Multiple-interval timing in rats: Performance on two-valued mixed fixed-interval schedules.

Three experiments studied timing in rats on 2-valued mixed-fixed-interval schedules, with equally probable components, Fixed-Interval S and Fixed-Interval L (FI S and FI L, respectively). When the L:S ratio was greater than 4, 2 distinct response peaks appeared close to FI S and FI L, and data could be well fitted by the sum of 2 Gaussian curves. When the L:S ratio was less than 4, only 1 response peak was usually visible, but nonlinear regression often identified separate sources of behavioral control, by FI S and FI L, although control by FI L dominated. Data were used to test ideas derived from scalar expectancy theory, the behavioral theory of timing, and learning to time. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Temporal Control of Conditioned Responding in Goldfish.

The peak procedure was used to characterize response timing during acquisition and maintenance of conditioned responding in goldfish. Subjects received light-shock pairings with a 5- or 15-s interstimulus interval. On interspersed peak trials, the conditioned stimulus light was presented for 45 s and no shock was delivered. Peaks in the conditioned response, general activity, occurred at about the time of the expected unconditioned stimulus, and variability in the activity distribution was scalar. Modeling of the changes in the activity distributions over sessions revealed that the temporal features of the conditioned response changed very little during acquisition. The data suggest that times are learned early in training, and, contrary to I. P. Pavlov's (1927/1960) concept of "inhibition of delay," that timing is learning when to respond rather than learning when not to respond. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Internal clock and memory processes in animal timing.

Two experiments, involving a total of 6 male pigeons, investigated temporal control of behavior within the framework of an internal clock model. Ss were exposed to signaled fixed interval (FI) 30-sec trials mixed with extended unreinforced (baseline) trials. On unreinforced break trials, the signal was interrupted for a period of time after trial onset. In Exp 1, comparisons between the peak time obtained on baseline and on break trials produced peak time shifts that were longer than those expected if the clock had stopped during the break but shorter than if the clock had reset. In Exp 2, systematic manipulations of duration and location of breaks produced peak time shifts that were nonlinear functions of break duration and that varied linearly with break location. The obtained peak times were more consistent with a continuous memory decay model than with the stop-retain or the reset hypotheses. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Stimulus compounding in interval timing: The modality–duration relationship of the anchor durations results in qualitatively different response patterns to the compound cue.

We have previously demonstrated that rats trained on a two-duration peak procedure in which two modal signals (i.e., tone and houselight) predicted probabilistic reinforcement availability at two times (10 s and 20 s) would respond in a scalar manner at a time between the trained durations in response to the simultaneous compound cue (tone + houselight). In these experiments, we evaluated whether this scalar response pattern would remain with greater relative separation between the anchor durations. Results revealed an effect of the modality–duration relationship, such that scalar responding was seen on compound trials in rats trained that the auditory stimulus signaled the shorter duration, whereas the visual stimulus signaled the longer duration, but not in the reverse condition. In rats showing scalar responding on compound trials, post hoc analyses demonstrated that the peak time of compound responding was most accurately predicted by the reinforcement probability weighted average of anchor peak times. In contrast, rats trained that the visual stimulus signaled the shorter duration, whereas the auditory stimulus signaled the longer duration, responded in a highly rightward skewed manner. In these rats, initiation of responding to the compound stimulus appeared to be controlled by the visual stimulus only, whereas response terminations reflected control by both modal stimuli. These latter data provide evidence of separate determinants of response initiation and termination. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Appetitively motivated instrumental learning in SynGAP heterozygous knockout mice.

The synaptic Ras/Rap-GTPase-activating protein (SynGAP) regulates specific intracellular events following N-methyl-d-aspartate receptor (NMDAR) activation. Here, the impact of SynGAP heterozygous knockout (SG+/?) on NMDAR-dependent functions was assessed using different positive reinforcement schedules in instrumental conditioning. The knockout did not affect the temporal control of operant responding under a fixed interval (FI) schedule, but led to a putative enhancement in response vigor and/or disinhibition. When examined on differential reinforcement of low rates of response (DRL) schedules, SG+/? mice showed increased responding under DRL-4s and DRL-8s, without impairing the response efficiency (total rewards/total lever presses) because both rewarded and nonrewarded presses were elevated. Motivation was unaffected as evaluated using a progressive ratio (PR) schedule. Yet, SG+/? mice persisted in responding during extinction at the end of PR training, although an equivalent phenotype was not evident in extinction learning following FI-20s training. This extinction phenotype is therefore schedule-specific and cannot be generalized to Pavlovian conditioning. In conclusion, constitutive SynGAP reduction increases vigor in the execution of learned operant behavior without compromising its temporal control, yielding effects readily distinguishable from NMDAR blockade. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Analysis of single trials in pigeons' timing performance.

Response distributions of pigeons on the peak procedure, averaged over many trials, were well fitted by a Gaussian function plus either a straight line or a ramp. But, each of 11 distributions analyzed showed a systematic positive skew. Response distributions on individual trials showed break–run–break patterns with abrupt transitions in the rate of responding. Within the run phase was an inner run phase at yet a higher rate. Intercorrelations among the starting time, ending time, middle, duration, and rate of the run showed a similar pattern in the outer and inner runs. A positive correlation between starting and ending times suggests variance across trials in clock speed, delay to start the clock, or expected time of reward. A negative correlation between starting time and duration suggests variance across trials in the threshold or thresholds to start and end the run. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Using the peak procedure to measure timing and counting processes in pigeons.

In 4 experiments, it was shown that the peak procedure can be used to study timing and counting in pigeons. In Experiment 1, pigeons were trained to peck at a green key that briefly flashed red every second. After training under a fixed interval 20 s/fixed number 20 flashes schedule of reinforcement, time and number were dissociated on empty test trials by varying the flash rate. The peak-time curves for different flash rates suggested that pigeons were using a strategy of responding to the first dimension, time or number, that crossed a threshold for initiating and terminating response. Experiments 2 and 3 showed that pigeons could be trained to time or count on empty trials by reinforcing response to only time or number. It was found in Experiment 4 that the set to time or count transferred to a new fixed interval 30 s/fixed number 30 flashes schedule of reinforcement. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The ethical problem of the health-environment relationship]

The effects of sustained stress on response rate and temporal patterning (quarter-life) of rats performing either a previously learned fixed-interval schedule (FI 60) or learning an FI 60 simultaneously with stress onset were determined. Rats lived 24 h/day in operant cages, where they earned all of their food via lever-pressing. During the stress portion of each experiment, one group of rats was able to avoid or escape signalled intermittent footshock (Avoidance/ Escape Group), a second group (Yoked) did not have control over shock termination, a third group never received shock (Control). Shock trials were presented around the clock at approximately 5-min intervals and the stress portion of each study lasted 1-2 weeks. We have previously reported that rats tolerate this paradigm well and avoid/escape 99% of the shock trials. In rats previously trained on the FI task, both rate of responding and quarter-life values were significantly decreased on the first day of stress for both the Avoidance/Escape and Yoked Groups. Food intakes and quarter-life values were not significantly different from the controls by stress Days 3 and 2, respectively. In the acquisition study, controls learned the F1 task by Day 4 as judged by quarter-life of responding. FI task acquisition was significantly impaired in stressed rats compared to controls, not reaching asymptotic performance until Day 9 of stress. There were no major differences between the 2 stress groups in either study. These data demonstrate that stress may impair both the rate and patterning of behavior, and suggest that this rodent paradigm may usefully model some aspects of the effects of stress in humans.     

Disrupted temporal control in the R6/2 mouse model of Huntington’s disease.

Huntington’s disease is characterized by corticostriatal dysfunction and degeneration of the striatum with progressive loss of the medium spiny neurons. These circuits are important for instrumental responding, interval timing, and temporal control over motor output. We investigated the acquisition of timed operant responding in two R6/2 Huntington’s Disease models, differing in CAG repeat length and genetic background (115 and 250 CAG repeats, and a mixed CBAxC57 or pure C57 background) and their corresponding wild type controls using the peak procedure. Both mouse lines exhibited similar response control deficits. In unreinforced peak trials, mice either did not learn to terminate an ongoing response past reinforcement time or required more trials to acquisition compared to the wild type mice. While transgenic and wild type mice did not exhibit differences in temporal accuracy, response curves were flatter in transgenic mice, suggesting decreased temporal control over operant responding. The results are discussed in terms of the neurobiology of interval timing, instrumental responding, and the neuropathology of HD and R6/2 mice. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Pigeons presented with sequences of false flashes use behavior to count but not to time.

On randomly ordered trials, pigeons were presented with either a blue or a white key that flashed red for 200 ms at a fast (2 flashes/s), medium (1 flash/s), or slow (0.5 flashes/s) rate. The blue key signaled a fixed-interval (FI) schedule in which the first response after 20 s was reinforced, and the white key signaled a fixed-number (FN) schedule in which the first response after 20 flashes was reinforced. In Experiments l and 2, pigeons showed depressed responding to the flash on FI-cued trials and accelerated responding to the flash on FN-cued trials. When the response key was periodically blacked out in Experiments 3 and 4, counting but not timing was eliminated. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Acquisition, maintenance and reinstatement of intravenous cocaine self-administration under a second-order schedule of reinforcement in rats: effects of conditioned cues and continuous access to cocaine

Second order schedules of IV cocaine reinforcement in rats provide a reliable method for evaluating the effects of conditioned stimuli on cocaine-seeking behaviour, and for measuring the motivational aspects of cocaine reinforcement. In the procedure established here, each infusion of cocaine (0.25 mg/infusion) was initially made contingent on a lever press and was paired with a 20-s light conditioned stimulus (CS). When rats acquired stable rates of cocaine self-administration, the response requirement for cocaine was increased progressively to a second-order schedule of the type FI15 min(FR10:S), whereby the IV cocaine infusion was self-administered following the completion of the first FR10 responses (and CS presentation) after a 15-min fixed interval (FI) had elapsed. Evaluation of the animals' responding during the first, drug-free interval of each daily session provided a measure of cocaine-seeking behaviour, independent of other pharmacological effects of the self-administered drug. Thus, a dose-response study (dose range: 0.083, 0.25 and 0.50 mg/infusion) revealed that responding under this schedule during the initial, drug-free interval changed monotonically with dose, whereas an inverse relationship between cocaine dose and response level tended to appear during the rest of the session, after rats had self-administered the drug. Responding under this schedule was also shown to occur under the control of the CS, which had acquired conditioned reinforcing properties. Thus, a decrease in responding and an increase in the latency to initiate responding followed the omission of the CS for 3 consecutive days. In addition, extinction of cocaine-seeking behaviour was slower when contingent CS presentations occurred compared to extinction when the CS was not present. Furthermore, the reinstatement of responding for cocaine, which followed a brief period of non-contingent CS presentations, was retarded when this conditioned reinforcer had been extinguished together with cocaine. Finally, cocaine-seeking behaviour decreased markedly for the first 6 h that followed a 12-h period of continuous access to cocaine, when compared to responding 6 h after a 90-min session of limited access to the drug. Responding subsequently increased to baseline levels within 72 h. These results emphasise the utility of second-order schedules for studying drug-seeking behaviour and the importance of drug-associated cues in maintaining such responding for cocaine.     

Estradiol modulation of the speed of an internal clock.

Estradiol significantly influences dopamine (DA) activity in the striatum (e.g., J. B. Becker, 1990b), and researchers have strongly implicated striatal DA in the regulation of temporal integration in the seconds-to-minutes range (e.g., W. H. Meck, 1996). In the current experiment, the author examines the effect of acute estradiol administered prior to testing on a peak-interval (PI) timing task. The administration of 5 μg of estradiol 30 min prior to testing resulted in an immediate and proportional leftward shift in the timing functions relative to the PI functions obtained following the administration of the oil vehicle. The precision of the response functions was increased in a manner commensurate with the scalar property of interval timing without significant alteration of peak response rates. When timing behavior was assessed 3 days following estradiol or oil administration, no differences were found in the peak time of responding or in the precision of responding between estradiol- and oil-treated rats, indicating that the effects of estradiol on these measures of interval timing are short lived. Together, these findings indicate that estradiol selectively increases the speed of an internal clock, perhaps through facilitating striatal DA activity. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Memory variance in the peak procedure of timing in pigeons.

Pigeons were rewarded on a fixed interval (FI) schedule. On occasional unrewarded tests, they usually showed a break–run–break pattern of responding. Across trials, the start of the run correlated positively with the end, suggesting variance across trials in (1) clock speed, (2) the delay to start the clock, or (3) a single criterion time used on a trial. If variance in criterion time exists, then a task in which the start and the end of the run are based on independent criterion times should produce a reduced start–end correlation. In 2 such tasks, the start–end correlation dropped to near zero, although other correlations were comparable to those found in the standard FI task. This provides evidence for variance across trials in a single criterion time used on a trial. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Influence of nonmemorial factors on manifestation of short-sample biases in choice and successive matching-to-duration tasks with pigeons.

The effects of procedural modifications of choice and successive matching tasks on retention of event duration (2- and 10-sec presentations of light) were examined. In accord with prior results, retention testing revealed that accuracy on short- and long-sample trials declined symmetrically in standard successive matching but asymmetrically (i.e., markedly on long-sample trials, and very little on short-sample trials) in standard choice matching. Moreover, asymmetrical retention functions were also obtained in (1) a modified successive task in which all trials ended in reinforcement and (2) a modified choice task in which the penalty for incorrect responding was substantially reduced. It was concluded that pigeons code duration analogically in both standard choice and successive matching tasks, and that such coding is manifest in asymmetrical retention functions only in the absence of a response bias engendered by the standard successive procedure. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of reinforcement omission on rats with lesions in the amygdala.

Trained 12 male Wistar rats with bilateral lesions in the amygdala to barpress on an FI schedule of reinforcement. During test trials, when reinforcement was occasionally omitted, response rates of 12 controls increased in the subsequent interval, whereas lesioned Ss showed no significant change. In Exp II Ss received fixed-ratio reinforcement on 1 lever, which was followed by a time-out period and fixed-ratio reinforcement on a 2nd lever. Results indicate that after reinforcement was withheld Ss with damage in the amygdala did not increase responding in the subsequent time-out period, whereas controls showed significantly higher rates. Differential latencies to initiation of response after nonreinforcement were also found. The deficits following brain damage are attributed to a reduction in nonreinforcement-induced frustration. (20 ref.) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of human self-assessment responding on learning.

Examined the effects of self-assessment (SA) responding on acquisition rate in paired-associates learning with 190 undergraduates. SA responding required the learner to indicate the degree of sureness in the correctness of each answer by pressing one of several buttons (representing different levels of sureness) either immediately before or after each answer. The number of trials required to learn the names of 8 tools by 20 Ss in each group, who made SA responses using either 2, 4, or 8 SA-response buttons, was compared with the number of trials required by Ss in 3 control groups, who either performed only the learning task or in addition pressed a single button labeled Record either before or after each answer. Results show that acquisition was expedited by as much as 25% by SA responding and was more rapid if the SA response was executed after the answer, rather than before it. The more rapid acquisition is tentatively attributed to a greater ability of the learner who engages in SA responding to identify a correct response. (6 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The temporal dynamics of a visual discrimination: The role of stimulus comparison and opponent processes.

The influence of trial spacing on simple conditioning is well established: When successive reinforced conditioned stimulus, CS+, trials are separated by a short interval (massed training), conditioned responding emerges less rapidly than when such trials are separated by longer intervals (spaced training). This study examined the influence of trial spacing on the acquisition of an appetitive visual discrimination in rats. Experiments 1 and 2 established that massed training facilitates the acquisition of such discriminations. The results of subsequent experiments demonstrated that this trial-spacing effect reflects the proximity of nonreinforced, CS-, trials to preceding (Experiment 3) and signaled (Experiment 4) presentations of the reinforcer. Experiment 5 showed that the facilitation of discrimination learning with massed training reflected an effect on learning rather than performance. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The pattern of responding in the peak-interval procedure with gaps: An individual-trials analysis.

Humans and lower animals time as if using a stopwatch that can be “stopped” or “reset” on command. This view is challenged by data from the peak-interval procedure with gaps: Unexpected retention intervals (gaps) delay the response function in a seemingly continuous fashion, from stop to reset. We evaluated whether these results are an artifact of averaging over trials, or whether subjects use discrete alternatives or a continuum of alternatives in individual-trials: A Probability-of-Reset hypothesis proposes that in individual gap trials subjects stochastically use discrete alternatives (stop/reset), such that when averaged over trials, the response distribution in gap trials falls in between “stop” and “reset.” Alternatively, a Resource Allocation hypothesis proposes that during individual gap trials working memory for the pregap duration decays, such that the response function in individual gap trials is shifted rightward in a continuous fashion. Both hypotheses provided very good fits with the observed individual-trial distributions, although the Resource Allocation hypothesis generated reliably better fits. Results provide support for the usefulness of individual-trial analyses in dissociating theoretical alternatives in interval timing tasks. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Acquisition With Partial and Continuous Reinforcement in Rat Magazine Approach.

Partial reinforcement, compared with continuous reinforcement, is widely considered detrimental to Pavlovian conditioned responding. However, time-accumulation models predict an invariance in acquisition when learning is assessed as a function of number of reinforcements, not trials, and intertrial interval is held constant. Three experiments examined this prediction in a rat magazine-approach procedure. All experiments showed superior responding with continuous reinforcement. Experiments 1 and 3 used common tests in between- and within-subject designs, respectively. Experiment 2 showed the same pattern in a discrimination. Earlier results are reanalyzed informally and in a meta-analysis. Contrary to previous summaries of the literature, evidence points to superior conditioned responding with continuous reinforcement in a number of procedures. Results are generally consistent with traditional associative models of learning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Overshadowing and stimulus duration.

In 3 experiments, the authors investigated the effect of stimulus duration on overshadowing. Experiments 1 and 2 examined responding to a target conditioned stimulus (CS1) when it was conditioned in compound with a coterminating overshadowing stimulus (CS2) that was longer, shorter, or of the same duration (the long, short, and matched groups, respectively). Equal overshadowing of conditioning to CS1 was obtained in all 3 conditions relative to a control group conditioned to the light alone. There were, however, differences in responding to CS2 as a function of its absolute duration. Experiment 3 examined the contribution of the food-food interval/CS onset-food interval ratio to these findings. In Experiments 1 and 2, the ratio differed for the overshadowing CS but not for the target CS. In Experiment 3, this arrangement was reversed, but the pattern of results remained the same. The implications of these findings for trial-based and real-time models of conditioning are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)