The control of keypecks during automaintenance by prekeypeck omission training

Measures of pigeons’ prekeypecking (pecking in front of the response key) during automaintenance sessions with keypeck omission contingencies indicate that prekeypecks occur frequently and are often followed by grain delivery. When the omission procedure is extended to prevent food delivery following any trial on which prekeypecks occur within 2 in. of the response key, keypecking is not maintained. These results are taken to suggest that the automaintenance keypeck is part of a sequence of approach behaviors, including prekeypecks. The persistence of keypecking during automaintenance schedules appears to result from the adventitious reinforcement of prekeypecks close to the response key, and the effectiveness of the omission procedure seems to depend upon the extent of the approach behaviors which result in omission.     

Classical conditioning of jaw movements in the pigeon: Acquisition and response topography

Previous studies have shown (1) that the form of the pigeon’s conditioned keypecking response resembles that of its ingestive pecking response, (2) that both ingestive and conditioned pecking in the pigeon are compound responses, including both transport (neck-movement) and gape (jaw-movement) components, and (3) that during operant conditioning or autoshaping of pecking behavior, the gape component comes under the associative control of the CS. In the present study, the gape component was experimentally isolated and a classical conditioning paradigm (water US) was used to bring jaw movements under the control of a CS (light). The results indicate that the topography of the jaw-movement CR is very similar to, though more variable than, that of the UR. They are consistent with the hypothesis that reported similarities in the form of ingestive and conditioned pecking responses reflect, in part, classical conditioning of the gape component.     

Some tests of the additivity (autoshaping) theory of behavioral contrast

Two experiments were performed to determine whether the location of the discriminative stimuli affects the amount of positive behavioral contrast exhibited during discrimination learning by pigeons. When signals for reinforcement and nonreinforcement of keypecking were situated directly on the response key (different line tilts), pecking rates during the positive stimulus were higher than when the source of the signals was located elsewhere (changes in chamber illumination or auditory click frequency). These results are in general agreement with the additivity theory of behavioral contrast, which attributes contrast to the combined effects of stimulus-reinforcer and response-reinforcer correlations on behavior directed at signals of reinforcement. Some shortcomings of the theory were discussed, and the notion that behavioral contrast is a basic, unitary phenomenon was criticized.     

The effect of the controllability of auditory discriminative stimuli in the performance of go/no-go discriminations by pigeons

Compared with their performance with localized (on-key) visual stimuli, pigeons are notoriously poor at performing go/no-go discriminations when keypecking for food in the presence of auditory discriminative stimuli. The difference might reflect the fact that an aversive visual onkey stimulus signaling nonreward can be escaped by looking away and not pecking, which contributes to the measure of good discriminative performance, while an auditory stimulus cannot be escaped. In Experiment 1, discriminative performance was significantly improved by providing pigeons with a response incompatible with keypecking by which they could escape a tone S+ and a tone S?. However, the pattern, frequency, and duration of escape responses were found to be insufficient to explain the improvement. In Experiment 2, it was found that the capacity to escape only S+ or only S? enhanced discriminative performance as much as the capacity to escape both. It is theorized that the Pavlovian relationship between the absence of the discriminative stimuli and the nonoccurrence of food might transfer to the instrumental relationships learned in a go/no-go discrimination. The possibility that intermittent stimuli command more attention than continuous stimuli is also considered.     

Reversing the reinforcement contingencies of eating and keypecking behaviors

Four of five pigeons were conditioned to peck a key at a high, stable rate on a VI schedule and then given concurrent access to free food. It was found, in replication of Neuringer’s results, that the pigeons pecked a key for grain in the presence of free grain. When availability of the response key (high-probability response) was made contingent on eating free grain (a lower probability response), there was a progressive increase in free-food eating, confirming Premack’s reinforcement principle. For two additional birds, when availability of the key was made contingent onnot eating the free food (a type of DRO schedule), the frequency of free-food eating declined. Thus, availability of the key. depending on the contingency, reinforced both the eating and noneating of free food.     

Required pecking and refraining from pecking alter judgments of time by pigeons

There is evidence that humans' perception of time is affected by the activity in which they are engaged while they are timing. The more demanding the task, the faster time appears to pass. A similar effect has been found in pigeons. Pigeons trained to discriminate between a short-duration (2-sec) and a long-duration (10-sec) stimulus were required to peck when the stimulus was one color and to refrain from pecking when it was a different color. On probe trials of intermediate durations, the bisection point (50% choice of the stimulus associated with both long and short stimuli) for trials in which the pigeons were required to peck was almost 1 sec longer than on trials in which the pigeons were required to refrain from pecking (Zentall, Friedrich, & Clement, 2006). In the present research, we replicated this effect and determined the relation between this effect and the typical bisection point that occurs when pecking is permitted but not required. Results indicated that the typical procedure results in a bisection point that is between required pecking and refraining from pecking. Furthermore, the rate of pecking when pecking is allowed but not required also falls between the rate of pecking for the required-pecking and refrain-from-pecking conditions. This result suggests that, similar to humans, pigeons underestimate the passage of time when they are active or when attention to time-related cues has to be shared with attention to satisfying the response requirement.     

A functional analysis of collateral behavior under differential-reinforcement-of-low-rate schedules

The collateral behavior of pigeons under differential-reinforcement-of-low-rate (DRL) schedules was evaluated for its role in controlling DRL performance. Two of three pigeons engaged in high rates of collateral keypecking under schedules up to DRL 28 sec. Rate of collateral pecking was positively correlated with DRL efficiency. Topographical features of the collateral behavior were inconsistent with the notion that collateral behavior mediates DRL performance through response-produced stimuli. Rather, the collateral behavior appeared to interfere with the operant response, delaying it long enough to meet the DRL requirement.     

Dissociation of the effect of reinforcer type and response strength on the force of a conditioned response

A dissociation between the effect of reinforcer type and response strength on the force of the pigeon’s keypeck response was shown in three experiments. In Experiment 1, pigeons were trained to peck two conditioned stimuli, one paired with water and another paired with grain. The pigeons made more forceful pecks for grain than for water and also showed a tendency, albeit an unreliable one, to respond on a higher percentage of food trials than water trials. In Experiment 2, the pigeons from Experiment 1 were satiated with either food or water and were then presented with the two conditioned stimuli in an extinction test. It was found that, regardless of the drive state, the pigeons made more forceful pecks to the stimulus that predicted food than to the stimulus that predicted water. In the thirsty group, however, this difference in force was not accompanied by a difference in the percentage of trials with a response. In Experiment 3, pigeons trained with a single reinforcer pecked more often on instrumentally reinforced trials than on Pavlovian conditioning trials, but there was no difference in the force of the pecks. Taken together, these results imply that differences in response strength cannot account for the difference between the force of food- and water-reinforced pecks. Instead, stimulus-substitution theory may provide the best account of the topography of the two types of pecks.     

The effect of localizing the stimulus at the response key on generalization along the hue dimension in the pigeon

After training with a variable-interval schedule of positive reinforcement, pigeons were tested for stimulus generalization along the hue dimension. For one group, the stimulus was located on the response key. For a second group, the stimulus was located on a surface adjacent to the response key. The stimulus-on-key group produced the typical steep gradients normally found with hue stimuli; the stimulus-off-key group produced flat gradients. After discrimination training between the presence and absence of the hue stimulus, both groups produced decremental gradients. In a second experiment, naive pigeons were trained to peck a transparent key with the stimulus surface located approximately 3.8 cm behind the key. When tested for generalization, the hue gradients were decremental. The results suggest that location of the stimulus in the line of sight with pecking is a necessary condition for stimulus control by hue after nondifferential training.     

Control of pigeons’ keypecking by the left-right arrangement of stimuli

Control of pigeons’ keypecking by conditionalities in the spatial arrangement of two element stimuli (designated A and B) was investigated. In Experiment 1, reinforcement for keypecking was made conditional upon the left-right location of A and B: Reinforcement was available when A was on the left and B was on the right (AB), but not on BA, AA, or BB trials. The pigeons successfully discriminated the rewarded AB configuration, but only after a stage in which a particular element in a particular location (e.g., A on left) primarily controlled pecking. Experiments 2 and 3 systematically replicated these findings and included controls to discount discrimination of the AB compound on the basis of the temporal order (e.g., A followed by B) rather than the spatial configuration of the elements. During a generalization test in Experiment 4, the elements were presented singly either in the left (AX, BX) or right (XA, XB) positions. As would be expected had the animals learned “A on the left, B on the right is rewarded,” responding on AX trials exceeded that on XA trials, and responding on XB trials exceeded that on BX trials.     

Food-anticipatory response to restricted food access based on the pigeon’s biological clock

Pecking at the food key was recorded for 4 pigeons given restricted access to food. The access period was set at a fixed time in a light-dark cycle, continuous dark, or continuous light. The pecking activity occurred a few hours before onset of the access period in all three conditions. When the bird was again given free access to food after being released from restricted access, its pecking rhythm free-ran in the continuous dark. The initial phase of the rhythm coincided with the onset of the food-anticipatory pecking in the previous condition. These results suggest that the bird anticipated food access, based on its biological clock mechanism. When the access period was set in the dark phase of the light-dark cycle, anticipatory pecking did not occur, although pecking actually occurred during the access period. The pigeon’s activity is reduced during the dark phase of the light-dark cycle. Therefore, the bird’s activity level was probably too low to shape the anticipatory response, even if the access period was stored in memory in the biological clock.     

Motivational mechanisms and schedule-induced behavioral stereotypy

In two experiments, behavioral stereotypies elicited by scheduled presentations of food and water were compared. In Experiment 1, pigeons were exposed to a fixed-time 30-sec (FT 30-sec) schedule of food or water deliveries with a brightening keylight stimulus signaling time to the unconditioned stimulus (UCS) on each trial. Food and water presentations both produced terminal autoshaped keypecking that was similarly distributed in the trial but differed in response topography and persistence. Locomotor interim behavior was different in the two motivational conditions: With food presentations, it consisted of a “retreat” to the rear of the chamber after UCS termination, followed by “pacing” in the midportion of trials. The water schedule produced very little locomotor activity with no regular distribution in the trial. Experiment 2, using a random-time 30-sec (RT 30-sec) schedule, showed that the differences in interim locomotor behavior persisted in the absence of temporal predictability of the UCS and the keypecking terminal response. The results are taken to support Timberlake’s (1983a) behavior-system theory.     

Midsession reversal learning: why do pigeons anticipate and perseverate?

Past research has shown that when given a simultaneous visual-discrimination midsession reversal task, pigeons typically anticipate the reversal well before it occurs and perseverate after it occurs. It appears that they use the estimation of time (or trial number) into the session, rather than (or in addition to) the more reliable cue, the outcome from the previous trial (i.e., a win–stay/lose–shift response rule), to determine which stimulus they should choose. In the present research, we investigated several variables that we thought might encourage pigeons to use a more efficient response strategy. In Experiment 1, we used a treadle-stepping response, rather than key pecking, to test the hypothesis that reflexive key pecking may have biased pigeons to estimate the time (or trial number) into the session at which the reversal would occur. In Experiment 2, we attempted to make the point of reversal in the session more salient by inserting irrelevant trials with stimuli different from the original discriminative stimuli, and for a separate group, we added a 5-s time-out penalty following incorrect choices. The use of a treadle-stepping response did not improve reversal performance, and although we found some improvement in reversal performance when the reversal was signaled and when errors resulted in a time-out, we found little evidence for performance that approached the win–stay/lose–shift accuracy shown by rats.     

Signal-food contingency and signal frequency in a continuous trials auto-shaping paradigm

Five groups of pigeons were studied in an auto-shaping procedure which programmed two types of trials represented by hues on the response key. Each signal was separated by a brief intertriai interval. Three groups were studied with a positive correlation between one of the signals and food (contingent groups). They differed with respect to the frequency with which the positive signal appeared. Two noncontingent groups were studied in which the correlation between the signals and food was eliminated by programming food with the same probability following either signal. One noncontingent group had a high density of reinforcement produced by adding reinforcement in the other signal, at the same rate as programmed in the positive signal for the contingent groups. The other noncontingent group experienced the same number of reinforcements in the session as the contingent group with the least frequent positive trial, but these reinforcements were distributed with equal probability across the signals. Birds in the contingent groups with intermediate or infrequent positive signals all acquired reliable pecking, with acquisition most rapid for the infrequent signal. Maintained responding covaried with the speed of acquisition. No birds in the noncontingent groups showed reliable responding. Birds in the contingent group with a frequent positive signal (approximately 3/4 of the session), also showed no reliable pecking. This result suggests that more than one noncontingent group is informative for assessing the role of differential reinforcement probability in the acquisition of auto-shaped keypecking. In particular, a noncontingent group which controls for the frequency of reinforced trials is an appropriate reference group.

     

On the role of trial outcomes in delayed discriminations

Delayed simple discriminations are typically retained more accurately over longer delays by pigeons than are delayed conditional discriminations (e.g., Honig & Wasserman, 1981). In two experiments, we investigated the extent to which trial outcomes contribute to this difference by comparing performances when all trials ended with food reinforcement versus when only half of the trials did. Experiment 1 showed that when food was presented on all trials, contingent upon either pecking or not pecking the test stimulus, levels of retention and rates of forgetting were comparable for these two tasks. By contrast, Experiment 2 showed better retention of delayed simple than delayed conditional discriminations when half of the trials ended with food and the other half in extinction. Furthermore, delayed simple discriminations were retained more accurately with food versus no-food outcomes than with food at the end of every trial, whereas the reverse was true for delayed conditional discriminations. These findings indicate that retention differences between these tasks are another instance of the differential outcomes effect.     

Effects of trial duration on overall and momentary rates of maintained autoshaped keypecking: Choice and single-stimulus trials

Pigeons were exposed to differentially cued autoshaping trials in which conditioned stimuli were followed by food after 6 or 14 sec. Average and momentary rates of keypecking were examined on two types of unreinforced test trials: single-stimulus probe trials and simultaneous choice trials, each 40 sec in duration. Rates averaged over the 40-sec test trials did not favor the cue associated with the shorter delay to food (the short-delay cue) on either type of test trial; however, average rates prior to the scheduled time of food delivery were reliably higher for the short-delay cue on choice trials. Momentary rates of keypecking during choice trials varied as a function of both cue and elapsed time from trial onset. At short elapsed trial times, rate of pecking was higher for the short-delay cue, with this difference reversing at longer times. A reversal of the programmed relation between key color and delay to food presentation for 5 birds confirmed the generality of these findings. Implications of these data for models of Pavlovian conditioning and for methods of assessing conditioned response strength are discussed.     

The effects of activity conditioned in random CS/US training on performance in autoshaping

Random presentations of keylights and food retarded acquisition and suppressed asymptotic rates of keypecking in autoshaping. Sequences of 10 sessions of random training alternated with 10 sessions of autoshaping resulted in poor performance (in terms of both the acquisition and asymptotic indices) relative to a group that received sequences of CS-only (rather than random) training alternating with autoshaping. When the birds that previously were trained with the random sequence were exposed to CS-alone extinction, retardation of acquisition was alleviated but the asymptotic suppression was not (Experiment 1). Pigeons with a history of autoshaping without prior random training showed no asymptotic suppression when exposed to the random procedure. These birds, when switched between two-session sequences of random training alternating with two-session sequences of autoshaping, were able to (1) surpass pigeons that received CS-only rather than random treatment in terms of asymptotic levels of responding in autoshaping, and (2) showed improvement in extinction performance with repeated random/autoshaping sequences (Experiment 2). Detailed observations of pigeons in random training showed that stereotypic activity behaviors were acquired, and these generally persisted in autoshaping; the degree of change in these behaviors was related to asymptotic rates of keypecking in autoshaping (Experiment 3). The same kinds of behaviors were observed when pigeons initially were autoshaped, and these persisted in subsequent random and fixed-interval 10-sec training. We suggest that the suppression effect is reflected in activity, conditioned in random training, which persists in autoshaping (especially if the activity is compatible with the kinds of behaviors elicited by the autoshaping contingency itself), and that the effect may be a deficit due to performance factors rather than to associative learning.     

Mechanisms of multidimensional grouping,fusion, and search in avian texture discrimination

The influence of dimensional organization on pigeon texture perception was examined in a simultaneous conditional discrimination procedure. Six experienced pigeons were reinforced for pecking at a small block oftarget elements randomly located within a larger array ofdistractor elements in each texture stimulus. Target/distractor differences in color, size, orientation, and combinations of these dimensions were examined. In Experiment 1, the influence of target/distractor similarity on performance was investigated by using different forms of unidimensional and conjunctively organized texture stimuli made of two and three dimensions. Targets infeature displays, in which the two regions consistently differed along a single dimension, were located more accurately than targets inconjunctive displays, where a combination of values from all dimensions defined each region. In Experiment 2, a tradeoff between response speed and accuracy was found in the pigeons’ processing of conjunctive displays. In Experiment 3, the number of distractors differentially influenced the localization of feature and conjunctive targets. Overall, the pigeons’ reactions to these feature and conjunctive stimuli paralleled those of humans, suggesting that functionally equivalent mechanisms may mediate the perceptual grouping, search, and discrimination of textured multidimensional stimuli in both species.     

More evidence of robust spatial associative memory in the pigeon,Columba livia

Willson and Wilkie (1993) developed a novel procedure for assessing pigeons’ memory for the spatial location of food. Only one of four locations (consisting of an illuminated pecking key and grain feeder) provided food each day. Over days, different locations provided food. The pigeons’ tendency to revisit the location that was profitable on the previous day demonstrated memory for food-spatiallocation associations over a period of 24 h, retention longer than previously reported for this species. This basic finding was replicated and extended in three experiments. Experiment 1 demonstrated that location-food discriminations were also remembered well when established with successive rather than concurrent procedures. Experiment 2 demonstrated that pigeons can remember two location-food associations over 24 h. Experiment 3 showed that the discrimination training inherent in this paradigm is important for retention; retention was impaired when only the rewarded location was presented. Overall, this research suggests that cross-species differences in spatial memory performance may be due to quantitative rather than qualitative differences in the memory system underlying performance.     

Conjunctive differentiation of gape during food-reinforced keypecking in the pigeon

The pigeon’s keypecking response includes both a head-transport (peck) and a jaw-movement (gape) component. Because the two components are mediated by different effector systems, they may potentially be viewed as orthogonal responses. A response differentiation procedure was used to bring gape amplitude under operant control. The procedure employed a conjunctive response requirement in which reinforcement was contingent upon both gaping and key contact. The key-contact requirement was held constant, while the gape contingency was systematically varied to reinforce either decreases or increases in gape amplitude with respect to baseline. The procedure was effective in shifting the gape distributions in both the upward and downward directions and in inducing new gape values that deviated from the baseline in the reinforced direction. These observations indicate that gape may be brought under operant control. However, subjects showed a bias in the differentiation of the gape response, such that larger gapes were more readily differentiated than smaller gapes. The results are discussed in relation to the methodological utility of the paradigm, the problem of biological constraints on learning, and the heuristic utility of a response components analysis.