Design and evaluation of a touchscreen apparatus for operant research with pigeons

We developed a touchscreen apparatus for pigeons and conducted a series of experiments that assessed its utility for free-operant procedures. The apparatus incorporated an on-board Windows computer, an electromechanical interface, an amplified speaker, and the touchscreen. We found that merely projecting a virtual key on the screen was insufficient; too many pecks missed the key. Adding a visual target in the center of the key and providing visual feedback for on-key pecks both failed to improve response accuracy. Accuracy was improved by imposing a timeout after off-key pecks or providing a physical boundary around the key. With the physical boundary, response accuracy was comparable to that obtained with conventional plastic keys, and response acquisition via autoshaping also was comparable. Mixing the color elements of the screen's pixels produced color stimuli, but the colors did not function as pure wavelengths of light in tests of stimulus generalization. Both colors and geometric shapes functioned as discriminative stimuli in multiple schedules with variable-interval and extinction components or rich and lean fixed-ratio components. In general, our touchscreen apparatus is a viable alternative to conventional pigeon chambers and increases the experimenter's options for visual stimuli, auditory stimuli, and the number and location of response keys.     

How pigeons discriminate the relative frequency of events.

This study examined how pigeons discriminate the relative frequencies of events when the events occur serially. In a discrete-trials procedure, 6 pigeons were shown one light nf times and then another nl times. Next, they received food for choosing the light that had occurred the least number of times during the sample. At issue were (a) how the discrimination was related to two variables, the difference between the frequencies of the two lights, D = nf - nl, and the total number of lights in the sample, T = nf + nl; and (b) whether a simple mathematical model of the discrimination process could account for the data. In contrast with models that assume that pigeons count the stimulus lights, engage in mental arithmetic on numerons, or remember the number of stimuli, the present model assumed only that the influence of a sample stimulus on choice increases linearly when the stimulus is presented, but decays exponentially when the stimulus is absent. The results showed that, overall, the pigeons discriminated the relative frequencies well. Their accuracy always increased with the absolute value of the difference D and, for D > 0, it decreased with T. Performance also showed clear recency, primacy, and contextual effects. The model accounted well for the major trends in the data.     

Temporal control in fixed-interval schedules

The peak procedure was used to study temporal control in pigeons exposed to seven fixed-interval schedules ranging from 7.5 to 480 s. The focus was on behavior in individual intervals. Quantitative properties of temporal control depended on whether the aspect of behavior considered was initial pause duration, the point of maximum acceleration in responding, the point of maximum deceleration, the point at which responding stopped, or several different statistical derivations of a point of maximum responding. Each aspect produced different conclusions about the nature of temporal control, and none conformed to what was known previously about the way ongoing responding was controlled by time under conditions of differential reinforcement. Existing theory does not explain why Weber's law so rarely fit the results or why each type of behavior seemed unique. These data fit with others suggesting that principles of temporal control may depend on the role played by the particular aspect of behavior in particular situations.     

Discrimination of response-reinforcer and response-stimulus contingencies in pigeons

For three pigeons (Experiment 1), the presentation of a red response key ended with a food presentation either following two responses separated by at least 10 seconds (a DRL contingency) or following a 10-second response-free period (a DRO contingency). For three other birds (Experiment 2), a brief stimulus presentation terminated the DRL and DRO contingencies. A white side key was presented next and ended with response-dependent food following one contingency and a timeout following the other. Since the contingency on the red key was unsignaled, differential responding on the white side key could indicate that the two response-reinforcer relations had been discriminated. In Experiment 1, the red-key duration and number of responses influenced white-key responding following the contingency that predicted the timeout. A response-initiated DRO was instated, and the influence of red-key duration and response number on white-key responding was diminished. In both experiments, the 10-second time criterion in both contingencies was varied from 0.34 second to 10 seconds. Even at short time intervals the DRO and DRL contingencies were readily discriminated. Pigeons tended to class the two contingencies according to a rule that did not involve simply stimulus duration, numbers of responses, or even the time between a response and its consequence.     

Timing, remembering, and discrimination

Four pigeons were first trained in a timing procedure. In one condition, each trial began with the presentation of an X on the center key, followed by a delay (short or long), after which two side keys were lit. If the delay was short, pecks to the red side key were reinforced. If the delay was long, pecks to the green side key were reinforced. In a second condition, the opposite contingencies applied following presentation of a square on the center key. Choice responses were then tested at 10 time intervals ranging from short to long (1 to 4 s and 4 to 7 s in different conditions). The two timing conditions were combined to create a remembering condition in which correct responding depended upon discrimination of both the sample stimulus (X or square) and the delay interval (short or long). Choices varied systematically across delay in timing conditions, but in remembering conditions, accurate choice at the training delays did not initially generalize to intermediate delays. However, with prolonged training in the remembering task, the response pattern began to resemble that of the timing conditions. Generalization gradients were asymmetrical, in accordance with Weber's Law, in that greater generalization occurred with longer delays than with shorter delays.     

Context effects in a temporal discrimination task" further tests of the Scalar Expectancy Theory and Learning-to-Time models

Pigeons were trained on two temporal bisection tasks, which alternated every two sessions. In the first task, they learned to choose a red key after a 1-s signal and a green key after a 4-s signal; in the second task, they learned to choose a blue key after a 4-s signal and a yellow key after a 16-s signal. Then the pigeons were exposed to a series of test trials in order to contrast two timing models, Learning-to-Time (LeT) and Scalar Expectancy Theory (SET). The models made substantially different predictions particularly for the test trials in which the sample duration ranged from 1 s to 16 s and the choice keys were Green and Blue, the keys associated with the same 4-s samples: LeT predicted that preference for Green should increase with sample duration, a context effect, but SET predicted that preference for Green should not vary with sample duration. The results were consistent with LeT. The present study adds to the literature the finding that the context effect occurs even when the two basic discriminations are never combined in the same session.     

Immediate reinforcement in delayed reward learning in pigeons

Pigeons were trained on simultaneous red-green discrimination procedures with delayed reward and sequences of stimuli during the delay. In Experiment 1, three stimuli appeared during the 60-second intervals between the correct responses and reward, and the incorrect responses and nonreward. The stimulus that immediately followed a correct response also preceded nonreward, and the stimulus that followed an incorrect response preceded reward. These stimuli were 10 or .33 second in duration for different groups. Stimuli during the remainder of the delay interval differed following correct and incorrect responses. Group 10 initially persisted in the nonrewarded choice, but shifted to a preponderance of rewarded responses after further training. Group .33 rapidly acquired the correct response. Similar results were obtained in Experiment 2 where delay intervals consisted of opposite sequences of two stimuli of equal duration and total delays were 6, 20, or 60 seconds. Early in training, generalization of differential conditioned-reinforcing properties from the conditions preceding reward and nonreward to postchoice conditions had a greater effect relative to backchaining than it did later. It was concluded that delayed-reward learning is best analyzed in terms of the conditioned-reinforcing value of the patterns of cues that follow immediately after rewarded and nonrewarded responses.     

Discriminated timeout avoidance in pigeons: the roles of added stimuli

Two experiments examined pigeons' postponement of a signaled extinction period, or timeout (TO), from an ongoing schedule of response-dependent food delivery. A concurrent-operant procedure was used in which responses on one (food) key produced food according to a variable-interval schedule and responses on a second (postponement) key delayed the next scheduled TO according to a response-TO (R-TO) interval. A series of response-independent stimulus changes on the food key temporally partitioned the R-TO into three equal segments (S1, S2, and S3). Postponement responses, in addition to postponing TO, also reinstated S1, the stimulus correlated with the greatest temporal distance from TO. In Experiment 1, the R-TO interval was manipulated systematically across blocks of sessions (conditions) at a given ratio of R-TO:TO duration. This R-TO:TO ratio was manipulated across blocks of conditions (phases). Postponement response rates varied inversely with R-TO interval in each phase. Changes in the R-TO:TO ratio did not produce consistent differences except at the 1:10 ratio for some pigeons, where it disrupted postponement responding in some conditions. Most of the postponement responses occurred in the presence of S2 and S3, the stimuli most proximal to TO, whereas most of the food-key responses occurred in S1. In Experiment 2, the R-TO contingencies were systematically manipulated in the presence of the time-correlated stimuli. In one set of conditions, the R-TO contingencies were made either ineffective or less effective in the presence of one or more stimuli. Postponement responses typically shifted to stimuli in the presence of which responses were relatively more effective. Postponement responses decreased markedly when the added stimuli were removed, and then recovered when the stimuli were reinstated. Results from both experiments indicate that the added stimuli in a discriminated TO-avoidance procedure serve predominately discriminative functions, delineating periods during which behavior is maximally effective. The results parallel those obtained in shock-avoidance procedures, providing further evidence that TO functions as an aversive stimulus.     

Cocaine's effects on food-reinforced pecking in pigeons depend on food-deprivation level.

Four pigeons deprived to 80% of their laboratory free-feeding weights pecked keys under a multiple fixed-ratio 30 fixed-interval 5-min schedule of food presentation. Components alternated strictly with 15-s timeouts separating them; each was presented six times. When rates of pecking were stable, 2 pigeons' weights were reduced to 70%, and the other 2 pigeons' weights were increased to 82.5% to 85% of free-feeding levels. Cocaine (1.0, 3.0, 5.6, and 10.0 mg/kg and saline) was administered 5 min prior to sessions. When each dose had been tested twice, pigeons' weights were adjusted to the level that they had not yet experienced, and cocaine was tested again. Cocaine reduced response rates in a dose-dependent manner under the fixed-ratio schedule and under the fixed-interval schedule at high doses, and increased rates under the fixed-interval schedule at low low doses. Reductions in pecking rates occurred at lower doses under both schedules in 3 of 4 pigeons when they were less food deprived compared to when they were more food deprived. Low doses of cocaine increased low baseline rates of pecking in the initial portions of the fixed-interval schedules by a greater magnitude when pigeons were more food deprived. Thus, food-deprivation levels altered both the rate-decreasing and rate-increasing effects of cocaine. The implications of these results for the mechanisms by which food deprivation increases cocaine self-administration and for the dependence of cocaine's effects on the baseline strength of operant behavior are discussed.     

Discrete-trial choice in pigeons: Effects of reinforcer magnitude

The preference of pigeons for large reinforcers which occasionally followed a response versus small reinforcers which invariably followed a response was studied in a discrete-trial situation. Two differently colored keys were associated with the two reinforcement alternatives, and preference was measured as the proportion of choice trials on which the key associated with uncertain reinforcement was pecked. A combination of choice and guidance trials insured that received distributions of reinforcement equalled the scheduled distributions. For five of six subjects, preference for the uncertain reinforcer appeared to be a linear function of the magnitude of the certain reinforcer. In addition, there was greater preference for the response alternative associated with uncertain reinforcement than would be expected on the basis of net reinforcer value.     

Stimulus control of delayed matching in pigeons: Directed forgetting

Pigeons were trained in delayed matching-to-sample with two postsample stimuli. A postsample R-cue signaled that a matching choice phase would follow. A postsample F-cue signaled that a matching choice phase would not follow. Previous research found reduced matching accuracy on F-cued probe trials when comparison stimuli were presented in the choice phase. The present four experiments systematically varied the events following an F-cue to determine the conditions under which the F-cue reduces delayed-matching accuracy. When F-cues and R-cues controlled different behavior, matching on probe trials was poor. When both cues controlled the same behavior, matching on probe trials was good. This result is best explained by the theory that comparison stimuli retrieve the sample representation, but only in the behavioral context established by the R-cue. The present research supports the view that response-produced stimuli serve a contextual role in animal short-term memory.     

The Timing Effects of Accent Production in Periodic Finger-Tapping Sequences

When subjects are required to produce short sequences of equally paced finger taps and to accentuate one of the taps, the interval preceding the forceful tap is shortened and the one that immediately follows the accent is lengthened. Assuming that the tapping movements are triggered by an internal clock, one explanation attributes the mistiming of the taps to central factors: The momentary rate of the clock is accelerated or decelerated as a function of motor preparation to, respectively, increase or decrease the movement force. This hypothesis predicts that the interre-sponse intervals measured between either tap movement onsets or movement terminations (taps) will show the same timing pattern. A second explanation for the observed interval effects is that the tapping movements are triggered by a regular internal clock but the timing of the successive taps is altered because the forceful movement is completed in less time than the other tap movements are. This “peripheral” hypothesis predicts regular timing of movement onsets but distorted timing of movement terminations. In the present study, the trajectories of the movements performed by subjects were recorded and the interresponse intervals were measured at the beginning and the end of the tapping movements. The results of Experiment 1 showed that neither model can fully explain the interval effects: The fast forceful movements were initiated with an additional delay that took into account the small execution time of these movements. Experiment 2 reproduced this finding and showed that the timing of the onset and contact intervals did not evolve with the repetition of trial blocks. Therefore, the assumption of an internal clock that would trigger the successive movements must be rejected. The results are discussed in the framework of a modified two-stage model in which the internal clock, instead of triggering the tapping movements, provides target time points at which the movements have to produce their meaningful effects, that is, contacts with the response key. The timing distortions are likely to reflect both peripheral and central components.     

Transfer of matching performance in pigeons.

Three pigeons were given extensive training on three-key simultaneous matching problems using geometric-form and hue stimuli. After acquisition of matching, the birds were tested with pairs of stimuli involving one or both novel members. Matching during the test stimuli occurred less often than during the later stages of the acquisition phase, but more often than would occur if no transfer had taken place. Greater positive transfer was observed for problems that involved one, rather than two, novel stimuli. In the second phase of the experiment, previously trained birds were shifted to problems that required symbolic matching, i.e., the pigeons had to associate a particular center-key stimulus with a particular side-key stimulus. On each trial, one of two simuli was presented on the center key, and two other stimuli, different from those used on the center key, were displayed on the side keys. When the problem shift was introduced, correct responding was impaired, but remained considerably above chance level and quickly recovered in following sessions. The results were interpreted as favoring a stimulus-response-chaining account of matching behavior.     

The discrimination of relative frequency by pigeons.

Five experiments addressed the issue of how pigeons learn to discriminate the relative frequency of stimuli. During a sampling period, three different stimuli (keylights) were presented serially, in mixed order, and with different frequencies. During a choice period, the stimuli were presented simultaneously, and reinforcement was arranged for choosing the stimulus that was presented the least number of times during the sample. The results showed that (a) the overall proportion of correct choices was always above chance levels; (b) the likelihood of a correct choice decreased with the serial position of the correct stimulus, a negative recency effect; (c) when the last three stimuli of the sample were constrained to be one of each kind, the negative recency effect decreased but errors became more likely when the correct stimulus occurred early in the sample, a negative primacy effect; (d) accurate performance generalized to new and larger samples; and (e) under some conditions the probability of a correct choice was independent of the serial position of the correct stimulus. The serial position curves suggest that in a least frequent discrimination task, two processes determine how the least frequent stimulus controls behavior: a passive decay process (the stimulus loses its effectiveness with time since its last occurrence), and a residual salience process (when the stimulus occurs in the first position it may decay to a higher asymptote than when it occurs in later positions.     

Discriminability of fixed-ratio schedules for pigeons: effects of payoff values

Three pigeons, previously trained to discriminate different numbers of responses (fixed ratios), were tested under different reinforcement contingencies (payoff matrices) at two levels of sensitivity. For one subject, relative reinforcement magnitude was varied—at first, across sessions and then, at midsession by reversing values—without exteroceptive cues. For another, relative reinforcement magnitude and/or probability was varied every 50 trials with cues by correlating different payoff matrices with different key colors. For the third subject, relative reinforcement probability was varied more frequently with cues—in the limit, at random—to demonstrate stimulus control of response bias on a trial-by-trial basis. A signal-detection analysis showed that bias changed with payoffs, for as many as seven different matrices, while sensitivity remained unchanged. The obtained functions (receiver operating characteristics) were similar under different payoff conditions, which suggests that a single mechanism controls bias. However, they differed enough in slope to require a relatively complex account (e.g., the general Gaussian model of detection theory).     

Response specificity in animal timing.

The stimuli that control responding in the peak procedure were investigated by training rats, in separate sessions, to make two different responses for food reinforcement. During one type of session, lever pressing was normally reinforced 32 s after the onset of a light. During the other type of session, chain pulling was normally reinforced either 8 s after the onset of one auditory cue or 128 s after the onset of a different auditory cue. For both types of sessions, only the appropriate manipulandum was available, and 20% of the trials lasted 240 s and involved no response-contingent consequences. Rats were then tested with the auditory cues in the presence of the lever and the light in the presence of the chain. If the time of reinforcement associated with each stimulus was learned, response rates should peak at these times during transfer testing. However, if a specific response pattern was learned for each stimulus, little transfer should occur. The results did not clearly support either prediction, leading to the conclusion that both a representation of the time of reinforcement and the rat's own behavior may control responding in this situation.     

Biasing the pacemaker in the behavioral theory of timing

In the behavioral theory of timing, pacemaker rate is determined by overall rate of reinforcement. A two-alternative free-operant psychophysical procedure was employed to investigate whether pacemaker period was also sensitive to the differential rate of reinforcement. Responding on a left key during the first 25 s and on a right key during the second 25 s of a 50-s trial was reinforced at variable intervals, and variable-interval schedule values during the two halves of the trials were varied systematically. Responding on the right key during the first 25 s and on the left key during the second 25 s was not reinforced. Estimates of pacemaker period were derived from fits of a function predicted by the behavioral theory of timing to right-key response proportions in consecutive 5-s bins of the 50-s trial. Estimates of pacemaker period were shortest when the differential reinforcer rate most strongly favored right-key responses, and were longest when the differential reinforcer rate most strongly favored left-key responses. The results were consistent with the conclusion that pacemaker rate is influenced by relative reinforcer rate.     

Disruption of temporally organized behavior by morphine

Four pigeons pecked keys in two different procedures commonly used in the study of timing, or temporal discrimination. Sessions consisted of 40 trials. During half of the trials, two keys were presented for 50 s. Left-key pecks were reinforced according to a variable-interval 67.86-s schedule during the first 25 s of the trial, and right-key pecks were not reinforced. During the second 25 s of the trial, right-key pecks were reinforced according to the same schedule, and left-key pecks were not reinforced. In the other half of the 40-trial session, the center key was presented. The majority of these trials arranged fixed-interval 2.5-s schedules. Occasionally a probe, or peak-interval, trial was presented. These trials were 100 s in duration and terminated without reinforcement. These two procedures were used to examine the effects of morphine on indexes of timing and on patterns of responding. Morphine altered behavior in a race-dependent manner in both procedures. Low baseline (saline) response rates were increased following morphine administration, and high baseline rates were either unaffected or decreased slightly. Rate-dependent effects appeared as leftward shifts in the timing index for two-key trials and decreases in the index of curvature for fixed-interval trials. Despite large changes in response rates, no consistent shift of the peak time was observed during peak-interval trials. These results are discussed primarily in terms of rate dependency; that is, rates of responding following drug administration tend to be determined in large part by rates of responding under baseline conditions.     

Waiting in pigeons: the effects of daily intercalation on temporal discrimination.

Pigeons trained on cyclic-interval schedules adjust their postfood pause from interval to interval within each experimental session. But on regular fixed-interval schedules, many sessions at a given parameter value are usually necessary before the typical fixed-interval "scallop" appears. In the first case, temporal control appears to act from one interfood interval to the next; in the second, it appears to act over hundreds of interfood intervals. The present experiments look at the intermediate case: daily variation in schedule parameters. In Experiments 1 and 2 we show that pauses proportional to interfood interval develop on short-valued response-initiated-delay schedules when parameters are changed daily, that additional experience under this regimen leads to little further improvement, and that pauses usually change as soon as the schedule parameter is changed. Experiment 3 demonstrates identical waiting behavior on fixed-interval and response-initiated-delay schedules when the food delays are short (less than 20 s) and conditions are changed daily. In Experiment 4 we show that daily intercalation prevents temporal control when interfood intervals are longer (25 to 60 s). The results of Experiment 5 suggest that downshifts in interfood interval produce more rapid waiting-time adjustments than upshifts. These and other results suggest that the effects of short interfood intervals seem to be more persistent than those of long intervals.     

The sunk cost effect with pigeons: some determinants of decisions about persistence

The sunk cost effect occurs when an individual persists following an initial investment, even when persisting is costly in the long run. The current study used a laboratory model of the sunk cost effect. Two response alternatives were available: Pigeons could persist by responding on a schedule key with mixed ratio requirements, or escape by responding on a second key. In Experiment 1, mean response requirements for persistence and escape were varied across conditions. Pigeons persisted (committing the sunk cost error) when persisting increased the mean response requirement only slightly but not when persisting was sufficiently nonoptimal. Experiment 2 explored more systematically combinations of ratios and probabilities assigned to the schedule key. Persistence varied with the ratio of the mean global response requirements for persistence and escape. In Experiment 3, transitions between ratios were signaled. This reduced nonoptimal persistence, and produced some instances of a reverse sunk cost error--escaping when persistence was optimal. In Experiment 4, it was optimal to escape after the second-smallest ratio ever presented. Pigeons escaped at approximately the optimal juncture, especially in conditions with added signals. Overall, this series of experiments suggests that the sunk cost error may arise in part because persistence is the default behavioral strategy in situations where the contingencies for escape and persistence are insufficiently disparate and/or it is relatively difficult to discriminate when to escape. The study also demonstrates the utility of animal models of complex decision making situations.