ON THE EFFECTS OF EXTENDED SAMPLE-OBSERVING RESPONSE REQUIREMENTS ON ADJUSTED DELAY IN A TITRATING DELAY MATCHING-TO-SAMPLE PROCEDURE WITH PIGEONS

ON THE EFFECTS OF EXTENDED SAMPLE-OBSERVING RESPONSE REQUIREMENTS ON ADJUSTED DELAY IN A TITRATING DELAY MATCHING-TO-SAMPLE PROCEDURE WITH PIGEONS Brian D. Kangas, B.A. Thesis Prepared for the Degree of MASTER OF SCIENCE UNIVERSITY OF NORTH TEXAS August 25 APPROVED: Manish Vaidya, Major Professor Sigrid Glenn, Committee Member Richard Smith, Committee Member Richard Smith, Chair of the Department of Behavior Analysis David W. Hartman, Dean of School of Community Service Sandra L. Terrell, Dean of the Robert B. Toulouse School of Graduate Studies

Kangas, Brian D. On the effects of extended sample-observing response requirements on adjusted delay in a titrating delay matching-to-sample procedure with pigeons. Master of Science (Behavior Analysis), August 25, 29 pp., 1 table, 7 illustrations, references, 1 titles. A common procedural variation that facilitates the acquisition of conditional discriminations is to increase the time an organism spends in the presence of the sample stimulus by programming extended sample-observing response requirements. Despite their common use, there has been little empirical investigation of the effects of extended sample-observing response requirements. In the current study, four pigeons worked on a titrating delay matching-to-sample procedure in which the delay between sample offset and comparison onset was adjusted as a function of the pigeons accuracy. The number of responses required to produce the comparison array was manipulated across conditions. Results show that all subjects were able to withstand longer delays between sample offset and comparison onset as sample-observing response requirements increased. These data show that the extent of the response requirement in the presence of the sample has systematic effects on conditional discrimination performances and should be considered in the design of experiments.

ACKNOWLEDGMENTS I would like to acknowledge Manish Vaidya for outstanding advising both inside and outside of the laboratory. His mentoring the last two years on both scientific and philosophical issues has given me a much richer appreciation of what a natural science approach to psychology could mean. I would also like to thank Sigrid Glenn and Richard Smith for sitting on my defense committee and for comments on a previous draft of this paper. Finally, I would like to thank Yusuke Hayashi for technical programming assistance. ii

TABLE OF CONTENTS Page ACKNOWLEDGMENTS...ii LIST OF TABLES AND FIGURES...iv Chapter 1. INTRODUCTION... 1 2. METHOD... 8 3. RESULTS... 12 4. DISCUSSION... 18 REFERENCE LIST... 29 iii

LIST OF TABLES AND FIGURES Page Table 1. Number of sessions to reach acquisition... 21 Figures 1. Session by session adjusted delay means... 22 2. Summary means of adjusted delays... 23 3. Mean adjusted delay for last 1 sessions... 24 4. Median sample latencies to first sample observing response... 25 5. Median duration between first and last sample response... 26 6. Median running rate of sample observing responses... 27 7. Percent correct as a function of adjusted delay...28 iv

CHAPTER 1 INTRODUCTION In a matching-to-sample procedure (MTS), the selection of a stimulus from an array of stimuli (hereafter, comparison stimuli) is reinforced conditionally upon the presence of another stimulus (hereafter, sample stimulus). Correct matching is determined either by some shared property of the sample and comparison stimuli, as in the case of identity or oddity matching (e.g., Weinstein, 1941), or by an arbitrary relation programmed by the experimenter, as in the case of symbolic or arbitrary matching (e.g., Carter & Eckerman, 1975). To further illustrate, a typical identity MTS trial begins with the presentation of a sample stimulus. A single response to the stimulus produces an array of comparison stimuli. Selection of the identical comparison stimulus produces food while selection of the non-identical comparison produces no programmed consequences. Both human and nonhuman animals typically acquire this performance with high accuracy via traditional trial-and-error contingencies. When they do not, arranging extended sample-observing response requirements seems to facilitate highly accurate performances. A report by Berryman, Cumming, and Nevin (1963) was the first published study to employ extended sample-observing response requirements as a procedural variable. The authors were interested in studying delayed matching in pigeons. The first phase of this study analyzed the acquisition of conditional discriminations using simultaneous, zero-delay, and variable delay MTS procedures. In the simultaneous MTS condition, a single response to the sample key illuminated both side comparison keys while the sample key remained on. In the zero-delay MTS condition, a single response to the 1

sample key concurrently turned off the sample key and turned on the side comparison keys. In the variable delay MTS condition, a single response to the sample key turned off the sample key and initiated one of several delays (1, 2, 4, 1, or 24-s) to the onset of the side comparison keys. The results indicate that when these conditions were alternated randomly, none of the seven birds acquired the matching performance. In Phase II of this study, each of the seven birds was exposed to the simultaneous condition only, and the programmed contingencies now required five responses to the sample stimulus before the comparison array could be produced. The authors stated that the extended response requirement was introduced to provide greater exposure to the sample stimulus (Berryman, Cumming, & Nevin, 1963, p. 12). The results indicate that all seven subjects quickly acquired the matching performance. However, noting the similar acquisition rates of the three subjects in a previous study on simultaneous matching employing a single sample response requirement (Cumming & Berryman, 1961), the authors concluded that additional exposure to the sample stimulus provided by the center key fixed-ratio requirement did not appear to facilitate acquisition of simultaneous matching (Berryman, Cumming, & Nevin, 1963, p. 14). In Phase III, three of the birds that participated in the previous phases were again exposed to the randomly alternating conditions described in Phase I. The FR 5 response requirement remained in effect. Results indicated high accuracy in the simultaneous condition, and moderate to low accuracy in the zero-and low delay value conditions, respectively. Performance approximated chance when the delay between sample offset and comparison onset was long. These results indicate that increasing the delay increases the difficulty of MTS tasks (lowers the accuracy). However, because 2

performance in the simultaneous MTS condition was already highly accurate at the onset of Phase III, the relative contributions of simultaneous MTS acquisition training and the FR 5 response requirement cannot be parsed with any certainty. Contrary to the conclusion drawn by Berryman et al. (1963), several systematic within-subject investigations have shown that extended sample-observing responses have an effect (e.g., Sacks, Kamil, and Mack, 1972; Roberts, 1972). For example, Sacks, Kamil, and Mack (1972) trained eight pigeons on a zero-delay MTS task similar to that described by Berryman et al. (1963); however, in this study, four different FR response requirements on the sample key were tested. Four groups of two pigeons experienced sample-observing response requirements of 1, 1, 2, or 4. The subjects in each group performed under a zero-delay MTS procedure until a criterion of 85% correct matching or better was maintained for three consecutive sessions. Results indicate that increasing the FR response requirement increased speed of acquisition; the two birds that experienced an FR 1 response requirement took 13 and 27 sessions to reach the 85% criterion, the two birds that experienced an FR 1 response requirement took 7 and 13 sessions, both birds that experienced the FR 2 response requirement took 11 sessions, and both birds that experienced the FR 4 response requirement took 6 sessions. Results from a condition in which delays were inserted between sample offset and comparison onset indicate that delayed matching performances improved with increased FR sample response requirements; the two birds that experienced the FR 1 requirement were performing at chance after 2 s delays, and both birds that experienced the FR 4 requirement were performing at levels above 75% after 8 s delays. Finally, in a condition in which novel hues replaced 3

some of the training hues, pigeons that experienced FR 1 or higher response requirements demonstrated above chance matching during the first transfer session. The authors note that transfer had never been observed in their laboratory; however, previous attempts had always employed an FR 1 response requirement. Roberts (1972) also assessed the effects of sample-observing response requirements on performance in a delayed MTS task. Ten pigeons were first trained using a simultaneous MTS procedure similar to that described by Berryman, et al. (1963), but with an FR 3 sample-observing response requirement. After performances exceeded 85% correct for 5 consecutive sessions, the conditions changed to a delayed MTS procedure in which delays of, 1, 3, or 6 s were initiated after the FR 3 observing response requirement was met. After 15 days of delayed MTS training with the FR 3 response requirement, FR response requirements of 1, 5, or 15 were programmed across sessions. Each subject cycled through the series of FR values six times, each time in a different order. Results from the condition in which FR response requirements were manipulated showed significant increases in accuracy across all delays for all subjects as the FR value increased. Despite the results described above, experimenters seem to have employed various extended sample-observing response requirements on the sample key with little or no stated rationale (e.g., Berryman, Cumming, and Nevin, 1963; Cumming and Berryman, 1965; Nordholm, Moore, and Wenger, 1995; Poling, Temple, and Foster, 1996; Dayer, Baron, Light, and Wenger, 2). For example, Poling, Temple, and Foster (1996) examined the differential outcomes effect (DOE) in chickens using a matching procedure with an FR 5 response requirement on the sample with no stated 4

rationale. Although the results indicated that when differential reinforcer magnitudes were programmed, the birds were able to perform accurately after longer delays between sample offset and comparison onset, the extended sample-observing response requirement may have interacted with the effects in unknown ways. For example, the extended observing response requirements may have facilitated performance on the problems with non-differential outcomes such that the overall effect of differential outcomes was understated. Similarly, Dayer, Baron, Light, and Wenger (2), examined the effect of ethanol on working memory and attention in the pigeon using an FR 15 response requirement on the sample, again, with no rationale stated. It remains unclear whether the extended sample-observing response requirement had an effect on the derived dose-response curves. For example, the extended sampleobserving response requirement could have served to reduce the effects of drugs in the delayed MTS task, relative to a preparation requiring only one observing response. In another study in the pharmacology literature, Nordholm, Moore, and Wenger (1995) analyzed the effects of the proposed cognitive enhancing agent Linopirdine on six pigeons and four squirrel monkeys. An FR 15 response requirement was programmed for the pigeons and an FR 2 was programmed for the squirrel monkeys. Not only was the extended sample-observing response requirement not discussed, but the basis for inclusion of between-species, within-experiment procedural differences was also not addressed. Again, without parametric data on the effects of extended sample-observing response requirements, arbitrarily programmed requirements may affect the magnitude of dose-response effects and even perhaps the shape of the derived curve. 5

The studies cited above illustrate an important point for current purposes: Extended sample-observing response requirements have been employed in a seemingly unsystematic manner despite some evidence that they may systematically influence performance in MTS preparations. In the studies described above, the ratio values varied between-experiments, within-labs (Wenger and colleagues), and even within-experiment (Nordholm et al., 1995). It may be that researchers have not paid much attention to this variable because there are few parametric data relating performance on conditional discriminations to extended sample-observing response requirements. One reason why parametric analyses of the effects of extended sampleobserving response requirements have not been conducted may involve the use of percent of correct trials as a dependent measure. This measure is problematic because performance may quickly reach near-1% accuracy making the facilitative role of extended sample-observing response requirements difficult to ascertain. Measures like trials to acquisition are better in that the data moves over a wider range than percent of correct trials; however, large differences across subjects may obscure the systematic nature of the control exerted by the independent variable. For example, Berryman et al. (1963) found up to a 4-trial difference in trials to acquisition across birds, and Sacks et al. (1972) found a difference of some 14 sessions. A procedure that avoids some of these limitations is delayed MTS. Systematic variation of delays between sample offset and comparison onset allows an analysis of the effects of extended sample-observing response requirements (e.g., Roberts, 1972) by systematically allowing an assessment of the role of sample observing on accuracy 6

on trials with varied delays. However, although delayed matching may show effects of extended observing, after the subject learns the task under a delay, performance will be highly accurate. A better procedure, in these regards, is a titrating delay MTS (TDMTS) procedure (Cumming and Berryman, 1965). In a TDMTS procedure, the delay between sample offset and comparison onset is adjusted as a function of the subject s performance. Typically in TDMTS preparations, correct responses increase the delay between sample offset and comparison onset and incorrect responses decrease the delay between sample offset and comparison onset. By adjusting delay values as a function of subjects performances, this preparation takes individual differences into account and also has the potential to detect systematic effects if they exist. The purpose of the present study was to use a TDMTS procedure to examine parametrically the effects of extended sample-observing response requirements on conditional discrimination performance. Evidence that the extent of sample-observing responses has systematic effects on behavior, paired with the continued use of extended sample-observing requirements in the absence of a thorough understanding of how they may interact with other independent variables, suggests that more research is needed. 7

CHAPTER 2 METHOD Subjects Four naïve White Carneux pigeons, obtained from Double T Farms, Glenwood, Iowa, were maintained at approximately 8% of their free-feeding weights by postsession feeding as needed. The animals were housed in individual cages, in a temperature- and humidity-controlled vivarium, with exposure to a 12-hour light/dark cycle (lights on at 7: a.m.). Water and grit were available continuously in the birds home cages. Apparatus The experiment was conducted in a standard operant chamber measuring 3 cm high, 8 cm long, and 3 cm deep. The chamber was sound- and light-attenuating with an exhaust fan to provide ventilation and masking noise. One side wall (the intelligence panel) contained a houselight, three horizontally arrayed response keys (2.5 cm in diameter and 1 cm apart from edge to edge) and a 5 cm square opening for access to a hopper (Model #ENV-25M) filled with mixed grain located 2 cm above the floor and centered below the center key. The center key was horizontally centered on the intelligence panel 16.25 cm above the floor. The two side keys were located 1 cm to the left and right of the center key. Each key could be transilluminated with a variety of colors and geometric forms using Industrial Electronics in-line projectors (IEEE Model #ENV-13M [11-1-OK21-182]). Scheduling of experimental events and data collection were controlled via computer using MED-PC software (Ver. 4., Med Associates, St. Albans, VT). 8

Procedure A parametric assessment of the effects of extended sample-observing response requirements (across conditions) on delays between sample offset and comparison onset that adjusted as a function of the subject s performance was conducted. Prior to the experiment, each pigeon was trained to eat from the food hopper and then hand shaped to peck each of the three keys (illuminated white) via differential reinforcement of successive approximations. After the pigeon pecked each of the keys reliably when lit, acquisition training began. Acquisition Training All four pigeons were trained on the MTS task using a simultaneous MTS procedure. Discrete trials began with the illumination of the houselight and the center (sample) key with either a red or yellow hue. A single response to the sample key illuminated the two side (comparison) keys with matching and non-matching hues. A single peck to the side key illuminated with the same color as the sample key (i.e. the correct match) turned off the houselight, the sample key, and both comparison keys, and raised the food hopper for 2 s followed by a 1-s intertrial-interval (ITI). A single peck to the non-matching comparison key (i.e. the incorrect response) turned off all lights in the chamber and initiated a 12-s ITI. The 1-s ITI (plus 2-s hopper access) following a correct match, and 12-s ITI following an incorrect match ensured equivalent ITIs following a correct or incorrect match. A two-color, two-comparison MTS procedure yields 4 possible configurations (RRY, YRR, YYR, RYY). The computer arranged the presentation of these 9

configurations on each trial in a quasi-random order. Specifically, each of the four configurations was presented before any configuration could be repeated (i.e. random selection without replacement). All subjects were exposed to daily sessions consisting of 72 trials. After 1 consecutive sessions with 85% or greater accuracy, each subject was exposed to a successive, or zero-delay, MTS procedure. In this condition, a single peck to the center key turned off the sample and simultaneously illuminated both side keys. The consequences for pecking the matching or non-matching key remained the same as before. After 1 consecutive sessions with 85% or greater accuracy, each pigeon was exposed to the TDMTS procedure. Titrating Delay Matching-to-Sample The TDMTS procedure was identical to the zero-delay procedure described above with the exception that the delay between sample stimulus offset and comparison stimuli onset was adjusted as a function of the pigeon s accuracy on immediately preceding trials. Specifically, every 2 consecutive correct matches increased the delay by 1 s, and every incorrect match decreased the delay by 1 s (regardless of trial type). The programmed contingencies of this titrating procedure will eventually hold accuracy around 67% as the delay between sample offset and comparison onset titrates. The first condition began as a zero-delay; thereafter, each daily session began with the delay value from the end of the previous session. Each subject was exposed to an ascending series of sample-observing response requirements (FR 1, 2, 4, 8, and 16) across conditions. For example, in the FR 16 1

condition, the 16 th peck on the center key turned off the sample key and initiated the delay interval to comparison onset. After a minimum of 2 sessions, conditions were changed when the mean delay values from the last 1 sessions were all within ± 25% of the mean of the 1-session means. Because more variability in performance was likely to occur at higher delay values, a percentage criterion was used to accommodate greater variability at higher adjusted delays. An absolute number would have made the criterion more stringent as a function of higher adjusted delays. Each subject was exposed to higher sample-observing response requirements until the mean delay value from the last 1 sessions was smaller than or equal to the value obtained in the previous FR value. 11

CHAPTER 3 RESULTS All pigeons learned to eat from the hopper and to peck lit keys reliably within 2 half-hour sessions. Table 1 presents the number of sessions in each of the two preliminary training conditions. Subjects 66, 659, 16, and 57 met the criterion after 33, 45, 27, and 31 sessions in the simultaneous and 27, 1, 28, and 4 sessions in the successive MTS training conditions, respectively. Figure 1 presents the mean adjusted delay values produced by 66, 659, 16, and 57 during daily sessions. Data from every session are included in these graphs. For Subject 66, the stable adjusted delay with an FR 1 response requirement was around.8 s. After the stability criterion was met, the FR 2 response requirement was implemented. There was a several session lag before increased variability was observed in adjusted delay values. A gradual increase was eventually observed, with a high peak midway through the condition. Between-session variability decreased in subsequent sessions, and delays eventually reached steady state. Delays in the FR 4 condition again exhibited day-to-day variability for roughly 2 days, but eventually reached stability at a value higher than the previous condition. Increased variability with a clear upward trend was observed in the FR 8 condition before the stability criterion was met. The highest variability observed for this subject was seen during the FR 16 response requirement condition. Following an initial ascending trend, a downward trend was observed, with day-to-day adjusted delay variation often exceeding 1 s. Eventually, decreasing levels of variability, and then steady state performance was observed. Adjusted delays decreased to near-zero levels almost immediately after the 12

return to the FR 1 sample-observing response requirement condition. This was followed by daily variability in adjusted delays before stability was reached. Like Subject 66, Subject 659 maintained the adjusted delay at low values during the FR 1 and FR 2 conditions. During the FR 4 condition, the adjusted delay values were slightly but consistently higher than the previous conditions, with variability remaining low. The variability in adjusted delays however, increased at the onset of the FR 8 and FR 16 sample-observing response requirements. A wave-like pattern was observed before adjusted delay values became stable during the FR 8 condition. More dramatic day-to-day variability was observed during the FR 16 condition before stability was observed. Subject 16 also maintained the adjusted delay at low values in the FR 1 condition. These low values persisted several sessions into the FR 2 condition, but an upward trend was eventually observed. This trend was initially highly variable, but eventually reached steady state at a value approximately 3 s higher than in the previous condition. The FR 4 condition was marked by a decrease in daily adjusted delay values for several sessions before an increasing trend that eventually leveled off at steady state. Some of the highest adjusted delay values were observed during the beginning of the FR 8 condition. However, values decreased within 15 sessions to levels similar to but slightly higher than in the previous condition. Subject 57 maintained consistently low adjusted delay values during the FR 1, 2, and 4 sample-observing response requirement conditions. At the onset of the FR 8 condition, higher delay values with increased daily variability were observed. Adjusted delays reached a peak of almost 2 s after about 3 sessions in the condition, but 13

steady state was eventually reached at lower values. High day-to-day variability was observed in the FR 16 condition, with the subject ultimately exhibiting the highest adjusted delays observed in this experiment. Figure 2 presents the summary means (bars) and the inter-quartile range (error bars) for the last 1 sessions of each condition for all 4 subjects. The sessions included are those that met the stability criteria described above. For 66, the delay between sample offset and comparison onset settled around.8 s under the FR1 observing response requirement, followed by an increase to almost 5-s delay under the FR 2 observing response requirement. A smaller increase to over 6 s was observed under the FR 4 response requirement, followed by a mean adjusted delay of 8.5 s in the FR 8 observing response requirement. Steady state summary data from the FR 16 condition showed a decrease in the mean adjusted delay (7.77 s). In accord with the rules for cessation of the ascending series, the FR1 condition was reinstated to assess the extent to which sample-observing response requirements alone were responsible for the observed increases in the mean adjusted delay values. As shown in Figure 1, the bird s adjusted delay value was closer to but lower than the FR2 condition and clearly higher than the first exposure to FR1 than the value obtained under the first exposure to this condition at the beginning of the experiment. For Subject 659 the delay between sample offset and comparison onset settled around.8 s under the FR 1 sample-observing response requirement. This was followed by a slightly higher mean adjusted delay (.9 s) during the FR 2 condition. An increase to around 2.3 s was observed in the FR 4 condition. A further increase to almost a 7 s adjusted delay was observed during the FR 8 sample-observing response 14

requirement, and the FR 16 condition produced the highest adjusted delays, averaging almost 18 s across the last 1 sessions. As with the previous two subjects, Subject 16 showed low measures of mean adjusted delay (.8 s) in the FR 1 condition. A 4.24 s adjusted delay was observed the FR 2 condition. A 5.37 s mean adjusted delay was observed under the FR 4 condition, and a slight increase to 5.74 s was observed in the FR 8 condition. Subject 57 s mean adjusted delay in the FR 1 condition was slightly lower than the previous 3 subjects, settling around.5 s. Mean adjusted delay in the FR 2 condition was only about.2 s higher, settling around.7 s. FR 4 showed a slightly higher mean adjusted delay of 1 s. Subsequently, an increase to almost 5 s was observed in the FR 8 condition. A very large increase in adjusted delay values was observed in the FR 16 condition, ultimately reaching over 38 s, a value that was over 8 times greater than in the previous condition. Figure 3 presents the individual session means (filled circles) taken from sessions that contributed to the means shown in Figure 2 (i.e., the last 1 sessions in each condition). The error bars indicate each session s inter-quartile range. Open triangles represent data from Subject 66 s FR 1 condition replication. This figure shows that variability in adjusted delay values increased as the sample-observing response requirements increased, suggesting that the choice to assess stability using relative rather than absolute terms was appropriate. Figure 4 presents the mean sample latencies (time between sample onset, and the first peck to the sample) for all 4 subjects when performance was stable. For Subject 66, sample latencies of about 1 s were observed during the FR 1, 2, and 4 15

conditions. An increase of about 1 s was observed during the FR 8 condition. A large increase in mean latency variability was observed during the FR 16 condition. The FR 1 condition replication (open triangles) showed slightly higher mean latencies, with a marked increase in between-session variability relative to the first FR 1 condition. Mean sample latencies for Subject 659 were about 1 s for the FR 1, 2, 4 and 8 response requirement conditions. However, during the FR 16 condition, considerably higher mean latencies were observed, ranging from roughly 1-23 s. Mean sample latencies for Subject 16 were consistently between.5 and 1.5 s during the FR 1, 2, 4, and 8 response requirement conditions. Near 1 s mean sample latencies were observed for Subject 57 under the FR 1, 2, 4, and 8 response requirements. However, a large increase in both sample latency and session-to-session variability was observed during the FR 16 condition. Figure 5 presents the median elapsed time (filled circles) between the first and last response to the sample stimulus in the conditions with an extended sampleobserving response requirement for all 4 subjects when performance was stable. Median values were chosen to provide a more representative description. This was necessary because there were a few trials, especially in the higher FR values (8 and 16), where the subject paused for an unusually long time during the ratio. Error bars indicate the inter-quartile range. As with the summary measures of adjusted delay values (Figures 2 and 3), duration between the first and last sample response increased as a function of the FR response requirement. This is not, of course, an unexpected outcome; higher FR ratios typically take longer to complete. However, increases in 16

durations are proportionately greater across conditions than the increase in FR sampleobserving response requirements. Figure 6 shows the median running rate for all 4 subjects when performance was stable. A running rate describes the rate of responding between the first and last sample-observing response during their programmed response requirement. The figure suggests that for all 4 subjects, response rate decreased as a function of the FR response requirement. In other words, the birds were pecking slower when FR values were higher. Figure 7 shows the percent of correct trials as a function of adjusted delay values for each condition. Although many of the curves overlap, close inspection of the figure suggests that the smaller FR response requirements (i.e., FR 1, 2, and 4) produced steeper slopes relative to the higher response requirements (i.e., FR 8, and 16). The smaller FR response requirements have fewer data points (shorter curves) because the subject adjusted the delay less and thus were exposed to fewer delay values. 17

CHAPTER 4 DISCUSSION The results of this study suggest that extended sample-observing response requirements affect adjusted delay values in a titrating MTS procedure. Figures 1, 2, and 3 indicate that, across all four subjects, as the sample-observing response requirement increased, adjusted delay between sample offset and comparison onset also increased. Although the magnitude of this effect varied between subjects, the overall function, which may be described as curvilinear, appears to be similar. The current data suggest that these effects are due to the sample-observing response requirements and not to extended practice. First, although the data from the last 1 sessions in each condition showed clear effects of the sample-observing response requirements, some low values were observed in almost every condition. If extended practice accounted for the obtained results, then a consistent rise in the adjusted delay values would be expected as the subject s performance becomes more and more accurate. Second, a replication of the FR1 condition with one subject showed that the mean adjusted delay value during the second exposure was higher than the original exposure but lower than the adjusted delay value from FR2 suggesting that the sample observing response requirements were important. The data presented in Figures 4, 5, and 6 however, suggest that the time spent in the presence of the sample may have been, at least in part, responsible for the increased adjusted delays. These data show that the time the birds spent in the presence of the sample stimulus was longer than the time necessary to emit the required sample-observing responses. Figure 4 shows that the subjects paused before 18

making the first observing response during higher response requirements. Figure 5 shows that durations to complete larger response requirements were proportionally longer than the programmed increases in those requirements. The longer durations are accounted for by slower running rates on higher response requirements, as shown in Figure 6. Moreover, a direct comparison of the elapsed time between first and last response with obtained titrated delays shows that the adjusted delays across response requirements seem to track changes in duration measures better than changes in the sample-observing response requirement. This suggests that the time spent in the presence of the sample may have been a more important determinant of adjusted delays than the actual number of responses required. Future research should examine whether extended sample-observing responses, additional time spent in the presence of the sample, or a combination of the two, is responsible for the increased adjusted delay values. This could be examined by programming a yoked response-initiated FI for the sample-observing response requirement. For example, the mean duration of responding on an FR 8 sample-observing response requirement could serve as the value of a response-initiated FI programmed on the sample key. Data on the number of pecks on the sample key together with the adjusted delay values might help elucidate the relative contributions of response requirements versus time spent in the presence of the sample. Whether response requirements or the time spent in the presence of the sample stimulus is directly responsible for improved MTS performances, extended sample-observing response requirements are likely to remain a very useful procedure to ensure that the organism attends to the sample stimulus for at least the amount of time it takes to complete the requirement. Without requiring observing responses that are 19

directed to the sample, subjects may engage in other behavior (e.g., preening, looking away), limiting contact with the sample stimulus. Figure 7 shows that the likelihood of accurate choices decreases as a function of the delay between sample offset and comparison onset. This figure further shows that extended sample-observing response requirements appear to attenuate that effect. Although there is considerable overlap in the curves, this display clearly shows the effects of the response requirement and, interestingly, weakening of the effect as a function of increased delay between sample offset and comparison onset. In conclusion, the results from this study provide a strong rationale for employing caution while designing MTS experiments using extended sample-observing response requirements. These requirements may have a greater effect on the dependent variable then simply providing greater exposure to the sample stimulus. For example, Nordholm and colleagues (1995) investigated the effects of the proposed cognitive enhancing agent Linopirdine with six pigeons using an FR 15 response requirement and four squirrel monkeys using an FR 2 response requirement. Results of the current study suggest that the outcomes of the prior investigation may have revealed only a fragment of the total possible effects of the drug on conditional discrimination performance. Latency to respond, duration to complete the ratio, response running rate, and adjusted delay values may be more significantly affected by extended sampleobserving response requirements than laboratory lore suggests. 2

Table 1 Number of Sessions to Reach Acquisition Training Criteria Subject Simultaneous MTS Successive MTS 66 33 27 659 45 1 16 27 28 57 31 4 21

2 FR 1 FR 2 FR 4 FR 8 FR 16 FR 1R 18 16 14 12 1 8 6 4 2 25 5 75 1 125 15 175 2 225 25 275 3 66 25 FR 1 FR 2 FR 4 FR 8 FR 16 2 15 Adjusted Delay (sec) 1 5 25 5 75 1 125 15 175 2 225 25 275 14 12 1 8 6 4 2 FR 1 FR 2 FR 4 FR 8 25 5 75 1 125 15 175 2 225 25 275 659 16 45 FR 1 FR 2 FR 4 FR 8 FR 16 4 35 3 25 2 15 1 5 25 5 75 1 125 15 175 2 225 25 57 Sessions Figure 1. Session by session adjusted delay means. 22

12 1 8 6 4 2 FR1 FR1R FR2 FR4 FR8 FR16 66 2 18 16 14 12 1 8 Adjusted Delay (sec) 6 4 2 8 6 FR1 FR2 FR4 FR8 FR16 659 4 2 16 FR1 FR2 FR4 FR8 FR16 4 35 3 25 2 15 1 5 57 FR1 FR2 FR4 FR8 FR16 Condition Figure 2. Summary means of adjusted delays. 23

14 12 1 8 6 4 2 66 25 2 15 Adjusted Delay (sec) 1 5 1 8 659 6 4 2 16 5 45 4 35 3 25 2 15 1 5 FR1 FR2 FR4 FR8 FR16 57 Condition Figure 3. Mean adjusted delay for last 1 sessions. 24

5 4.5 4 3.5 3 2.5 2 1.5 1.5 66 25 2 15 1 Seconds 5 1.6 1.4 1.2 1.8.6.4.2 659 16 35 3 25 2 15 1 5 FR1 FR2 FR4 FR8 FR16 57 Condition Figure 4. Median sample latencies to first sample observing response. 25

6 5 4 3 2 1 66 Seconds 14 12 1 8 6 4 2 2.5 659 2 1.5 1.5 16 35 3 25 2 15 1 5 FR2 FR4 FR8 FR16 57 Condition Figure 5. Median duration between first and last sample response. 26

7 6 5 4 3 2 1 66 6 5 4 3 2 Seconds 1 6 659 5 4 3 2 1 16 7 6 5 4 3 2 1 FR2 FR4 FR8 FR16 57 Condition Figure 6. Median running rate of sample observing responses. 27

1% 8% 6% 4% 2% FR1 FR2 FR4 FR8 FR16 FR1R % 2 4 6 8 1 12 14 66 1% 8% 6% Percent Correct 4% 2% % 5 1 15 2 25 3 1% 659 8% 6% 4% 2% % 2 4 6 8 1 12 14 16 1% 8% 6% 4% 2% % 5 1 15 2 25 3 35 4 45 5 Delay Value 57 Figure 7. Percent correct as a function of adjusted delay. 28

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