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One of the major challenges for computational models of timing and time perception is to identify a neurobiological plausible implementation that predicts various behavioral properties, including the scalar property and retrospective timing. The available timing models primarily focus on the scalar property and prospective timing, while virtually ignoring the computational accessibility. Here, we first selectively review timing models based on ramping activity, oscillatory pattern, and time cells, and discuss potential challenges for the existing models. We then propose a multifrequency oscillatory model that offers computational accessibility, which could account for a much broader range of timing features, including both retrospective and prospective timing.

The major tenets of beat-frequency/coincidence-detection models of reward-related timing are reviewed in light of recent behavioral and neurobiological findings. This includes the emphasis on a core timing network embedded in the motor system that is comprised of a corticothalamic-basal ganglia circuit. Therein, a central hub provides timing pulses (i.e., predictive signals) to the entire brain, including a set of distributed satellite regions in the cerebellum, cortex, amygdala, and hippocampus that are selectively engaged in timing in a manner that is more dependent upon the specific sensory, behavioral, and contextual requirements of the task. Oscillation/coincidence-detection models also emphasize the importance of a tuned ‘perception’ learning and memory system whereby target durations are detected by striatal networks of medium spiny neurons (MSNs) through the coincidental activation of different neural populations, typically utilizing patterns of oscillatory input from the cortex and thalamus or derivations thereof (e.g., population coding) as a time base. The measure of success of beat-frequency/coincidence-detection accounts, such as the Striatal Beat-Frequency model of reward-related timing (SBF), is their ability to accommodate new experimental findings while maintaining their original framework, thereby making testable experimental predictions concerning diagnosis and treatment of issues related to a variety of dopamine-dependent basal ganglia disorders, including Huntington’s and Parkinson’s disease.

Accurate and precise timing is crucial for complex and purposeful behaviors, such as foraging for food or playing a musical instrument. The brain is capable of processing temporal information in a coordinated manner, as if it contains an ‘internal clock’. Similar to the need for the brain to orient itself in space in order to understand its surroundings, temporal orientation and tracking is an essential component of cognition as well. While there have been multiple models explaining the neural correlates of timing, independent lines of research appear to converge on the conclusion that populations of neurons in the dorsal striatum encode information relating to where a subject is in time relative to an anticipated goal. Similar to other learning processes, acquisition and maintenance of this temporal information is dependent on synaptic plasticity. Microtubules are cytoskeletal proteins that have been implicated in synaptic plasticity mechanisms and therefore are considered key elements in learning and memory. In this study, we investigated the role of microtubule dynamics in temporal learning by local infusions of microtubule stabilizing and destabilizing agents into the dorsolateral striatum. Our results suggested a bidirectional role for microtubules in timing, such that microtubule stabilization improves the maintenance of learned target durations, but impairs the acquisition of a novel duration. On the other hand, microtubule destabilization enhances the acquisition of novel target durations, while compromising the maintenance of previously learned durations. These findings suggest that microtubule dynamics plays an important role in synaptic plasticity mechanisms in the dorsolateral striatum, which in turn modulates temporal learning and time perception.

Male Sprague–Dawley rats were exposed to social defeat and subordination by aggressive male Long–Evans rats. The social defeat procedure involved the continuous exposure to an aggressive resident for 10 days, while living in a protective cage within the resident’s home cage with daily brief confrontations. These stress experiences resulted in 1) reduced body weight; 2) decreased social interaction; 3) increased ultrasonic vocalizations; 4) reduced sucrose preference (anhedonia); and 5) decreased clock speed while timing 15-s and 45-s target durations in a bi-peak procedure. Treatment with ketamine (15 mg/kg, i.p.) produced a rapid reversal of anhedonia and overproduction of duration. Taken together, these data provide the first evaluation of the effects of continuous social defeat and its associated depression-like symptoms on timing and time perception using a ‘state change’ design.

Basic mechanisms of interval timing and associative learning are shared by many animal species, and develop quickly in early life, particularly across infancy, and childhood. Indeed, John Wearden in his book “The Psychology of Time Perception”, which is based on decades of his own research with colleagues, and which our commentary serves to primarily review, has been instrumental in implementing animal models and methods in children and adults, and has revealed important similarities (and differences) between human timing (and that of animals) when considered within the context of scalar timing theory. These seminal studies provide a firm foundation upon which the contemporary multifaceted field of timing and time perception has since advanced. The contents of the book are arguably one piece of a larger puzzle, and as Wearden cautions, “The reader is warned that my own contribution to the field has been exaggerated here, but if you are not interested in your own work, why would anyone else be?” Surely there will be many interested readers, however the book is noticeably lacking in it neurobiological perspective. The mind (however it is conceived) needs a brain (even if behaviorists tend to say “the brain behaves”, and most neuroscientists currently have a tenuous grasp on the neural mechanisms of temporal cognition), and to truly understand the psychology of time, brain and behavior must go hand in hand regardless of the twists, turns, and detours along the way.

Bilateral intratympanic sodium arsenate injections (100 mg/ml in isotonic saline) in adult male Long Evans rats produced impairments in allocentric navigation using a 12-arm radial maze procedure as well as a motor test battery designed to evaluate vestibular function. In contrast, no impairments in the accuracy or precision of duration reproduction using 20-s and 80-s peak-interval procedures were observed when both target durations were associated with the same lever response, but distinguished by signal modality (e.g., light or sound). In contrast, an ordinal-reproduction procedure with 800, 3200, and 12,800 ms standards requiring the timing of self-initiated movements during the production phase revealed large impairments in the accuracy and precision of timing for vestibular lesioned rats. These impairments were greater on trials in which self-initiated body movements (e.g., holding down the response lever for a fixed duration) were required without the support of external stimuli signaling the onset and offset of the reproduced duration in contrast to trials in which such external support was provided. The conclusion is that space and time are separable entities and not simply the product of a generalized system, but they can be integrated into a common metric using gravity and self-initiated movement as a reference.

We present an integrated view of interval timing and reinforcement learning (RL) in the brain. The computational goal of RL is to maximize future rewards, and this depends crucially on a representation of time. Different RL systems in the brain process time in distinct ways. A model-based system learns 'what happens when', employing this internal model to generate action plans, while a model-free system learns to predict reward directly from a set of temporal basis functions. We describe how these systems are subserved by a computational division of labor between several brain regions, with a focus on the basal ganglia and the hippocampus, as well as how these regions are influenced by the neuromodulator dopamine.

Ordinal comparison of successively presented signal durations requires (a) the encoding of the first signal duration (standard), (b) maintenance of temporal information specific to the standard duration in memory, and (c) timing of the second signal duration (comparison) during which a comparison is made of the first and second durations. Rats were first trained to make ordinal comparisons of signal durations within three time ranges using 0.5, 1.0, and 3.0-s standard durations. Local field potentials were then recorded from the dorsal striatum and sensorimotor cortex in order to investigate the pattern of neural oscillations during each phase of the ordinal-comparison process. Increased power in delta and theta frequency ranges was observed during both the encoding and comparison stages. Active maintenance of a selected response, "shorter" or "longer" (counter-balanced across left and right levers), was represented by an increase of theta and delta oscillations in the contralateral striatum and cortex. Taken together, these data suggest that neural oscillations in the delta-theta range play an important role in the encoding, maintenance, and comparison of signal durations.

The overlap of neural circuits involved in episodic memory, relational learning, trace conditioning, and interval timing suggests the importance of hippocampal-dependent processes. Identifying the functional and neural mechanisms whereby the hippocampus plays a role in timing and decision-making, however, has been elusive. In this article we describe recent neurobiological findings, including the discovery of hippocampal ‘time cells’, dependency of duration discriminations in the minutes range on hippocampal function, and the correlation of hippocampal theta rhythm with specific features of temporal processing. These results provide novel insights into the ways in which the hippocampus might interact with the striatum in order to support both retrospective and prospective timing. Suggestions are also provided for future research on the role of the hippocampus in memory for elapsed time.

Animals, including fish, birds, rodents, non-human primates, and pre-verbal infants are able to discriminate the duration and number of events without the use of language. In this paper, we present the results of six experiments exploring the capability of adult rats to count 2–6 sequentially presented white-noise stimuli. The investigation focuses on the animal’s ability to exhibit spontaneous subtraction following the presentation of novel stimulus inversions in the auditory signals being counted. Results suggest that a subtraction operation between two opposite sensory representations may be a general processing strategy used for the comparison of stimulus magnitudes. These findings are discussed within the context of a mode-control model of timing and counting that relies on an analog temporal-integration process for the addition and subtraction of sequential events.

Temporal and social processing are intricately linked. The temporal extent and organization of interactional behaviors both within and between individuals critically determine interaction success. Conversely, social signals and social context influence time perception by, for example, altering subjective duration and making an event seem ‘out of sync'. An ‘internal clock' involving subcortically orchestrated cortical oscillations that represent temporal information, such as duration and rhythm, as well as insular projections linking temporal information with internal and external experiences is proposed as the core of these reciprocal interactions. The timing of social relative to non-social stimuli augments right insular activity and recruits right superior temporal cortex. Together, these reciprocal pathways may enable the exchange and respective modulation of temporal and social computations.

Although fear-producing treatments (e.g., electric shock) and pleasure-inducing treatments (e.g., methamphetamine) have different emotional valences, they both produce physiological arousal and lead to effects on timing and time perception that have been interpreted as reflecting an increase in speed of an internal clock. In this commentary, we review the results reported by Fayolle et al. (2015):Behav. Process., 120, 135–140) and Meck (1983: J. Exp. Psychol. Anim. Behav. Process., 9, 171–201) using electric shock and by Maricq et al. (1981: J. Exp. Psychol. Anim. Behav. Process., 7, 18–30) using methamphetamine in a duration-bisection procedure across multiple duration ranges. The psychometric functions obtained from this procedure relate the proportion ‘long’ responses to signal durations spaced between a pair of ‘short’ and ‘long’ anchor durations. Horizontal shifts in these functions can be described in terms of attention or arousal processes depending upon whether they are a fixed number of seconds independent of the timed durations (additive) or proportional to the durations being timed (multiplicative). Multiplicative effects are thought to result from a change in clock speed that is regulated by dopamine activity in the medial prefrontal cortex. These dopaminergic effects are discussed within the context of the striatal beat frequency model of interval timing (Matell & Meck, 2004:Cogn. Brain Res.,21, 139–170) and clinical implications for the effects of emotional reactivity on temporal cognition (Parker et al., 2013:Front. Integr. Neurosci., 7, 75).

Discriminative fear conditioning requires learning to dissociate between safety cues and cues that predict negative outcomes yet little is known about what processes contribute to discriminative fear learning. According to attentional models of time perception, processes that distract from timing result in temporal underestimation. If discriminative fear learning only requires learning what cues predict what outcomes, and threatening stimuli distract attention from timing, then better discriminative fear learning should predict greater temporal distortion on threat trials. Alternatively, if discriminative fear learning also reflects a more accurate perceptual experience of time in threatening contexts, discriminative fear learning scores would predict less temporal distortion on threat trials, as time is perceived more veridically. Healthy young adults completed discriminative fear conditioning in which they learned to associate one stimulus (CS+) with aversive electrical stimulation and another stimulus (CS−) with non-aversive tactile stimulation and then an ordinal-comparison timing task during which CSs were presented as task-irrelevant distractors. Consistent with predictions, we found an overall temporal underestimation bias on CS+ relative to CS− trials. Differential skin conductance responses to the CS+ versus the CS− during conditioning served as a physiological index of discriminative fear conditioning and this measure predicted the magnitude of the underestimation bias, such that individuals exhibiting greater discriminative fear conditioning showed less underestimation on CS+ versus CS− trials. These results are discussed with respect to the nature of discriminative fear learning and the relationship between temporal distortions and maladaptive threat processing in anxiety.

Our sense of time is altered by our emotions to such an extent that time seems to fly when we are having fun and drags when we are bored. Recent studies using standardized emotional material provide a unique opportunity for understanding the neurocognitive mechanisms that underlie the effects of emotion on timing and time perception in the milliseconds-to-hours range. We outline how these new findings can be explained within the framework of internal-clock models and describe how emotional arousal and valence interact to produce both increases and decreases in attentional time sharing and clock speed. The study of time and emotion is at a crossroads, and we outline possible examples for future directions.

The present study examined the clock-speed modulating effects of acute cocaine administration in groups of male rats that received different amounts of baseline training on a 36-s peak-interval procedure prior to initial drug injection. After injection of cocaine (10, 15, or 20mg/kg, ip), rats that had received a minimal amount of training (e.g.,or=30 sessions) prior to drug administration displayed a horizontal leftward shift in their timing functions indicating that the speed of the internal clock was increased. In contrast, rats that had received an extended amount of training (e.g.,or=180 sessions) prior to cocaine (15 mg/kg, ip) administration did not produce this "classic" curve-shift effect, but instead displayed a general disruption of temporal control following drug administration. Importantly, when co-administered with a behaviorally ineffective dose of ketamine (10mg/kg, ip) the ability of cocaine to modulate clock speed in rats receiving extended training was restored. A glutamate "lock/unlock" hypothesis is used to explain the observed dopamine-glutamate interactions as a function of timing behaviors becoming learned habits.

Gaps and distracters were presented during the timed signal to examine whether the stop/reset mechanism is activated by (a) changes in the timed signal (switch hypothesis), (b) ITI-like events (ambiguity hypothesis), or (c) processes concurrent with the timing process (time-sharing hypothesis). While the switch and ambiguity hypotheses predict that rats should time through (ignore) distracters, the time-sharing hypothesis predicts that extraneous events (e.g., gaps and distracters) delay timing by causing working memory to decay in proportion to the events' salience. The authors found that response functions were displaced by both gaps and distracters, in accord with the time-sharing hypothesis. Computer simulations show that the time-sharing and memory-decay hypotheses can mechanistically address present data, and reflect different levels of the same model.

Patients with unilateral (left or right) medial temporal lobe lesions and normal control (NC) volunteers participated in two experiments, both using a duration bisection procedure. Experiment 1 assessed discrimination of auditory and visual signal durations ranging from 2 to 8 s, in the same test session. Patients and NC participants judged auditory signals as longer than equivalent duration visual signals. The difference between auditory and visual time discrimination was equivalent for the three groups, suggesting that a unilateral temporal lobe resection does not modulate the modality effect. To document interval-timing abilities after temporal lobe resection for different duration ranges, Experiment 2 investigated the discrimination of brief, 50-200 ms, auditory durations in the same patients. Overall, patients with right temporal lobe resection were found to have more variable duration judgments across both signal modality and duration range. These findings suggest the involvement of the right temporal lobe at the level of the decision process in temporal discriminations.

The authors hypothesized that during a gap in a timed signal, the time accumulated during the pregap interval decays at a rate proportional to the perceived salience of the gap, influenced by sensory acuity and signal intensity. When timing visual signals, albino (Sprague-Dawley) rats, which have poor visual acuity, stopped timing irrespective of gap duration, whereas pigmented (Long-Evans) rats, which have good visual acuity, stopped timing for short gaps but reset timing for long gaps. Pigmented rats stopped timing during a gap in a low-intensity visual signal and reset after a gap in a high-intensity visual signal, suggesting that memory for time in the gap procedure varies with the perceived salience of the gap, possibly through an attentional mechanism.

The relationship between the magnitude of the error in the content of temporal memory and sodium-dependent high-affinity choline uptake (SDHACU) in the frontal cortex and hippocampus was examined in mature (10- to 16-month-old) and aged (24- to 30-month-old) male rats. The peak time of the response rate distribution that relates the probability of a response to signal duration in a 20-s peak-interval timing procedure was used to index the remembered time of reinforcement. Regression analyses indicated that SDHACU in the frontal cortex of both mature and aged rats and in the hippocampus of aged rats is proportional to the absolute error in the content of temporal memory. These biochemical effects of peak-interval training were also compared with biochemical measures taken from control rats that received random-interval training. This comparison indicated that the observed changes in SDHACU were dependent upon the predictability of the programmed time of reinforcement and age-related changes in memory encoding and retrieval.

The effects of signal modality on duration classification in college students were studied with the duration bisection task. When auditory and visual signals were presented in the same test session and shared common anchor durations, visual signals were classified as shorter than equivalent duration auditory signals. This occurred when auditory and visual signals were presented sequentially in the same test session and when presented simultaneously but asynchronously. Presentation of a single modality signal within a test session, or both modalities but with different anchor durations did not result in classification differences. The authors posit a model in which auditory and visual signals drive an internal clock at different rates. The clock rate difference is due to an attentional effect on the mode switch and is revealed only when the memories for the short and long anchor durations consist of a mix of contributions from accumulations generated by both the fast auditory and slower visual clock rates. When this occurs auditory signals seem longer than visual signals relative to the composite memory representation.

Contrary to data showing sensitivity to nontemporal properties of timed signals, current theories of interval timing assume that animals can use the presence or absence of a signal as equally valid cues as long as duration is the most predictive feature. Consequently, the authors examined rats' behavior when timing the absence of a visual or auditory stimulus in trace conditioning and in a "reversed" gap procedure. Memory for timing was tested by presenting the stimulus as a reversed gap into its timed absence. Results suggest that in trace conditioning (Experiment 1), rats time for the absence of a stimulus by using its offset as a time marker. As in the standard gap procedure, the insertion of a reversed gap was expected to "stop" rats' internal clock. In contrast, a reversed gap of 1-, 5-, or 15-s duration "reset" the timing process in both trace conditioning (Experiment 2) and the reversed gap procedure (Experiment 3). A direct comparison of the standard and reversed gap procedures (Experiment 4) supported these findings. Results suggest that attentional mechanisms involving the salience or content of the gap might contribute to the response rule adopted in a gap procedure. When an ongoing timed interval is interrupted by a break or gap, animals seem to suspend their temporal processing and choose a response rule that falls between two extremes: They may restart the entire timing process, a phenomenon usually called "reset" (Church, 1978; W. A. Roberts, Cheng, & Cohen, 1989) or they may "stop" timing for the duration of the gap and resume it after the gap (Church, 1978; Meek, Church, & Olton, 1984; S. Roberts

The properties of the internal clock, temporal memory, and decision processes used to time short durations were investigated. The peak-interval procedure was used to evaluate the timing of 8-, 12-, and 21-s intervals, and analyses were conducted on the mean response functions and on individual trials. A distractor task prevented counting, and visual feedback on accuracy and precision was provided after each trial. Mean response distributions were (a) centered at the appropriate real-time criteria, (b) highly symmetrical, and (c) scalar in their variability. Analysis of individual trials indicated more memory variability relative to response threshold variability. Taken together, these results demonstrate that humans show the same qualitative timing properties that other animals do, but with some quantitative differences.

Precise timing is crucial to decision-making and behavioral control, yet subjective time can be easily distorted by various temporal contexts. Application of a Bayesian framework to various forms of contextual calibration reveals that, contrary to popular belief, contextual biases in timing help to optimize overall performance under noisy conditions. Here, we review recent progress in understanding these forms of temporal calibration, and integrate a Bayesian framework with information-processing models of timing. We show that the essential components of a Bayesian framework are closely related to the clock, memory, and decision stages used by these models, and that such an integrated framework offers a new perspective on distortions in timing and time perception that are otherwise difficult to explain.

An emerging corpus of clinical and neuroimaging data suggests that subsecond and suprasecond durations are represented via 2 distinct mechanisms in humans; however, surprisingly, behavioral data to this effect are lacking. In our first experiment, we perform the first systematic exploration of subsecond and suprasecond timing within the same session in nonhuman subjects. Rats were trained to judge the relative duration of 2 sequential stimuli, responding on one lever if the first stimulus was longer or on a second lever if the converse was true. Our data provide strong evidence of an abstract understanding of longer and shorter for durations in the suprasecond range, whereas responding was at chance levels for durations in the subsecond range. Data from a second experiment reveal that this pattern is not due to an inability to time subsecond signals, as rats respond systematically in subsecond and suprasecond bisection tasks. Together, our results provide the first clear behavioral evidence of a discontinuity in the mental time line. These data from rats are discussed in light of similar findings of a discontinuity in the mental number line in human infants.

Variations in both pitch and time are important in conveying meaning through speech and music, however, research is scant on perceptual interactions between these two domains. Using an ordinal comparison procedure, we explored how different pitch levels of flanker tones influenced the perceived duration of empty interstimulus intervals (ISIs). Participants heard monotonic, isochronous tone sequences (ISIs of 300, 600, or 1200 ms) composed of either one or five standard ISIs flanked by 500 Hz tones, followed by a final interval (FI) flanked by tones of either the same (500 Hz), higher (625 Hz), or lower (400 Hz) pitch. The FI varied in duration around the standard ISI duration. Participants were asked to determine if the FI was longer or shorter in duration than the preceding intervals. We found that an increase in FI flanker tone pitch level led to the underestimation of FI durations while a decrease in FI flanker tone pitch led to the overestimation of FI durations. The magnitude of these pitch-level effects decreased as the duration of the standard interval was increased, suggesting that the effect was driven by differences in mode-switch latencies to start/stop timing. Temporal context (One vs. Five Standard ISIs) did not have a consistent effect on performance. We propose that the interaction between pitch and time may have important consequences in understanding the ways in which meaning and emotion are communicated.

Timing is ever-present in our daily lives — from the ringing of the alarm clock to our ability to walk, dance, remember, and communicate with others. This intimate relationship has led scientists from different disciplines to investigate timing mechanisms and to explore how individuals perceive, process, and effectively use timing in their daily activities. With the launch of Timing & Time Perception (brill.com/time) the field of timing and time perception enters a new and exciting phase of development in which the multi-dimensions of this rapidly emerging field can be explored and contemplatedwith the support of a journal that provides a platform for researchers to announce their latest discoveries and theoretical formulations. In this sense, Timing & Time Perception aims to become the forum for all psychophysical, neuroimaging, pharmacological, computational, and theoretical advances on the topic of timing and timeperception in humans andnonhuman animals.We envision amultidisciplinary approach to the topics covered, including the synergy of neuroscience andphilosophy for understanding the concept of time, cognitive science and artificial intelligence for adapting basic research to artificial agents, as well as psychiatric, neurological, behavioral, cognitive, and computational sciences for neuro-rehabilitation and modeling of normal and disordered brains, to name just a few. Given the ubiquity of interval timing, this journal will host all basic studies, including interdisciplinary and multidisciplinary works on timing and time perception and serve as a forum for discussion and extension of current knowledge on the topic. The title of the new journal is reminiscent of the influential Annals of the New York Academy of Sciences volume edited by John Gibbon and Lorraine Allan (Gibbon & Allan, 1984). That volume had the important goal of helping to integrate the study of timing in humans and other animals. Our current endeavor has even broader ambitions to integrate it with neuroscience, computational analysis, and other topics. We think that Timing & Time Perception

Healthy volunteers were tested on 7-s and 17-s peak-interval timing procedures following d-amphetamine (20mg-oral), haloperidol (2mg-oral), and placebo treatments in order to assess the dopaminergic regulation of temporal processing. Individual differences were observed in the drug effects such that two different patterns of timing behavior emerged. In the first pattern, d-amphetamine produced proportional leftward shifts of the timing functions while haloperidol produced proportional rightward shifts. This symmetrical pattern of results suggests that clock speed is regulated by the effective level of dopamine, i.e., d-amphetamine increases clock speed and haloperidol decreases clock speed. The second pattern was the opposite of the first pattern and was revealed by d-amphetamine producing proportional rightward shifts of the timing functions while haloperidol produced no reliable effect. This asymmetrical pattern of results is consistent with an explanation in which attention toward the stimulant-induced euphoria produced by d-amphetamine diminishes the attentional resources available for temporal processing, thereby diluting any drug-induced changes in clock speed. The result of increased competition and time-sharing between these two dimensions (e.g., attention towards feelings of euphoria versus attention towards the passage of time) leads to the underestimation/overproduction of temporal intervals. Interestingly, participants that displayed the 'clock-speed' pattern liked d-amphetamine significantly less than participants that displayed the 'attention' pattern and were more variable in a simple reaction time task than other participants. These results suggest that individuals with a higher degree of sensitivity to time are also more sensitive to their feelings of stimulant-induced euphoria and drug liking-suggesting that internal clock and reward pathways share common dopaminergic pathways.

The perception of time is heavily influenced by attention and memory, both of which change over the lifespan. In the current study, children (8 yrs), young adults (18–25 yrs), and older adults (60–75 yrs) were tested on a duration bisection procedure using 3 and 6-s auditory and visual signals as anchor durations. During test, participants were exposed to a range of intermediate durations, and the task was to indicate whether test durations were closer to the “short” or “long” anchor. All groups reproduced the classic finding that “sounds are judged longer than lights”. This effect was greater for older adults and children than for young adults, but for different reasons. Replicating previous results, older adults made similar auditory judgments as young adults, but underestimated the duration of visual test stimuli. Children showed the opposite pattern, with similar visual judgments as young adults but overestimation of auditory stimuli. Psychometric functions were analyzed using the Sample Known Exactly-Mixed Memory quantitative model of the Scalar Timing Theory of interval timing. Results indicate that children show an auditory-specific deficit in reference memory for the anchors, rather than a general bias to overestimate time and that aged adults show an exaggerated tendency to judge visual stimuli as “short” due to a reduction in the availability of controlled attention.

In two experiments, the peak-interval procedure was used with humans to test effects related to gaps in multisecond timing. In Experiment 1, peak times of response distributions were shorter when the gap occurred later during the encoding of the criterion time to be reproduced, suggesting that gap expectancy shortened perceived durations. Peak times were also positively related to objective target durations. Spreads of response distributions were generally related to estimated durations. In Experiment 2, peak times were shortest when gaps were expected but did not occur, confirming that the shortening effect of gap expectancy is independent of the gap occurrence. High positive start-stop correlations and moderate positive peak-time-spread correlations showed strong memory variability, whereas low and negative start-spread correlations suggest small response-threshold variability. Correlations seemed not to be influenced by expectancy. Overall, the peak-interval procedure with gaps provided relevant information on similarities and differences in timing in humans and other animals.

Ten rats were trained in a temporal generalization task (the peak procedure) with variations in the time of reinforcement, the intertrial interval, and the mean and variance of the duration of nonfood trials. There were three types of asymmetry in the temporal generalization gradients: positive skew, secondary rise, and positive asymptote. Asymmetrical gradients can occur as a result of asymmetrical sources of variance, multiplicative combinations of symmetrical sources of variance, and effects of anticipation of the end of a trial and the conditions of the next trial. Ten additional rats were trained with a single time of reinforcement, a limited time of reinforcement availability, long and fixed durations of nonfood trials, and a nonresponse requirement for ending a trial. These conditions markedly reduced all asymmetrical sources of variance and led to very symmetrical gradients. These results demonstrate that none of the asymmetrical sources of variance necessarily has a substantial influence on observed temporal generalization gradients.

Two groups of male Sprague-Dawley rats (n's = 10) were trained to discriminate between 2-s and 8-s signal durations at one of two different phases of a 14:10 light:dark (LD) cycle with lights on at 7:00 am . One group of rats was trained beginning at 8:00 am and the other group of rats was trained beginning at 8:00 pm . Signals were composed of either a 2-s or an 8-s duration of light (6-or 7.5-W lamp onset) or sound (80-dB white noise onset) presented with equal probability in a random order during a 2-h session. Five intermediate signal durations, in addition to the two anchor points, were presented by means of the bisection method with discrete trials separated by a random-interval 45-s intertrial interval (ITI). After baseline response functions for both auditory and visual signals were obtained at the two training phases, test sessions were conducted at eight different times during the LD cycle. These test sessions were conducted in order to determine whether any of the psychological processes involved in the discrimination of signal duration in the seconds to minutes' range exhibit a cirdadian rhythm. The results obtained from the test stage revealed no circadian rhythmicity in clock speed (Λ) for either signal modality; whereas, the variability or temporal memory for sound signals ( γ K ∗ | S ) and the probability of attention to light signals (p(A) | L)—as estimated from the application of a scalar expectancy model of temporal bisection—showed circadian rhythmicities that interacted with the circadian phase of initial baseline training.

Three experiments assessed the ability of male Sprague-Dawley rats to organize the spatial locations of different food types in a hierarchical manner to maximize the efficiency of working memory. Independent groups were exposed, on a 12-arm radial maze, to baiting arrangements varying in the stability of the pattern and type of food used as bait. Training rats with stable, differentiable baiting arrangements produced increased accuracy in choice performance, hierarchically ordered patterns of choice selection, slower growth of proactive interference when trials were massed, and the learning of the geometrical relations among food types independent of other extramaze cues. Such findings are strong evidence of the rat's ability to encode and use local cues for navigation, based on properties of the reinforcer. The application of a chunking strategy may provide for more efficient use of working memory by facilitating information storage, recall, or resetting mechanisms.

A duration-bisection procedure was used to study the effects of signal modality and divided attention on duration classification in participants at high genetic risk for schizophrenia (HrSz), major affective disorder (HrAff), and normal controls (NC). Participants learned short and long target durations during training and classified probe durations during test. All groups classified visual signals as shorter than equivalent duration auditory signals. However, the difference between auditory and visual signal classification was significantly larger for the HrSz group than for the NC group. We posit a model in which there is a clock rate difference between auditory and visual signals due to an attentional effect at the level of a mode switch that gates pulses into an accumulator. This attentionally mediated clock rate difference was larger for the HrSz participants than for the NC participants, resulting in a larger auditory/visual difference for the HrSz group.

Normal participants (n=5) having no experience with antipsychotic drugs and medicated participants (n=5) with clinical experience with chronic low doses of haloperidol (3-10 mg/day for 2-4 months) in the treatment of neuroses were evaluated for the effects of inter-trial interval (ITI) feedback on a discrete-trials peak-interval timing procedure. Feedback was presented during the ITI in the form of a histogram showing the distribution of the responses participants made on the previous trial plotted on a relative time scale. As feedback concerning the accuracy and precision of a reproduced duration (e.g., 7- and 14-s visual signals) became more remote in time, reproduced intervals gradually lengthened in duration. This rightward horizontal shift in peak time increased as a function of the probability of feedback and was enhanced by chronic treatment with haloperidol in a manner that was proportional to the duration of the signal. Our data suggest a gradual change in the underlying representation of the signal duration as a function of the remoteness of ITI feedback that is dependent upon both changes in working memory and the speed of the internal clock used to time durations in the seconds-to-minutes range.

Interval timing in the range of milliseconds to minutes is affected in a variety of neurological and psychiatric populations involving disruption of the frontal cortex, hippocampus, basal ganglia, and cerebellum. Our understanding of these distortions in timing and time perception are aided by the analysis of the sources of variance attributable to clock, memory, decision, and motor-control processes. The conclusion is that the representation of time depends on the integration of multiple neural systems that can be fruitfully studied in selected patient populations.

Functional magnetic resonance imaging (fMRI) was used to measure the location and intensity of brain activations when participants time an 11-s signal duration. The experiment evaluated six healthy adult male participants who performed the peak-interval timing procedure in variants of stimulus modality (auditory or visual) and condition (foreground or background: i.e., whether the presence or absence of the stimulus is the signal to be timed). The complete experimental design called for each signal variant to be used across four behavioral tasks presented in the following order: control, timing+motor, timing, and motor. In the control task, participants passively experienced the stimuli. The timing+motor and timing tasks were preceded by five fixed-time training trials in which participants learned the 11-s signal they would subsequently reproduce. In the timing+motor task, participants made two motor responses centered around their subjective estimate of the criterion time. For the timing task, participants were instructed to time internally without making a motor response. The motor task had participants make two cued responses that were not determined by the participant's sense of the passage of time. Neuroimaging data from the timing+motor and timing tasks showed activation of the frontal cortex, striatum and thalamus--none of which was apparent in the control or motor tasks. These results, combined with other peak-interval procedure data from drug and lesion studies in animals as well as behavioral results in human patient populations with striatal damage, support the involvement of frontal-striatal circuitry in human interval timing.

Exactly how the brain is able to measure the durations of events lasting from seconds to minutes while maintaining time-scale invariance remains largely a mystery. Neuroimaging studies are only now beginning to unravel the nature of interval timing and reveal whether different timing mechanisms are required for the perception and production of sub- and supra-second intervals that can be defined by different stimulus modalities. We here review the impact that neuroimaging studies have had on the field of timing and time perception and outline the major challenges that remain to be addressed before a physiologically realistic theory of interval timing can be established involving cortico-striatal circuits.

Humans and other animals demonstrate the ability to perceive and respond to temporally relevant information with characteristic behavioral properties. For example, the response time distributions in peak-interval timing tasks are well described by Gaussian functions, and superimpose when scaled by the criterion duration. This superimposition has been referred to as the scalar property and results from the fact that the standard deviation of a temporal estimate is proportional to the duration being timed. Various psychological models have been proposed to account for such responding. These models vary in their success in predicting the temporal control of behavior as well as in the neurobiological feasibility of the mechanisms they postulate. A review of the major interval timing models reveals that no current model is successful on both counts. The neurobiological properties of the basal ganglia, an area known to be necessary for interval timing and motor control, suggests that this set of structures act as a coincidence detector of cortical and thalamic input. The hypothesized functioning of the basal ganglia is similar to the mechanisms proposed in the beat frequency timing model [R.C. Miall, Neural Computation 1 (1989) 359-371], leading to a reevaluation of its capabilities in terms of behavioral prediction. By implementing a probabilistic firing rule, a dynamic response threshold, and adding variance to a number of its components, simulations of the striatal beat frequency model were able to produce output that is functionally equivalent to the expected behavioral response form of peak-interval timing procedures.

The ability of organisms to time and coordinate temporal sequences of events and to select particular aspects of their internal and external environments to which they will attend is vital to the organism's ability to adapt to the world around them. Numerous psychological theories have been proposed that describe how organisms might accomplish such stimulus selection and represent discrete temporal events as well as rhythm production. In addition, a large number of studies have demonstrated that damage to the frontostriatal circuitry appears to compromise the ability of organisms to successfully shift attention and behavior to adapt to changing temporal contexts. This suggests that frontostriatal circuitry is involved in the ability to make such shifts and to process temporal intervals. A selective review is accomplished in this article which focuses upon the specific neural mechanisms that may be involved in interval timing and set shifting. It is concluded that prefrontal cortex, substantia nigra pars compacta, pedunculopontine nucleus, and the direct and indirect pathways from the caudate to the thalamus may provide the neuroanatomical and neurophysiological substrates that underlie the organism's ability to shift its attention from one temporal context to another.

Time is a guiding force in the behavior of all organisms. For both a rat in an experimental setting (e.g. Skinner box) trying to predict when reinforcement will be delivered and a human in a restaurant waiting for his dinner to be served an accurate perception of time is an important determinant of behavior. Recent research has used a combination of pharmacological and behavioral manipulations to gain a fuller understanding of how temporal information is processed. A psychological model of duration discrimination that differentiates the speed of an internal clock used for the registration of current sensory input from the speed of the memory-storage process used for the representation of the durations of prior stimulus events has proven useful in integrating these findings. Current pharmacological research suggests that different stages of temporal processing may involve separate brain regions and be modified by different neurotransmitter systems. For example, the internal clock used to time durations in the seconds-to-minutes range appears linked to dopamine (DA) function in the basal ganglia, while temporal memory and attentional mechanisms appear linked to acetylcholine (ACh) function in the frontal cortex. These two systems are connected by frontal-striatal loops, thus allowing for the completion of the timing sequences involved in duration discrimination.

The article contains cross-referencing and other citation errors that were introduced during the final editing process. In particular, on page 2, the third paragraph should read: As suggested by Santamaria (2002), there is ample reason to believe that humans and other animals have a well-developed time sense in the seconds-to-minutes range. Humans are excellent at interval timing and sequencing, they can make fine temporal discriminations and are sensitive to small perturbations in rhythm and musical structure (e.g., Epstein, 1989; Janata & Grafton, 2003; McAuley & Jones, 2003; Naatanen, Syssoeva, & Takegata, 2004). ‘‘Estimating time intervals is an important adaptive skill, vital for making predictions and for motor control’’ (e.g., Diedrichsen, Ivry, & Pressing, 2003; Ivry, 1996; Ivry & Richardson, 2002; Spencer, Zelaznik, Diedrichsen, & Ivry, 2003). Evolutionarily, time estimations in the seconds-to-minutes range ‘‘are important for making predictions about one’s environment, for example, about the appearance of predators or prey’’ (e.g., Bateson, 2003; Gallistel & Gibbon, 2000). Millisecond time estimations ‘‘are important for motor control and for rapid sequencing of cognitive operations’’, such as updating working memory and language processing (e.g., Justus & Ivry, 2001; Lustig, Matell, & Meck, 2004; Meck & Benson, 2002; Schirmer, 2004).

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.

Seven experiments assessed the ability of rats to process temporal information from two internal clocks simultaneously and independently. In the first six experiments a light stimulus signalled an overall interval between the beginning of a trial and the availability of food reinforcement (e.g., a 50-s fixed interval). During the overall interval a sound stimulus was used to signal shorter intervals that divided the overall interval into equal segments. When there was a fixed temporal relation between the final segment signal and the availability of reinforcement, there was a double-scallop pattern of responding throughout the segmented overall interval; the function relating response rate to time during segment intervals was similar to the function relating response rate to time in unsegmented overall intervals; a change in response rate occurred at the time that a normally presented segment signal was omitted. Taken together, the results indicate that rats timed the overall interval and the segment intervals simultaneously and independently without interference. In Experiment 7 a light stimulus was used on some trials, and a sound stimulus was used on other trials to signal a discrete-trial 50-s peak procedure. When these two signals were presented in compound, there was a leftward shift of the response function, which suggests that rats timed both signals simultaneously. For all of the experiments a scalar timing model with specific stimulus integration rules is used to explain the results. The stimulus integration rule used in the first six experiments, in which there were two signals for the same reinforcement, was to respond if both the segment and the overall interval had exceeded a response threshold. The stimulus integration rule used in Experiment 7, in which there were two signals for different reinforcements, was to respond if the response threshold for either interval had been exceeded.

ion of Temporal Attributes Warren H. Meek and Russell M. ChurchBrown University Cross-modal transfer of classification rules for temporal intervals was demon-strated in four experiments. In Experiment 1, rats learned a temporal discrim-ination between a 2-sec and an 8-sec signal in one modality (vision or audition).There was positive transfer to a temporal discrimination between a 2-sec andan 8-sec signal in the other modality when the response rule was maintained,and negative transfer when the response rule was reversed. Experiment 2 dem-onstrated positive cross-modal transfer in a temporal generalization procedure.Experiment 3 demonstrated cross-modal transfer of both duration arid temporallocation in a procedure in which rats were exposed to three successive signaldurations. Experiment 4 demonstrated cross-modal transfer of both duration andtemporal order in a procedure in which rats were exposed to simultaneouslypresented signal durations, one auditory and one visual. The conclusion is thatrats can abstract temporal attributes from modality-specific aspects of a signal.

The similarity of animal counting and timing processes was demonstrated in four experiments that used a psychophysical choice procedure. In Experiment 1, rats initially learned a discrimination between a two-cycle auditory signal of 2-sec duration and an eight-cycle auditory signal of 8-sec duration. For the number discrimination test, the number of cycles was varied, and the signal duration was held constant at an intermediate value. For the duration discrimination test, the signal duration was varied, and the number of cycles was held constant at an intermediate value. Rats were equally sensitive to a 4:1 ratio of counts (with duration controlled) and a 4:1 ratio of times (with number controlled). The point of subjective equality for the psychophysical functions that related response classification to signal value was near the geometric mean of the extreme values for both number and duration discriminations. Experiment 2 demonstrated that 1.5 mg/kg of methamphetamine administered intraperitoneally shifted the psychophysical functions for both number and duration leftward by approximately 10%. Experiment 3 demonstrated that the magnitude of cross-modal transfer from auditory signals to cutaneous signals was similar for number and duration. In Experiment 4 the mapping of number onto duration demonstrated that a count was approximately equal to 200 msec. The psychophysical functions for number and duration were fit with a scalar expectancy model with the same parameter values for each attribute. The conclusion was that the same internal mechanism is used for counting and timing. This mechanism can be used in several modes: the "event" mode for counting or the "run" and the "stop" modes for timing.

Four experiments studied the scaling of time by rats. The purpose was to determine if internal clock and memory processes could be selectively adjusted by pharmacological manipulations. All of the experiments used a temporal discrimination procedure in which one response ("short") was reinforced following a 2-sec noise signal and a different response ("long") was reinforced following an 8-sec noise signal; unreinforced signals of intermediate duration were also presented. The proportion of "long" responses increased as a function of signal duration. All drugs were administered intraperitoneally (ip) and their effect on clock or memory processes was inferred from the observed pattern of change in the point of subjective equality of the psychophysical functions under training and testing conditions. Experiment 1 demonstrated that methamphetamine (1.5 mg/kg) can selectively increase clock speed and that haloperidol (.12 mg/kg) can selectively decrease clock speed. Experiment 2 demonstrated that footshock stress (.2 mA) can selectively increase clock speed during continuous administration but leads to a decrease in clock speed below control values when the footshock is abruptly terminated. Experiment 3 demonstrated that vasopressin (.07 pressor units/kg) and oxytocin (.02 pressor units/kg) can selectively decrease the remembered durations of reinforced times, which suggests that memory storage speed increased. Experiment 4 demonstrated that physostigmine (.01 mg/kg) can selectively decrease the remembered durations of reinforced times and that atropine (.05 mg/kg) can selectively increase these remembered durations, which suggests that memory storage speed was differentially affected. The conclusion is that internal clock and memory processes can be dissociated by selectively adjusting their speed of operation and that these changes can be quantitatively modeled by a scalar timing theory.

Temporal integration in duration and number discrimination by rats was investigated with the use ofa psychophysical choice procedure. A response on one lever ("short" response) following a l-s white-noise signal was followed by food reinforcement, and a response on the other lever ("long" response) following a 2-s white-noise signal was also followed by food reinforcement. Either response following a signal of one of five intermediate durations was unreinforced. This led to a psychophysical function in which the probability of a long response was related to signal duration in an ogival manner. On 2 test days, a white-noise signal wit h 5, 6, 7, 8, or 10 segments of either 0.5-s on and 0.5-s off or l-s on and l-s off was presented, and a choice response following these signals was unreinforced. The probability of a long response was the same function of a segmented signal and a continuous signal if each segment was considered equivalent to 200 ms. A quantitative fit of a scalar estimation theory suggested that the latencies to initiate temporal integration and to terminate the process are both about 200 ms, and that the same internal accumulation process can be used for counting and timing. Rats and other animals have the ability to discriminate between the number of events. The events may either be stimuli or responses. Davis and Memmott (1982) reviewed this evidence. Not all studies that attempted to train rats to discriminate between the number of events have been successful (Imada, Shuku, & Moriya, 1983), and in many cases the discrimination may have been stimulus durations rather than stimulus number. Even when all temporal cues are controlled, however, animals can learn to discriminate between the number of events (Fernandes & Church, 1982). There are many similarities between animal counting and timing processes. (In this article counting refers to number discrimination, and timing refers to duration discrimination; some authors prefer to restrict the term counting further, e.g., Davis & Memott, 1982, and Gelman & Gallistel, 1978.) Meck and Church