Subjective and Real Time: Coding Under Different Drug States.

Organisms are constantly extracting information from the temporal structure of the environment, which allows them to select appropriate actions and predict impending changes. Several lines of research have suggested that interval timing is modulated by the dopaminergic system. It has been proposed that higher levels of dopamine cause an internal clock to speed up, whereas less dopamine causes a deceleration of the clock. In most experiments the subjects are first trained to perform a timing task while drug free. Consequently, most of what is known about the influence of dopaminergic modulation of timing is on well-established timing performance. In the current study the impact of altered DA on the acquisition of temporal control was the focal question. Thirty male Sprague-Dawley rats were distributed randomly into three different groups (haloperidol, d-amphetamine or vehicle). Each animal received an injection 15 min prior to the start of every session from the beginning of interval training. The subjects were trained in a Fixed Interval (FI) 16s schedule followed by training on a peak procedure in which 64s non-reinforced peak trials were intermixed with FI trials. In a final test session all subjects were given vehicle injections and 10 consecutive non-reinforced peak trials to see if training under drug conditions altered the encoding of time. The current study suggests that administration of drugs that modulate dopamine do not alter the encoding temporal durations but do acutely affect the initiation of responding.

Temporal maps in appetitive Pavlovian conditioning.

Previous research suggests animals may integrate temporal information into mental representations, or temporal maps. We examined the parameters under which animals integrate temporal information in three appetitive conditioning experiments. In Experiment 1 the temporal relationship between 2 auditory cues was established during sensory preconditioning (SPC). Subsequently, rats were given first order conditioning (FOC) with one of the cues. Results showed integration of the order of cues between the SPC and FOC training phases. In subsequent experiments we tested the hypothesis that quantitative temporal information can be integrated across phases. In Experiment 2, SPC of two short auditory cues superimposed on a longer auditory cue was followed by FOC of either one of the short cues, or of the long cue at different times in the cue. Contrary to our predictions we did not find evidence of integration of temporal information across the phases of the experiment and instead responding to the SPC cues in Experiment 2 appeared to be dominated by generalization from the FOC cues. In Experiment 3 shorter auditory cues were superimposed on a longer duration light cue but with asynchronous onset and offset of the superimposed cues. There is some evidence consistent with the hypothesis that quantitative discrimination of whether reward should be expected during the early or later parts of a cue could be integrated across experiences. However, the pattern of responding within cues was not indicative of integration of quantitative temporal information. Generalization of expected times of reward during FOC seems to be the dominant determinant of within-cue response patterns in these experiments. Consequently, while we clearly demonstrated the integration of temporal order in the modulation of this dominant pattern we did not find strong evidence of integration of precise quantitative temporal information. This article is part of a Special Issue entitled: Associative and Temporal Learning.

Conditioned taste aversion from neostigmine or methyl-naloxonium in the nucleus accumbens.

An opioid antagonist injected in the nucleus accumbens of a morphine-dependent rat will lower extracellular dopamine and release acetylcholine (ACh), as also seen in opiate withdrawal. It was hypothesized that raising extracellular ACh experimentally would be aversive as reflected by the induction of a conditioned taste aversion. Rats were implanted with cannulas aimed above the nucleus accumbens (NAc) for injection of the opiate antagonist methyl-naloxonium in morphine-dependent animals or neostigmine to increase ACh in drug naïve animals. Experiment 1 in addicted rats showed that local morphine withdrawal by local injection of methyl-naloxonium paired with the taste of saccharin induces a conditioned taste aversion. Experiment 2 in non-addicted rats demonstrated the same learned aversion after local administration of the cholinergic agonist neostigmine in the NAc. These results suggest that ACh released in the NAc during opiate withdrawal contributes to the dysphoric, aversive state characteristic of withdrawal. This accumbens system is implicated in the mechanism for generating the memory of an aversive event that is expressed as learned taste aversion.

Relative temporal representations in Pavlovian conditioning.

The transfer of relative temporal representations was assessed in a series of three experiments. In each experiment, rats (Rattus norvegicus) received one set of conditioned stimulus (CS) and intertrial interval (ITI) durations in Phase 1 and another set in Phase 2. The ratio between the CS and ITI intervals was either changed or maintained across phases. On the hypothesis that relative temporal representations are learned, groups receiving maintained temporal ratios across phases were expected to display greater change in responding upon encountering the new intervals. When the CS duration decreased across phases, maintaining the temporal ratio did lead to greater change in Day 1 of Phase 2 towards the final pattern of responding. However, when the CS increased across phases, maintaining the temporal ratio across phases did not facilitate adjustment to the new intervals, suggesting that extinction of previously reinforced times induced new learning. These results provide evidence that under some conditions, relative relationships in temporal maps may survive transformation-of-scale, like relative relationships in spatial maps.

Target-absent controls in blocking experiments with rats.

In three between-groups blocking experiments with rats, two concurrent and one forward, several common control procedures were employed: Reinforced trials with the putative blocking stimulus were either omitted entirely (Kamin control), replaced by unsignaled reinforcements (Wagner control), or replaced by reinforced trials with a different stimulus (C+ control). In each experiment, parallel treatments with the target stimulus absent during training served to examine the possibility that differential responding in tests with the target stimulus might be traced solely to differential exposure to the nontarget stimuli. In Experiment 1, responding by a concurrent blocking group during the test was no different than responding by a Kamin control group, and responding by a Wagner control group was greater than that of either of the other groups--a pattern of results, mirrored in the performance of the target-absent groups, that could be attributed to the elevation of contextual excitation by unsignaled reinforcement. In Experiment 2, responding in the test by a concurrent blocking group was no different than that by a C+ control group. In Experiment 3, a finding of less responding by a forward blocking group than by a C+ control group when the target stimulus was present during training, but not when it was absent, provided plausible evidence of blocking.

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.