Training delays reduce the choose-short effect with keylight duration samples in pigeons

Pigeons were trained to matching 2- and 8-s keylight samples. The delay on training trials was either 0s (group 0sF), 2s (group 2sF), or varied between 1 and 3s (M=2s, group 2sV). Testing at delays of 10 and 20s revealed a choose-short tendency in all three groups. The magnitude of this tendency was largest in group 0sF and was highly similar in magnitude in groups 2sF and 2sV. In Experiment 2, the training delay remained at 0s in group 0sF and was increased to 5s in group 2sF (now group 5sF). Group 2sV (now group 5sV) received variable training delays ranging from 2 to 8s (M=5s). Testing at a 20-s delay and at a delay that exceeded the training delay by 15s for each group revealed a robust choose-short effect only in group 0sF. Groups 5sF and 5sV both demonstrated a weak and statistically nonsignificant choose-short tendency. When different durations of keylight are employed as samples, training with a nonzero delay, whether fixed or variable, reduces the magnitude of the choose-short effect, with longer training delays producing a greater reduction in the choose-short effect.     

The choose-short effect in rat memory for event duration: the subjective-shortening model

Two predictions derived from the subjective-shortening model were tested in rats. The predictions concerned the temporary occurrence of the choose-short effect with extended training at a given retention interval (RI) and the occurrence of a temporary choose-long effect, when RIs shorter than those used during training were applied. In a first experiment, using a stepwise delay procedure with training 0-s RI sessions interpolated between each series of increasing RIs, results showed: (1) a choose-short effect during the stepwise increase in the delay procedure, (2) a temporary occurrence of the choose-short effect during testing at a given RI and (3) a choose-long effect in half of the animals, when a RI shorter than that used previously was applied. These contrasting results suggest that the disappearance of the choose-short effect could be, as proposed by the model, either the consequence of the foreshortening of the reference memory (for rats choosing-long) or the consequence of an adaptation of the working memory (for rats which did not choose long). Results were discussed in relation with the procedure which could have contributed, by the interposition of 0-s RI sessions, to maintain a stable reference memory. In order to test this interpretation, a second experiment, using the classical stepwise delay procedure without training sessions interpolated, was carried out. In these conditions, rats did never present a significant choose-long effect when the RI was shortened. These results suggest that rats maintained a stable reference memory and could improve their performances during retention testing sessions either by an adaptation of their working memory or by the adoption of an alternative strategy which consisted in learning to maintain an orientation towards the location of the correct lever.     

Effects of delays on human performance on a temporal discrimination procedure: evidence of a choose-short effect

Studies of temporal discrimination in non-human subjects have reliably shown a choose-short effect: higher matching accuracy on short-duration-sample trials than on long-duration-sample trials. This effect occurs as a function of increasing the delay between the onset of sample and comparison stimuli in a delayed matching-to-sample procedure. The present experiment investigated whether the choose-short effect could be demonstrated in human subjects under conditions which paralleled those used with non-human subjects. Subjects responded under a discrete-trial procedure in which they were required to push one of two buttons depending on the duration of a sample stimulus (a blue square on a computer monitor). Delays (0, 8, 16, and 32s) separated sample and comparison stimuli (two white boxes) and were tested both within and across several sessions. Intermediate durations (probe stimuli between 2 and 4s) were also presented. The addition of a delay between the sample and comparison stimuli produced a bias to judge intervals as short when the 8 and 32-s delays were tested across sessions and when the 0, 16, and 32-s delays were tested within the same session. Thus, the choose-short effect was produced in human subjects using the interval bisection procedure regardless of delay length.     

Training delays reduce the choose-short effect with keylight, but not with food, duration samples in pigeons

Pigeons were trained to match 2- and 8-s food samples. The delay on training trials was either 0s (group 0sF), 5s (group 5sF), or varied between 2 and 8s (M=5s, group 5sV). Testing at a delay that exceeded the training delay by 15s in each group revealed a robust choose-short effect in each group. The same pigeons then reacquired a previously trained matching task involving 2- and 8-s keylight samples. Different comparison stimuli were used on food-sample and keylight-sample trials. The delay on training trials was the same on both food- and keylight-sample trials. Extended-delay testing revealed a robust choose-short effect in all three groups when the durations were conveyed by food presentations, but only group 0sF revealed a choose-short effect when the durations were conveyed by keylight presentations. Hence, training with a nonzero delay, whether fixed or variable, reduces the choose-short effect with keylight durations but not with food durations. It was concluded that at least some of the psychological processes mediating performance differ as a function of the event that conveys the duration.     

Pigeons' memory for sequences of light flashes: reliance on temporal properties and evidence for delay interval/gap confusion

In Experiment 1, pigeons were trained to discriminate between sequences of two and four light flashes (illumination of the feeder). Retention functions obtained with dark delays exhibited a choose-many bias at a 1-s delay and a choose-few bias at delays of 4 and 8s. Retention functions obtained with illuminated delays only displayed a slight choose-few bias. In Experiment 2, additional birds were trained with the same sample sequences. However, the intertrial interval was illuminated by the houselight for Group Light, and it was dark for Group Dark. The acquisition data suggested that multiple temporal features of the light flash sequences controlled choice responding. For both groups, the retention functions were similar to those obtained in Experiment 1. In Experiment 3, baseline training with a 1-s dark delay eliminated the choose-many bias, but a significant choose-few bias at extended dark delays was still observed. Pigeons discriminate light flash sequences by relying on temporal properties of the sequence rather than using an event switch to count flashes. The biased-forgetting effects obtained in these studies appear to be primarily due to confusion between the delay interval and the gap between light flashes.     

Dopamine agonists and antagonists can produce an attenuation of response bias in a temporal discrimination task depending on discriminability of target duration

The current study examined the effects of the D2 agonist (quinpirole) and D2 antagonist (eticlopride) on temporal discrimination performance in a conditional discrimination task (Experiment I) and a delayed conditional discrimination task (Experiment II). In both experiments rats discriminated between a scheduled stimulus duration of 3 s versus 9 s. Consistent with previous reports, overall discrimination performance decreased in a dose-dependent manner with both drugs. Changes in response bias (the tendency to choose-short or choose-long irrespective of actual stimulus duration), however, were best characterized in terms of both drugs tending to attenuate the bias effects normally observed during baseline drug-free performance. Specifically, the 'choose-short' bias observed in Experiment I and at a relatively short, 0.1 s, delay in Experiment II became less extreme with increasing doses. In addition, the 'choose-long' bias observed at a relatively long, 6 s, delay in Experiment II also became less extreme with increasing doses. Thus, whether there was an apparent shift from a short response bias to long, or vice versa, was the product of the delay interval between stimulus presentation and choice rather than whether the drug in question was a D2 agonist or antagonist. Such an attenuation of bias may have arisen because of subjects confounding the delay interval with the actual discriminative stimulus duration.     

Effect of training delays and start and stop markers on the choose-short effect in pigeons

Pigeons were trained in a matching-to-duration task using short and long filled intervals. Group 2/8 was trained with 2- and 8-s intervals, Group 4/10 with 4- and 10-s intervals, and group marker with 2- and 8-s intervals presented between 1-s start and stop markers. Extended-delay testing showed no significant choose-short effect (CSE) in any group. It was hypothesized that the lack of a CSE may have resulted from use of a variable delay (range 1-3s) during training. The same subjects were employed in Experiment 2 and were trained with a new set of comparisons and one of the alternate types of samples employed in Experiment 1. All training trials involved a 0-s delay. Extended-delay testing revealed a significant CSE in Groups 2/8 and 4/10 but only a weak, and statistically nonsignificant, CSE in group marker. It was concluded that use of a variable delay during training reduced the CSE. The notion that subjective shortening underlies the CSE provided an adequate account of these findings.     

Effects of unsignaled delays of reinforcement on fixed-interval schedule performance

Key pecking of pigeons was maintained by a fixed-interval (FI) 61-s schedule. The effects of resetting and nonresetting unsignaled delays of reinforcement then were examined. The resetting delay was programmed as a differential-reinforcement-of-other-behavior schedule, and the nonresetting delay as a fixed-time schedule. Three delay durations (0.5, 1 and 10 s) were examined. Overall response rates were decreased by one and 10-s delays and increased by 0.5-s delays. Response patterns changed from positively accelerated to more linear when resetting or nonresetting 10-s delays were imposed, but remained predominantly positively accelerated when resetting and nonresetting 0.5- and 1-s delays were in effect. In general, temporal control, as measured by quarter-life values, changed less than overall response rates when delays of reinforcement were in effect. The response patterns controlled by FI schedules are more resilient to the nominally disruptive effects of delays of reinforcement than are corresponding overall response rates.     

Rats' memory for event duration in delayed matching-to-sample with nonspatial comparison response alternatives

Previous research has suggested that using stationary and moving levers as nonspatial response alternatives can significantly enhance the speed of acquiring a temporal discrimination in rats. In Experiment 1, rats were trained to discriminate 2 and 8s of magazine light illumination by responding to either a stationary lever or a moving lever with a cue light illuminated above it. Rats learned to discriminate event durations at a high level of accuracy after 25 sessions of training. During subsequent delay tests, rats exhibited a strong choose-long bias, indicating that they were timing from the onset of the magazine light until the entry of levers into the chamber. This occurred regardless of whether intertrial intervals and delay intervals were dark or illuminated. On test trials in which the sample was omitted, rats responded as if the short sample had been presented. In Experiment 2, the rats received extensive training with dark and illuminated variable delay intervals (1-4 s). However, they continued to exhibit a tendency to time from the onset of the magazine light until entry of the levers into the chamber. Although the use of stationary/uncued and moving/cued levers as response alternatives enhanced the speed of acquisition of the event duration discrimination in rats, additional procedural modifications will be necessary to prevent rats from timing during the delay interval.     

Effects of retention interval on performance in a numerical reproduction task

The retention interval (RI) between the sample and production phase in a numerical reproduction task was varied to determine whether a "produce-small" effect would be obtained with increased delays. Four pigeons were trained with a retention interval of 2s, and then tested with intervals of 0.5s and 8s. Results showed a number-dependent, "produce-large" effect-response number increased when RI was increased-analyses of average response number and accuracy suggested RI affected responding most on the 2-flash trials with an 8-s RI. Additionally, discrimination between trial types decreased as RI increased. Existing explanations for the "choose-short/small" effect appear unable to account for these results; however the "produce-large" effect may be attributed to a disruption in stimulus control over responding.     

Rats' memory for event duration: differential effects of delaying the discriminative choice cue as opposed to the opportunity to execute the choice response

Rats were trained to discriminate short or long durations of houselight illumination using a choice procedure. During the test phase of each trial, the left and right levers were presented with an auditory cue above one of them on (cued lever) while the other was off (uncued lever). The auditory cue was presented immediately after sample offset and the levers were inserted after the auditory cue had been presented for 2 s. For half of the rats, the correct response following the short sample was to press the cued lever, while following the long sample, it was to press the uncued lever. This was reversed for the remaining rats. Following acquisition of the discrimination, two different types of delay tests were administered. In the first set, the delay between offset of the sample and onset of the auditory cue was manipulated (Cue Delay Test). In the second set, the delay between onset of the auditory cue and entry of the levers into the chamber was manipulated (Response Delay Test). Cue Delay testing resulted in a choose-long bias at the longer delays. Response Delay testing did not result in a systematic response bias and there was little forgetting over the delay interval. These data suggest that the rats did not stop the internal clock when the nominal sample was offset, but allowed it to keep running until the auditory cue was presented. The data from the Response Delay Test indicate that either a response decision was made based on the clock reading as soon as the auditory cue was presented, or the clock reading itself was retained over the delay with no subjective shortening and little forgetting.     

The effects of changes in consequences on hens' performance in delayed-matching-to-sample tasks

A delayed-matching-to-sample (DMTS) task was used to investigate remembering with domestic hens. In Conditions 1 and 3 of Experiment 1, six hens responded under a mixed-delay procedure with delays of 0.25, 2, and 8 s. In Condition 2, the reinforcer for correct responding was delayed for 6 s after each correct matching response on 2-s delay trials. In Condition 1, discrimination performance decreased monotonically over the three delays. With the delay to the reinforcer, the decreases were non-monotonic as a result of the considerable drop in the accuracy of discrimination on the 2-s delay trials. Performance at the 2-s delay did not recover completely in Condition 3. In Conditions 1 and 3 of Experiment 2, five hens responded under a mixed-delay procedure with delays of 0, 4, and 16 s. In Condition 2 no reinforcers were provided for correct responding on 0- and 16-s delay trials. When reinforcers were available on all trials discrimination performance decreased monotonically with delay. There were non-monotonic changes in discrimination with delay when there was extinction at two delays resulting mainly from a large drop in discrimination performance at 0 s. In addition, response latencies increased markedly at the two delays associated with extinction. Performance recovered completely in Condition 3. The data support the ideas that remembering involves a temporal discrimination that the effects of delaying reinforcement and removing reinforcement may differ, and that the measurement of response latencies may be a useful tool in DMTS procedures.     

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.     

Temporal discrimination learning by pigeons

Memory for time by animals appears to undergo a systematic shortening. This so-called choose-short effect can be seen in a conditional temporal discrimination when a delay is inserted between the sample and comparison stimuli. We have proposed that this temporal shortening may result from a procedural artifact in which the delay appears similar to the intertrial interval and thus, produces an inadvertent ambiguity or 'instructional failure'. When this ambiguity is avoided by distinguishing the intertrial interval from the delay, as well as the samples from the delay, the temporal shortening effect and other asymmetries often disappear. By avoiding artifacts that can lead to a misinterpretation of results, we may understand better how animals represent time. An alternative procedure for studying temporal discriminations is with the psychophysical bisection procedure in which following conditional discrimination training, intermediate durations are presented and the point of subjective equality is determined. Research using the bisection procedure has shown that pigeons represent temporal durations not only as their absolute value but also relative to durations from which they must be discriminated. Using this procedure, we have also found that time passes subjectively slower when animals are required to respond to the to-be-timed stimulus.     

Resistance to change varies inversely with reinforcement context

We report two experiments which test whether resistance to prefeeding and satiation for a variable-interval (VI) schedule that delivers a constant rate of reinforcement varies inversely with the reinforcement rate for an alternative schedule. In Experiment 1, eight pigeons responded in a multiple schedule in which the red key was always associated with a VI 90-s schedule and the green key with either a richer (VI 18s) or leaner (VI 540s) schedule in different conditions. After baseline training in each condition, prefeeding test sessions were conducted in which 10g, 20g, 30g, 40g, and 50g food were provided one-hour prior to test. Additional baseline training was given between each test session. In Experiment 2, two groups of pigeons responded in a multiple schedule similar to Experiment 1. After baseline training, pigeons were exposed to a 5-h satiation test session in which the VI 90-s schedule was available continuously. Test sessions were conducted when pigeons were maintained at 85%, 95%, and 85% of their body weights in an ABA design. Results of both experiments showed that responding in the VI 90-s schedule that alternated with a leaner schedule during baseline was more resistant to prefeeding and satiation. These data rule out alternative explanations for results of previous studies, and confirm that resistance to change varies inversely with reinforcement context.     

Effects of unpredictable changes in initial-link duration on choice and timing

Four pigeons responded in a concurrent-chains procedure in which terminal-link schedules were fixed-interval (FI) 10 s and FI 20 s. Across sessions, the location of the shorter terminal-link changed according to a pseudorandom binary sequence. Each session, the variable-interval initial-link schedule value was sampled from a uniform distribution that ranged from 0.01 to 30 s. On some terminal links, food was withheld to obtain measures of temporal control. Terminal-link delays determined choice (log initial-link response ratios) and timing (start and stop times on no-food trials) measures, which stabilized within the 1st half of each session. Preference for the shorter terminal-link delay was a monotonically decreasing function of initial-link duration. There was no evidence of control by initial-link durations from previous sessions.     

Effect of unsignaled delays between stimuli in a chain schedule on responding and resistance to change

Behavioral momentum theory is an evolving theoretical account of the strength of behavior. One challenge for the theory is determining the role of signal stimuli in determining response strength. This study evaluated the effect of an unsignaled delay between the initial link and terminal link of a two-link chain schedule on resistance to change using a multiple schedule of reinforcement. Pigeons were presented two different signaled delay to reinforcement schedules. Both schedules employed a two-link chain schedule with a variable interval 120-s initial link followed by a 5-s fixed time terminal link schedule. One of the schedules included a 5-s unsignaled delay between the initial link and the terminal link. Resistance to change was assessed with two separate disruption procedures: extinction and adding a variable time 20-s schedule of reinforcement to the inter-component interval. Baseline responding was lower in the schedule with the unsignaled delay but resistance to change for the initial link was unaffected by the unsignaled delay. The results suggest that not all unsignaled delays are equal in their effect on resistance to change.     

Nicotine and the behavioral mechanisms of intertemporal choice

Nicotine has been found to produce dose-dependent increases in impulsive choice (preference for smaller, sooner reinforcers relative to larger, later reinforcers) in rats. Such increases could be produced by either of two behavioral mechanisms: (1) an increase in delay discounting (i.e., exacerbating the impact of differences in reinforcer delays) which would increase the value of a sooner reinforcer relative to a later one, or (2) a decrease in magnitude sensitivity (i.e., diminishing the impact of differences in reinforcer magnitudes) which would increase the value of a smaller reinforcer relative to a larger one. To isolate which of these two behavioral mechanisms was likely responsible for nicotine's effect on impulsive choice, we manipulated reinforcer delay and magnitude using a concurrent, variable interval (VI 30 s, VI 30 s) schedule of reinforcement with 2 groups of Long-Evans rats (n = 6 per group). For one group, choices were made between a 1-s delay and a 9-s delay to 2 food pellets. For a second group, choices were made between 1 pellet and 3 pellets. Nicotine (vehicle, 0.03, 0.1, 0.3, 0.56 and 0.74 mg/kg) produced dose-dependent decreases in preference for large versus small magnitude reinforcers and had no consistent effect on preference for short versus long delays. This suggests that nicotine decreases sensitivity to reinforcer magnitude.     

Discrimination of the dynamic properties of sound source spatial location in humans: Electrophysiology and psychophysics

The spatial resolution of the human auditory system was studied under conditions, where the location of the sound source was changed according to different temporal patterns of interaural time delay. Two experimental procedures were run in the same group of subjects: a psychophysical procedure (the transformed staircase method) and an electrophysiological one (which requires recording of mismatch negativity, the auditory evoked response component). It was established that (1) the value of the mismatch negativity reflected the degree of spatial deviation of the sound source; (2) the mismatch negativity was elicited even at minimum (20μs) interaural time delays under both temporal patterns (abrupt azimuth change and gradual sound movement at different velocities); (3) an abrupt change of the sound source azimuth resulted in a greater mismatch negativity than gradual sound movement did if the interaural time delay exceeded 40 μs; (4) the discrimination threshold values of the interaural delay obtained in the psychophysical procedure were greater than the minimum interaural delays that elicited mismatch negativity, with the exception of the expert listeners, who exhibited no significant difference.     

Photic and nonphotic circadian phase resetting in a diurnal primate, the common marmoset

Despite the considerable literature on circadian entrainment, there is little information on this subject in diurnal mammals. Contributing to this lack of understanding is the problem of separating photic from nonphotic (behavioral) phase-resetting events in diurnal species. In the present study, photic phase resetting was obtained in diurnal common marmosets held under constant dim light (DimDim; <0.5 lx) by using a 20-s pulse of bright light to minimize time available for behavioral arousal. This stimulus elicited phase advances at circadian time (CT) 18-22 and phase delays at CT9-12. Daily presentation of these 20-s pulses produced entrainment with a phase angle of approximately 11 h (0 h = activity onset). Nonphotic phase resetting was obtained under DimDim with the use of a 1-h-induced activity pulse, consisting of intermittent cage agitation and water sprinkling, delivered in total darkness to minimize photic effects. This stimulus caused phase delays at CT20-24, and entrainment to a scheduled daily regimen of these pulses occurred with a phase angle of approximately 0 h. These results indicate that photic and nonphotic phase-response curves (PRCs) of marmosets are similar to those of nocturnal rodents and that nonphotic PRCs are keyed to the phase of the suprachiasmatic nucleus pacemaker, not to the phase of the activity-rest cycle.