Attention, the presolution period, and choice accuracy in pigeons

Six pigeons were trained first on a color then on a form discrimination; four other pigeons were trained first on form and then on color. One of two colors or one of two forms (sample stimuli) appeared in the center of a touch sensitive monitor for 5 pigeons and in the center and in 16 other locations for 5 other pigeons. A peck anywhere within the region in which the sample stimuli appeared produced two white disks (comparison stimuli), one on the left and one on the lower right corners of the screen. Correct left-right choices provided food. Although of no consequence, the location of pecks in presence of the sample was predictive of the pigeon's subsequent choice. Accuracy, choice of the correct comparison stimulus, was greater when the sample stimuli appeared in the center as well as 16 other locations than when it appeared only in the center. The presolution period, the period of chance accuracy prior to evidence of discrimination learning, was decreased on each task following training on the other task. This evidence of facilitation following an extra-dimensional shift was attributed to continued relevance of the conditions under which the first task was learned. The duration of the presolution period was inversely related to asymptotic accuracy-data accounted for by Heinemann's (1983) theory of information processing during the presolution period.     

Representation of time intervals in a double bisection task: Relative or absolute?

We examined if the representation of time intervals in a temporal discrimination task is based not only on their absolute but also on their relative durations. Six pigeons learned two temporal discriminations. In the first, red and green choices were correct following 2-s and 8-s samples, respectively. In the second, vertical and horizontal bar choices were correct following 4-s and 16-s samples, respectively. In a previous study [Zentall, T.R., Weaver, J.E., Clement, T.S., 2004. Pigeons group time intervals according to their relative duration. Psychon. Bull. Rev. 11, 113-117.], tests with 4-s samples and red/green comparisons revealed a bias for red, whereas tests with 8-s samples with vertical/horizontal comparisons revealed a bias for horizontal. These results were interpreted in terms of relative encoding of sample durations. We attempted to replicate this finding but instead of testing with only 4-s or 8-s samples, we tested with several other sample durations to obtain a psychometric function. Results were inconsistent with the relative encoding hypothesis.     

Species differences between rats and pigeons in choices with probabilistic and delayed reinforcers

An adjusting-delay procedure was used to study rats' choices with probabilistic and delayed reinforcers, and to compare them with previous results from pigeons. A left lever press led to a 5-s delay signaled by a light and a tone, followed by a food pellet on 50% of the trials. A right lever press led to an adjusting delay signaled by a light followed by a food pellet on 100% of the trials. In some conditions, the light and tone for the probabilistic reinforcer were present only on trials that delivered food. In other conditions, the light and tone were present on all trials that the left lever was chosen. Similar studies with pigeons [Mazur, J.E., 1989. Theories of probabilistic reinforcement. J. Exp. Anal. Behav. 51, 87-99; Mazur, J.E., 1991. Conditioned reinforcement and choice with delayed and uncertain primary reinforcers. J. Exp. Anal. Behav. 63, 139-150] found that choice of the probabilistic reinforcer increased dramatically when the delay-interval stimuli were omitted on no-food trials, but this study found no such effect with the rats. In other conditions, the probability of food was varied, and comparisons to previous studies with pigeons indicated that rats showed greater sensitivity to decreasing reinforcer probabilities. The results support the hypothesis that rats' choices in these situations depend on the total time between a choice response and a reinforcer, whereas pigeons' choices are strongly influenced by the presence of delay-interval stimuli.     

The role of common reinforced comparison responses in acquired sample equivalence

Different samples occasioning the same reinforced comparison response in matching-to-sample are interchangeable for one another outside of original training. The present studies were designed to verify the role of these common responses in producing acquired sample equivalence by explicitly varying the presence or absence of this commonality during training. In each of the two experiments, one group of pigeons made the same reinforced choice response following multiple sample stimuli, whereas controls either made different reinforced choices following each sample (Experiment 1) or reinforced choices after only two of four center-key stimuli (Experiment 2). Later, two of the original samples/center-key stimuli were established as conditional cues for new comparison responses, after which the ability of the remaining samples/center-key stimuli to occasion those new responses was assessed. Following common-response training, matching accuracy was higher on class-consistent than on class-inconsistent transfer tests, whereas accuracy in the controls was generally intermediate between these two extremes, a pattern similar to that reported in the human paired-associate literature. These findings confirm that occasioning the same reinforced choice response is one means by which disparate samples become functionally equivalent.     

Need probability effects in animal short-term memory

Five pigeons performed in a delayed matching-to-sample (DMTS) procedure with five delay durations (0.5, 2.5, 5, 10 and 20 s) mixed within sessions. Contrary to the predictions of need probability theory, discriminability decreased when fewer short than long delays were included in each session. To test whether the decrease in discriminability was due to a decrease in obtained reinforcement at short delays, the number of trials at each delay was held constant and reinforcer probability was increased with increasing delay. This manipulation produced a similar decrease in discriminability as when the frequency of delays was manipulated. It was concluded that the effect of delay frequency on the forgetting function is mediated by the effect of the reinforcer distribution, which influences discriminability by weakening stimulus control.     

Effect of stimulus orderability and reinforcement history on transitive responding in pigeons

Transitive responding in humans and non-human animals has attracted considerable attention because of its presumably inferential nature. In an attempt to replicate our earlier study with crows [Lazareva, O.F., Smirnova, A.A., Bagozkaja, M.S., Zorina, Z.A., Rayevsky, V.V., Wasserman, E.A., 2004. Transitive responding in hooded crows requires linearly ordered stimuli. J. Exp. Anal. Behav. 82, 1-19], we trained pigeons to discriminate overlapping pairs of colored squares (A+ B-, B+ C-, C+ D-, and D+ E-). For some birds, the colored squares, or primary stimuli, were followed by a circle of the same color (feedback stimuli) whose diameter decreased from A to E (Ordered Feedback group); these circles were made available to help order the stimuli along a physical dimension. For other birds, all of the feedback stimuli had the same diameter (Constant Feedback group). In later testing, novel choice pairs were presented, including the critical BD pair. The pigeons' reinforcement history with Stimuli B and D was controlled, so that the birds should not have chosen Stimulus B during the BD test. Unlike the crows, the pigeons selected Stimulus B over Stimulus D in both ordered and Constant Feedback groups, suggesting that the orderability of the post-choice feedback stimuli did not affect pigeons' transitive responding. Post hoc simulations showed that associative models [Wynne, C.D.L., 1995. Reinforcement accounts for transitive inference (TI) performance. Anim. Learn. Behav. 23, 207-217; Siemann, M., Delius, J.D., 1998. Algebraic learning and neural network models for transitive and non-transitive responding. Eur. J. Cogn. Psychol. 10, 307-334] failed to predict pigeons' responding in the BD test.     

Toward a framework for the evaluation of feature binding in pigeons

Pigeons were trained in a new procedure to test for visual binding errors between the dimensions of color and shape. In Experiment 1, pigeons learned to discriminate a target compound from 15 non-target compounds (constructed from four colors and shapes) by choosing one of two hoppers in a two-hopper choice task. The similarity of the target to non-target stimuli influenced choice responding. In Experiment 2, pigeons learned to detect a target compound when presented with a non-target compound within the same trial under conditions of simultaneity and sequentiality. Non-target trials were arranged to allow for the testing of binding errors (i.e., false identifications of the target on certain non-target trials). Transient evidence for binding errors in two of the birds occurred at the start of two-item training, but decreased with training. The experiments represent an important step toward developing a framework for the evaluation of visual feature binding in nonhumans.     

Comparison of Scalar Expectancy Theory (SET) and the Learning-to-Time (LeT) model in a successive temporal bisection task

The present research tested the generality of the "context effect" previously reported in experiments using temporal double bisection tasks [e.g., Arantes, J., Machado, A. Context effects in a temporal discrimination task: Further tests of the Scalar Expectancy Theory and Learning-to-Time models. J. Exp. Anal. Behav., in press]. Pigeons learned two temporal discriminations in which all the stimuli appear successively: 1s (red) vs. 4s (green) and 4s (blue) vs. 16s (yellow). Then, two tests were conducted to compare predictions of two timing models, Scalar Expectancy Theory (SET) and the Learning-to-Time (LeT) model. In one test, two psychometric functions were obtained by presenting pigeons with intermediate signal durations (1-4s and 4-16s). Results were mixed. In the critical test, pigeons were exposed to signals ranging from 1 to 16s and followed by the green or the blue key. Whereas SET predicted that the relative response rate to each of these keys should be independent of the signal duration, LeT predicted that the relative response rate to the green key (compared with the blue key) should increase with the signal duration. Results were consistent with LeT's predictions, showing that the context effect is obtained even when subjects do not need to make a choice between two keys presented simultaneously.     

Relative numerosity discrimination in the pigeon: further tests of the linear-exponential-ratio model

This study tested a model of how animals discriminate the relative numerosity of stimuli in successive or sequential presentation tasks. In a discrete-trials procedure, pigeons were shown one light for nf times and then another for 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 performance varies with the interval between the two stimulus sets (the interblock interval) and the interval between the end of the sample and the beginning of the choice period (the retention interval); and (b) whether a simple mathematical model of the discrimination process could account for the data. The model assumed that the influence of a stimulus on choice increases linearly when the stimulus is presented, but decays exponentially when the stimulus is absent; choice probability is given by the ratio of the influence values of the two stimuli. The model also assumed that as the retention interval elapses there is an increasing probability that the ongoing discriminative process be disrupted and then the animal responds randomly. Results showed that increasing the interblock intervals reduced the probability of choosing the last stimulus of the sample as the least-frequent one. Increasing the retention interval reduced accuracy without inducing any stimulus bias. The model accounted well for the major trends in the data.     

Role of stimulus-stimulus pairing in matching-to-sample procedure: cross-species comparison of humans and pigeons

The present experiment examined whether, in a matching-to-sample (MTS) procedure, a relation between two stimuli, a sample and a comparison, could be established as a result of just stimulus–stimulus pairing, even if back up reinforcers were never provided for the conditional relation between the sample and comparison stimuli, but rather only for the comparison stimulus. A procedure called “pseudo matching-to-sample” was used in which, when S1 was presented as a sample stimulus, two comparison stimuli (C1 and C2) were presented, and only responses to C1 were reinforced. Conversely, when S2 was presented, only responses to C3 (and not C4) were reinforced. In other words, organisms experiencing this procedure could discriminate C1 from C2, and C3 from C4, by simple discrimination without regard to the conditional sample stimuli. In order to examine cross-species differences, responding by humans in this procedure was compared to that by pigeons. Although the humans developed a discriminative function for the sample stimuli, that is, the humans’ responding was affected by both the sample stimuli and the reinforcers, responding by the pigeons was affected solely by the reinforcers. These data suggest that, in this procedure, humans (but not pigeons) are able to learn relations among stimuli simply as a result of stimulus–stimulus pairing.     

Choice and timing in concurrent chains: effects of initial-link duration

Theories of timing have been applied to choice between delayed rewards by assuming that delays are represented in memory and that subjects sample from memory when choosing between alternatives. To search for covariation in single-trial measures of performance that might confirm this assumption, we used a procedure that allowed for convergent measurement of choice and timing behavior. Four pigeons responded in a concurrent chains/peak procedure in which the terminal links were fixed-interval (FI) 8s and FI 16s, across conditions the duration of the initial-link schedule was either short or long, and one quarter of the terminal links lasted for 48 s and ended without reinforcer delivery. Preference for the FI 8-s alternative was stronger with shorter initial links, replicating the 'initial-link effect'. Responding on no-food trials was unaffected by initial-link duration, and aggregated across trials, was typical of the peak procedure: response distributions were approximately Gaussian, with modes near the FI schedule durations, and variance was greater for the FI 16-s terminal link. Analysis of local measures of initial-link performance (e.g., pause to begin responding, time spent responding, number and duration of visits to each alternative, etc.) found that the initial-link effect was associated with an increase in the number and duration of visits per cycle to the nonpreferred alternative. Regression analyses showed that local initial-link measures contributed relatively little additional variance in predicting performance on individual no-food trials beyond that accounted for by FI schedule. Our results provide no clear evidence that initial- and terminal-link responding in concurrent chains are mediated by a common representation of terminal-link delays.     

Effects of prefeeding, extinction, and distraction during sample and comparison presentation on sensitivity to reinforcer frequency in matching to sample

The present experiment examined the effects of several test manipulations on discrimination, accuracy and sensitivity to reinforcer frequency in a conditional discrimination. Four pigeons responded on a multiple schedule of matching to sample procedures in which the reinforcer-frequency ratio for correct comparison choice responding was varied across components within session from 1:9 to 9:1. Following stability, the effects of prefeeding, extinction, and distraction during sample and comparison presentation were assessed. Discrimination accuracy decreased under prefeeding, extinction, and distraction during sample presentation. Sensitivity to reinforcer frequency decreased under prefeeding and extinction. Decreases in sensitivity were positively related to decreases in discrimination accuracy. The decreases in discrimination accuracy and sensitivity under prefeeding and extinction are interpreted as being due to decreases in attending to the sample and comparison stimuli, respectively, possibly mediated by motivational effects of these manipulations. This interpretation is consistent with current conceptualizations of the contingencies that govern conditional-discrimination performance.     

Within-trial contrast: The effect of probability of reinforcement in training

There is evidence that pigeons prefer conditioned reinforcers that are preceded by greater effort over those that are preceded by less effort (an effect that has been attributed to within-trial contrast). In past research the probability of reinforcement for correct choice of the conditioned reinforcer has been 100%, however, the high level of reinforcement for both alternatives in training may result in a performance ceiling when choice between those alternatives is provided on test trials. In the present study we tested this hypothesis by including a group for which the probability of reinforcement in training was only 50%. Pigeons were trained on two simultaneous discriminations, one that was preceded by a 30 peck requirement the other by a single peck requirement. On test trials, we found a significant preference for the S+ that required the greater effort in training for pigeons trained with 100% and a small but nonsignificant effect for pigeons trained with 50% reinforcement. Although the hypothesis that the within-trial contrast effect was constrained by a performance ceiling was not confirmed, we did find a reliable within-trial contrast effect with 100% reinforcement.     

Visual discrimination of objects differing in spatial depth by goldfish

Training experiments were performed to investigate the ability of goldfish to discriminate objects differing in spatial depth. Tests on size constancy should give insight into the mechanisms of distance estimation. Goldfish were successfully trained to discriminate between two black disk stimuli of equal size but different distance from the tank wall. Each stimulus was presented in a white tube so that the fish could see only one stimulus at a time. For each of eight training stimulus distances, the just noticeable difference in distance was determined at a threshold criterion of 70% choice frequency. The ratio of the retinal image sizes between training stimulus and comparison stimulus at threshold was about constant. However, in contrast to Douglas et al. (Behav Brain Res 30:37–42, 1988), goldfish did not show size constancy in tests with stimuli of the same visual angle. This indicates that they did not estimate distance, but simply compared the retinal images under our experimental conditions. We did not find any indication for the use of accommodation as a depth cue. A patterned background at the rear end of the tubes did not have any effect, which, however, does not exclude the possibility that motion parallax is used as a depth cue under natural conditions.     

Models for the pressing rate response of rats and pigeons on a certain class of variable interval schedules of behavior

Two mathematical models are proposed for a certain class of variable interval schedules. These schedules are derived from the fixed interval schedule by adding (with probabilityp on any interval) reinforcing stimuli at a fixed point within the interval. The first model is a deterministic one in which the excitation producing the response is treated as a superposition of an excitation generated by the added reinforcements upon that generated by the fixed interval reinforcements. A method for estimating some of the parameters is presented. The model is applied with slight variations to data from performances by pigeons and by normal and brain-operated rats. The second model is a probabilistic one in which the basic entity is the interval between successive responses. This quantity must be represented by a probability distribution which is changing throughout the intervals between reinforcements. A solution is presented for a class of interresponse time distributions. Results from a simulation of a variant of this model are discussed briefly.     

The effect of food deprivation on self-control

Two experiments examined the effect of food deprivation on choice in a discrete-trials self-control paradigm, choice between a larger, more-delayed reinforcer and a smaller, less-delayed reinforcer. In Experiment 1, four pigeons were each deprived to 65%, 80%, and 90% of their free-feeding weights, and the delay to the smaller reinforcer was varied. Deprivation level did not affect choice, but the rate of ineffective key pecks made during the reinforcer delays increased as deprivation increased. In Experiment 2, four pigeons were exposed to conditions in which they were fed up to their 80% free-feeding weights following experimental sessions, and in which they were given no postsession feedings. Both the pigeons' weights and their latencies to insert their heads into the food hopper when food was available were lower when the pigeons were not fed following experimental sessions. Choice showed no change. Deprivation level affects response rate and eating behavior in these procedures, but not choice.     

Pigeon's recognition of cartoons: effects of fragmentation, scrambling, and deletion of elements

J. Cerella [Pattern Recognit. 12 (1980) 1] and more recently S. Watanabe [Behav. Proc. 53 (2001) 3] demonstrated that pigeons showed no decrement in recognizing cartoons that were spatially scrambled, indicating that pigeons' discriminative responding is controlled by local features alone. In contrast Kirkpatrick-Steger et al. [J. Exp. Psychol. Anim. Behav. Proc. 24 (1998) 34] used line drawings as stimuli and demonstrated the importance of spatial organization for picture recognition by pigeons, confirming related findings reported in their previous studies. The present study revisited the recognition of cartoons by pigeons. In Experiment 1, pigeons were trained to discriminate cartoon people on a variety of background scenes. Subsequent tests revealed that discriminative performances with both familiar and novel instances decreased as the objects and object-like parts were progressively fragmented, indicating that search for the targeted cartoon people in the stimulus array might have enhanced the pigeons to attend to global aspects of cartoon people. Experiment 2 used line drawings of cartoon faces as stimuli and examined effects of scrambling and deletion of components. A set of components (eyes and eyebrows) exerted strong control over behavior and scrambling only moderately suppressed responding. The results suggest that pigeons use both global and local aspects, with different mixtures of these types of information depending on the particular perceptual context.     

Context effect in a temporal bisection task with the choice keys available during the sample

In a symbolic matching to sample task, six pigeons learned to associate different sample durations with different comparison stimuli. On “Short” trials, choice of Red and Green keylight comparisons were reinforced following 3-s and 9-s samples, respectively; on “Long” trials, Blue and Yellow keylight comparisons were reinforced following 9-s and 27-s samples, respectively. In contrast with previous studies, the comparison keys were available during the samples. After the temporal discriminations were learned, new pairs of comparison keys were presented and the preference for each was assessed during 27-s samples. One pair in particular, Green and Blue, was critical because it tested the predictions of two timing models, Scalar Expectancy Theory (SET) and the Learning-to-Time (LeT) model. The results showed that preference for Green increased during the sample, a result consistent with LeT but not with SET. Other test results, however, were predicted by neither model.     

Discrete-trial vs. continuous free-operant procedures in assessing whether reinforcement context affects reinforcement value

Pigeons were trained to respond to two alternating concurrent reinforcement schedules. The reinforcement probabilities were .05 and .10 in one component, and .10 and .20 in the other. In one condition, the pigeons received training on a discrete-trial procedure in which the keylights remained illuminated for 5s or until a response occurred. In another condition, pigeons received training on a procedure in which the reinforcement contingencies were the same as in the discrete-trial procedure, but the stimuli were not turned off after 5s or after a response. Following training in each condition, probe tests were presented. In both conditions, the .20 alternative was, overall, preferred to the .05 alternative during probe tests. Following discrete-trial training, there was no reliable preference between the two .10 alternatives. However, when the stimuli remained illuminated during the intertrial interval periods during training, probe tests results showed preference for the .10 alternative that had been presented in the leaner context during training. The pattern of results is consistent with the notion that probe preference can be influenced both by the absolute reinforcement schedules associated with each alternative, as well as changeover behavior developed during training.