-proportion correct. Blocks 1

2

Sample/distractor Bird 1 .891 .80 Bird 2 .848 .60 Bird 3 .978 .90 Sample-only Bird 4 .806 .70 Bird 5 .735 .70 Bird 6 .415 .40

3 -

4

5

6

sample-only group

.719 .60

.910 .80

.748 .60

.513 .50

.500 .40

.954 .90

.966 .90

.657 .80

.466 .30

.579 .40

.914 .80

.934 .80

.599 .587 .80 .50 - sample/distractor group

.673 .50

.748 .60

.732 .70

.955 .90

.963 1.00

.726 .60

.528 .40

.779 .90

.962 1.00

.899 .70

.767 .70

.500 .40

.824 .50

.914 .70

.964 1.00

.652 .80

light by a projector mounted on its posterior side. A standard grain dispenser was mounted on the 11th side, and a small opening (6.25 cm by 7.5 cm) for food access was centered on this wall, 7.5 cm above the floor. The apparatus was covered by a removable plastic mesh lid (90 cm by 90 cm). The gauge of the mesh was 2.5 cm by 5 cm and did not obstruct the subjects' view of the surrounding room, which was lit with overhead fluorescent lamps. The wooden base of the chamber was overlaid with identical mesh. The apparatus was placed on the floor of a room (150 cm by 180 cm by 270 cm) that had several prominent landmarks, all of which were clearly visible from the chamber. The location of these landmarks remained constant throughout the experiments. Data collection and experimental control were carried out on a Data General NOVA 3 @ minicomputer operating under RDOS and the MANX programming language (Gilbert & Rice, 1979). Procedure

Preliminary training. Because all subjects responded to illuminated pecking keys, relatively little preliminary training was necessary. Key

pecking was shaped until it occurred at a steady rate on all 10 keys on a variable-interval (VI) 30-s schedule. The subjects were then exposed to the conditions of the experiment proper on the following day. Trial types. There were 10 types of possible trials. On each trial (day), one of five key pairs was selected. The key pairs consisted of keys that were equidistant from the grain dispenser. For example, Keys 1 and 10, which bracketed the grain dispenser, made up Key Pair 1, and Keys 5 and 6, the keys farthest from the grain dispenser and adjacent to each other, made up Key Pair 5 (see Figure 1). Each key pair was used twice during each block of 10 sessions. A particular key served as the sample in one instance and as the distractor in the other. Initial phase of each trial. In the sample/ distractor condition both members of the quasirandomly chosen key pair were illuminated and remained lit for 15 min. Pecks to the sample key produced 8 s of access to grain on a VI 30-s schedule. Pecks to the distractor key had no scheduled consequences. At the end of 15 min, the next peck to the sample produced a final reinforcement. The experimenter then removed the subject from the apparatus and placed it in a small holding cage in the hallway outside the testing room where it remained for the duration of the retention interval. The sample-only condition was identical in all respects except that the distractor key- was not illuminated. Test phase of each trial. Following the retention interval, the subject was carefully placed in front of the grain dispenser and, as soon as the experimenter had closed the door of the test room, the sample and distractor keys were lit. The keys remained lit for 1 min. Pecks made during this time were recorded, as was the location of the pigeon's initial choice. If the subject had made more pecks to the sample at the end of the 1 -min period, the distractor key was extinguished and food was made available on the VI 30-s schedule for an additional 15 min. If, on the other hand, the pigeon made more pecks to the distractor, both keys were extinguished and the bird was returned to its home cage. (If a subject happened to respond equally to both keys during the 1 -min test, the next peck determined whether or not the trial was continued.) Sequence of conditions. Three subjects (see Table 1) received three 10-day blocks of the sample/distractor condition followed by three

KEY LOCATION MATCHING m SAMPLE/DISTRACTOR O SAMPLE ONLY

1.

-

o 0.8F

z

0 F z (A)

a

0.6-~

I

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D1

BIRD 2

BIRD 4 BIRD 3 SUBJECTS

BIRD 5

BIRD 6

Fig. 2. Average discrimination ratio for each subject in each condition in Experiment 1.

10-day blocks of the sample-only condition. The other 3 pigeons received the conditions in reversed order. Data analysis. Two performance measures were computed for each subject. A discrimination ratio (DR) based on responding during the 1-min test period was calculated for each block of 10 trials. A DR is a ratio of correct responses to total responses and is computed by dividing the total number of responses to the sample key by the total number of responses. Thus, if a subject pecks only the sample key, the DR will be 1.0. If the subject responds equally often to the sample and distractor, the DR will be .50, and so forth. The proportion of correct first choices was also calculated for each block of 10 sessions. RESULTS AND DISCUSSION Table 1 shows the two performance measures for each subject over the six 1 0-day blocks of the experiment. Both measures of performance showed a similar trend. The DR scores

tended to be slightly higher than the proportion of correct first responses per block. A 2 (Measures) by 6 (Blocks) analysis of variance of the data in Table 1 revealed a significant Measures effect, F(1, 5) = 52.8, p = .0011. The proportion correct (M = .683) was lower than the DR (M = .744). Matching performance was generally better in the sample/distractor condition than in the sample-only condition. Figure 2 summarizes this effect. This figure shows DRs for each condition for each subject, averaged over the three blocks in a condition. The differences in the two conditions were statistically significant when assessed by a repeated measures t test, t(5) = 6.70, p = .0011. The one-trial-per-day procedure was designed to minimize proactive interference produced by item repetition within a session. The procedure does not preclude proactive interference between sessions (trials). Key locations that were correct were later incorrect, and vice versa, within blocks of 10 days. We examined

ROBERT J. WILLSON and DONALD M. WILKIE

206

V 0.9 -

2

0.8 -

x

I I

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A

0.7-

O

X

0 0

0

a

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A

!-

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cs 0.6-

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0

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x

z

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0

0.2V

0.1x 1 1

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I

2

3

4

5

6

7

9

8

10

DAYS BETWEEN REPEATS Fig. 3. Daily discrimination ratios for all sample-only sessions in Experiment 1 days elapsing between key repetitions.

whether such interference occurred by examining daily DRs as a function of the number of days separating instances in which a key was correct and then later incorrect. These data are shown in Figure 3. Because keys were selected in a random manner, not all subjects experienced all of the possible numbers of days between repetitions. Evidence of proactive interference was found for the sample-only condition. Performance tended to increase as the number of days separating repetition of a key location increased. The correlation between the DR values and days was .526. The linear regression equation, DR = 0.359 + 0.0558 days, was significant, F(1, 39) = 14.91, p < .001. The regression equation intersects chance level of performance (i.e., a DR of .5) at 2.53 days, suggesting memory for key location that is long lasting (see Figure 3). Comparable data for the sample/ distractor condition are not shown. No evi-

as a

function of the number of

dence of proactive interference was found in these sessions. We also calculated a daily DR for the initial phase as well as the test phase of each trial in the sample/distractor condition. Table 2 shows these DRs for each of the 10 keys (numbered as in Figure 1). The DRs for the initial phase (M = .70) were generally lower than those for the test phase (M = .88), a difference that was confirmed in a DR (initial and test) by Key (1 to 10) analysis of variance, F(1, 5) = 95.4, p = .0004. The initial and test DR values were positively, but only rather weakly, correlated, a finding that is not surprising given that the DR from the initial phase of a trial includes responses emitted before the subject had learned which of the two illuminated keys was the sample. The Pearson r correlation coefficients for Subjects 1 to 6 were .402, .637, .163, .371, .1 13, and .472, respectively. There was no systematic or statistically significant effect of key

KEY LOCATION MATCHING

207

Table 2 Initial phase (DR1) and test phase (DR2) discrimination ratios for each key, for each subject, in the sample/distractor condition in Experiment 1. Key

Bird 1 DR1 DR2 Bird 2 DR1 DR2 Bird 3 DR1 DR2 Bird 4 DR1 DR2 Bird 5 DR1 DR2 Bird 6 DR1 DR2 Average DR1 DR2

1

2

.73 .93

.57 .72

1

.70 .77

.55 .94

1

.75 .86

.57 .96

.69 .89

.68 .82

.77 .61

.60 .96

.52 .92

.78 .83

.33 .60

.54 .75

.71 .96

.67 .92

.46 .98

.53 1

.77 .96

.66 .93

.71 .92

1

.67 .89

.61 .89

3

4

.81

.80 .92

.91

5

6

7

8

9

10

.60

.69 .96

.84 .98

.76 .50

.92 .70

.81 .87

.88

.82 .99

.69 1

.73 .92

.76 .99

.79 .93

.80 1

.60 .98

.72 .77

.33 .94

.66 .87

.75 .78

.66 .96

.83

.87 .94

.98

.83 .88

.63 .74

1

.93

.49 .52

1

.76 1

.81 1

1

1

1

.60

.44 .76

.72 .83

.62 .95

.74 .78

.73 .65

.69 .99

.67 .75

.65 .92

.69 .87

.68 .82

.70 .94

.74 .94

.79 .81

.78 .86

.68 .93

location on the DRs, F(9, 45) = 0.58, p > .05 for Key effect, F(9, 45) = 1.34, p > .05 for the DR by Key interaction. EXPERIMENT 2 In Experiment 1 we found some evidence that memory for contingencies on a previous trial has deleterious effects on a current trial. Statistical analysis suggested that such carryover effects may have lasted for about 2 days, a finding that implies considerably longer memory for key location than found in previous research. The purpose of the present experiment was to map out the forgetting function for key location in some detail. Because the sample/distractor condition produced the best matching in Experiment 1, it was used in the present experiment. METHOD and Subjects Apparatus The same apparatus as used in Experiment 1 was employed. Subjects 1, 3, and 6 from

Experiment 1 were used. In this experiment they were renumbered as 1, 2, and 3, respectively. Three new subjects (4, 5, and 6) were also used. These subjects received preliminary training similar to that given to the subjects in Experiment 1, followed by several sessions on the sample/distractor condition. Procedure All subjects were studied in the sample/ distractor condition for 30 sessions with a delay of 30 s in order to establish a baseline. In subsequent sessions, the retention interval was manipulated. Retention interval manipulation. The initial value of the retention interval (RI) was 30 s, approximately the time that it took the experimenter to remove the subject from the apparatus, place it in the holding cage, remove it from the holding cage, and carefully replace it in the apparatus facing the grain hopper. After these sessions the RI remained constant for a block of 10 trials (days), at which time performance was evaluated. If the DR was .70 or larger, the RI was increased. All subjects

208

ROBERT I. WILLSON and DONALD M. WILKIE 10.9 -

0.8 -

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0.7 -

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II II

0.6 -

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0

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BIRD 1 BIRD 2

II II II I

BIRD_3 BIRD 4

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24

10

24

RETENTION INTERVAL (HRS) Fig. 4. The final discrimination ratio for each retention interval tested in Experiment 2. All data points, the final one for each subject, represent blocks in which criterion was met. Note log scaling of x axis.

except

RI. Note that the RI in this figure is plotted log scale. All birds met criterion at 30 s, 5 min, and 30 min. Five of the 6 birds met criterion at the 2-hr RI. However, the majority of birds did much better than this. Birds 1, 3, and 4 met criterion at 4 hr, 8 hr, and 24 hr, respectively. Birds 2 and 5 attained criterion at 2 hr. Bird 6, who performed the worst of all subjects, met criterion at the 30-min RI. Bird 4 performed best. He attained criterion at all retention intervals, including 24 hr. During the final block of trials, his DR was .823. In general, our subjects' performance as the retention intervals were lengthened was comparable to RESULTS AND DISCUSSION or better than that of pigeons tested on other Table 3 shows DRs and proportion of cor- spatial memory tasks such as the open-field rect first response for each subject for each 10- "walking" maze (e.g., Spetch & Edwards, day block of sessions at each RI. Figure 4 1986). Shettleworth and Plowright (1989) reshows the DR data from the final block at each cently found evidence of pigeons' retention of

given a maximum of three blocks of 10 sessions to reach this conservative criterion at each interval. (Due to an extended layoff at Christmas, subjects received more than three blocks at some RIs.) If a subject failed to reach criterion, no further data were collected. Due to logistical constraints, Bird 1 was not tested at RIs longer than 8 hr, even though it met criterion. Table 2 shows the number of blocks received by each subject. Following the 30-s RI baseline, the RI was increased in the following sequence: 5, 15, and 30 min, and 1, 2, 4, 8, 12, and 24 hr. were

on a

KEY LOCATION MATCHING location information over many hours. In their study, pigeons were tested in a shuttlebox that had a key and feeder at each end. Each day one key produced more food than the other, but the location of this key varied from day to day. They found that pigeons tended to remember the location of the richer alternative from the previous session, sometimes for as long as 48 hr. GENERAL DISCUSSION In these experiments several modifications to the basic delayed-matching-of-key-location task were made, and improved matching accuracy at longer retention intervals was observed. One change was the one-trial-per-day procedure. It is difficult to determine whether this change, by itself, contributed to accurate matching. It may have, by reducing item repetition within sessions. However, we did find some evidence for interference between sessions, which suggests that proactive effects were still present in the one-trial-per-day procedure. One modification that seems to have been important was the use of a discrimination ratio rather than simply recording whether the first response on a trial was correct or incorrect. The latter measure was typically lower than the discrimination ratio and was much more variable, especially as the retention interval was extended. Some subjects would not have met criterion at some of the longer retention intervals in Experiment 2 based on this measure. Choice errors seem not always to reflect forgetting (see, e.g., Devenport, 1989). The influence of nonmemorial errors is minimized when animals are allowed several choices on a trial and a discrimination ratio is then calculated on these responses. A change that seemed to have been very important was the concurrent presentation of the sample and distractor key during the initial phase of the trial. The performance of subjects exposed to the sample-only condition in Experiment 1 was at near-chance levels, suggesting that the one-trial-per-day variable, the extended sample presentation, and the presentation of multiple reinforcers during the sample, by themselves, did not facilitate delayed matching of location. These subjects' perfor-

209

Table 3 Cumulative discrimination ratios for each block in Experiment 2. First entry on a line is the first block, second entry is the second block, etc. Retention interval =

Bird 30 5 15 30 1 2

1 s

min

hr

4 8 Bird 2 30 s

5 min 15 30 1 hr 2 4 Bird 3 30 s 5 min 15 30 1 hr 2 4 8 12 Bird 4 30 s 5 min 15 30 1 hr 2 4 8 12 24 Bird 30 5 15 30 1 2 4 Bird 30 5 15 30 1

.860 .857 .720 .643 .577 .610 .637 .438

.754

.862

.737 .795 .787 .824 .612 .612

.750 .750

.501 .825 .746 .804 .495 .601 .603

.543

.904

.742 .771 .640

.585

.724 .805 .911 .871 .574 .824 .730 .677 .593

.856

.774

.684 .634

.769 .676

.876 .931 .808 .733 .717 .812 .719 .638 .746 .681

.708

.855

.685 .709 .713

.677 .793 .823

.962 .828 .869 .814 .555 .794 .591

.914

.984

.637

.516

.582

.531

.865 .579 .962 .626 .484

.939 .841

.888

.605 .515

.665 .351

.870

.558

.589

.827

.748

5 s

min hr

.840

6 s

min hr

.743

.860

.653

ROBERTJ. WILLSON and DONALD M. WILKIE mance immediately improved when they were switched to the sample/distractor condition. The facilitation of delayed matching by discrimination training between the sample and distractor is consistent with the findings of Urcuioli and Callender (1989). They found that off-baseline training to discriminate between the stimuli employed as samples facilitated matching to sample. They interpreted this effect as the result of enhanced attention to the sample cues produced by the discrimination training. A similar interpretation could be applied to the present findings. The sample/distractor discrimination training may have facilitated attention to these cues, and the enhanced attention may have facilitated remembering over the retention intervals. However, given that the discrimination training during the sample/distractor condition was, in a sense, on-baseline, there are other possible reasons for the better performance under the sample/distractor condition. One of these is that the sample/distractor discrimination training may have eliminated proactive interference. An example may be useful in illustrating this possibility. Assume that Key 3 was correct yesterday but is wrong today. When the sample and distractor appear in the initial phase of a trial in the sample/distractor condition, subjects may initially choose Key 3. However, these choices are not reinforced, and subjects will likely switch to the rewarded Key 8. Thus, although the sample/distractor condition facilitates delayed location matching, the mechanism by which it does so is not yet clear. The improved performance in the sample/ distractor condition may be a direct result of differential conditioning to the two stimuli presented during the initial phase of the trial. Subjects may have learned that one stimulus was associated with reinforcement and the other with extinction. When reexposed to the sample stimuli following the RI, the subjects continued to respond to the S + and avoided responding to the S -. However, recent work in our laboratory (Willson & Wilkie, unpublished data) using the more traditional delayedmatching-of-key-location apparatus (i.e., a 3 by 3 matrix of pecking keys) suggests that the improved performance observed in the sample/distractor condition is not easily explained in terms of the simple strengthening or weakening of responding to the correct or incorrect location.

In this research subjects received four daily trials. In one trial, subjects were exposed to the sample alone for 10 min. Following a brief RI, the sample and a randomly chosen distractor were presented for 1 min. In the other three trials, two stimuli were presented during the initial part of the trial; responding to one stimulus, but not the other, was reinforced. Following the RI, two stimuli were again presented. In one trial, these stimuli were identical to those presented during the initial part of the trial. In the other two trials, the identity of one of the stimuli changed following the RI. In one case, the previously reinforced stimulus was presented together with a novel distractor. In the other case, the previously nonreinforced distractor was presented with a novel stimulus. As in the present research, matching performance was poor following sample-only training and improved following sample/distractor training. However, performance was also higher on the other two trial types. The subjects avoided the novel distractor and responded to the previously reinforced sample in one trial type and avoided the previously nonreinforced distractor and responded to the novel stimulus in the other. If the enhanced performance following sample/distractor training was a result of simple conditioning, performance in the latter two trial types discussed above should be similar to sample-only training. In one case, responding to the novel distractor was never extinguished (as is also the case following sample-only training), but subjects correctly chose the sample. In the other case, responses to the novel sample were never reinforced, but the subjects chose that stimulus rather than the familiar distractor. Although it seems likely that conditioning plays an important role in mediating the enhanced performance following sample/distractor training, it is clearly not solely responsible. Some other factor, possibly enhanced attention to the location of both the sample and distractor (e.g., Urcuioli & Callender, 1989), also plays an important role. In conclusion, it seems prudent to offer some comments on the distinction between working and reference memory and how the present findings fit within that framework. Previous research has suggested that pigeons possess an excellent reference memory (cf. Vaughan & Greene, 1984) but have a poor working memory relative to other species (cf. Bond et al.,

KEY LOCATION MATCHING 1981). As initially outlined by Honig (1978; see also Olton, 1978), the critical distinction between working and reference memory is that, in the former case, different stimuli govern the criterion response in different trials and the information that a subject must remember varies from trial to trial. On the other hand, when the same information is useful on many different trials, the task can be viewed as one requiring reference memory. Operationally, the procedures used in the present research fit the criterion for working memory; the subject needs to remember unique information in each trial to perform correctly during the test phase of the procedure. However, it is also the case that a subject and an experimenter do not necessarily define a trial in the same way. One might argue that the 15-min sample presentation in the present case consisted of a series of trials involving the same contingencies and that therefore the present procedure is a test of reference memory. The results from the present research are interesting in either case. If the present task constitutes a test of working memory, then the results show clear evidence of performance beyond that demonstrated previously in this species (Spetch & Honig, 1988). Alternatively, the present procedure may be viewed as one that tests preasymptotic reference memory. In such a case, the present procedure permits investigation of how much training is needed to result in a certain degree of remembering. Our results suggest that a brief period of discrimination training (15 min) produces remembering for many hours. However, at this point, it is impossible to say for certain whether the present procedures test working or reference memory. This problem stems from treating the working/reference memory distinction as a true dichotomy rather than as a conceptual framework. Organisms need to detect and remember unique events as well as commonalities in their environment. Traditionally, we have viewed the former as the domain of working memory and the latter as the domain of reference memory. However, uniqueness and commonality represent different ends of the same continuum (between variance and invariance). To date, the vast majority of research has focused on studying the end points of this continuum. The present procedure (and procedures such as Shettleworth & Plowright's two-armed ban-

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dit, 1989) offers a means of examining other points on the continuum and thus represents a powerful tool for furthering our understanding of nonhuman memory.

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memory capacity. Journal of Experimental Psychology: Animal Behavior Processes, 10, 256-271. Wilkie, D. M. (1983a). Pigeons' spatial memory: II. Acquisition of delayed matching of key location and transfer to new locations. Journal of the Experimental Analysis of Behavior, 39, 69-76. Wilkie, D. M. (1983b). Pigeons' spatial memory: III. Effect of distractors on delayed matching of key location. Journal of the Experimental Analysis of Behavior, 40, 143-151. Wilkie, D. M. (1983c). Reinforcement for pecking the sample facilitates pigeons' delayed matching to sample. Behaviour Analysis Letters, 3, 311-316. Wilkie, D. M. (1984). Pigeons' spatial memory: IV. Effects of intertrial interval manipulations on delayed matching of key location. Canadian Journal of Psychology, 38, 178-195. Wilkie, D. M. (1986). Pigeons' spatial memory: V. Proactive interference in the delayed matching of key location paradigm occurs only under restricted conditions. Animal Learning & Behavior, 14, 257-266.

Wilkie, D. M. (1989). Evidence that pigeons represent Euclidean properties of space. Journal of Experimental Psychology: Animal Behavior Processes, 15, 114-123. Wilkie, D. M., & Kennedy, D. J. (1987). Computer simulation of pigeons' performance on a spatial memory task. Behavioural Processes, 14, 105-122. Wilkie, D. M., & Spetch, M. L. (1981). Pigeons' delayed matching to sample errors are not always due to forgetting. Behaviour Analysis Letters, 1, 317-323. Wilkie, D. M., & Summers, R. J. (1982). Pigeons' spatial memory: Factors affecting delayed matching of key location. Journal of the Experimental Analysis of Behavior, 37, 45-56. Wright, A. A., Urcuioli, P. J., & Sands, S. F. (1986). Proactive interference in animal memory. In D. F. Kendrick, M. E. Rilling, & M. R. Denny (Eds.), Theories of animal memory (pp. 101-125). Hillsdale, NJ: Erlbaum. Received April 24, 1990 Final acceptance October 26, 1990