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.     

Verbal self-reports of delayed matching to sample by humans

Undergraduates participated in two experiments to develop methods for the experimental analysis of self-reports about behavior. The target behavior was the choice response in a delayed-matching-to-sample task in which monetary reinforcement was contingent upon both speed and accuracy of the choice. In Experiment 1, the temporal portion of the contingency was manipulated within each session, and the presence and absence of feedback about reinforcement was manipulated across sessions. As the time limits became stricter, target response speeds increased, but accuracy and reinforcement rates decreased. When feedback was withheld, further reductions in speed and reinforcement occurred, but only at the strictest time limit. Thus, the procedures were successful in producing systematic variation in the speed, accuracy, and reinforcement of the target behavior. Experiment 2 was designed to assess the influence of these characteristics on self-reports. In self-report conditions, each target response was followed by a computer-generated query: “Did you earn points?” The subject reported by pressing “Yes” or “No” buttons, with the sole consequence of advancing the session. In some cases, feedback about reinforcement of the target response followed the reports; in other cases it was withheld. Self-reports were less accurate when the target responses occurred under greater time pressure. When feedback was withheld, the speed of the target response influenced reports, in that the probability of a “Yes” report increased directly with the speed of accurate target responses. In addition, imposing the self-report procedure disrupted target performance by reducing response speeds at the strictest time limit. These results allow investigation of issues in both behavioral and cognitive psychology. More important, the overall order in the data suggests promise for the experimental analysis of self-reports by human subjects.     

Sample-specific ratio effects in matching to sample

In a symbolic matching-to-sample task, pigeons were trained using sample-specific, fixed-ratio “observing responses.” Subsequently, in a mixed condition, each sample was presented equally often with each ratio requirement, i.e., the ratios were no longer correlated with the samples. In a second experiment, pigeons were trained initially in the mixed condition and subsequently shifted to the sample-specific condition in which the required ratios were correlated with the samples. Results of both experiments suggested joint control of choices by ratio value and by the exteroceptive stimuli. The discriminative properties of the ratios appeared to outweigh absolute ratio-size effects.     

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.     

Base rates versus sample accuracy: competition for control in human matching to sample

People often place undue weight on specific sources of information (case cues) and insufficient weight on more global sources (base rates) even when the latter are highly predictive, a phenomenon termed base-rate neglect. This phenomenon was first demonstrated with paper-and-pencil tasks, and also occurs in a matching-to-sample procedure in which subjects directly experience case sample (cue) accuracy and base rates, and in which discrete, nonverbal choices are made. In two nonverbal experiments, subjects were exposed to hundreds of trials in which they chose between two response options that were both probabilistically reinforced. In Experiment 1, following one of two possible samples (the unpredictive sample), either response was reinforced with a .5 probability. The other sample (predictive) provided reinforcement for matching on 80% of the trials in one condition but in only 20% of the trials in another condition. Subjects' choices following the unpredictive sample were determined primarily by the contingencies in effect for the predictive sample: If matching was reinforced following the predictive sample, subjects tended to match the unpredictive sample as well; if countermatching the predictive sample was generally reinforced, subjects tended to countermatch the unpredictive sample. These results demonstrate only weak control by base rates. In Experiment 2, base rates and sample accuracy were simultaneously varied in opposite directions to keep one set of conditional probabilities constant. Subjects' choices were determined primarily by the overall accuracy of the sample, again demonstrating only weak control by base rates. The same pattern of choice occurred whether this pattern increased or decreased rate of reinforcement. Together, the results of the two experiments provide a clear empirical demonstration of base-rate neglect.     

The effects of chlorpromazine and imipramine on rate and stimulus control of matching to sample.

Pigeons were trained to perform simultaneous, two-color matching to sample under a multiple fixed-ratio fixed-interval schedule of food presentation. The sequence terminating with a peck on the matching key (a "match") was treated as a unit, analogous to a single key peck in conventional schedules. Except for intermittent reinforcement of matches, no consequent stimulus distinguished matches from mismatches (sequences terminating with pecks on the nonmatching key). The pattern of matches during nondrug sessions resembled that of simpler operants maintained by similar schedules. Matches increased in rate toward the end of both components; mismatch rates increased more slowly. Imipramine increased the rate of mismatches, disrupted schedule patterning, and lowered accuracy in a dose-dependent fashion. Chlorpromazine lowered the overall rate of matches but affected schedule patterns and accuracy less than imipramine. The types of errors during drug sessions were not systematically related to the types of errors that appeared during nondrug sessions. Stimulus control was evaluated for each of the four possible color configurations and was found to be by the entire configuration of colors, not simply by the color of the sample.     

Effectiveness and persistence of precurrent mediating behavior in delayed matching to sample and oddity matching with children.

Two kinds of mediating behavior were compared with respect to their effectiveness in variable-delay matching-to-sample and oddity-matching tasks. Each of four 5-year-old children was trained to emit either differential or common mediating responses. The differential mediating response consisted of pressing a specific computer key corresponding to either of two possible sample stimuli (a red or a green square). The common mediating response consisted of pressing one of the two response keys regardless of the sample. The differential-response subjects did not show the typical, delay-related decrease in matching-to-sample performance that characterized the behavior of common-response subjects. An oddity-matching task was then introduced, and subjects were instructed to use the mediating keys however they preferred, including not at all. Differential-response subjects continued to respond on the originally trained mediating keys in response to sample presentation and later reversed their choice responding, thus accommodating the oddity-matching requirements. Common-response subjects continued to emit the previously trained mediating response and experienced limited success in oddity matching. Results were interpreted in terms of stimulus control, instructional control, and experimental history.     

Assessing control by elements of complex stimuli in delayed matching to sample.

A series of six experiments examined delayed identity matching-to-sample performances of subjects with mental retardation. The stimuli were either one or two simultaneously displayed forms. When the reinforcement contingencies required that only one form exert discriminative control, all subjects achieved high accuracy scores. However, accuracy scores were substantially lower when the contingencies required discriminative control by two forms, suggesting restricted stimulus control. The decline in matching accuracy appeared to reflect selective losses of conditional control by sample stimuli and shifts in control to features of the comparison stimulus displays. The experiments suggest improved techniques for assessing control by complex stimuli and for evaluating the effects of procedures that seek to broaden restricted stimulus control. The results challenge interpretations based on stimulus-generalization decrement or shared attention.     

Local proactive interference in delayed matching to sample: the role of reinforcement

Discriminability in delayed matching to sample was lower when the samples on consecutive trials differed compared with when samples on consecutive trials were the same. This local proactive interference occurred when correct choices on the previous trial were reinforced but not when correct choices on the previous trial were not reinforced. When the choice on the previous trial was incorrect, discriminability was higher on different consecutive trials than on same trials. These effects were amplified by varying the ratio of reinforcers for correct choices, as predicted by a model that attributes local proactive interference to an interaction between control by the sample on the current trial and the influence of reinforcers for correct choices on previous trials.     

Effect of delay-interval stimuli on delayed symbolic matching to sample in the pigeon

In Experiment 1, food-deprived pigeons received delayed symbolic matching to sample training in a darkened Skinner box. Trials began with the illumination of the grain feeder lamp (no food sample), or illumination of this lamp, accompanied by the raising of the feeder tray (food sample). After a delay of a few seconds, the two side response keys were illuminated, one with red and one with green light, with positions counterbalanced over trials. Pecking the red (green) comparison produced grain reinforcement if the trial had started with food (no food); pecking red after a no-food sample or green after a food sample was not reinforced. Once matching performance was stable, four stimuli were presented during the delay interval, and their effects on matching accuracy were evaluated. Both illumination of the houselight and the center key with white geometric forms decreased matching accuracy, whereas presentation of a tone and vibration of the test chamber did not. In Experiment 2, pecking the red center key was reinforced with food according to a variable interval schedule. The effects of occasional brief presentations of the four stimuli used in the first experiment on ongoing pecking were assessed. The houselight and form disturbed key pecking, but the tone and vibration did not. Thus, stimuli that interfered with delayed matching also interfered with simple operant behavior. Implications of these results for theories of remembering are discussed.     

Vasopressin injections impair working memory in a delayed matching to sample task in rats

Effects of vasopressin were measured in a nonspatial working memory task: food-reinforced, delayed matching to sample. Subcutaneous injections of 0.2 microgram of lysine vasopressin (LVP) or saline were alternately administered to Sprague-Dawley rats after the presentation of the sample and compared to the effect of the same treatments given to a yoked control group of rats. Different durations of sample presentation (5 and 30 min) and various retention intervals (10 min, 3 h, 24 h) were selected. The results showed that LVP never facilitated retention performance: there was no improvement under conditions of weak memory (short presentation of the sample and long retention interval); moreover LVP abolished the facilitation normally obtained when either the length of the sample presentation was longer or the duration of the retention interval was shorter. The performance after the injection of the peptide was differentially impaired, according to brightness of the sample which had been presented: after the presentation of the white box, LVP injections lead to more errors and after that of the black one the treatment induced an increase in latencies of response. All these data may suggest that the physiological consequences of hormonal modifications triggered by the LVP injection are processed along with the stimuli of the learning episode and interfere with the learned positive value of the sample.     

Delayed reinforcement and delayed choice in symbolic matching to sample: Effects on stimulus discriminability

Six pigeons were trained to peck a red side key when the brighter of two white lights (S₁) had been presented on the center key, and to peck a green side key when the dimmer of two white lights (S₂) had been presented on the center key. Equal frequencies of reinforcers were provided for the two types of correct choice. Incorrect choices, red side-key pecks following S₂ presentations and green side-key pecks following S₁ presentations, resulted in blackout. With 0-s delay between choice and reinforcement, the delay between sample presentation and choice was varied from 0 to 20 s. Then, with 0-s delay between sample presentation and choice, the delay between choice and reinforcement was varied from 0 to 20 s. Both types of delay resulted in decreased discriminability (defined in terms of a signal-detection analysis) of the center-key stimuli, but delayed choice had more effect on discriminability than did delayed reinforcement. These data are consistent with the view that the two kinds of delay operate differently. The effect of a sample-choice delay may result from a degradation of the conditional discriminative stimuli during the delay; the effect of a choice-reinforcer delay may result from a decrement in control by differential reinforcement.     

Memory without awareness: pigeons do not show metamemory in delayed matching to sample

Metamemory, the ability to report on memory strength, is clearly established in rhesus macaques (Macaca mulatta) by converging evidence from several paradigms. In contrast, A. Inman and S. J. Shettleworth (1999) found no conclusive evidence of metamemory in pigeons. The authors studied pigeons further in 3 paradigms, with multiple tests of metamemory in each. Pigeons encountered a safe alternative to a matching-to-sample test either before (Experiment 2) or concurrently with (Experiment 3) the matching test. Choices of the safe option did not vary consistently with matching accuracy or change in trials with omitted samples in the way predicted for an animal with metamemory. In Experiment 4, confidence ratings following completion of the matching test also did not vary consistently as predicted by metamemory.     

Serial order effects in auditory discrimination by oddity and matching to sample

The oddity method and several matching-to-sample procedures were used for frequency and intensity discrimination by adult Ss. For all sequence tasks involving three choices, the Ss employed a decision strategy which resulted in a pattern of correctness that differed from the pattern of discriminability among the choice sounds. It was concluded that the use of signal detection theory for analysis of results had added an important dimension to the study, and it was recommended that this method of analysis be used in further attempts to relate sequential processes to various forms of verbal impairment.     

Cholinergic lesions produce task-selective effects on delayed matching to position and configural association learning related to response pattern and strategy

192IgG-saporin (SAP) was used to selectively destroy cholinergic neurons in the rostral basal forebrain (e.g., medial septum (MS) and vertical limb of the diagonal band of Broca (VDB)) and/or the caudal basal forebrain (e.g., nucleus basalis magnocellularis (NBM)) of ovariectomized Sprague-Dawley rats. The effects of these lesions on two different cognitive tasks, a delayed matching to position (DMP) T-maze task, and a configural association (CA) operant conditioning task, were evaluated and compared. Injecting SAP into either the MS or NBM significantly impaired acquisition of the DMP task. Analysis showed that the effects were due largely to an affect on response patterns adopted by the rats during training, as opposed to an effect on working memory performance. Notably, the impairment in DMP acquisition did not correlate with the degree of cholinergic denervation of the hippocampus. Despite the deficit, most animals eventually learned the task and reached criterion; however by the end of training, controls and animals that received SAP into either the MS or NBM appeared more likely to use an allocentric place strategy to solve the task, whereas animals that received SAP into both the MS and NBM were more likely to use an egocentric response strategy. Cholinergic lesions also produced a small but significant affect on acquisition of the CA task, but only with respect to response time, and only in the SAP-NBM-treated animals. SAP-NBM lesions also produced small but significant impairments in both the number of responses and response time during the acquisition of simple associations, possibly reflecting an effect on alertness or attention. Notably, the effects on CA acquisition were small, and like the effects on DMP acquisition did not correlate with the degree of cholinergic denervation of the hippocampus. We conclude that selective basal forebrain cholinergic lesions produce learning deficits that are task specific, and that cholinergic denervation of either the frontal cortex or hippocampus can affect response patterns and strategy in ways that affect learning, without necessarily reflecting deficits in working memory performance.     

Programming stimuli in matching to sample

In these investigations, a “teaching machine” was used to train pre-school and first-grade children in a series of progressively difficult discrimination tasks, leading up to matching to sample. Such training was much more efficient than training in the final discrimination alone. The errors the subjects made were found to be a functon both of the differences between consecutive discriminations (the “size of the steps” in the program) and the length of training on each discrimination. Theoretical and practical implications of these findings are discussed.     

Learning mechanisms in matching to sample.

A model system and an experiment on early learning and decision processes in matching-to-sample and oddity-from-sample tasks are presented. The model system is based, in part, on videotaped records of pigeons' looking responses before they chose 1 of 2 comparison stimuli. In order to see the wavelength stimuli recessed behind the pecking keys, the pigeons had to move in front of them. Although there were slight increases in the acceptance probability with switches between the stimuli before a choice response, the overall decision strategy was close to a Markov choice process in which choice proportions could be predicted by the product of each rejection probability and the final acceptance probability. Learning involved learning to discriminate rather than learning to adopt a stricter criterion for an acceptable sample match.     

Effects of fixed-ratio sample and choice response requirements upon oddity matching

Three pigeons were trained on oddity matching in which either 1, 4, 8, 16, or 32 sample-key observing responses were required to turn off the sample stimuli and turn on the comparison stimuli. Oddity accuracy increased when the observing-response requirement was raised and decreased when the requirement was lowered. Next, while the observing requirement was maintained at one response, the number of responses required to the comparison stimuli was either 1, 4, 8, 16, or 32. Under these conditions, choice was defined as the comparison that first accumulated the required number of responses. In general, increasing the comparison-response requirement decreased accuracy and lowering the comparison requirement increased accuracy. The fixed-ratio observing requirements appeared to facilitate control by stimuli serving an instructional function.     

Voice recognition by matching to sample

Voice recognition was assessed by a matching to sample procedure in 30 right-handed adults with normal hearing. The subject was required to indicate which of three voices speaking a nonsense syllable matched the speaker of a sample vowel. Subjects were able to recognize voices with reasonable accuracy, but there were no significant differences as a function of ears or practice, and performance was not markedly affected by knowledge of results or mode of response. There was a significant difference as a function of the temporal position of the matching voice, with recognition being most accurate when the matching voice was first and least accurate when it was third. Further research is necessary to determine whether voice recognition should be classified as the type of verbal ability associated with the cerebral hemisphere dominant for speech, or whether it is the type of nonverbal auditory ability associated with the nonspeech hemisphere.This research was supported in part by grant MA-1652 from the Medical Research Council of Canada.