Seasonal variation in pigeon body weight and delayed matching-to-sample performance

The weights of 5 pigeons with free access to food, monitored over 3 calendar years in the laboratory, were found to fluctuate with season. All pigeons were at their heaviest in the winter and were lightest in the summer. Five different pigeons performed a standard delayed matching-to-sample task for 44 weeks from January to November. Their weights were held at 85% of their summer free-feeding weights, making their predicted deprivation level higher in the winter relative to predicted winter free-feeding weights. Slopes of forgetting functions fit to weekly response totals for each pigeon were shallower in winter, showing an improvement in accuracy with longer delays. Thus, delayed matching-to-sample performance may have been affected by the practice of maintaining the pigeons at a constant body weight throughout the calendar year.     

Three-configuration matching-to-sample in the pigeon

Pigeons were trained on a zero-delay matching-to-sample procedure during which only three of the four possible stimulus configurations were presented. Subsequently, all birds were exposed to all four configurations as a transfer test. A high degree of negative transfer from the three training configurations was obtained in Experiment 1. The results of Experiment 2 indicated that three-configuration training produced differential position-preference effects. During the transfer test, responding after one sample stimulus was apparently based on position, while responding after the other sample was based on color.     

Titrating-delay matching-to-sample in the pigeon

The titrating-delay matching-to-sample (TDMTS) procedure offers researchers an additional behavioral task thought to capture some important features of remembering. In this procedure, the delay between sample offset and comparison onset adjusts as a function of the subject's performance. Specifically, correct matches increase the delay and incorrect matches decrease the delay, and steady-state titrated delays serve as the primary dependent measure. The present series of experiments investigated the effects of several procedural variables on performance in TDMTS procedures in an effort to elucidate better its features to allow for more precision in future use. Experiment 1 reports results from a parametric analysis of fixed-ratio response requirements on the sample key that indicated improved remembering in the form of higher daily titrated delay values as the requirement was increased. Experiment 2 investigated the extent to which the initial delay value in each session affected session-wide delay values. Results indicated that regardless of value of the initial delay, the subjects' performances adjusted the delay values in the direction of the known baseline delay-value levels. Experiment 3 manipulated the step size by which delay values were adjusted and the results indicated that larger step sizes increased both session-to-session variability and within-session range of titrated delay values, although the average values remained approximately the same. These results suggest that the TDMTS task serves as a promising procedure to study what many refer to as memory.     

Signal-detection analyses of conditional discrimination and delayed matching-to-sample performance

Quantitative analyses of stimulus control and reinforcer control in conditional discriminations and delayed matching-to-sample procedures often encounter a problem; it is not clear how to analyze data when subjects have not made errors. The present article examines two common methods for overcoming this problem. Monte Carlo simulations of performance demonstrated that both methods introduced systematic deviations into the results, and that there were genuine risks of drawing misleading conclusions concerning behavioral models of signal detection and animal short-term memory.     

On the development and mechanics of delayed matching-to-sample performance

Despite its frequent use to assess effects of environmental and pharmacological variables on short-term memory, little is known about the development of delayed matching-to-sample (DMTS) performance. This study was designed to examine the dimensions and dynamics of DMTS performance development over a long period of exposure to provide a more secure foundation for assessing stability in future research. Six pigeons were exposed to a DMTS task with variable delays for 300 sessions (i.e., 18,000 total trials; 3,600 trials per retention interval). Percent-correct and log-d measures used to quantify the development of conditional stimulus control under the procedure generally and at each of five retention intervals (0, 2, 4, 8 and 16-s) individually revealed that high levels of accuracy developed relatively quickly under the shorter retention intervals, but increases in accuracy under the longer retention intervals sometimes were not observed until 100-150 sessions had passed, with some still increasing at Session 300. Analyses of errors suggested that retention intervals induced biases by shifting control from the sample stimulus to control by position, something that was predicted by observed response biases during initial training. These results suggest that although it may require a great deal of exposure to DMTS prior to obtaining asymptotic steady state, quantification of model parameters may help predict trends when extended exposure is not feasible.     

Intertrial sources of stimulus control and delayed matching-to-sample performance in humans

Two experiments compared delayed matching-to-sample (DMTS) accuracy under 2 procedures in adults with mental retardation. In the trial-unique procedure, every trial in a session contained different stimuli. Thus, comparison stimuli that were correct on one trial were never incorrect on other trials in that session (or vice versa). In the 2-sample DMTS procedure, the same 2 comparison stimuli were presented on each trial, and their function changed quasi-randomly across trials conditional upon the sample stimulus. Across 2 experiments, 7 of 8 subjects showed the highest overall accuracy under the trial-unique procedure, and no subject showed consistently higher accuracy under the 2-sample procedure. Negative, exponential decay functions fit to logit p values showed that this difference was due largely to the steeper delay-mediated decline in sample control for the 2-sample procedure. Stimulus-control analyses indicated that, under the 2-sample procedure, the selection of the comparison stimulus on Trial N was often controlled by the comparison stimulus selection on Trial N-1 rather than the Trial-N sample stimulus. This source of competing stimulus control is not present in trial-unique procedures. Experiment 2 manipulated intertrial interval duration. There was a small but consistent increase in accuracy as a function of intertrial interval duration under the 2-sample procedure, but not under the trial-unique procedure.     

Instructional effects on performance in a matching-to-sample study

Conducting studies using an undergraduate participant pool is fraught with difficulties. Among them are problems with adequately motivating subjects both to come to the study, and once there, to actively engage the experimental task. Thirty-one college students participated in a matching-to-sample (MTS) study involving substantial training, testing, retraining, and retesting of conditional discriminations and equivalence relations among four 4-member classes of nonsensical words. The study was conducted during the end of the semester, when performance often had been observed to be poorer than at other points in the semester. Eleven of the participants, in addition to standard instructions about the task, received additional instructions specifying molar consequences for high rates of "correct" responses throughout the procedure. This subset of participants displayed markedly improved performance as compared to those who did not receive the additional instructions. Results suggest that specification of molar contingencies improves participants' sensitivity to molecular contingencies within the study. Instructions that specify and increase the consequential functions of feedback provided during MTS trials may be one means of reducing unwanted variability in human MTS performance.     

Retrospective coding in pigeons' delayed matching-to-sample

In this study we examined how coding processes in pigeons' delayed matching-to-sample were affected by the stimuli to be remembered. In Experiment 1, two groups of pigeons initially learned 0-delay matching-to-sample with identical comparison stimuli (vertical and horizontal lines) but with different sample stimuli (red and green hues or vertical and horizontal lines). Longer delays were then introduced between sample offset and comparison onset to assess whether pigeons were prospectively coding the same events (viz., the correct line comparisons) or retrospectively coding different events (viz., their respective sample stimuli). The hue-sample group matched more accurately and showed a slower rate of forgetting than the line-sample group. In Experiment 2, pigeons were trained with either hues or lines as both sample and comparison stimuli, or with hue samples and line comparisons or vice versa. Subsequent delay tests revealed that the hue-sample groups remembered more accurately and generally showed slower rates of forgetting than the line-sample groups. Comparison dimension had little or no effect on performance. Together, these data suggest that pigeons retrospectively code the samples in delayed matching-to-sample.     

Associative symmetry in the pigeon after successive matching-to-sample training

If an organism is explicitly taught an A→B association, then might it also spontaneously learn the symmetrical B→A association? Little evidence attests to such “associative symmetry” in nonhuman animals. We report for the first time a clear case of associative symmetry in the pigeon. Experiment 1 used a successive go/no go matching‐to‐sample procedure, which showed all of the training and testing stimuli in one location and intermixed arbitrary and identity matching trials. We found symmetrical responding that was as robust during testing (B→A) as during training (A→B). In Experiment 2, we trained different pigeons using only arbitrary matching trials before symmetry testing. No symmetrical responding was found. In Experiment 3, we trained other pigeons with only arbitrary matching trials and then tested for symmetry. When these pigeons, too, did not exhibit symmetrical responding, we retrained them with intermixed identity and arbitrary matching trials. Less robust symmetrical responding was obtained here than in Experiment 1. Collectively, these results suggest that identity matching may have to be learned concurrently with arbitrary matching from the outset of training for symmetry to emerge.     

Generalization gradients and stimulus control in delayed matching-to-sample

Neurological patients were subjects in delayed visual matching-to-sample. The sample and choice stimuli were ellipses of varying size. By measuring the difference in size between the sample on a given trial and the ellipse the subject chose on that trial, gradients of differences between samples and choice stimuli could be plotted. These difference gradients broadened with increasing delays. Sharp gradients were controlled by the samples. Flat gradients were controlled by features of the choice display, independently of the samples. Intermediate gradients reflected combined control by the samples and by the choice displays.     

Reinforcer efficacy in a delayed matching-to-sample task.

Five domestic hens were exposed to a delayed matching-to-sample task. Conditions 1, 5, and 8 were variable-delay conditions in which five delays (0.25, 1, 2, 4, and 8 s) from the red or green sample to the presentation of the red and green comparison stimuli were presented a number of times during each session. In the fixed-delay condition (Condition 3), each delay was presented for 15 sessions under a Latin square design across birds. When improvements in accuracy across the variable-delay conditions are taken into account, the data were similar under both the variable and fixed delays. In Conditions 2, 4, 6, and 7 sample-reinforcer intervals were held at 8, 8, 4, and 2 s, respectively, while sample-choice intervals were varied within these during each session. With increasing sample-reinforcer interval, both initial discriminability (i.e., with sample-choice delay = 0) and rate of decrement in discriminability decreased. Although the former would be predicted if accuracy depends of the average sample-reinforcer interval, the latter would not. These data show that increasing the sample-choice interval had less effect on matching accuracy than increasing the sample-reinforcer interval did.     

Sources of visual interference in delayed matching-to-sample with pigeons

In two experiments, independent groups of pigeons were trained on an identity matching task involving line orientations as sample and comparison stimuli. For some birds an overhead houselight was illuminated continuously throughout each training session. For other birds the houselight was never illuminated during training sessions. During subsequent testing, the lighting conditions during the delay were the same as in training on some trials, but on other trials they were opposite those of training during either the entire delay (Experiment 1) or during a portion of the delay (Experiment 2). In birds trained with the houselight off, turning the houselight on during the delay produced a large and enduring disruption in matching accuracy. On the other hand, in birds trained with the houselight on, turning the houselight off during the delay produced only a moderate and temporary disruption in matching accuracy. These findings are inconsistent with the prevailing view that retroactive interference in pigeons is a function of a change in illumination level relative to that which prevailed during training. In pigeons, as in monkeys, sustained retoactive interference effects obtain only when the level of illumination is increase during the delay interval.     

Quantitative analyses of matching-to-sample performance

Six pigeons performed a simultaneous matching-to-sample (MTS) task involving patterns of dots on a liquid-crystal display. Two samples and two comparisons differed in terms of the density of pixels visible through pecking keys mounted in front of the display. Selections of Comparison 1 after Sample 1, and of Comparison 2 after Sample 2, produced intermittent access to food, and errors always produced a time-out. The disparity between the samples and between the comparisons varied across sets of conditions. The ratio of food deliveries for the two correct responses varied over a wide range within each set of conditions, and one condition arranged extinction for correct responses following Sample 1. The quantitative models proposed by Davison and Tustin (1978), Alsop (1991), and Davison (1991) failed to predict performance in some extreme reinforcer-ratio conditions because comparison choice approached indifference (and strong position biases emerged) when the sample clearly signaled a low (or zero) rate of reinforcement. An alternative conceptualization of the reinforcement contingencies operating in MTS tasks is advanced and was supported by further analyses of the data. This model relates the differential responding between the comparisons following each sample to the differential reinforcement for correct responses following that sample.     

Successive matching-to-sample in the pigeon: Variations on a theme by Konorski

In 1959, Konorski proposed a successive matching-to-sample paradigm with which to study short-term memory in animals. Although the technique has been little used, it affords several distinct advantages over more commonly employed matching-to-sample procedures. Konorski’s paradigm involves the successive presentation of a pair of discriminative stimuli with a brief interstimulus interval between them. Reinforcement is scheduled to occur only when the second stimulus of a pair matches the first; otherwise, nonreinforcement follows. An investigation of the pigeon’s food-reinforced keypecking behavior is reported using a variant of Konorski’s technique. Pigeons rapidly learned to differentiate matching and nonmatching stimulus pairs when brief (5-sec) color stimuli were separated by a 1-sec interstimulus interval. No such differentiation arose when control subjects were trained with reinforcement equiprobable on matching and nonmatching trials. No support was found for the notion that correct performance under this successive-matching procedure was due to overt mediating behaviors.     

Acquisition and maintenance of delayed matching-to-sample in tufted capuchin monkeys

Delayed matching-to-sample (DMTS) is a commonly used procedure to investigate short-term memory. For the study of functions of forgetting, the delay between the disappearance of the sample stimulus and appearance of choices is manipulated. The intertrial interval (ITI) is also varied to assess interference effects. Performance decrements have been observed as delay increases and, in some cases, performance recovery occurs when ITIs are increased. Other studies indicate that the higher the ITI/delay ratio, the greater the accuracy in DMTS. In this study, 2 experiments investigated DMTS performances of 3 tufted capuchin monkeys as function of delay and ITI. In Experiment 1, alternation of gradual increases of delay and ITI was effective in producing ≥90% accuracy at delays as long as 90 s. Individual monkeys differed in the highest value of delay at which this criterion was met. In Experiment 2, the monkeys were exposed to 5-s DMTS with different ITIs to assess the effects of various ITI/delay ratios on accuracy. Highest accuracy tended to occur at the higher ITI/delay ratios.     

Acquisition of delayed matching in the pigeon

Pigeons were exposed to three successive matching-to-sample procedures. On a given trial, the sample (red, green or blue light) appeared on a center key; observing responses to this key produced the comparison stimuli on two side keys. Seven different experimental conditions could govern the temporal relations between the sample and comparison stimuli. In the "simultaneous" condition, the center key response was followed immediately by illumination of the side key comparison stimuli, with the center key remaining on. In "zero delay" the center key response simultaneously turned the side keys on and the center key off, while in the "variable delay" conditions, intervals of 1, 2, 4, 10, and 24 sec were interposed between the offset of the sample and the appearance of the comparison stimuli on the side keys. In all conditions, a response to the side key of matching hue produced reinforcement, while a response to the non-matching side key was followed by a blackout. In procedure I all seven experimental conditions were presented in randomly permutated order. After nine sessions of exposure (at 191 trials per session, for a total of 1719 trials) the birds gave no evidence of acquisition in any of the conditions. They were therefore transferred to Procedure II, which required them to match only in the "simultaneous" condition, with both the sample and comparison stimuli present at the same time. With the exception of one bird, all subjects acquired this performance to near 100% levels. Next, in Procedure III, they were once more exposed to presentation of all seven experimental conditions in random order. In contrast to Procedure I, they now acquired the delay performance, and were able to match effectively at delays of about 4 sec.     

Non-spatial delayed alternation by the pigeon

Pigeons were trained on a non-spatial delayed alternation task in which the color not pecked on the previous trial was correct. When varying delays were interposed between trials, alternation accuracy decreased as a function of delay, but remained greater than chance with a 45-sec delay. Successful alternation on the longer delays was accomplished without behavioral mediation of the delay intervals. Also, during initial testing when a position cue was available in addition to the color cue after incorrect trials, alternation accuracy was greater after a preceding incorrect trial than after a correct trial. When the position cue was removed, no differences occurred as a function of the outcome of the preceding trial.     

Teaching serial position sequences to monkeys with a delayed matching-to-sample procedure

Comparison was made of two methods for training monkeys to “observe” a two-member serial position sequence by pressing two consecutively lighted keys and then to “report” the sequence by pressing the same two keys in the same order but without the lights. A fading technique involving gradual elimination of brightness cues from “reporting” keys was found more effective than a no-fading procedure in which the cues remained bright during training and then were suddenly removed. Animals that failed to learn to report a new sequence with the no-fading procedure sometimes developed behavior incompatible with that desired. They made repeated and specific errors that prematurely terminated trials of the sequence to-be-learned, even though the correct key was cued by a bright light. They behaved appropriately, however, on succeeding trials of other sequences. Thus, the errors were followed by trials on which reinforcement occurred. Manipulation of this contingency indicated its importance in maintaining the stereotyped error patterns.     

Reinforcer control by comparison-stimulus color and location in a delayed matching-to-sample task

Six pigeons were trained in a delayed matching-to-sample task involving bright- and dim-yellow samples on a central key, a five-peck response requirement to either sample, a constant 1.5-s delay, and the presentation of comparison stimuli composed of red on the left key and green on the right key or vice versa. Green-key responses were occasionally reinforced following the dimmer-yellow sample, and red-key responses were occasionally reinforced following the brighter-yellow sample. Reinforcer delivery was controlled such that the distribution of reinforcers across both comparison-stimulus color and comparison-stimulus location could be varied systematically and independently across conditions. Matching accuracy was high throughout. The ratio of left to right side-key responses increased as the ratio of left to right reinforcers increased, the ratio of red to green responses increased as the ratio of red to green reinforcers increased, and there was no interaction between these variables. However, side-key biases were more sensitive to the distribution of reinforcers across key location than were comparison-color biases to the distribution of reinforcers across key color. An extension of Davison and Tustin's (1978) model of DMTS performance fit the data well, but the results were also consistent with an alternative theory of conditional discrimination performance (Jones, 2003) that calls for a conceptually distinct quantitative model.     

Evidence of precurrent responses expanding equivalence classes in a delayed matching-to-sample task

Delayed matching to sample (DMTS) increases the probability of equivalence class formation. Precurrent responses can mediate the retention interval in DMTS trials and control the selection of comparisons. In human participants, precurrent responses usually consist of naming the experimental stimuli based on their similarities to meaningful stimuli with preexperimental history. We tested whether precurrents expand classes by serving as nodes between experimental and meaningful stimuli. A DMTS (2 s) was used throughout the entire experiment. Eleven undergraduates learned A1B1 and A2B2 relations and then were submitted to ArC trials that required them to answer math problems presented during the DMTS interval: when the sample was A1, the problems resulted in 12 and C1 was correct; when the sample was A2, they resulted in 9 and C2 was correct. Response-as-node tests assessed whether participants would relate B1 and C1 to the printed number 12 and B2 and C2 to the printed number 9. Ten participants responded accordingly to this pattern, showing that the responses to the problems expanded the classes. Parity tests using the words “even” and “odd” further confirmed this hypothesis. These results contribute to understanding why DMTS enhances equivalence performances. Implications of using this procedure in stimulus-equivalence studies are discussed.