Response bias and the discrimination of stimulus duration

Pigeons discriminated stimulus duration in a psychophysical choice situation. Following presentation of any duration from a set of short duration (11 to 15 sec), responses on a red key were reinforced intermittently. Following presentation of any duration from a set of long durations (16 to 22 sec), responses on a green key were reinforced intermittently. Relative reinforcement rates were manipulated for choice responses across conditions. As relative reinforcement rates were varied, psychometric functions showed shifts in green-key responses at all durations. A signal-detection analysis showed that sensitivity remained roughly constant across conditions while response bias changed as a function of changes in relative reinforcement rate. Relative error rates tended to match relative reinforcement rates.     

Relative judgments affect assessments of stimulus duration

Humans were trained on two independent temporal discriminations, with correct choice dependent on the initial stimulus duration. In Experiment 1, the durations were 1.0 and 4.0 sec, with one set of choice stimuli, and 2.0 and 8.0 sec, with a different set of choice stimuli. The 2.0- and 4.0-sec values were selected to be the geometric mean of the two values in the other discrimination. In Experiment 2, the durations were 2.0 and 5.0 sec for one discrimination and 3.5 and 6.5 sec for the other. The 3.5- and 5.0-sec values were selected to be the arithmetic mean of the two values in the other discrimination. In both experiments, participants showed evidence for relational coding of the duration pairs. That is, the test durations were selected to be at the presumed bisection point (i.e., they should have produced indifferent choice), but instead the shorter test duration from the longer duration pair produced a “short” bias (in both experiments), whereas the longer duration from the shorter duration pair produced a “long” bias (in the second experiment). Results were similar to those from Zentall, Weaver, and Clement (2004) with pigeons.     

Discrimination of temporal relations by pigeons

In four experiments, pigeons were tested on a duration comparison task involving the successive presentation of two visual stimuli that varied in duration from trial to trial. Following presentation of the durations, two choice keys were lit, and reinforcement for choices was based on the temporal relation between duration of the pair. In Experiment 1, the range of durations was varied over conditions. Responding changed as an orderly function of the ratio of the two durations. There was a decrease in discrimination accuracy as average duration increased over condition but no difference in accuracy between shorter and longer problems within a duration range. There was no systematic response bias over conditions for all problems within a range, but there was a bias to report the second duration longer than the first for "long" problems within a range. In Experiment 2, the pigeons were transferred from a task involving spatially differentiated choices to one involving hue-differentiated choices. Performance was similar to that of the spatial procedure of Experiment 1. Additional analyses revealed that although information provided by a single duration of the pair was sometimes predictive of the temporal relation between pair members, responding was also based on the relation and comparison of both durations. In Experiment 3, the pigeons were exposed to a single duration range that included many durations from the four ranges of Experiment 1. Discrimination accuracy was comparable in the fourth and longest category. Manipulation of absolute reinforcement rate in Experiment 4 resulted in no chang in discrimination accuracy, suggesting that the decline in accuracy over conditions of Experiment 1 could not be attributed to decreases in reinforcement rate that accompanied lengthier durations. The results are discussed in terms of theories of animal timing, with Staddon's (1983, 1984) temporal perspective model providing the most systematic account of all aspects of performance.     

On the effects of signaling reinforcer probability and magnitude in delayed matching to sample

Two experiments examined whether postsample signals of reinforcer probability or magnitude affected the accuracy of delayed matching to sample in pigeons. On each trial, red or green choice responses that matched red or green stimuli seen shortly before a variable retention interval were reinforced with wheat access. In Experiment 1, the reinforcer probability was either 0.2 or 1.0 for both red and green responses. Reinforcer probability was signaled by line or cross symbols that appeared after the sample had been presented. In Experiment 2, all correct responses were reinforced, and the signaled reinforcer durations were 1.0 s and 4.5 s. Matching was more accurate when larger or more probable reinforcers were signaled, independently of retention interval duration. Because signals were presented postsample, the effects were not the result of differential attention to the sample.     

Duration discrimination is better with food access as the signal than with light as the signal

In Experiment 1 a go/no-go discrimination procedure was used to compare control of five pigeons' keypecking by food-access duration with control by light duration. Pecks to an illuminated key were reinforced with grain following 10-sec presentations of food access or houselight, but not after 5-sec presentations of either stimulus. Each subject discriminated food-access duration faster and to a greater degree than light duration. In four between-subject replications, pigeons discriminated food-access duration better than the duration of a localized light, the feeder light and a keylight, and with either water or food as reinforcement. In Experiment 2 control by durations of food access and light was compared using a conditional right-left choice procedure (two pigeons), and a delayed symbolic matching-to-sample procedure (six pigeons). Under both, choice accuracy again was higher on food-access trials. The results of Experiment 3, in which two pigeons received generalization trials with durations of food access and light that were intermediate to the training values, confirmed that responding was controlled by the duration dimension of both food access and light. The superior control by food access is consistent with previous evidence that food is an effective and memorable stimulus, possibly because of its biological importance. These results also provided empirical support for the commonly made assumption that stimuli differ in effectiveness. As well, the results show that the stimulus to be discriminated can play an important role in the accuracy of duration disciminations, a fact which has implications for the study of temporal discriminations in animals.     

The perception of empty and filled time intervals by pigeons

Pigeons were trained in a within-subjects design to discriminate durations of a filled interval (2 s and 8 s of light) and durations of an empty interval (2 s and 8 s bound by two 500-ms light markers). Filled intervals required a response to one set of comparisons (e.g., blue vs. yellow), whereas empty intervals required a response to a different set of comparisons (e.g., red vs. green). Psychophysical testing indicated that empty intervals were judged to be longer than equivalent durations of a filled interval. This finding was replicated when the anchor durations used during training were changed to 1 s and 4 s, or 4 s and 16 s. The difference between the point of subjective equality (PSE) for the empty intervals and the PSE for filled intervals increased as the magnitude of the anchor duration pairs increased. In addition, the difference limens (DL) for empty intervals were smaller than those for filled intervals, and they also increased as the magnitude of anchor duration pairs increased. An analysis of the Weber fractions (WF; i.e., DL/PSE) provided evidence for superimposition of the empty and filled timing functions across the different sets of anchor durations. These results suggest that the accumulation of subjective time was greater for empty intervals than for filled intervals. Within the framework of scalar timing theory, this difference in timing appeared to be the result of a clock rate difference rather than a switch latency difference.     

Autoshaped responding: A baseline for studying stimulus preference

In an autoshaping procedure with pigeons, trials consisted of the illumination of two keys, each with a different color, and then a response-independent feeder operation. Over successive conditions, all key-color pairs were arranged from the set of amber, red, green, and blue lamps. During sessions with a given pair, the left-right configuration of the colors varied irregularly, and the two colors alternated in illuminating the feeder. With one red and one green key, for example, red appeared sometimes on the left and sometimes on the right, and the feeder was alternately lit red or green on successive trials. Both total pecks and proportion of trials with at least one peck on a key of a given color were generally greater for red and amber than for green and blue, and relations among preferences were generally transitive across different color pairs. Repeating the procedure with decreased red and amber intensities and increased green and blue intensities reduced red and amber pecking relative to green and blue pecking, implying that differences in responding were determined more strongly by intensive than by chromatic properties of the stimuli.     

Discrimination of differences in digitally manipulated phoneme length during speech

The present study investigated differences in the thresholds of phoneme duration in word and sentence contexts by participants' sex. In the word condition, 27 participants listened to two pairs of words in a dual pair discrimination task. One pair contained the same durations of /s/, and the other pair contained different durations of /s/. Participants were to select the pair which contained different durations of /s/ in the words. In the sentence condition, the other set of 27 participants listened to two pairs of sentences and selected the pair containing different durations of /s/ in the sentences. Throughout these tasks, the participants' just-noticeable-difference and trials-to-completion were analyzed. The results showed that the participants demonstrated better performance in detecting just-noticeable-difference in the sentence condition than in the word condition. In addition, a sex difference was found in just-noticeable-difference in both conditions with better performance in men than women. No significant differences in trials-to-completion were found in either condition.     

Stimulus control during conditional discrimination

Pigeons were used to assess stimulus control during the development of a conditional discrimination. The training consisted of three stages. In Stage 1, key pecks were reinforced in the presence of a white line tilted 40 degrees to the right of vertical on a green background and non-reinforced when the same line appeared on a red background. In Stage 2, key pecks were reinforced when a white vertical line appeared on a red background and were non-reinforced in the presence of a 40 degrees slanted line on a red background. In Stage 3, key pecks were reinforced in the presence of the green background regardless of the line tilt, but were differentially reinforced in the presence of the red background (as in Stage 2). Generalization tests were conducted after each stage of training and consisted of five white lines on backgrounds that were green, red, or dark. The effects of the differential reinforcement contingencies on control by line orientation were restricted to the condition in which the red light appeared and resulted in behavioral control that could be characterized as: if red, pay closer attention to line tilt than if not red.     

Delayed choice responding by pigeons when the correct response is not predictable from the sample stimulus.

Food-deprived pigeons were presented with a row of four response keys situated above a grain hopper aperture. At the start of a trial, three of four keys were randomly selected and illuminated white for six seconds. After a variable blackout period, one of the three previously white keys and the previously dark key were illuminated green, and the remaining white keys were reilluminated as before. A response to the green key that was previously white was reinforced with three-second access to gain, a response to any other key resulted in a three-second blackout and the start of a new trial. Five of six subjects responded to the correct green key more often than chance at an interstimulus interval of 1.5 seconds, and they displayed maximal performance at different intertrial interval values ranging from 15 to 60 seconds. Choice accuracy decreased for all but one subject as the interstimulus interval was increased. For the range of interstimulus interval durations employed, decrements in choice accuracy were qualitatively similar to, but lower than those typically obtained from, delayed-matching-to-sample or delayed-pair comparison procedures.     

Sensitivity to reinforcer duration in a self-control procedure

In a concurrent-chains procedure, pigeons' responses on left and right keys were followed by reinforcers of different durations at different delays following the choice responses. Three pairs of reinforcer delays were arranged in each session, and reinforcer durations were varied over conditions. In Experiment 1 reinforcer delays were unequal, and in Experiment 2 reinforcer delays were equal. In Experiment 1 preference reversal was demonstrated in that an immediate short reinforcer was chosen more frequently than a longer reinforcer delayed 6 s from the choice, whereas the longer reinforcer was chosen more frequently when delays to both reinforcers were lengthened. In both experiments, choice responding was more sensitive to variations in reinforcer duration at overall longer reinforcer delays than at overall shorter reinforcer delays, independently of whether fixed-interval or variable-interval schedules were arranged in the choice phase. We concluded that preference reversal results from a change in sensitivity of choice responding to ratios of reinforcer duration as the delays to both reinforcers are lengthened.     

Dual-Element Effects in Pigeons’ Matching-to-Sample with Temporal and Visual Stimuli

The present study examined whether the dual-element effect occurs when temporal and visual stimuli appear simultaneously in a zero-delayed, symbolic matching-to-sample task. Two groups of pigeons were first exposed to either a red or green sample stimulus, for either 30 s or 5 s. The sample was followed by the presentation of yellow and blue comparisons. For pigeons in one group, the duration of the sample was the relevant cue. Responses to the yellow comparison were reinforced if the sample was 30 s, and responses to the blue comparison were reinforced if the sample was 5 s. For the other group, sample duration was irrelevant. Responses to the yellow comparison were reinforced if the sample had been green and responses to the blue comparison were reinforced if the sample had been red. Both groups then learned a second matching task in which the sample and comparison stimuli were vertical and horizontal lines. Finally, matching performance was examined when the lines appeared together with the temporal or color elements. The results showed that the line matching task was acquired more slowly for pigeons that were first trained to attend to duration. More importantly, matching was reduced when the temporal and line elements appeared simultaneously, and the effects were similar to those obtained when visual elements are combined.

     

Control of responding by stimulus duration

Pigeons were trained on a procedure in which the key was white for 30 sec, alternating with periods of darkness, or timeout. In a nondifferential training procedure, timeout duration was held constant at either 9 or 21 sec for different animals, and pecks on the white key were reinforced on a variable-interval 36-sec schedule. After 30 sessions an extinction generalization test was conducted where the duration of the timeout was varied from 3 to 27 sec. This test showed no differences in responding following timeouts of different durations. In a differential training procedure, timeout durations of either 9 or 21 sec were randomly scheduled for each animal. The variable-internal schedule was in effect following the same timeout duration as in the prior nondifferential procedure. No pecks were reinforced after the other timeout duration. In 40 sessions, differences in response rates following the two durations gradually developed. A maintained generalization procedure was then imposed in which timeout durations were varied from 3 to 27 sec, with the variable-interval schedule in effect following only the same duration as in the previous procedures. The first maintained generalization session showed that the prior differential training had established control of the animals' behavior by the timeout duration. In continued training on the maintained generalization procedure, control by the timeout duration decreased.     

Colored aftereffects produced with moving edges

Colored aftereffects that lasted as long as 6 weeks were produced with moving patterns of parallel black and white stripes or with black and white spirals. During adaptation, the patterns moved periodically in opposite directions, each direction paired with one illuminant, red or green. When the moving patterns were later viewed in white light, S saw the red and green colors, but they were related in the opposite way to the direction of motion. The red and green aftereffects were also produced by other pairs of illuminants, red and white, white and green, reddish-yellow and white, and white and greenish-yellow. The aftereffects did not occur unless, during adaptation, the stripes moved in both directions, each direction paired with a different color. The aftereffect was elicited by stripe motion over the retina—it was seen when the eye swept over a pattern of stationary stripes. The aftereffect desaturated when the retinal orientation of the stripes was changed from the adaptation orientation. Saturation was increased by longer exposure and slower speed during adaptation and by faster speed and a more rapid rate of altemation during the test. The luminance of the adaptation light seemed to have little effect. The aftereffect did not transfer from one eye to the other, and it did not change retinal locus, as was shown when clear images of a colored square that lasted several days were produced with a spiral. S ftxated the spiral’s center. The spiral rotated altemately in opposite directions. A red square with a green surround was projected on the center of the spiral when it rotated in one direction; a green square with a red surround was used when it rotated in the other direction. Following 50 min of adaptation, colored images of the squares were seen when the center of the spiral was ftxated and the direction of     

Punishment of observing by the negative discriminative stimulus

To determine the effect of a negative discriminative stimulus on the response producing it, two pigeons were each studied in a three-key conditioning chamber. During alternating periods of unpredictable duration, pecking the center (food) key either was reinforced with grain on a variable-interval schedule or was never reinforced. On equal but independent variable-interval schedules, pecking either of the side (observing) keys changed the color of all keys for 30 sec from yellow to either green or red. When the schedule on the center key was variable-interval reinforcement, the color was green (positive discriminative stimulus); when no reinforcements were scheduled, the color was red (negative discriminative stimulus). Since pecking the side keys did not affect grain deliveries, changes in the rate of pecking could not be ascribed to changes in the frequency of primary reinforcement. In subsequent sessions, red was withheld as one of the possible consequences of pecking a given side key. When red was omitted, the rate on that key increased, and when red was restored, the rate decreased. It was concluded that red illumination of the keys, the negative discriminative stimulus, had a suppressive effect on the response that produced it.     

Choice based on separately established response tendencies: The effect of recency of reinforcement

The responding of pigeons was studied in a chamber with two adjacent keys that could each be lit with either a red or a green dot. Fixed-length trials were used, with pecking on positive trials reinforced by presenting food at the end of the trial. In each session the first 40 trials were one-key trials with one key lit on each trial, either red or green, and the next four trials were choice trials with both keys lit, one red and the other green. When blocks of sessions were presented in which all one-key trials were positive trials of a single color, choice responding gradually shifted to the color presented most recently during one-key trials. When all one-key trials were nonreinforced trials of a single color, responding on choice trials shifted toward the other color. When positive one-key trials of one color were intermingled with negative one-key trials of the other color, choice responding shifted exclusively to the reinforced color. These shifts in choice responding occurred even when responding on choice trials was never reinforced, and were viewed as based on separate response tendencies for pecking red and pecking green built up during one-key trials.     

Reciprocal response interactions in concurrent variable-interval and discrete-trial fixed-ratio schedules

Abstract.— Pecking a red key by pigeons was reinforced with grain on a continuously accessible variable-interval schedule. Pecking a second key was reinforced on a discrete-trial fixed-ratio schedule; occasionally the second key was illuminated green and after a single run on the fixed-ratio schedule a reinforcer was presented and the green light was turned off. The experiment investigated the effects of acquisition, extinction, and re-acquisition of pecking the second key. All pigeons changed over immediately from pecking the red key to pecking the green key whenever the green light controlled a high rate of pecking this key. Pecking the red key was completely suppressed during pecking the green key. The experiment shows that a changeover from one response to a second response can come under discriminative control of a stimulus during which the second response is intermittently reinforced. All pigeons frequently emitted observing and orienting behaviors towards the dark key that was occasionally lit green.     

Memory for recent behavior in the pigeon

Variations of the symbolic delayed-matching-to-sample procedure were used to study a pigeon's memory for a small number of pecks. In the first experiment a choice of a left or right sidekey after a delay or retention interval was reinforced if a bird had not pecked at all or had pecked exactly once, before the delay, respectively. In the second experiment a choice of a red or green sidekey, regardless of its position, was reinforced if a bird had not pecked at all or had pecked exactly twice, respectively. In the first experiment a bird could orient toward the correct choice during the delay, whereas it could not in the second experiment. In a third experiment a feature-probing method was used to study a pigeon's memory for a number of pecks in the context of certain other pecks. The results showed that a pigeon can remember a small number of pecks for one-half to one minute or more and that the percent correct is a decreasing function of the log retention interval. When a second number of pecks is different from the first number, memory for the first number lasts only a few seconds. When a second number is the same, memory lasts considerably longer. The more recent number of pecks is remembered better. The results are interpreted in terms of a theory which holds that a reinforcer, in general, may act on a subjects' memory for recent behavior to generate patterns of behavior.     

Rats’ memory for time and relational responding in the duration-comparison procedure

Rats were trained in a duration-comparison task to press one lever if the comparison duration (c) was 1.2-s shorter than a standard duration (s), and another lever if c was 1.2-s longer than s. The interval between s and c duration was 1 s. The 10 duration pairs used during training controlled for the absolute duration of c and the total duration of an s-c pair. The total duration of an s-c pair was not predictive of the correct response. In Experiment 1, during equal-duration pair test trials, rats increasingly responded long (i.e., c > s) as the s-c delay was lengthened. In Experiment 2, long responding increased as the s-c delay was lengthened, even when the illumination condition during the s-c delay differed from that during the intertrial interval (ITI). In Experiment 3, transfer to novel duration pairs was assessed. Overall accuracy for the novel duration pairs was significantly above chance, but transfer performance was also affected by the absolute value of the novel c durations. This is the first study to demonstrate that rats can acquire relational duration discriminations. As in previous studies with pigeons the evidence was consistent with subjective-shortening of the standard duration and there was also evidence of a reliance on a mixture of absolute and relational strategies in responding.     

Inhibitory stimulus control following errorless discrimination learning

Three generalization procedures were used to investigate inhibitory stimulus control following discrimination learning with few errors. Three groups of pigeons acquired a discrimination between a green stimulus (the positive stimulus) and a vertical or horizontal line (the negative stimulus) through differential autoshaping followed by multiple schedule presentation of the two stimuli with gradually increasing stimulus durations. Genereralization testing was along a line-tilt continuum. For one group, the test involved a resistance-to-reinforcement procedure in which responses to all line tilts were reinforced on a variable-interval schedule. For a second group, also tested with the resistance-to-reinforcement procedure, the lines were superimposed on the green field that formerly served as the positive stimulus. A third group was tested in extinction with the combined stimuli. Control groups had no discrimination training but responding to green was nondifferentially reinforced. The control subjects responded more to all line tilts during testing than did the comparable experimental subjects, indicating that the negative stimulus had become an inhibitory stimulus. Both resistance-to-reinforcement groups revealed inhibitory gradients around the negative stimulus, but the gradient for the extinction group was relatively flat. These data are consistent with others that modify Terrace's early conclusion concerning the failure of inhibition to develop during errorless training.