The effect of intruded events on peak time: the role of reinforcement history during the intruded event

Pigeons were studied in an extension of a study by Aum et al. [Aum, S., Brown, B.L., Hemmes, N.S. 2004. The effects of concurrent task and gap events on peak time in the peak procedure. Behav. Process. 65, 43-56] on timing behavior under a discrete-trial fixed-interval (FI) procedure during which 6-s intruded events were superimposed on peak-interval (PI) test trials. In Aum et al., one event consisted in termination of the timing cue (gap trial); the other was a stimulus in the presence of which subjects had been trained to respond under an independent random-interval (RI) schedule of reinforcement (concurrent task trial). Aum et al. found a disruption of timing on concurrent task trials that was greater than that on gap trials. The present study investigated history of reinforcement associated with intruded events as a possible explanation of this earlier finding. After training to peck a side key on a 30-s PI procedure, discrimination training was conducted on the center key in separate sessions; red or green 6-s stimuli were associated with RI 24s or EXT (extinction) schedules. During testing under the PI procedure, three types of intruded events were presented during probe trials--the stimulus associated with the RI (S+) or EXT (S-) schedule during discrimination training, or a gap (termination of the side-keylight). Intruded events occurred 3, 9, or 15s after PI trial onset. Effects of reinforcement history were revealed as substantial disruption of timing during the S+ event and relatively little disruption during the S- event. Intermediate effects were found for the gap event. Results indicate that postcue effects are at least partially responsible for the disruptive effects of the S+ event.     

Investigations of timing during the schedule and reinforcement intervals with wheel-running reinforcement

Across two experiments, a peak procedure was used to assess the timing of the onset and offset of an opportunity to run as a reinforcer. The first experiment investigated the effect of reinforcer duration on temporal discrimination of the onset of the reinforcement interval. Three male Wistar rats were exposed to fixed-interval (FI) 30-s schedules of wheel-running reinforcement and the duration of the opportunity to run was varied across values of 15, 30, and 60s. Each session consisted of 50 reinforcers and 10 probe trials. Results showed that as reinforcer duration increased, the percentage of postreinforcement pauses longer than the 30-s schedule interval increased. On probe trials, peak response rates occurred near the time of reinforcer delivery and peak times varied with reinforcer duration. In a second experiment, seven female Long-Evans rats were exposed to FI 30-s schedules leading to 30-s opportunities to run. Timing of the onset and offset of the reinforcement period was assessed by probe trials during the schedule interval and during the reinforcement interval in separate conditions. The results provided evidence of timing of the onset, but not the offset of the wheel-running reinforcement period. Further research is required to assess if timing occurs during a wheel-running reinforcement period.     

The effect of an intruded event on peak-interval timing in rats: isolation of a postcue effect

The present experiment employed the peak-interval (PI) procedure to study the effect of an intruded cue on timing behavior. Rats were trained on a 30-s PI procedure with a tone cue. Subsequently, a 6-s flashing light was paired off-baseline with foot shock (Experiment 1) or presented alone (Experiment 2). Then, in test trials, the light cue was presented 9s prior to (before) or 3s after (during) the onset of the timing cue, or the light was omitted (probe). Results showed rightward shifts in peak time occurring on both before and during trials in both experiments. Peak shifts on during trials exceeded the reset prediction in Experiment 1. When PI functions for before and probe trials were normalized in peak rate and peak time, they superimposed better than when functions were adjusted additively along the time axis, suggesting that the light cue may engender a decrease in functional clock rate. The findings suggested that the intruded cue produced both intracue and postcue interference with timing that was enhanced by fear conditioning.     

The proportion of fixed interval trials to probe trials affects acquisition of the peak procedure fixed interval timing task

A common procedure for studying the ability of animals to time is the peak procedure. With the peak procedure, animals are first trained on a fixed interval schedule (i.e., 30s). After the animals have been well trained on the fixed interval schedule, probe trials are introduced. On probe trials, the stimulus is presented longer (i.e., 90s) and the animal does not receive reinforcement for responding. When animals are first presented with probe trials responding remains flat following the point that reinforcement normally occurs on fixed interval trials. The descending slope that eventually emerges is acquired with experience with probe trials. The present experiments manipulated the percentage of probe trials compared to FI trials across groups of rats. It was hypothesized that the descending limb of peak responding would be acquired more quickly when there were many probe trials per session as this might facilitate extinction of responding beyond the interval that reinforcement normally occurs. It was found, however, that acquisition of peak responding occurred best when there were few probe trials per session.     

Fewer peak trials per session facilitate acquisition of peak responding despite elimination of response rate differences

It has been shown in previous research [Kaiser, D.H., 2008. The proportion of fixed interval trials to probe trials affects acquisition of the peak procedure fixed interval timing task. Behav. Process., 77 (1), 100-108] that rats acquired peak responding sooner when fewer peak trials were presented during sessions of training with the peak procedure timing task. One potential problem with that research was that there were large differences in response rates among the groups. The present experiment attempted to examine the effect of proportion of peak trials when differences in response rate were controlled. Two groups of rats were each simultaneously tested with two versions of the peak procedure. One group was tested with 10% peak trials per session, and the other group was tested with 50% peak trials per session. For both of the groups, one of the panel lights and levers was associated with the traditional peak procedure. The other panel light and lever was associated with a similar peak procedure; however, reinforcement was provided at the end of each peak trial. This manipulation eliminated differences in response rate among the groups, however, Group 10% acquired peak responding more quickly than Group 50%, effectively replicating previous work in the absence of a response bias.     

Feature-short and feature-long discrimination learning in the pigeon: conditional control of a two-event temporal map

Eight pigeons were trained to peck an illuminated target key on discrete-trial fixed-interval schedules of reinforcement by food. Four birds were exposed to a feature-short (FS) task where a feature light signaled shortening of the forthcoming target-outcome interval from 30 to 15s, while the other four birds were exposed to a feature-long (FL) task where a feature light signaled extension of the forthcoming target-outcome interval from 15 to 30s. The discrimination performance measured by differential temporal distributions of pecks between featured and non-featured target trials suggested that the target-food temporal map was under conditional control of the feature light in both groups. The FS discrimination was more difficult to learn than the FL discrimination. This FS inferiority implies that our birds did not resort on the simple temporal discrimination by timing from the trial onset. The simple temporal discrimination account was also negated by the finding that increasing the feature-target gap did not have a predicted effect on the response distribution.     

Choice and timing in concurrent chains: effects of initial-link duration

Theories of timing have been applied to choice between delayed rewards by assuming that delays are represented in memory and that subjects sample from memory when choosing between alternatives. To search for covariation in single-trial measures of performance that might confirm this assumption, we used a procedure that allowed for convergent measurement of choice and timing behavior. Four pigeons responded in a concurrent chains/peak procedure in which the terminal links were fixed-interval (FI) 8s and FI 16s, across conditions the duration of the initial-link schedule was either short or long, and one quarter of the terminal links lasted for 48 s and ended without reinforcer delivery. Preference for the FI 8-s alternative was stronger with shorter initial links, replicating the 'initial-link effect'. Responding on no-food trials was unaffected by initial-link duration, and aggregated across trials, was typical of the peak procedure: response distributions were approximately Gaussian, with modes near the FI schedule durations, and variance was greater for the FI 16-s terminal link. Analysis of local measures of initial-link performance (e.g., pause to begin responding, time spent responding, number and duration of visits to each alternative, etc.) found that the initial-link effect was associated with an increase in the number and duration of visits per cycle to the nonpreferred alternative. Regression analyses showed that local initial-link measures contributed relatively little additional variance in predicting performance on individual no-food trials beyond that accounted for by FI schedule. Our results provide no clear evidence that initial- and terminal-link responding in concurrent chains are mediated by a common representation of terminal-link delays.     

Temporal generalization accounts for response resurgence in the peak procedure

The peak interval (PI) procedure is commonly used to evaluate animals' ability to produce timed intervals. It consists of presenting fixed interval (FI) schedules in which some of the trials are replaced by extended non-reinforced trials. Responding will often resume (resurge) at the end of the non-reinforced trials unless precautions are taken to prevent it. Response resurgence was replicated in rats and pigeons. Variation of the durations of the FI and the non-reinforced probe trials showed it to be dependent on the time when reinforcement is expected. Timing of both the normal time to reinforcement, and the subsequent time to reinforcement during the probe trials followed Weber's law. A quantitative model of resurgence is described, suggesting how animals respond to the signaling properties of reinforcement omission. Model results were simulated using a stochastic binary counter.     

Effects of 5-HT1A and 5-HT2A receptor stimulation on temporal differentiation performance in the fixed-interval peak procedure

We examined the effects of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and 5-HT(2A/2C) receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) on performance on the fixed-interval peak procedure, and the sensitivity of these effects to 5-HT1A and 5-HT2A receptor antagonists (N-[2-(4-[2-methoxyphenyl]-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide [WAY-100635] and ketanserin). Rats were trained to press a lever for food reinforcement in 50 min sessions consisting of 32 trials in which the lever was continuously available, separated by 10 s inter-trial intervals. In 16 trials, reinforcement was delivered following the first response after 30 s had elapsed since trial onset (fixed-interval 30 s). In 16 randomly interposed (peak/probe) trials, reinforcement was omitted, and the lever remained in the operant chamber for 120 s. Response rate in probe trials was plotted against time from trial onset. Time to peak response rate (t(peak)) and the Weber fraction were derived from modified Gaussian curves fitted to each rat's data. 8-OH-DPAT (0.05 mg kg(-1)) reduced t(peak) and increased the Weber fraction; the effect on t(peak) was antagonized by WAY-100635 (0.1 mg kg(-1)). DOI (0.25 mg kg(-1)) also reduced t(peak) and increased the Weber fraction; the reduction of t(peak) was antagonized by ketanserin (2 mg kg(-1)). Stimulation of 5-HT1A and 5-HT2A receptors alters temporal differentiation in qualitatively similar ways.     

Amphetamine affects the start of responding in the peak interval timing task

In this paper we investigate how amphetamine affects performance in a PI task by comparing two analyses of responding during peak trials. After training on 24 s fixed interval (FI-24) with 96 s peak trials, rats were given amphetamine for 4 consecutive days at doses of .5 and 1.0 mg/kg. Responses during peak trials were fitted with a Gaussian distribution to estimate the expected time of reinforcement from the peak time. A single trials analysis was also performed to determine the start time and stop time of the transition into and out of a high rate of responding on each peak trial. Amphetamine significantly decreased peak times as measured with the Gaussian curve fitting. However, in the single trials analysis, animals initiated responding significantly earlier, but did not stop responding earlier. Thus, fitting a Gaussian to the average performance across trials sometimes provides a different characterization of the timing process than does analyzing the start and stop of responding on individual trials. In the current experiment, the latter approach provided a more precise characterization of the effects of amphetamine on response timing.     

Oscillations following periodic reinforcement

Three experiments examined behavior in extinction following periodic reinforcement. During the first phase of Experiment 1, four groups of pigeons were exposed to fixed interval (FI 16 s or FI 48 s) or variable interval (VI 16 s or VI 48 s) reinforcement schedules. Next, during the second phase, each session started with reinforcement trials and ended with an extinction segment. Experiment 2 was similar except that the extinction segment was considerably longer. Experiment 3 replaced the FI schedules with a peak procedure, with FI trials interspersed with non-food peak interval (PI) trials that were four times longer. One group of pigeons was exposed to FI 20 s PI 80 s trials, and another to FI 40 s PI 160 s trials. Results showed that, during the extinction segment, most pigeons trained with FI schedules, but not with VI schedules, displayed pause-peck oscillations with a period close to, but slightly greater than the FI parameter. These oscillations did not start immediately after the onset of extinction. Comparing the oscillations from Experiments 1 and 2 suggested that the alternation of reconditioning and re-extinction increases the reliability and earlier onset of the oscillations. In Experiment 3 the pigeons exhibited well-defined pause-peck cycles since the onset of extinction. These cycles had periods close to twice the value of the FI and lasted for long intervals of time. We discuss some hypotheses concerning the processes underlying behavioral oscillations following periodic reinforcement.     

Learning to stop or reset the internal clock

In the present experiments, after training rats in a standard fixed interval (FI) 30 s schedule, we induced a change in the strategy employed during gap trials, by presenting during FI with gaps training, 9-s interruptions of the FI discriminative stimulus in 40% of the trials; in one type of interruption, after the discriminative stimulus resumed, the FI was re-started; in the second type of interruption, the FI had to be completed considering the time before the interruption. The effect of these manipulations was tested in a peak-interval with gaps procedure. The main result was that the strategy employed during gap trials depended on the type of interruption experienced during the training phase, both in a comparison between subjects (experiment 1) and within subjects (experiment 2).     

The effects of cue competition on timing in pigeons

We studied the effects of cue competition on timing in both overshadowing and blocking operant procedures with pigeons. A white center key delivered reward when pecked 30 s after a red or green sidekey was presented and 10 s after presentation of the alternate color on the other sidekey. In Experiment 1, key presentations were concurrent during training trials for overshadow-condition pigeons, while side key presentations were separated across training trials for control birds. In Experiment 2, half of the birds (Blocking group) were given pre-exposure trials to either the 10-s or 30-s sidekey condition. Both blocking-condition and control birds were then given trials of concurrent side key presentations. Peak time curves were compared between experimental and control conditions. The results showed blocking of timing accuracy of a long (30-s) stimulus by a short (10-s) stimulus, but no evidence for overshadowing of timing accuracy.     

Time estimation by pigeons on a fixed interval: the effect of pre-feeding

Pigeons well trained on a fixed interval 10-s schedule of reinforcement were tested on the peak procedure. In a successive conditions design, they were either pre-fed or not in the experimental setting. Pre-feeding decreased the rate of responding. It also led to a maximum rate of responding that occurred 2-3 s later than in the control condition, where the maximum occurred at the usual time of reinforcement. The shift in peak time in response to pre-feeding shows that peak time may not be a pure measure of timing. The results are also interpreted in terms of timing theories.     

Reinforcer concentration effects on a fixed-interval schedule

Four rats received training on a mixed FI 30-s FI 150-s schedule, where the different FI values were associated with different levers. During baseline, the reinforcer was a 30% concentration of condensed milk. During subsequent testing sessions, the reinforcer concentration was varied within sessions over values of 10, 30, 50, and 70%. Measures of behaviour were taken from the FI 30-s lever during trials where the reinforcer was delivered for responses on the other lever. Increasing the reinforcer concentration which began the interval (a) increased the time to start responding in the interval, and (b) increased the location of the response peak on the FI 30-s lever (often to values well above 30s). Response rate at the peak, and spread of the response rate versus time function, changed much less with reinforcer concentration. The data are discussed relative to predictions derived from Scalar Expectancy Theory, the Behavioural Theory of Timing, and the Tuned-trace model.     

An Assessment of Fixed Interval Timing in Free-Flying Honey Bees (Apis mellifera ligustica): An Analysis of Individual Performance

Interval timing is a key element of foraging theory, models of predator avoidance, and competitive interactions. Although interval timing is well documented in vertebrate species, it is virtually unstudied in invertebrates. In the present experiment, we used free-flying honey bees (Apis mellifera ligustica) as a model for timing behaviors. Subjects were trained to enter a hole in an automated artificial flower to receive a nectar reinforcer (i.e. reward). Responses were continuously reinforced prior to exposure to either a fixed interval (FI) 15-sec, FI 30-sec, FI 60-sec, or FI 120-sec reinforcement schedule. We measured response rate and post-reinforcement pause within each fixed interval trial between reinforcers. Honey bees responded at higher frequencies earlier in the fixed interval suggesting subject responding did not come under traditional forms of temporal control. Response rates were lower during FI conditions compared to performance on continuous reinforcement schedules, and responding was more resistant to extinction when previously reinforced on FI schedules. However, no “scalloped” or “break-and-run” patterns of group or individual responses reinforced on FI schedules were observed; no traditional evidence of temporal control was found. Finally, longer FI schedules eventually caused all subjects to cease returning to the operant chamber indicating subjects did not tolerate the longer FI schedules.     

Within-session modulation of timed anticipatory responding: When to start responding

The peak procedure is widely used in the study of interval timing with animals. Multiple timing measures can be derived from peak responding. These measures are typically presented as averages across many trials based on the implicit assumption that peak responding is stable throughout the session. We tested this assumption by examining whether peak responding changed over the course of the session in 45 mice that were trained on a fixed-interval 30-s schedule. All common measures of peak responding, except stop times, changed over the course of the session: start times increased, response rates and spreads decreased, and, although less reliably, peak times also shifted rightward. These results are congruent with a motivational interpretation, whereby increased satiety leads to the observed behavioral signature of within-session modulation of timed anticipatory responding.     

Fatigue profile: a numerical method to examine fatigue in cycle ergometry

Fatigue Profile, a new numerical method for characterising fatigue in isokinetic cycle ergometry is presented and compared with the conventional fatigue index (FI). The new method describes the temporal development of muscle fatigue based on the decline of peak power output throughout a whole trial. The advantage of this method is demonstrated by the analysis of two 25 s maximum trials, separated by 90 s recovery, performed by a well-trained athlete at a pedal frequency of 120 revolutions per minute. A fourth degree polynomial was fitted to model the peak power data. Using the polynomial model coefficients the first derivative represented the rate of changing peak power which represented the Fatigue Profile. The conventional FI was calculated as -35 Ws(-1) and -32 Ws(-1) for trials 1 and 2 respectively, indicating minor differences in fatigue between trials. In contrast the Fatigue Profile revealed important numeric and temporal differences between the trials. For trial 1 a maximum rate of peak power decline of -65 Ws(-1) was reached at approximately 6 s into the trial. In marked contrast, in trial 2, maximum rate of peak power decline (-146 Ws(-1)) occurred immediately. The Fatigue Profile approach allows the characterisation of the temporal development of fatigue under different experimental conditions and in combination with other techniques may yield further insight into the underlying mechanisms of fatigue.