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The modality of a stimulus and its intermittency affect time estimation. The present experiment explores the effect of a combination of modality and intermittency, and its implications for internal clock explanations. Twenty-four participants were tested on a temporal bisection task with durations of 200-800ms. Durations were signaled by visual steady stimuli, auditory steady stimuli, visual flickering stimuli, and auditory clicks. Psychophysical functions and bisection points indicated that the durations of visual steady stimuli were classified as shorter and more variable than the durations signaled by the auditory stimuli (steady and clicks), and that the durations of the visual flickering stimuli were classified as longer than the durations signaled by the auditory stimuli (steady and clicks). An interpretation of the results is that there are different speeds for the internal clock, which are mediated by the perceptual features of the stimuli timed, such as differences in time of processing.

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During the 20th century there has been considerable progress in the study of animal cognition, based on essential ideas that were developed earlier. These essential ideas include the metaphors of the animal, the questions, the sources of hypotheses, and the types of explanations. The progress cannot be generally characterized as continuous improvement, cyclic changes, or discrete steps, but the quality of the empirical results has increased substantially since the first experiments in the field. Improvements in research methods have been primarily responsible for this progress. These have included improvements in animal husbandry, stimulus control, apparatus design, the measurement of behavior, experimental procedures, and data analysis. In the last third of the century the computer has led to major improvements in all stages of the research process in animal cognition. Although they have greatly improved the quality of the empirical results, they have not led to a general theory of animal cognition. In the near future the available research methods are likely to lead to further progress in the developments of quantitative theories of animal cognition.

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In 2 experiments, separate groups of rats were given stimulus conditioning, temporal conditioning, untreated control and (in Experiment 2) learned irrelevance control procedures, followed by a compound with both stimulus and temporal cues. Stimulus conditioning consisted of a random 15-s duration conditioned stimulus (CS) followed by food; temporal conditioning consisted of food-food intervals of fixed 90 s (Experiment 1) or fixed 75 + random 15 s (Experiment 2). The stimulus group abruptly increased responding after CS onset, and the temporal group gradually increased responding over the food-food interval. When the food-food interval was fixed 90 s, the temporal cue exerted stronger control in the compound, whereas when the food-food interval was fixed 75 + random 15 s, the stimulus cue exerted stronger control. The strength of conditioning, temporal gradients of responding, and cue competition effects appear to reflect simultaneous timing of multiple intervals.

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Scalar timing theory is a clear, complete, modular, and precise theory of timing that explains much of the data from many timing procedures, but not all of the data from all of the procedures. The multiple-time-scale theory of timing provides an alternative representation of time that has not yet been tested with respect to its fit to timing data.

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We attempted to determine whether timing theories developed primarily to explain performance in fixed-interval reinforcement schedules are also applicable to variable intervals. Groups of rats were trained in lever boxes on peak procedures with a 30-, 45-, or 60-s interval, or a 30- to 60-s uniform distribution (Experiment 1); a 60-s fixed and 1- to 121-s uniform distribution between and within animals (Experiment 2); and a procedure in which the interval between food and next available food gradually changed from a fixed 60 s to a uniform distribution between 0 and 120 s (Experiment 3). In uniform interval schedules rats made lever responses at particular times since food, as measured by the distribution of food-food intervals, the distribution of postreinforcement pauses, and the mean response rate as a function of time since food. Qualitative features of this performance are described by a multiple-oscillator connectionist theory of timing.

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Conditioning and timing studies have evolved under separate traditions, which is exemplified in both traditional theories (e.g. the Rescorla-Wagner model of conditioning vs. Scalar Timing Theory) and in a dual process model (Gibbon, J., Balsam, P., 1981. In: Autoshaping and Conditioning Theory. Academic Press, New York.). Other lines of theoretical development in both timing and conditioning fields have resulted in the emergence of 'hybrid' theories in which conditioning and timing processes are integrated. Simulations were conducted with a recent hybrid theory of timing (Machado, A., 1997. Psychol. Rev. 104, 241-265). The simulations were of classical conditioning procedures in which the local or global predictability of food was varied by manipulating the variability of the CS-US relationship, variability of the CS duration, and variability of the intertrial interval. The hybrid model provided good qualitative fits to indices of conditioning (discrimination ratios) and timing (local rates of responding), indicating that it may be possible to model both conditioning and timing results with a single process in which an internal representation of time and a strength of association are integrated. However, the failure of the model to provide good quantitative fits of the data indicates the need for a consideration of alternative perceptual representations of time and/or principles of association within the framework of the hybrid model.

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This article reviews the prerequisites for quantitative models of animal learning and cognition, describes the types of models, provides a rationale for the development of such quantitative models, describes criteria for their evaluation, and makes recommendations for the next generation of quantitative models. A modular approach to the development of models is described in which a procedure is considered as a generator of stimuli and a model is considered as a generator of responses. The goal is to develop models that, in combination with many different procedures, produce sequences of times of occurrence of events (stimuli and responses) that are indistinguishable from those produced by the animal under many experimental procedures and data analysis techniques.

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The representation of time was investigated by testing rats with intervals that changed by 2 s across trials. In Experiment 1, 2 ranges (20-150 s, 30-160 s; n = 10 rats per group) were examined. The times at which response bursts occurred (start time) were approximately proportional to interval durations. However, systematic departures from linearity were observed. Nonlinearities were related to the absolute duration of intervals, rather than to durations relative to the range. In Experiment 2, 660-s trials were inserted into the sequence of intervals (10-140 s, n = 20). Start and end times of response bursts were approximately proportional to intervals, but nonlinearities in start and end times were correlated, indicating that the source of nonlinearity was in the memory representation of time rather than in a decision process. These results indicate that the representation of time is nonlinearly related to physical time.

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In a constrained finger-tapping task, in which a subject attempts to match the rate of tapping responses to the rate of a pacer stimulus, interresponse interval (IRI) was a nonlinear function of interstimulus interval (ISI), in agreement with the results of Collyer, Broadbent, and Church (1992). In an unconstrained task, the subjects were not given an ISI to match, but were instructed to tap at their preferred rate, one that seemed not too fast or too slow for comfortable production. The distribution of preferred IRIs was bimodal rather than unimodal, with modes at 272 and 450 msec. Preferred IRIs also tended to become shorter over successive sessions. Time intervals that were preferred in the unconstrained task tended to be intervals that were overproduced (IRI > ISI) when they were used as ISIs in the constrained task. A multiple-oscillator model of timing developed by Church and Broadbent (1990) was used to simulate the two tasks. The nonlinearity in constrained tapping, termed the oscillator signature, and the bimodal distribution in unconstrained tapping were both exhibited by the model. The nature of the experimental results and the success of the simulation in capturing them both provide further support for a multiple-oscillator view of timing.

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Our purpose was to infer the characteristics of the internal clock, temporal memory, and decision processes involved in temporal generalization behavior on the basis of the analysis of individual trials. Three groups of 10 rats each were trained on a peak procedure with reinforcement at 15, 30, or 60 s, with several nonfood trial durations. On nonfood trials, the mean response rate gradually increased to a maximum near the time that reinforcement sometimes occurred and then gradually decreased. Individual trials were characterized by a period of high response rate, preceded and followed by a low response rate. The covariance pattern among measures of the temporal characteristics of the high response rate (start, stop, middle, and spread) supported a parallel, scalar timing model in which animals used on each trial a single sample from memory of the time of reinforcement and separate response thresholds to decide when to start and stop responding. An alternative model, the quasi-serial model (J. Gibbon & R. M. Church, 1992), was not consistent with the obtained relationships between covariances or with the scalar property seen across different nonfood signal durations.

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Parallel and serial timing processes are analyzed for their account of the dynamics of intertrial responding in the peak procedure. A strictly serial model, such as the behavioral theory of timing (Killeen & Fetterman, 1988), does not fit the dynamic correlation pattern in the location and duration of the middle high-rate responding portion of peak trials. In contrast, the parallel scalar expectancy theory model, with a sample for memory and threshold, does fit this pattern. A modification of the serial model is presented that also accommodates the within-trial covariance pattern. The modification, which is formally equivalent to a model for human tapping (Wing & Kristofferson, 1973), entails the addition of concurrent processes operating in parallel with serial timing.

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Subjects performed a repetitive manual tapping task, attempting to match a given rate of auditory stimulus pulses, first with the pulses audible (synchronization) and then with the pulses turned off (continuation). In different sessions, the interstimulus interval (ISI) was selected from the range 175 to 825 msec in steps of 25 msec, with different ISI values presented in a random order. Across this range of ISI conditions, interresponse intervals (IRIs) exhibited alternating positive bias (too slow) and negative bias (too fast). We interpret this pattern of bias in terms of a discrete, or categorical, timing mechanism in motor timing. Categorical time production can be viewed as extending our conception of the timekeeper in Wing's (Wing & Kristofferson, 1973a, 1973b) two-process model of motor timing and may be related to the system of multiple clocks proposed by Kristofferson (1980) to explain a categorical pattern of variability measures in duration discrimination.

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Memory representation for time was studied in two settings. First, an analysis of timing in a laboratory analog of a foraging situation revealed that departure times from a patchy resource followed a Weber Law-like property implied by scalar timing. A trial-by-trial analysis was then pursued in a similar but more structured experimental paradigm, the Peak procedure. Study of covariance structures in the data implicated scalar variance in the memory for time as well as in the decision process, but the correlation pattern ruled out multiple access to memory within a trial.

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Three facts of time perception are described based upon a temporal generalization task for rats (the peak procedure) in which food reinforcement is delivered on half the trials following the first lever-press response after some fixed interval after signal onset. (1) The mean response rate as a function of time is a smooth, slightly asymmetric, function with a maximum near the time of reinforcement; (2) the response rate on individual trials is characterized by an abrupt change from a state of low responding to a state of high responding and finally another state of low responding (break-run-break pattern); and (3) the mean response rate in 12-s and 20-s peak procedures is similar when plotted against time relative to the time of reinforcement (superposition). An information-processing version of scalar timing theory is described and compared to an alternative connectionist version of scalar timing theory that involves multiple oscillators and an autoassociation network. Psychological, mathematical and biological descriptions of the two versions are described and some possible extensions of the connectionist version are proposed to deal with perception of number, rate, and spatial orientation.

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Pharmacists in the US Public Health Service (PHS) provide direct patient-care support activities at the National Institutes of Health, Food and Drug Administration, and the Indian Health Service (IHS). This is the first of a series of articles describing pharmacists' work in the IHS, where most of the pharmacists in the PHS serve. The IHS operates medical centers, hospitals, and ambulatory health centers. Many innovative pharmacy services have been developed within the IHS since its founding in 1955. IHS pharmacists have long performed clinical and educational duties that many of their private sector colleagues are just now assuming. This paper introduces pharmaceutical services within the IHS and describes several of the important professional innovations that have been developed by IHS pharmacists.

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The brain mechanisms involved in attention and memory were examined by testing rats in temporal discriminations designed to emphasize these cognitive processes. Normal rats were able to time each of two stimuli whether they were presented alone or together. Rats with lesions of the frontal cortex (FC) or nucleus basalis magnocellularis (NBM) were able to time each stimulus when it was presented alone, but not when it was presented together with another stimulus. Rather, these rats timed only the intruding stimulus and ignored the other, demonstrating a failure of divided attention. Rats with lesions of the fimbria-fornix (FF) or medial septal area (MSA) performed the divided attention task normally, but failed to remember the duration of a stimulus that had been terminated temporarily earlier in the trial, demonstrating a failure of working memory. These results provide another informative dissociation between the functions of the frontal and hippocampal systems, emphasizing frontal involvement in attention, and hippocampal involvement in working memory.

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Functional dissociations between the medial septal area (MSA) and the nucleus basalis magnocellularis (NBM) were examined using the concepts and experimental procedures developed by scalar timing theory. Rats were tested in variations of a signalled discrete-trial peak-interval schedule of reinforcement in which the response rate functions identified the time when the rats expected reinforcement. The variations assessed aspects of both reference and working memory for information obtained from prior trials and from the current trial. A double dissociation was found in reference memory. Rats with NBM lesions, like those with frontal cortex (FC) lesions, remembered the time of reinforcement as having occurred later than it actually did; rats with MSA lesions, like those with fimbria-fornix (FF) lesions, remembered the time of reinforcement as having occurred earlier than it did. A single dissociation was found in working memory. MSA lesions and FF lesions impaired working memory, while NBM and FC lesions had no effect on it. These data begin to identify the brain mechanisms underlying temporal memory; they indicate that the frontal and hippocampal systems are both involved, but in complementary ways; and they provide information that helps specify more clearly the functions of the frontal and hippocampal systems.

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The pharmacological effects of anticholinesterases (physostigmine and neostigmine) and cholinergic receptor blockers (atropine and methylatropine) on the content of temporal memory in the rat were studied with the use of a 20-s peak-interval procedure with auditory signals. Physostigmine administered ip decreased the variability of the temporal discrimination and shifted peak times permanently leftward on the time scale in a dose-dependent fashion (0.01, 0.03, & 0.09 mg/kg). Neostigmine (0.03 mg/kg) did not produce any of these effects. Atropine administered ip increased the variability of the temporal discrimination and shifted peak times permanently rightward on the time scale in a dose-dependent fashion (0.05, 0.15, & 0.45 mg/kg). Methylatropine (0.15 mg/kg) did not produce any of these effects. Application of a scalar timing model indicated that physostigmine decreased the remembered times of reinforcement and increased sensitivity to time, whereas atropine increased the remembered times of reinforcement and decreased sensitivity to time. These results suggest that the effective level of brain acetylcholine sets the communication speed for the translation of durations measured by the internal clock into values stored in temporal memory.

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Two experiments assessed the effects of nutrients on timing behavior by rats. The nutrients were laced with saccharin and given to rats as a snack before training on a 20-s peak-interval procedure. The primary component of the snacks for four groups of 10 rats was lecithin (phosphatidylcholine), protein (casein), carbohydrate (sucrose), or a nonnutrient (saccharin). The primary measure of behavior was the time of the rat's highest response rate during a trial (peak time), which represented the interval during which the rat maximally expected food. With a lecithin snack, peak time was gradually shifted over sessions to a shorter time, remained shifted to the left of the normal function with additional testing, and then remained at the shorter time on two sessions after the snack was discontinued; with the protein snack, peak time was abruptly shifted to a shorter time, returned to normal with additional testing, and then rebounded to a longer time when the snack was discontinued; with a carbohydrate, snack peak time was abruptly shifted to a longer time, returned to normal with additional testing, and then rebounded to a shorter time when the snack was discontinued. The behavioral patterns produced by the nutrients were interpreted in terms of precursor effects on central neurotransmitter synthesis and release, psychological stages of an information-processing model, and mathematical parameters of a scalar timing theory.

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