Characteristics of forgetting functions in delayed matching to sample

Performance of pigeons in delayed matching-to-sample procedures was measured in terms of an index of discriminability derived from the difference between logarithms of ratios of choice responses to comparison stimuli following the different sample stimuli. Forgetting functions that plotted discriminability as a function of delay-interval duration were well described by a simple negative exponential function with two parameters, one describing initial discriminability of sample stimuli at zero delay (log d₀) and the other describing rate of decrement in discriminability with increasing delay-interval duration (b). With the difference between wavelength values of the comparison stimuli held constant, a large difference between wavelengths of the sample stimuli resulted in a higher log d₀ value than that for a small difference between sample stimuli, without changing the rate of decrement in discriminability, b. An increase in the fixed-ratio requirement for sample-key responding produced an increase in log d₀ without affecting b, and interpolation of ambient illumination in the delay interval increased b without influencing log d₀. Both parameters changed when intertrial-interval duration was varied. The result of variation in the point of interpolation of ambient illumination in the delay interval indicated that levels of discriminability at longer delays were independent of discriminability levels at earlier delays, consistent with the properties of the exponential function. Functions relating performance to delay-interval duration were suggested to have two characteristics: discriminability of the sample stimuli in the absence of a delay between the stimuli and the behavior they occasion, and rate of attenuation in discriminability with increasing delay-interval duration.     

Most directed forgetting in pigeons can be attributed to the absence of reinforcement on forget trials during training or to other procedural artifacts.

In research on directed forgetting in pigeons using delayed matching procedures, remember cues, presented in the delay interval between sample and comparisons, have been followed by comparisons (i.e., a memory test), whereas forget cues have been followed by one of a number of different sample-independent events. The source of directed forgetting in delayed matching to sample in pigeons was examined in a 2 x 2 design by independently manipulating whether or not forget-cue trials in training ended with reinforcement and whether or not forget-cue trials in training included a simultaneous discrimination (involving stimuli other than those used in the matching task). Results were consistent with the hypothesis that reinforced responding following forget cues is sufficient to eliminate performance deficits on forget-cue probe trials. Only when reinforcement was omitted on forget-cue trials in training (whether a discrimination was required or not) was there a decrement in accuracy on forget-cue probe trials. When reinforcement is present, however, the pattern of responding established during and following a forget cue in training may also play a role in the directed forgetting effect. These findings support the view that much of the evidence for directed forgetting using matching procedures may result from motivational and behavioral artifacts rather than the loss of memory.     

Quantification of the effects of chlorpromazine on performance under delayed matching to sample in pigeons.

The effects of four doses of chlorpromazine (dose range 0.5 to 12.5 mg/kg) on performance under a delayed matching-to-sample procedure in pigeons was investigated, using the exponential model of memory (White, 1985). Performance was measured using a bias-free measure of discriminability, log d (Davison & Tustin, 1978), and negative exponential functions were fitted to individual-subject and group data at each dose level. A decrease in matching accuracy was found to be caused by an increase in the rate of forgetting, b, and a decrease in the initial discriminability, log d0. Changes in rate of forgetting and discriminability occurred at doses that had no statistically significant effect on response latency. The exponential model of memory accounted well for the data and provided a useful way of quantifying the effects of chlorpromazine on the processes involved in delayed matching-to-sample performance.     

Use of an ambiguous-sample procedure to establish a cue to forget in pigeons

Pigeons were trained on a variation of the matching-to-sample task in which on double-sample trials two samples, one associated with each of the comparison stimuli, were presented successively. Responding to the comparison associated with the first sample was reinforced on half the double-sample trials, and responding to the comparison associated with the second sample was reinforced on the remaining half. One of two postsample stimuli was presented following the termination of each colored sample. A vertical line was presented after a correct or target sample, and a horizontal line was presented after an incorrect or interfering sample. With extended training, each bird demonstrated above-chance accuracy on double-sample trials, providing prima facie evidence that one or both of the postsample stimuli exerted control over matching behavior. Experiment 2 provided evidence that the horizontal line functioned as a cue to forget the code activated by the preceding sample stimulus. It was concluded that a condition sufficient to establish a postsample stimulus as a cue to forget is that the postsample immediately follow presentation of a sample that, if it were to control test responding, would lead to nonreinforcement.     

Delayed stimulus control: recall for single and relational stimuli.

In a discrete-trial symbolic matching-to-sample procedure, pigeons' left-key responses were reinforced following presentation of one center-key sample, and right-key responses were reinforced following presentation of another. Recallability was measured by the difference between log ratios of left to right responses following each sample. In Experiment 1, samples were successively presented same or different wavelengths in the relational discrimination, or individual wavelengths in the single discrimination. The rate at which recallability decreased with increasing delay since sample presentation was the same for single and relational discriminations, but the initial level of performance differed, indicating that the relational discrimination was more difficult. In Experiment 2, recall functions for easy and difficult discriminations between individual wavelengths also differed in levels of initial performance but not in rate of decrement of recallability over time. Recall for stimuli differing in complexity may therefore reflect differences in discrimination difficulty.     

Free-operant forgetting: Delayed stimulus control of multiple-schedule performance

Pigeons' responses were reinforced in two components of several multiple variable-interval variable-interval schedules of food reinforcement. In one component, the key was illuminated green for 15 seconds and white for 45 seconds. In the other component, the key was illuminated red for 15 seconds and white for 45 seconds. Values for the exponent of the power functions relating response ratios to reinforcement ratios were higher in the presence of the discriminative stimuli (green or red) than in the presence of white. Sensitivity of response ratios to changes in reinforcement ratios provided an index of the extent to which responding was under delayed stimulus control by prior discriminative stimuli.     

Delayed matching-to-sample performance of hens: Effects of sample duration and response requirements during the sample

Six domestic hens were trained under a delayed matching-to-sample procedure with red and green keylights as sample and comparison stimuli and a 1.5-s delay interval. The hens were trained to stop pecking the sample stimuli when a tone sounded. Duration of the sample stimuli (2 to 10 s) and the number of pecks required on the key on which these stimuli were presented (0 to 10) were altered across conditions. Both the response requirement on the sample key and the duration of sample presentations affected accuracy. These findings are in agreement with those of earlier studies using other species and somewhat different procedures.     

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.     

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.     

Discriminability between alternatives in a switching-key concurrent schedule

Six pigeons were trained to discriminate between two intensities of white light in a symbolic matching-to-sample procedure. These stimuli were then used to signal which schedule was available on the main key in a switching-key concurrent schedule. The concurrent schedules led to a symbolic matching-to-sample phase in which the subject identified the concurrent schedule to which it last responded before a reinforcer could be obtained. The concurrent schedules were varied across conditions. Discriminability, measured during the symbolic matching-to-sample performance, was high throughout and did not differ across the two procedures. Performance in the concurrent schedules was like that typically obtained using these schedules. Delays were then arranged between completion of the concurrent schedules and presentations of the symbolic matching-to-sample phase. A series of conditions with an intervening delay of 10 s showed that both concurrent-schedule performance and symbolic matching-to-sample performance were affected by the delay in a similar way; that is, choice responding was closer to indifference.     

Pigeons' spatial memory: factors affecting delayed matching of key location.

The delayed-matching-to-sample procedure was modified to study pigeons' spatial memory. Nine pecking keys, arranged as a three-by-three matrix, served as the spatial cues. Trials began with a brief "ready" stimulus (dimming of the houselight). Then a randomly chosen key was lit briefly as a sample. After a short delay the sample key was lit again along with one of the other eight keys. A peck at the key that had served as the sample produced grain reinforcement, where as a peck to the other key produced only the intertrial interval. After delayed matching of key location was learned, the effects of sample and delay duration, number of keys illuminated as sample and comparisons, and organization of three-key samples were studied. Matching accuracy decreased as sample duration decreased, delay increased, the number of locations serving as samples increased, the number and proximity of comparisons increased, and when the three-key samples were "discontinuous" rather than "lines".