EFFECTS OF A LIMITED HOLD ON PIGEONS MATCH-TO-SAMPLE PERFORMANCE UNDER FIXED-RATIO SCHEDULING. Joseph Leland Cermak, B.A.

Transcription

1 EFFECTS OF A LIMITED HOLD ON PIGEONS MATCH-TO-SAMPLE PERFORMANCE UNDER FIXED-RATIO SCHEDULING Joseph Leland Cermak, B.A. Thesis Prepared for the Degree of MASTER OF SCIENCE UNIVERSITY OF NORTH TEXAS December 2005 APPROVED: Manish Vaidya, Thesis Advisor Cloyd Hyten, Committee Member Sigrid Glenn, Committee Member Richard Smith, Chair of the Department of Behavior Analysis David Hartman, Dean of the College of Public Affairs and Community Service Sandra L. Terrell, Dean of the Robert B. Toulouse School of Graduate Studies

2 Cermak, Joseph Leland. Effects of a limited hold on pigeons match-to-sample performance under fixed-ratio scheduling. Master of Science (Behavior Analysis), December 2005, 39 pp., 1 table, 15 illustrations, references, 25 titles. Pigeons were trained on a zero-delay identity match-to-sample task. Experiment 1 started with every correct match reinforced with grain access and subsequent conditions include higher fixed-ratio values. Experiment 2 included the same fixed-ratio values as experiment 1 with and without a limited hold (LH) on the opportunity to select a comparison stimulus. Prior research suggested that trials after reinforcement would have an increased likelihood of error, and that these errors would be reduced in LH conditions. Results confirmed this expected error pattern and in most LH conditions errors were reduced early in the ratio.

3 TABLE OF CONTENTS Page LIST OF TABLES AND ILLUSTRATIONS...iii INTRODUCTION... 1 EXPERIMENT Method... 5 Results... 9 Discussion EXPERIMENT Method Results Discussion GENERAL DISCUSSION REFERENCE LIST ii

4 LIST OF TABLES AND ILLUSTRATIONS Page Tables 1. Limited Hold Values Figures 1. Percent correct summary measures FR Within-ratio percent correct FR Latency summary measures FR Within-ratio observing response latency FR Within-ratio selection response latency FR LH condition percent correct summary measures with errors LH condition within-ratio errors and errors plus skips Latency summary measures LH and non-lh conditions Within-ratio observing response latencies in LH and non-lh conditions Within-ratio selection response latencies in LH and non-lh conditions Within-ratio errors and skips separated LH condition percent correct summary measures without errors correct and incorrect latencies correct and incorrect latencies correct and incorrect latencies iii

5 INTRODUCTION The phrase schedule of reinforcement refers to the specification of a relation between the occurrence of responses and the delivery of consequences contingent on those responses. For example, an experimenter may arrange the environment such that every 3 rd lever press by a rat is followed by a food pellet. Or, a parent may inadvertently present cookies or candy to a child only after a child has asked for the item several times. In the above examples the experimenter and the parent are maintaining particular schedules of reinforcement for the rat s and the child s behavior, respectively. Research over the last 70 years shows that schedules of reinforcement produce schedule-specific, characteristic patterns of responding that are relatively independent of other features that characterize the situation. For example, characteristic patterns of responding have been documented for responses as varied as pigeons key pecking (Skinner & Morse, 1958a), rats wheel running (Skinner & Morse, 1958b), monkeys eye movement (Berger, 1968), vocalization (Lane & Shinkman, 1963), and hand-writing (Gonzalez & Waller, 1974). Similarly, characteristic patterns of responding have been observed when reinforcement consisted of: grain (Schuster, 1959), sweetenedcondensed milk (Clark, 1959), tokens exchangeable for food (Kelleher, 1958) or toys (Staats, Staats, Schultz, & Wolf, 1962) and, even, electrical brain stimulation (Hawkins & Pliskoff, 1964). The reliability and regularity with which schedules organize behavior led Dews (1963) to comment it is suggested that schedule influences operate generally in psychology; that when these influences can operate, they will; and that a student of any problem in psychology motivation, generalization, discrimination, or the functions of the frontal lobes ignores the consequences of the precise scheduling 1

6 arrangements of his experiments at his peril. (p. 148) In a similar vein, Zeiler (1984) asserted that schedules of reinforcement were the most powerful independent variables ever seen in psychology. (p. 485) Despite the generality of the effects of schedules described above, the effects of schedules appeared to be limited to discrete simple responses. Ferster (1960) sought to understand the role of schedules of reinforcement on responses that were more complex than lever presses or key pecks. Toward that end, he arranged an identity matching-to-sample procedure in which two pigeons were first taught to select a stimulus from an array of stimuli that was physically identical to another stimulus presented earlier. Correct matching responses produced 4-s access to the food hopper and incorrect responses led to a brief period during which all lights in the chamber were turned off. After the pigeons had learned this task, Ferster began manipulating the schedule by which food was delivered following correct matches. Specifically, he used schedules in which reinforcement followed after a fixed number of correct matches (also called fixed ratio or FR); the first matching response after a fixed period of time had elapsed (fixed interval or FI); and the first matching response after variable periods of time had elapsed (variable interval or VI). These schedules were used alone and in various combinations in mixed schedules of reinforcement. In addition to systematic effects of the various schedules on response patterns, Ferster (1960) also found systematic effects on the accuracy of responses. For current purposes, one important finding was that accuracy was lowest in the 2 nd quarter of a fixed-interval 10-min schedule and increased during the 3 rd and 4 th quarters of the interval. 2

7 In a follow up study, Nevin, Cumming & Berryman (1963) compared the effects of fixed- and variable-ratio reinforcement of correct matching responses on the accuracy of performance. They found that session-wide accuracy decreased as the FR value increased. In addition, they found that accuracy was lowest immediately after reinforcement and increased as the subjects progressed through the schedule requirements. Interestingly, the use of variable-ratio schedules (with the same average correct matches required) produced an overall increase in response accuracy by considerably attenuating the likelihood of errors early in the schedule. These data suggest that the decreases in accuracy observed during the fixed ratio conditions were not due to differing densities of reinforcement at the session level. Rather, the effects appeared to be more directly related to the schedule by which responses were reinforced. In another study, Mintz, Mourer & Weinberg (1966) studied the frequency of pigeons errors as a function of the distance from terminal reinforcement or, alternatively, ordinal position in the ratio. In a systematic replication of Ferster s (1960) original experiment, Mintz et al. (1966) used a token reinforcement preparation. The conditioned-reinforcement system consisted of ten white lamps arrayed vertically on the right side of the panel. During FR9 conditions, the lowest of these lamps was lit, and each successively higher lamp was illuminated after the next correct match, and reset after the grain presentation that followed the ninth correct match. They replicated the within-ratio pattern of errors that Nevin et al. found and showed in probe conditions that the number of lamps exerted some stimulus control over the likelihood of errors. Vaidya & Hackenberg (1997) replicated some of these procedures. Using 3

8 pigeons and red LEDs, they compared fixed and variable ratio reinforcement and found that fixed ratio schedules decreased overall accuracy. They found further that the likelihood of error and the latency to initiate trials varied systematically as a function of ordinal position in the ratio and that these effects were attenuated with variable ratio schedules of equal average value. In sum, the major findings of these studies that overall session accuracy decreases as schedule values (the number of correct matches required) increase and that fixed schedules produce errors early in the schedule period which are attenuated in variable schedules with the same average requirement have been replicated many times with nonhuman subjects (Boren & Gollub, 1972; Clark & Sherman, 1970; Ferster, 1960; Mintz, Mourer, & Weinberg, 1966; Nevin, Cumming & Berryman, 1963; Stubbs, 1968; Vaidya & Hackenberg, 1997). Finally, the effect has also been documented in children and adult humans (Davidson & Osborne, 1974; Osborne & Burns, 1975). Schedules of reinforcement have thus been shown to organize behavior in quite predictable ways. In addition, schedules of reinforcement have been shown to affect the accuracy of an otherwise well-established response of a complex nature (identity matching-to-sample). The present experiment sought to investigate schedule typical effects (in Experiment 1) and manipulate some of the variables that may be responsible for producing the level and pattern of responding (Experiment 2) typically seen when correct matching responses are reinforced on a schedule of reinforcement in matchingto-sample procedures. 4

9 EXPERIMENT 1 Method Subjects Three experimentally naïve White Carneaux pigeons (Columba livia) were maintained within 15 grams of 80% of their free-feeding weights for the duration of the experiment via supplemental feeding when necessary. The pigeons were housed in stainless steel rabbit chambers in a room maintained at approximately 72 degrees Fahrenheit with 24-hour access to water and grit and a 12-hour cycle of alternating light and dark periods. Sessions were typically conducted six days a week for each pigeon unless its pre-session weight was outside of the established range. Apparatus Sessions were conducted in a custom-built operant chamber in which the running space was 51 cm wide by 36 cm deep by 33.5 cm tall. The floor consisted of a rubbercoated wire grate. Three of the four walls were covered with acoustic tiling to minimize sound inside the chamber. The fourth wall was made out of aluminum and measured 36 cm (wide) by 33.5 cm (tall) and served as the intelligence panel. The panel consisted of a houselight mounted 2.8 cm below the ceiling, a square opening 6.1 cm wide to allow access to grain located 4 cm above the floor and in the horizontal middle of the panel, and three 2.54 cm circular openings to allow for key peck responses. Each key consisted of a plexiglass panel, a microswitch and an inline projector by which the keys could be backlit with colors or geometric forms. Each key was outfitted with a relay that produced an audible click each time the microswitch was closed. The bottom edge of the magazine opening was located centrally 4 cm above the 5

10 chamber s wire footing. The response keys were located in a horizontal array, with the lower edge of the center key located in the center of the panel 10.5 cm above the upper edge of the magazine opening, and each side key was separated from the center key by 6.5 cm. The lower edge of the houselight was located centrally 3.4 cm above the upper edge of the center response key. A one-way mirror on the right wall of the chamber allowed for observation of the running space and a small fan mounted on the top of the chamber served as both ventilation and masking noise. Procedures Preliminary Training Preliminary training included three phases: magazine training, key peck shaping, and matching performance. The first two sessions were general adaptation sessions in which a five minute dark period was followed by 30 minutes in which the houselight was illuminated. No other stimulus presentations occurred in these sessions. This was followed by magazine training in which a five minute dark period was followed by brief manual presentations of the food magazine independent of any responses for a minute session. This training continued until the pigeons were reliably eating whenever access to the hopper became available. Key peck shaping began in the next session during which each pigeon received 4-5 seconds of grain access for key pecks or responses that approximated key pecks. Each pigeon learned to peck the key in the first session of training. Matching performance was developed in two stages: discrimination training and matching-to-sample. During discrimination training, the center key was illuminated only if 10 seconds elapsed without a key peck. Once illuminated, a single peck on the lit key 6

11 produced three seconds of grain access. Matching-to-sample (hereafter, MTS) training began once the pigeons were reliably pecking only lit keys. MTS trials began with the center key lit white. A peck to this key turned off the key light and illuminated one of the side keys (randomly selected) with white. A response to the lit key resulted in three seconds of grain access. Over the next 17 to 21 sessions the following parameters were added or changed: The side with the lit key could be the same no more than three times in a row, a 10 second ITI was added, the houselight was turned off at all times except when one of the three keys was lit, and the session ended after second grain presentations instead of 50 3-second presentations. All of these changes were in place for at least the last six sessions of preliminary training. Matching-to-Sample Training MTS training sessions began with a five minute dark period in which all lights were off and all keys inoperative. At the end of the five minutes, the first trial began with presentation of the houselight and the illumination of the center key (hereafter, the sample key) with one of three hues: red, green, or yellow. A single response on the sample turned off the center key light and illuminated the two side keys (hereafter, comparisons), one with a hue that was physically identical to the center key and the other with one of the two remaining hues. A peck to the matching comparison stimulus turned off all lights in the chamber and raised the food hopper for 2.5 s. A peck to the non-matching comparison stimulus produced a 2.5 second period during which all lights in the chamber were off. Regardless of outcome, trials were separated by a 10 s intertrial interval (ITI). Each session had a minimum of 72 trials, and with the exception of the first 5 7

12 sessions for 1410, the first 6 sessions for 1073, and the first 3 sessions for 1411, the trial configurations consisted of all of the twelve possible combinations of trial types given three colors. For example, the four possible combinations of trial types with the red sample were red sample with red on the left and green on the right (R-RG), or green left and red right (R-GR), or red left and yellow right (R-RY), or yellow left and red right (R-YR). In the initial sessions indicated above, only the following six trial types were used: R-RG, R-GR, G-GY, G-YG, Y-YR, and Y-RY. The twelve trial types were presented randomly without replacement with the restriction that the matching comparison stimulus could be on the same side more than three times in a row. Sessions could not end unless a ratio was completed; if after 72 trials a ratio was incomplete the pigeon was required to complete enough correct trials to complete it. Experimental Conditions In the initial condition of the experiment, every correct response resulted in grain access (hereafter, FR1 condition). Subjects were exposed to a minimum of 20 sessions in this and subsequent conditions (described below). Conditions were changed when the overall accuracy was stable for 10 sessions (as determined by visual inspection of plotted data) and neither the highest or the lowest accuracy was observed in the prior 10 sessions. Each subject was exposed to an ascending series of schedule requirements FR1, FR2, FR4, and FR8. In the FR2 condition, for example, every second trial with a correct response resulted in grain access. Errors (i.e., responses to non-matching stimuli) did not advance the ratio counter. 8

13 Pigeons 1410 and 1411 developed strong position biases which had to be remediated via exposure to sessions in which the trial failed to advance until the matching stimulus was pecked. Results Figure 1 presents average percent of correct trials and standard deviations of the last 10 sessions in each condition. These figures show that, for all pigeons, accuracy generally decreased as schedule value was increased, with three exceptions. There were slight increases in mean accuracy for Pigeon 1410 from FR2 and FR4, and Pigeon 1411 from FR4 to FR8. Figure 2 presents the average number of incorrect matches as a function of ordinal position in the ratio. Errors from the first ordinal position consist of all errors before the first correct response of each ratio; errors in second position are those errors between the first and second correct responses, and so forth. These figures show that errors were most likely in the early parts of the schedule and that the likelihood of errors diminished systematically with decreasing distance to the terminal reinforcer. The most notable exceptions to this include the FR2 condition of Pigeon Additionally, Pigeon 1410 s errors in the FR8 condition were higher in the second ordinal position than in the first and in 1073 s FR4 condition the fewest number of errors was in the second ordinal position. Figure 3 presents the average latencies (with +1 standard deviations) to peck the sample stimulus (filled circles) and to peck the comparison stimulus (open circles). Each data point represents the mean of the median latencies from the last 10 sessions in each condition. This figure shows that, with the exception of Pigeon 1411, the 9

14 observing response latencies increased as the schedule values increased. Average latencies to select comparison stimuli, however, remained generally constant across schedule requirements. Figures 4 and 5 present latencies to peck the sample and comparison stimuli, respectively. As before, each data point represents the mean of the median latencies from the last 10 sessions in each condition. Figure 4 shows that observing responses latencies were highest in the first ordinal position and decreased in later ordinal positions. Comparing an individual pigeon s data across conditions shows that increases in schedule requirements produced large changes in the latency to peck the sample stimulus. In contrast, Figure 5 shows that the latencies to select comparison stimuli were not as strongly affected. The exception was 1410 for whom latencies to peck comparison stimuli changed as schedule requirements were changed. These changes, however, were not as substantial as the changes observed in the latencies to peck the sample stimuli. Figure 5 further shows that the latencies to select comparison stimuli appeared to be related to the ordinal position in the schedule. Again, however, the effects were not nearly as pronounced as they were for the sample stimulus. Discussion These data largely replicate earlier findings on the role of schedules of reinforcement on accuracy and latency in identity matching-to-sample procedures. For example, the highest overall accuracy was seen in the FR1 condition for all pigeons and accuracy was generally lower in the FR2, FR4, and FR8 conditions. In some cases, the accuracy did not decrease further once a particular ratio value was reached. Pigeon 1410 s overall accuracy did not decrease further after FR2, and 1411 s accuracy 10

15 remained constant at FR4. This finding is not surprising considering the limited range of FR values used in this experiment. Given a higher range of FR values, session accuracy would likely have decreased further, especially for Pigeon Ferster s (1960) results revealed the highest FR accuracy in FR20 and reduced accuracy up to FR85. The lack of an ITI in Ferster s study probably allowed higher FR values to be used; in the present study, with a 10-second ITI the inter-reinforcement interval might have been too long to maintain consistent responding at large ratio values. An analysis of the within-ratio error patterns showed that errors were most likely at the beginning of a ratio and that the likelihood of errors diminished systematically at higher ordinal positions in the schedule. In addition, these patterns were highly consistent across all pigeons in the experiment. These data are consistent with the suggestion that the temporal distance to food was an important controlling variable. This pattern of results has been documented many times (Davidson and Osborne, 1974; Lydersen, 1977; Mintz et al., Nevin et al., 1963; 1966; Nevin, 1967; Stubbs, 1968; Vaidya & Hackenberg, 1997; Zeiler, 1968). Analysis of the median latency to peck the sample stimulus showed that the latency was always longest at the beginning of a ratio and lower at higher ordinal positions. These data also showed that, for all pigeons, latency to peck the sample stimulus at beginning of a ratio increased as the schedule value increased (when comparing across conditions for the same subjects). Although not identical, these data bear some similarity to the obtained within-ratio pattern of errors. Nevin, et al., (1963) also showed a strong relation between the likelihood of errors and observing response latencies. The present data are consistent with Nevin et al. s results in that the first 11

16 ordinal position contained the longest latencies and the most errors. Together these findings lend support to the notion that variables that contribute to post-reinforcement pausing may also be related to the likelihood of error. 12

17 EXPERIMENT 2 Although previous studies and the data from Experiment 1 find some commonalities in the ways in which latencies and accuracies in an identity MTS procedure are affected by fixed ratio schedules of reinforcement, the relation between the two phenomena is not well understood. However, data from a study reported by Nevin (1967) may shed some light on the nature of the relation. These experiments were concerned with the effects of reinforcement schedules on simple discriminations. Nevin (1967) trained three pigeons to peck the brighter of two simultaneously presented stimuli with a FR1 schedule of grain delivery. After the accuracy of responding had become stable, Nevin increased the schedule requirements. The schedules used were FT 5, FT 10, and FR5, in that order, with five sessions of FR1 in between each condition. In FT conditions every nth trial was scheduled to deliver grain access, as long as the brighter key was pecked, regardless of performance on previous trials. This was done to create a schedule analogous to a fixed-interval schedule. Across three experiments, Nevin (1967) investigated the effects of the presence or absence of a limited hold contingency on the simple discrimination task. He found that the addition of a limited hold (LH) contingency such that the selection was required to occur within 2 seconds or the trial ended and a 6-second ITI began substantially attenuated the characteristic drop in accuracy seen following reinforcement (or at the beginning of a ratio). Specifically, given 3 pigeons and 3 conditions in Experiment 2, the accuracy remained at high levels across trials after reinforcement in eight out of nine cases. Nevin also found, however, that the likelihood of responding was low early in the schedule and increased as a function of ordinal position in the schedule. Experiments 1 13

18 and 3 demonstrated that the attenuation in likelihood of error was due to the LH contingency -- when a response was required on all trials, characteristic changes in accuracy across ordinal positions in the schedule reemerged. Would the effects of a limited hold in a simple discrimination procedure be seen in the context of a MTS procedure? MTS tasks comprise two main components - in a typical trial, the subject must first make an observing response to produce the array of comparison stimuli and then peck one of the several comparison stimuli simultaneously available. Since Nevin (1967) was using a simple discrimination procedure, there was only one response pecking one of several simultaneously available stimuli. This experiment sought to investigate the generality of Nevin s (1967) findings by assessing the effects of a LH contingency on the comparison-selection responses on accuracy. Method Subjects and Apparatus The subjects and apparatus were identical to those used in Experiment 1. Procedures The general plan of this experiment involved exposing the subjects to three fixed ratio conditions, FR2, FR4, and FR8, with and without the limited hold contingency. Each subject was exposed to the condition without limited hold first and the average latency to peck comparison stimuli during the last 10 sessions of the condition served as the limited hold value during the LH condition (see Table 1). 1 During LH conditions, a limited hold was in effect while the comparisons were present. If a response occurred 1 In two cases for Pigeon 1411 the LH calculation only included the final nine sessions instead of ten; the FR8-LH value would have been 1.57 instead of 1.61 and the FR4-LH value would have been 1.8 instead 14

19 before the LH expired, the trial outcomes were as described in Experiment 1. If no response occurred before the end of the LH, all lights in the chamber were turned off and there was a 2.5 second black period in addition to the 10 s ITI. The pigeons were exposed to the following sequence of conditions: FR8-LH, FR4, FR4-LH, FR2, and FR2- LH. The FR8-LH condition immediately followed the final FR8 condition of experiment 1 without interruption to allow for comparisons between that condition and the FR8-LH condition of the present experiment. Following this sequence, some further replications were completed including an FR1 and FR8 replication for each pigeon, and a second FR4 replication for Pigeon Results Figure 6 presents percent of correct trials from the last 10 sessions in each of the six conditions. For the LH conditions, skipping a trial or pecking the nonmatching comparison both counted as errors. Counted this way, a considerable drop in accuracy is seen in nearly every LH condition when compared to similar non-lh conditions. Figure 7 presents the mean likelihood of error as a function of ordinal position in the ratio from the last 10 sessions of each condition (error bars represent one standard deviation in either direction). Both skipped trials and selection of nonmatching comparison were counted as errors. Patterns of errors in both conditions were generally similar although the subjects consistently made more errors in the LH condition. In both conditions, however, the likelihood of error was highest at the beginning of a ratio requirement and decreased at higher ordinal positions. The mean and standard deviation of the median observing response and of

20 comparison selection latencies from the last 10 sessions of each condition are shown in Figure 8. Observing response latencies tended to increase as a function of the FR value, and they also tended to be slightly lower in LH conditions as compared to similar non-lh conditions. Latencies to peck the comparison tended to be low and constant as in Experiment 1. Figures 9 and 10 present the latency data by ordinal position. Overall the mean observing response latencies (Figure 9) continued to be much higher in the initial ordinal positions than in the second positions, and did not reduce much further in subsequent ordinal positions. The difference in observing response latencies between LH and non-lh conditions is noticeable at the first ordinal position, with the LH latencies shorter except for 1411 s FR8 condition. Mean latencies to peck the comparison stimuli (Figure 10) were consistently lower in the first ordinal position of the LH conditions. This difference was the most pronounced for bird 1410 in both the first and second ordinal positions in the FR8 condition. There was very little difference between any of the other ordinal position data points in the second and later positions except in 1411 s FR8 condition and 1073 s FR4 condition. Figure 11 includes the ordinal position data previously classified as errors separated into skipped trials and trials in which the nonmatching comparison was pecked (errors). Across subjects and conditions with only a few exceptions, errors were most common in the first ordinal position and reduced as a function of ordinal position. The error levels in limited hold conditions presented here tended to be lower than in comparable non-lh conditions (Figure 2), especially in the first ordinal position. Skipped trials were much more likely to occur in the early part of the ratio, and the number of 16

21 trials skipped varied greatly across ratio values. Many more first ordinal position trials were skipped in the FR2-LH conditions than in FR4-LH conditions, and fewer still in the FR8-LH condition. Nevertheless, skipped trials tended to occur at all ordinal positions in each condition. If response accuracy is examined in the LH conditions independently of the skipped trials, systematic accuracy effects are observed. Figure 12 presents the same data as Figure 6, although skipped trials are not included. Graphed in this way, differences are most pronounced between FR2 and FR2-LH conditions, in which trials that contained responses were more likely to be correct. Figures 13, 14, & 15 present the latencies to select comparison stimuli on correct and error trials separately for Pigeons 1410, 1411, and 1073, respectively. Each figure shows the median and inter-quartile range of the comparison selection response latencies at each ordinal position from the last 10 sessions in each condition. The dark circles show the latencies from trials with matching responses and the open circles show the latencies from trials with errors, and the bar in each LH graph shows the LH value used in that condition. There was a general tendency for the latencies from nonmatching responses to be longer than the matching latencies, and the most discrepancy was observed again at the first ordinal position within the ratio. Regardless of whether or not the LH was present, in only a few cases did the matching latency exceed the error latency, and when it did the difference was very slight. Discussion These data show that, when trials without a response were not counted as errors, the introduction of a LH contingency served to increase the accuracy of responding relative to conditions in which the LH contingency was not available. These 17

22 effects generally replicate the results reported by Nevin (1967). In general, then, the current set of data combine with earlier results in suggesting that the introduction of a limited-hold contingency on the selection response serves to increase the accuracy of responding. The more detailed analyses of latencies on correct and incorrect trials suggest a possible mechanism for this effect. Introducing the limited hold at the particular values used resulted in a reduction in the overall number of trials that contained comparison selection responses, and this effect appears when errors are combined with skipped trials as in Figure 6. However, there was a slight increase in the likelihood of corrects across ordinal positions in these conditions when trials did contain selection responses. These data suggest that the LH contingency may have been effective in ending trials that were more likely to end in errors. 18

23 GENERAL DISCUSSION The data from Experiment 1 generally replicate effects previously reported. The overall accuracy of the birds performances on identity matching-to-sample trials decreased as the schedule value was increased. In addition to the accuracy, schedule requirements influenced the birds latencies to initiate trials (by pecking the sample stimulus) and, to a lesser extent, influenced the latencies to peck comparison stimuli. The current set of data also show that the most consistent effects were seen in the changes in latencies and accuracy as a function of ordinal position in the ratio. The complexity of the task may be enhancing the likelihood of errors by interacting with the effects of the FR scheduling. Comparisons of FR and VR conditions from previous experiments suggest that the period of time following grain delivery is more likely to contain errors if it is a period that is correlated with extinction (as is the case in FR conditions). Additionally, stimulus control is shown to be intact since the errors occur in cyclical patterns in FR conditions. Therefore the errors may best be described as motivational in the sense that the pigeon s behavior is discriminated between periods of high and low probability of grain delivery. The effects of the limited hold in increasing accuracy in Nevin s (1967) study and modestly in this one has been explained with a forced responding account. This states that non-lh trials can be thought of as ones that force a response even when the contingencies result in high error likelihood. This may be supported by the finding that incorrect latencies tend to be longer than correct ones; in these trials the LH contingency may prevent incorrect responses from occurring. If the forced responding account is correct, an additional limited hold contingency 19

24 on the observing response portion of the trial might yield further improvement since it would allow more trials to be skipped. However, the additional LH might not help the performance, and in fact might reduce accuracy since it would restrict the amount of time in the presence of the sample. One interesting effect was the change in the latency to peck the sample. In the LH conditions, these latencies tended to be shorter than in the non-lh conditions even though there was no LH requirement on this response. This suggests that the limited hold affected the entire match-to-sample sequence, providing evidence that both pecking the sample and pecking the comparison were two components of a unitary response sequence. The differences in the obtained accuracy levels in LH and non-lh conditions does not appear to be related systematically to the particular value of the limited hold; Figure 12 shows that the effect occurs across FR2 conditions regardless of LH value. However, additional research is required to determine the best methods for determining the optimal LH value. The limited hold duration should be long enough to allow a response to occur but not so long that the contingency is never encountered. Other procedures could include a systematic study of methods for producing LH values that result in the highest levels of accuracy, whether this means determining a standardized LH value that is constant across all conditions or individuals, or a dynamic value that adjusts according to ratio position. One possible avenue to explore is the use of a titrating LH procedure in which the LH value adjusts according to correct and incorrect responses. Such work could have significant implications for applied behavior analysts and educators who are required to engineer highly accurate performance. 20

25 Another possible direction for future research suggested by the work of Perone and Courtney (1992) is an examination of the effects of upcoming and past reinforcer size on matching accuracy under FR scheduling. They studied the effects of reinforcer amount on the duration of the pause in an FR80 task with pigeons. In conditions with multiple schedules of reinforcement, in which a stimulus was correlated with either a small or large upcoming reinforcer amount, they found that pause duration depended on both the upcoming and past reinforcer amounts. An upcoming reinforcer amount that s small along with a past reinforcer amount that s large produced the longest pause duration. If the same variables that control pausing also control accuracy, error likelihood may be reduced if larger grain deliveries are used with the same ratio amounts. Manipulating the reinforcer amount is one way of assessing the effects of the cost-benefit ratio of working for the reinforcer. This assessment could occur in two ways; hold either the cost (ratio size) constant and manipulate the reinforcer amount, or vice versa. Future research could determine if cost-benefit ratio manipulation is the same regardless of whether the cost or the benefit is being manipulated. If alternating long and short ratio requirements were used along with schedule-correlated stimuli, error patterns might be expected to occur as a function of increased response effort in addition to reduced payoff. 21

26 Table 1 Limited Hold Values Condition FR2-LH FR4-LH FR8-LH Pigeon Number

27 Figure 1. Mean and standard deviation of the percent correct from the last 10 sessions per condition. 23

28 Figure 2. Mean and standard deviation of the number of errors in each ordinal position of the ratio. 24

29 Figure 3. Mean and standard deviation of the median latency to make observing and selection responses from the last 10 sessions in each condition. 25

30 Figure 4. Mean and standard deviation of the median observing response latencies in each ordinal position of the ratio from the last 10 sessions of each condition. 26

31 Figure 5. Mean and standard deviation of the median selection response latencies in each ordinal position of the ratio from the last 10 sessions of each condition. 27

32 Figure 6. Mean and standard deviation of the percent correct in LH and non-lh conditions. Errors in LH conditions include skipped trials and non-matching choices. 28

33 Figure 7. Mean and standard deviation of the number of errors in non-lh conditions and errors and skipped trials in LH conditions. 29

34 Figure 8. Mean and standard deviation of the median observing and selection response latencies from the last 10 sessions of LH and non-lh conditions. 30

35 Figure 9. Mean and standard deviation of the median observing response latency at each ordinal position from the last 10 sessions in LH and non-lh conditions. 31

36 Figure 10. Mean and standard deviation of the median selection response latency at each ordinal position from the last 10 sessions in LH and non-lh conditions. 32

37 Figure 11. Mean and standard deviation of the number of errors and skipped trials at each ordinal position from LH conditions. 33

38 Figure 12. Mean and standard deviation of the percent correct in LH and non-lh conditions. Errors in LH conditions do not include skipped trials. 34

39 Figure 13. Median and inter-quartile range of the matching and non-matching selection response latencies at each ordinal position for Pigeon

40 Figure 14. Median and inter-quartile range of the matching and non-matching selection response latencies at each ordinal position for Pigeon

41 Figure 15. Median and inter-quartile range of the matching and non-matching selection response latencies at each ordinal position for Pigeon

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