OBSERVING AND ATTENDING IN A DELAYED MATCHING-TO-SAMPLE PREPARATION IN PIGEONS. Bryan S. Lovelace, B.S. Thesis Prepared for the Degree of

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1 OBSERVING AND ATTENDING IN A DELAYED MATCHING-TO-SAMPLE PREPARATION IN PIGEONS Bryan S. Lovelace, B.S. Thesis Prepared for the Degree of MASTER OF SCIENCE UNIVERSITY OF NORTH TEXAS December 2008 APPROVED: Manish Vaidya, Major Professor Sigrid Glenn, Committee Member Richard Smith, Committee Member and Chair of the Department of Behavior Analysis Thomas Evenson, Dean of the College of Public Affairs and Community Service Sandra L. Terrell, Dean of the Robert B. Toulouse School of Graduate Studies

2 Lovelace, Bryan S. Observing and Attending in a Delayed Matching-to-Sample Preparation in Pigeons. Master of Science (Behavior Analysis), December 2008, 25 pp., 5 figures, references, 9 titles. Pigeons worked in a titrating delay match-to-sample (TDMTS) procedure in which selecting the correct comparison stimulus increased the delay between sample offset and comparison-array onset and incorrect comparison selections decreased that delay. Prior research in our lab has shown that the stable adjusted value of the retention interval is a curvilinear function of the observing response requirement. The current study examined the effect of the distribution and predictability of observing response requirements on adjusted retention interval values. The data show that unpredictable observing response requirements were more effective in attenuating the deleterious effects of delay on matching accuracy. The data have implications for our understanding of attending and encoding in performances involving remembering over short temporal durations.

3 Copyright 2008 By Bryan S. Lovelace ii

4 ACKNOWLEDGEMENTS First and foremost, I would like to acknowledge Manish Vaidya. Because of him I experienced the joy of learning so many new things. For example, as a result of his guidance, I learned how to program computers; which, it turns out, is something I take hedonistic pleasure in doing. Manish deserves a special thank you for trusting me to teach his class over the summer. Of all the things that I will be leaving behind after I graduate, I will miss teaching his class the most. Next I would like to thank Rick Smith who deserves recognition for standing by my side when I faced those challenges at the Denton State School. I would not have made it through those events unharmed if he had not been there to support me. Rick has always set a good example for me to follow and because of his leadership I learned how to behave ethically and to have patience in the face of adversity. Finally, I would like to thank Sigrid Glenn for her role in founding this department for if not for her hard work I would not have discovered behavior analysis and would still be stuck in the dark ages of my life. Manish, Rick, and Sigrid have given me something to live for. Because of them, my life has meaning. I will never forget what they have done for me. iii

5 TABLE OF CONTENTS Page ACKNOWLEDGEMENTS... iii LIST OF FIGURES... v Chapters 1. INTRODUCTION METHOD RESULTS DISCUSSION REFERENCES iv

6 LIST OF FIGURES Page 1. Flow chart for the TDMTS procedure The rectangular distribution of observing responses Stable adjusted retention interval by condition Retention interval as a function of sample engagement Error probability as a function of observing responses v

7 CHAPTER 1 INTRODUCTION Attention, memory, and related phenomena constitute a large part of the professional interest of experimental psychology. The issues have been addressed from as many perspectives as the clearly identifiable schools of psychological thought and maybe more. Behavioral psychologists have had a long standing interest in the dynamics of attention and memory; however, behaviorists have found themselves less interested in underlying structural features and more interested in the functional characteristics of attention and memory. The behavioral approach can be summarized as follows: Operant behavior often occurs under the control of the presence of antecedent events that have historically been correlated with particular types of consequences. Behavior so controlled is called discriminated and the controlling stimuli are called discriminative. Instances of discriminated behavior that occur in the absence of relevant discriminative stimuli may be conceptualized as acts of remembering. By extension, the decay in stimulus control as a function of the passage of time in the absence of the discriminative stimulus is one way to conceptualize the process of forgetting (McCarthy & Davison, 1979; White, 1985). Remembering and forgetting have been studied experimentally using variations of matching-to-sample (MTS) procedures. A typical trial in an MTS procedure begins with the presentation of a visual or auditory stimulus (hereafter, the sample stimulus). A response to the sample stimulus (hereafter, an observing response) results in the presentation of an array of stimuli (hereafter, comparison stimuli). Selection of the comparison stimulus that is physically identical to the sample stimulus typically produce 1

8 a reinforcing consequence. Selection of the non-matching comparison stimulus produces a time-out from the opportunity to earn reinforcement or ends the trial with no programmed consequences. In delayed matching-to-sample (DMTS), the sample stimulus is removed following the observing response and a delay is inserted between the offset of the sample stimulus and the subsequent onset of comparison stimuli (hereafter, the retention interval). In everyday terms, control by an antecedent event over an organism s behavior requires sensory contact with that event (cf. Dinsmoor, 1985). This common sense notion has been subjected to experimental scrutiny in behavioral laboratories. Several researchers, for example, have investigated the effects of time or effort allocated toward the sample stimulus on performance accuracy in DMTS procedure (e.g. Roberts, 1972, Sacks, Kamil, & Mack, 1972, White 1985.) For example, Roberts (1972) using pigeons as subjects, studied the effects of repeated pecks to the sample stimulus projected onto the center key of a horizontal three key array. In Part 1 of the study, the experimenter investigated the effects of repeated pecks to the sample stimulus on short-term remembering. Accuracy was compared across trials with a combination of one of three observing response requirements (1, 5, or 10) and one of four retention intervals (0, 2, 4, or 6 s). The results showed that accuracy was greater on trials with more observing responses required relative to trials with fewer observing responses required at all values of the retention interval. These data further showed that performance on the trials with 0 s retention intervals was a good predictor of the rate at which stimulus control would decay as the retention interval increased (p. 75, 80). 2

9 Sacks, Kamil & Mack (1972) used DMTS procedures to investigate the effects of observing response requirements on acquisition, delayed matching, and performance with completely novel stimuli using pigeons as subjects. The researchers presented trials that combined one of four observing response requirements (1, 10, 20, or 40 s) with one of five retention intervals (.5, 1, 2, 5, or 8 s). Results showed that increased sample-observing response requirements improved the rate of acquisition as well as the accuracy of stable performance on DMTS trials. They also found that when novel stimuli were used, birds with extended observing response requirements were more likely to match on the basis of identity (see also, Wright, 1997). These outcomes demonstrate a direct relation between the extent of observing requirements to the sample stimulus and accuracy on DMTS tasks; however, there are limitations to these procedures that must be addressed if a more comprehensive understanding of the role of observing is to be achieved. The typical measure of performance in MTS research is percent of correct trials, a measure that may not be sufficiently sensitive to observe subtle effects that higher values of observing response requirements may exert on behavior. This may be due in part to a ceiling effect that is encountered when accuracy is used as the dependent variable. For example, Roberts et al. (1972) reported highly accurate DMTS performance in pigeons with just 3 pecks required to the sample. Such high accuracy leaves little room to see what effects, if any result from extended observing response requirements. Previous research on the role of sample observing response requirements in DMTS task typically selects the value of the retention interval arbitrarily which may be problematic. In general, the accuracy of matching decreases as the duration of the 3

10 retention interval increases. This decrease has been found across a variety of species and stimulus modalities and is well described by an exponential decay function (White 1985). Despite these regularities, individual subjects differ with regard to the specific parameters of the function. That is, although forgetting can be well described as exponential decay of stimulus control, the rate at which that decay occurs varies across individual animals. These factors further interact with the discriminability of different stimuli which also varies across individual subjects. Taken together, these factors suggest that the choice of fixed arbitrary values of observing requirements, delay values, and stimuli typically used in DMTS procedures may result in selective sampling of the steep part of the decay function for some subjects but the shallow part for other subjects. In the final analysis, these procedural features are likely to produce an inaccurate or at least incomplete estimation of remembering capacities (Wenger & Wright, 1990). One potential solution to these limitations is to adjust the retention intervals based on the accuracy of a subject s performance on previous trials (Cumming & Berryman, 1965). Using a titrating delay matching-to-sample (TDMTS) procedure, the retention interval is adjusted upward following a specified number of correct matching responses and downward following a specified number of errors. These procedures hold accuracy in a narrow constant range across a variety of conditions. Instead, the titrating value of the retention interval can serve as a dependent measure. Because the titrating retention interval is unbounded, the TDMTS procedure can be useful for resolving problems related to the ceiling effect commonly encountered when percent of correct trials is employed as the primary dependent measure. 4

11 Prior research in our lab using the TDMTS procedure showed that the stable titrated retention interval value was a direct function of the observing response requirement (Kangas, 2006 unpublished Master s thesis). That is, the stable titrated retention interval value was seen to increase as sample observing-response requirements increased. These data systematically replicated findings from other laboratories showing extended sample observing response requirements attenuates the deleterious effects of retention intervals in DMTS procedures. In addition, these data confirmed the utility of the TDMTS procedure as a sensitive measure of the effects of sample observing response requirements. A close look at Kangas data reveals that increases in sample observing response requirements (at condition change) did not immediately produce increases in titrating retention interval values. Rather, it was common to observe a lag of a few sessions before the retention interval values began increasing. These effects could be interpreted as showing that the effects of extended sample observing response requirements are cumulative in nature. Kangas data also showed however, that large reductions in observing response requirements reliably produce immediate decreases in the stable titrating retention interval value. This outcome is consistent with the interpretation that sample observing response requirements were relevant on individual trials. These divergent outcomes raise questions about the precise nature of the effects were the effects best understood as cumulative and molar (i.e. stable titrating retention interval values were a function of cumulative exposure to contingencies) or local and molecular (i.e. stable titrating retention interval values were a function of variables operating within individual trials)? A procedure that held molar observing 5

12 requirements constant and permitted variability in molecular observing requirements might provide a way to disentangle the molecular versus molar effects of observing response requirements. The current experiment attempted to compare conditions in which a fixed number of observing responses (FR conditions) were required on all trials within sessions against conditions in which a variable number of observing responses were required on all trials within a session. The session-wide sample observing response requirements were identical in the two conditions. Thus, session-wide response requirements were held constant across the two conditions while trial-by-trial response requirements varied in the VR condition, permitting an analysis of the effects of molar (FR) versus molecular (VR) effects of changes in observing response requirements. 6

13 CHAPTER 2 METHOD Subjects Four white Carneau pigeons with experience in MTS procedures were maintained at 80% of their free feeding weight, via post-session feeding if necessary, for the duration of the experiment. Each bird experienced one session per day, six days per week as long as its running weight was within + 5% of its established running weight. Water and grit were available in the home cages. Apparatus and Stimuli The experimental chamber measured 30cm high, 80cm long and 30cm deep. The intelligence panel contained three horizontally arrayed keys (Med Associated, ENV- 135) behind which uniform hues could be projected via inline projectors (IEEE Model #ENV-130M). The intelligence panel also contained a house light to provide diffuse illumination in the chamber and an opening through which access to mixed grain could be made available via a coil-operated food hopper. The chamber was sound attenuating and a fan provided ventilation and a masking noise. All stimulus presentation, response detection and contingency management were accomplished via Med PC Version 4 running on a standard PC clone with a Pentium processor and the Windows XP operating system. General Procedure The pigeons worked on a 2-color MTS task involving red and yellow hues for the 7

14 duration of the experiment. Taking position of comparison stimuli into account, a 2-color MTS task where the sample stimulus always appears on the center key, yields four possible trial configurations (RRY, YRR, YYR, RYY). Each trial configuration was presented 18 times during a session in a quasi-randomized fashion. Sessions ended after 72 trials had been completed. Preliminary Training Each pigeon had previous experience with the MTS procedure and, therefore, did not require training relating to the food hopper or key pecking. The preliminary training condition for these birds involved a simultaneous matching to sample (SMTS) procedure. SMTS trials began with the presentation of the sample stimulus on the center key. One response on the sample key immediately added the comparison stimuli while the sample stimulus remained lit. A single peck to the matching (identical) comparison stimulus produced access to the food hopper followed by an inter-trial interval (ITI) during which all lights in the chamber were off. A peck to the non-matching stimulus turned off all lights in the chamber and initiated the ITI with no other programmed consequences. This condition continued until performance accuracy exceeded 90% for 10 consecutive sessions. The next training condition involved changing the procedure from the SMTS to a 0 s delayed matching-to-sample (DMTS) procedure. This condition was identical to the preceding condition except that the one observing response on the sample key turned off the sample and immediately presented the comparison array of stimuli. The experimental conditions began after 8

15 performance accuracy exceeded 90% for 10 consecutive sessions in the 0 s DMTS condition. Titrating Delay Matching-to-Sample The experiment proper began with the introduction of the titrating delay matching-to-sample (TDMTS) procedure. Figure 1 presents a schematic of the procedure. Each session contained 72 trials. After a 5-min warm-up period during which all lights in the chamber were off, sessions began with the illumination of the onset of the house light and the presentation of one of the hues on the center key. After the sample was displayed on the center key, the bird was required to emit one or a series of sample-observing responses in order to proceed to the comparison presentation phase of the trial. After the observing response requirement was met, the offset of the sample stimulus marked the beginning of the retention interval and the onset of the comparison array of stimuli marked its end. When the comparison stimuli were displayed, the pigeon was required to make a choice by pecking either the right or left key in order to move the trial forward. Choice of the matching comparison stimulus turned off all lights in the chamber and simultaneously operated the food hopper and food hopper light for 2 s. Conversely, choice of the non-matching comparison stimulus turned off all lights in the chamber for 2 s, after which an inter-trial interval, the value of which was determined as a function of the accuracy of the subject s prior performance, was initiated. Specifically, two consecutive correct matches increased the value of the retention interval by 1 s and one incorrect match decreased the value of the retention interval by the same amount (see Figure 1). Data from individual sessions were summarized as means of the RI 9

16 values experienced during that session and conditions were changed when the means from individual sessions fell within + 25% of the running 10-session mean (i.e., when stable titrating retention interval values were obtained). All pigeons began the experiment proper in the TDMTS condition with one sample observing-response required. The retention interval was set at 0 s at the beginning of the first session only. For the remainder of the experiment, including condition changes, each session began with an initial delay value equal to the titrated delay value from the last trial of the previous session. The main design of the experiment involved comparing conditions in which observing response requirements remained constant for all 72 trials in the session against conditions in which the sample observing response requirements varied from trial to trial (see Figure 2). Each of the four fixed observing conditions (FR 2, FR 4, FR 8, and FR 16) was compared against variable observing conditions (VR 2, VR 4, VR 8, and VR 16) which were matched for the overall number of observing responses required per session. For example, individual sessions during both FR2 and VR2 conditions required a total of 144 sample-observing responses. In the FR2 condition, two sample observing responses were required on each trial. In the VR2 condition, the number of sample observing responses required per trial ranged from 1 to 3. All values that comprised the distribution in a VR condition were presented equally often. In the case of VR2, for example, equal numbers of sample observing-response requirements of 1, 2, and 3 were presented during an individual session. The fixed observing conditions always were presented first. Conditions were changed when the stability criteria described above were met. Figure 2 presents a schematic for the plan overall 10

17 plan of the study. Observing response requirements for each of the paired comparisons were doubled in ascending order resulting in four distinct experimental comparisons: FR 2-VR 2, FR 4-VR 4, FR 8-VR 8, and FR 16-VR

18 CHAPTER 3 RESULTS Figure 3 shows the mean titrated retention interval, derived from the individual mean titrated retention intervals of the last 10 sessions (i.e., the stable titrated retention interval value), for each condition. The Y-ordinate is scaled linearly. The black bars present data from the FR conditions and the grey bars present data from the VR conditions. The data from replicated conditions are displayed as superimposed open or closed circles representing data from the VR and FR conditions, respectively. The data for P1411 show that the value of the stable titrated retention interval increases as the observing response requirement increased. The data for each FR-VR comparison suggest that for P1411, VR conditions resulted in higher tolerance to delay over the FR conditions. The mean stable titrated retention intervals per condition were: FR2 =.57 s, VR2 = 1.25 s; FR4 = 1.69 s, VR4 = 2.79 s; FR8 = 6.98 s, VR8 = 7.71 s; FR16 = s, VR16 = s. P1411 replicated three conditions; FR16R = s, VR16R = s; FR4R = 3.67s. The data for P1076 show that the stable titrated retention interval value remained largely unchanged across all conditions of the experiment. The mean stable titrated retention interval per condition (based on the last 10 sessions in each condition) was: FR2 =.5 s, VR2 =.4 s; FR4 =.613 s, VR4 =.50 s; FR8 =.652 s, VR8 =.804 s, FR16 =.631 s, VR16 =.588 s. Following the completion of all planned comparisons, P1076 was placed into a correction procedure to correct prominent position biases. The data from the post-correction replications are presented as filled and open circles for the FR8 & VR8 replications respectively. The stable titrated retention intervals during each 12

19 replicated condition were: FR8 rep = 4.74 s, VR8 rep = s. This constitutes an improvement of 726% and 446% over the original FR8 & VR8 conditions, respectively. The data for P1073 show that, similar to P1076, the stable titrated retention interval value remained largely unchanged until the FR8 condition. The mean stable titrated retention interval per condition was: FR2 =.881 s, VR2 =.898 s; FR4 = s, VR4 = s; FR8 = 2.73 s, VR8 = 1.08 s; FR16 = 6.97 s. Prior to the FR8 condition, P1073 was placed into a correction procedure to correct prominent position biases - the data were all collected after the correction procedure had been implemented. P1410 engaged in some self-injurious behavior during the study and, as a result, completed the fewest number of comparisons. The mean stable titrated retention interval per condition was: FR2 =.775 s, VR2 =.558 s; FR4 =.709 s, VR4 =.741 s; FR8 = 3.74 s. Once again, the data collected post-correction showed increases in the stable titrated retention interval value relative to the pre-correction data. A total of 15 comparisons were conducted across four birds. Of the 15, only 6 comparisons showed a higher stable titrated retention interval value under the VR conditions, 2 comparisons showed a higher stable titrated retention interval value under the FR conditions, and 7 comparisons failed to find any difference in the stable titrated retention interval value for the FR and VR conditions. Figure 4 plots the retention interval as a function of the time subjects spent completing the sample observing-response requirement or actively engaging the sample stimulus. This measure is derived by subtracting the latency to the 1 st observing response from the overall sample duration on any given trial. The titrated retention interval for that trial was then plotted as a function of the duration of sample 13

20 engagement. Data points consisting of dark (FR) and open circles (VR) represent data from conditions in which position and stimulus preferences were left uncorrected. Data points consisting of dark (FR) and open triangles (VR) represent data from conditions in which prominent stimulus and position preferences had been corrected. The figure shows that, for P1411, the amount of time spent pecking the sample stimulus was directly related to the value of the stable titrated retention interval. The figure also shows that, for three of the four birds (P1410, P1073, and P1076) the amount of time spent pecking the sample has little effect on the titrated retention interval value when strong position biases characterized the MTS performance. Probability of Error and Observing Responses Figure 5 shows the probability of error plotted as a function of the sample observing response requirements in all the VR conditions (see Figure 2). The probability of error was calculated by sorting the data for the last 10 sessions in each VR condition, into separate bins defined by the number of observing responses. For each response requirement bin, the number of incorrect trials was divided by the total number of trials for that bin. Note that the denominators were similar across calculations because the different observing response requirements occurred equally often during individual sessions. The probability of error for the FR comparison conditions is represented by the symbol FR. The specific sample-observing response requirements for each of the four VR conditions are displayed on the abscissa and the probability of error is represented on the ordinate. The data from the original exposure are represented by filled circles and the data for the replications are represented by clear 14

21 circles. The diagonal broken lines represent the best fitting linear regression (determined by minimizing sum of squared errors) for each VR condition. The figure shows that, for all birds, the likelihood of an error decreased as the number of sample-observing responses made increased. This pattern was observed for all conditions for P1411, except the VR2 condition. The other three birds (P1073, P1076, and P1410) also showed a decreased likelihood of error as the number of observing responses required increased. The relation between likelihood of error and the local sample observing response requirement was also found for P1073, P1076, and P1410 even though their performances were not consistently accurate enough to increase the titrated delay. For P1411, the data from the replicated conditions further confirm the relation observed between observing-response requirement and the likelihood of error. For P1076, the replicated condition occurred after an attempt to remediate a position bias. These data show that the correction procedure increased the range of error probabilities, which showed orderly decreases with increases in the number of observing responses. The figure also shows another interesting dynamic. Every VR condition included trials comprising a distribution of observing response requirements (see Figure 2) ranging from one to 2(n) 1 (where n = the average schedule value). Figure 5 suggests that, during the VR conditions, the effect of a given observing response requirement was partly a function of other response requirements that comprised the distribution. For example, for all four birds the likelihood of error on trials where a single observing response was required increased as the range of the distribution comprising the VR schedule increased. That is, a single observing response was more effective at 15

22 reducing error likelihood in the VR2 condition than it was in the VR8 or VR16 condition. This pattern was consistent for all four birds in nearly all conditions. 16

23 CHAPTER 4 DISCUSSION The results from this study support three broad conclusions: first, extended sample observing response requirements appear to facilitate short-term remembering of visual information; second, extended sample observing appears to influence performance locally, and third, the titrating delay match-to-sample (TDMTS) procedure provides a sensitive preparation for investigating the effects of variables that may influence short-term remembering and forgetting. Observing and Attending in Delayed Matching Performance Previous research supports the general conclusion that extended contact with sample stimuli facilitates performance in DMTS task (White, 1985, Sacks et al., 1972, Nelson & Wasserman, 1978; Wenger et al., 1972). The data from this study suggest that performance in a TDMTS procedure is similarly facilitated by extended sample contact. All four birds showed increases in the stable titrated delays as sample observing-response requirements were increased. The initial performance of three of the four birds, however, did not show an effect of extended observing response requirements. A closer look at the trial-by-trial choices of these birds showed that substantial position and color biases had come to characterize their performances. For each bird, exposure to correction procedures, designed to remediate these biases, resulted in increases in the stable titrated delay values. The post-correction data for three birds (P1073, P1076, and P1410) and the 17

24 data for P1411 support the conclusion that extended contact with the sample stimulus facilitates short-term remembering. The current data raise the intriguing possibility that the observing response procedures employed in the current study did not always insure control over responding by the sensory properties of the color of the sample stimulus and that increases in response requirements alone were not always sufficient to establish such control. Perhaps extended exposure to sample stimuli did not necessarily result in extended contact with those stimuli for these birds. Support for this interpretation is derived from the fact that the correction procedure, designed to establish or reestablish conditional control by sample stimuli (i.e., to insure contact with those stimuli), appears to have resulted in higher titrated delays, as well as more orderly trial-specific effects of response requirements. Trial Specific Effects of Observing One question of interest to researchers and theoreticians alike has been the level of resolution at which the effects of titrated delays can be seen. White and colleagues, for example, have asked whether the effects of observing requirements in DMTS procedures are based on extended exposure to the independent variables or on more local dynamics. The data presented in Figure 5 may offer a partial response to this question. These data show that the likelihood of errors, regardless of the retention interval values, was a decreasing function of observing response requirements. If the effects of extended observing requirements were molar, one would not expect to see a close linear relation between error probability and response requirement. The close 18

25 connection found between response requirements and error probability suggests that the error-attenuating effects of extended observing are realized at the level of individual trials. For P1411, the VR conditions appeared to result in systematically higher stable titrated retention interval values in relation to the FR conditions. This finding is interesting because the frequency of observing across similar FR-VR conditions was held constant. One interpretation for this difference is that the unpredictability of the observing response schedule during the VR condition evokes more attending during that condition compared to FR condition. These findings were reliable across response requirement values for P1411, but were not observed consistently with the other three subjects. To summarize, the current results support three broad conclusions: first, extended contact with sample stimuli appears to facilitate short-term remembering of visual information; second, extended sample observing appears to influence performance locally, and third, the TDMTS procedure provides a sensitive preparation for investigating variables that may influence short-term remembering and forgetting. 19

26 Figure 1. Flow chart for the titrating-delay matching-to-sample (TDMTS) procedure. 20

27 Figure 2. The rectangular distribution of observing responses during each VR condition. 21

28 Figure 3. Stable titrated retention intervals for the last 10 session in each condition. 22

29 Figure 4. Retention interval as a function of sample engagement. 23

30 Figure 5. Error probability as a function of observing responses across VR conditions. 24

31 REFERENCES Cumming, W. W., & Berryman, R. (1965). The complex discriminated operant: Studies of matching-to-sample and related problems. In D. I. Mostofsky (Ed.), Stimulus generalization (pp ). Stanford, CA: Stanford University Press. Dinsmoor, J. (1985). The role of observing and attention in establishing stimulus control. Journal of the Experimental Analysis of Behavior, 43, McCarthy, D. & Davison, M. (1979). Signal probability, reinforcement and signal detection. Journal of the Experimental Analysis of Behavior, 32, Nelson, K, & Wasserman, E. (1978). Temporal factors influencing the pigeon s successive matching-to-sample performance: sample duration, intertrial interval, and retention interval. Journal of the Experimental Analysis of Behavior, 30, Roberts, W. A. (1972). Short-term memory in the pigeon: Effects of repetition and spacing. Journal of Experimental Psychology, 94, Sacks, R. A.. Kamil, A. C., & Mack, R. (1972). The effects of fixed-ratio sample requirements on matching to sample in the pigeon. Psychonomic Science, 26, Wenger, G. & Wright, D. (1990). Disruption of performance under a titrating matchingto-sample schedule of reinforcement by drugs of abuse. Journal of Pharmachology and Experimental Theraputics, 254(1), Wright, A. (1992). Learning Mechanisms in Matching to Sample. Journal of the Experimental Analysis of Behavior, 18, White, G. (1985). Characteristics of forgetting functions in delayed matching-to-sample. Journal of the Experimental Analysis of Behavior, 44,