Interval Timing as a Computational Pathway From Early Life Adversity to Affective Disorders

Adverse early life experiences can have remarkably enduring negative consequences on mental health, with numerous, varied psychiatric conditions sharing this developmental origin. Yet, the mechanisms linking adverse experiences to these conditions remain poorly understood. Here, we draw on a principled model of interval timing to propose that statistically optimal adaptation of temporal representations to an unpredictable early life environment can produce key characteristics of anhedonia, a transdiagnostic symptom associated with affective disorders like depression and anxiety. The core observation is that early temporal unpredictability produces broader, more imprecise temporal expectations. As a result, reward anticipation is diminished, and associative learning is slowed. When agents with such representations are later introduced to more stable environments, they demonstrate a negativity bias, responding more to the omission of reward than its receipt. Increased encoding of negative events has been proposed to contribute to disorders with anhedonia as a symptom. We then examined how unpredictability interacts with another form of adversity, low reward availability. We found that unpredictability's effect was most strongly felt in richer environments, potentially leading to categorically different phenotypic expressions. In sum, our formalization suggests a single mechanism can help to link early life adversity to a range of behaviors associated with anhedonia, and offers novel insights into the interactive impacts of multiple adversities.

The timing database: An open-access, live repository for interval timing studies

Interval timing refers to the ability to perceive and remember intervals in the seconds to minutes range. Our contemporary understanding of interval timing is derived from relatively small-scale, isolated studies that investigate a limited range of intervals with a small sample size, usually based on a single task. Consequently, the conclusions drawn from individual studies are not readily generalizable to other tasks, conditions, and task parameters. The current paper presents a live database that presents raw data from interval timing studies (currently composed of 68 datasets from eight different tasks incorporating various interval and temporal order judgments) with an online graphical user interface to easily select, compile, and download the data organized in a standard format. The Timing Database aims to promote and cultivate key and novel analyses of our timing ability by making published and future datasets accessible as open-source resources for the entire research community. In the current paper, we showcase the use of the database by testing various core ideas based on data compiled across studies (i.e., temporal accuracy, scalar property, location of the point of subjective equality, malleability of timing precision). The Timing Database will serve as the repository for interval timing studies through the submission of new datasets.

Core clock gene, Bmal1, is required for optimal second-level interval production

Perception and production of second-level temporal intervals are critical in several behavioral and cognitive processes, including adaptive anticipation, motor control, and social communication. These processes are impaired in several neurological and psychological disorders, such as Parkinson's disease and attention-deficit hyperactivity disorder. Although evidence indicates that second-level interval timing exhibit circadian patterns, it remains unclear whether the core clock machinery controls the circadian pattern of interval timing. To investigate the role of core clock molecules in interval timing capacity, we devised a behavioral assay called the interval timing task to examine prospective motor interval timing ability. In this task, the mouse produces two separate nose pokes in a pretrained second-level interval to obtain a sucrose solution as a reward. We discovered that interval perception in wild-type mice displayed a circadian pattern, with the best performance observed during the late active phase. To investigate whether the core molecular clock is involved in the circadian control of interval timing, we employed Bmal1 knockout mice (BKO) in the interval timing task. The interval production of BKO did not display any difference between early and late active phase, without reaching the optimal interval production level observed in wild-type. In summary, we report that the core clock gene Bmal1 is required for the optimal performance of prospective motor timing typically observed during the late part of the active period.

Working memory for time intervals: Another manifestation of the central tendency effect

The relationship between working memory and time perception has been typically investigated using dual-task paradigms (e.g., testing timing performance during a concurrent task). To our knowledge, none of these studies used time intervals as the target stimulus to be remembered. The current study investigated the working memory for time intervals by asking participants to reproduce durations they experienced at different orders in a series of experienced intervals (n-back task). One of the experiments was conducted online and the other one in the lab setting. Results showed a central tendency bias and additive elongation of time reproductions with increasing working memory load. Our results also showed that participants assigned different weights to experienced intervals based on their order of presentation (higher weight to the target interval). We conclude that the recall of intervals from working memory under high cognitive load leads to a central tendency effect, which is known to be induced by the temporal context and present particularly in aging and in those with Parkinson's disease.

Mechanisms of impulsive choice: Experiments to explore and models to map the empirical terrain

Impulsive choice is preference for a smaller-sooner (SS) outcome over a larger-later (LL) outcome when LL choices result in greater reinforcement maximization. Delay discounting is a model of impulsive choice that describes the decaying value of a reinforcer over time, with impulsive choice evident when the empirical choice-delay function is steep. Steep discounting is correlated with multiple diseases and disorders. Thus, understanding the processes underlying impulsive choice is a popular topic for investigation. Experimental research has explored the conditions that moderate impulsive choice, and quantitative models of impulsive choice have been developed that elegantly represent the underlying processes. This review spotlights experimental research in impulsive choice covering human and nonhuman animals across the domains of learning, motivation, and cognition. Contemporary models of delay discounting designed to explain the underlying mechanisms of impulsive choice are discussed. These models focus on potential candidate mechanisms, which include perception, delay and/or reinforcer sensitivity, reinforcement maximization, motivation, and cognitive systems. Although the models collectively explain multiple mechanistic phenomena, there are several cognitive processes, such as attention and working memory, that are overlooked. Future research and model development should focus on bridging the gap between quantitative models and empirical phenomena.

Lost in time: Relocating the perception of duration outside the brain

It is well-accepted in neuroscience that animals process time internally to estimate the duration of intervals lasting between one and several seconds. More than 100 years ago, Henri Bergson nevertheless remarked that, because animals have memory, their inner experience of time is ever-changing, making duration impossible to measure internally and time a source of change. Bergson proposed that quantifying the inner experience of time requires its externalization in movements (observed or self-generated), as their unfolding leaves measurable traces in space. Here, studies across species are reviewed and collectively suggest that, in line with Bergson's ideas, animals spontaneously solve time estimation tasks through a movement-based spatialization of time. Moreover, the well-known scalable anticipatory responses of animals to regularly spaced rewards can be explained by the variable pressure of time on reward-oriented actions. Finally, the brain regions linked with time perception overlap with those implicated in motor control, spatial navigation and motivation. Thus, instead of considering time as static information processed by the brain, it might be fruitful to conceptualize it as a kind of force to which animals are more or less sensitive depending on their internal state and environment.

Time perception and pain: Can a temporal illusion reduce the intensity of pain?

It is commonly known-and previous studies have indicated-that time appears to last longer during unpleasant situations. This study examined whether a reciprocal statement can be made-that is, whether changes in the perception of time can influence our judgment (or rating) of a negative event. We used a temporal illusion method (Pomares et al. Pain 152, 230-234, 2011) to induce distortions in the perception of time. Two stimuli were presented for a constant time: a full clock, which stayed on the screen until its clock hand completed a full rotation (360°); and a short clock, in which the clock hand moved just three-quarters of the way (270°), thus suggesting a reduced interval duration. However, both stimuli were shown for the same amount of time. We specifically investigated (a) whether we could induce a temporal illusion with this simple visual manipulation, and (b) whether this illusion could change participants' ratings of a painful stimulus. In Experiment I (n = 22), to answer (a) above, participants were asked to reproduce the duration in which the different clocks were presented. In Experiment II (n = 30), a painful thermal stimulation was applied on participants' hands while the clocks were shown. Participants were asked to rate the perceived intensity of their pain, and to reproduce its duration. Results showed that, for both experiments, participants reproduced a longer interval after watching the full clock compared with the short clock, confirming that the clock manipulation was able to induce a temporal illusion. Furthermore, the second experiment showed that participants rated the thermal stimuli as less painful when delivered with the short clock than with the full clock. These findings suggest that temporal distortions can modulate the experience of pain.

Motivated to time: Effects of reinforcer devaluation and opportunity cost on interval timing

Prior research suggests that interval timing performance is sensitive to reinforcer devaluation effects and to the rate of competing sources of reinforcement. The present study sought to replicate and account for these findings in rats. A self-paced concurrent fixed-interval (FI) random-ratio (RR) schedule of reinforcement was implemented in which the FI requirement varied across training conditions (12, 24, 48 s). The RR requirement-which imposed an opportunity cost to responding on the FI component-was adjusted so that it took about twice the FI requirement, on average, to complete it. Probe reinforcer devaluation (prefeeding) sessions were conducted at the end of each condition. To assess the effect of contextual reinforcement on timing performance, the RR requirement was removed before the end of the experiment. Consistent with prior findings, performance on the FI component tracked schedule requirement and displayed scalar invariance; the removal of the RR component yielded more premature FI responses. For some rats, prefeeding reduced the number of trials initiated without affecting timing performance; for other rats, prefeeding delayed responding on the FI component but had a weaker effect on trial initiation. These results support the notion that timing and motivational processes are separable, suggesting novel explanations for ostensible motivational effects on timing performance.

Keep your finger on the pulse: Better rate perception and gap detection with vibrotactile compared to visual stimuli

Important characteristics of the environment can be represented in the temporal pattern of sensory stimulation. In two experiments, we compared accuracy of temporal processing by different modalities. Experiment 1 examined binary categorization of rate for visual (V) or vibrotactile (T) stimulus pulses presented at either 4 or 6 Hz. Inter-pulse intervals were either constant or variable, perturbed by random Gaussian variates. Subjects categorized the rate of T pulse sequences more accurately than V sequences. In V conditions only, subjects disproportionately tended to mis-categorize 4-Hz pulse rates, for all but the most variable sequences. In Experiment 2, we compared gap detection thresholds across modalities, using the same V and T pulses from Experiment 1, as well as with bimodal (VT) pulses. Visual gap detection thresholds were larger (3[Formula: see text]) than tactile thresholds. Additionally, performance with VT stimuli seemed to be nearly completely dominated by their T components. Together, these results suggest (i) that vibrotactile temporal acuity surpasses visual temporal acuity, and (ii) that vibrotactile stimulation has considerable, untapped potential to convey temporal information like that needed for eyes-free alerting signals.

An open-source behavior controller for associative learning and memory (B-CALM)

Associative learning and memory, i.e., learning and remembering the associations between environmental stimuli, self-generated actions, and outcomes such as rewards or punishments, are critical for the well-being of animals. Hence, the neural mechanisms underlying these processes are extensively studied using behavioral tasks in laboratory animals. Traditionally, these tasks have been controlled using commercial hardware and software, which limits scalability and accessibility due to their cost. More recently, due to the revolution in microcontrollers or microcomputers, several general-purpose and open-source solutions have been advanced for controlling neuroscientific behavioral tasks. While these solutions have great strength due to their flexibility and general-purpose nature, for the same reasons, they suffer from some disadvantages including the need for considerable programming expertise, limited online visualization, or slower than optimal response latencies for any specific task. Here, to mitigate these concerns, we present an open-source behavior controller for associative learning and memory (B-CALM). B-CALM provides an integrated suite that can control a host of associative learning and memory behaviors. As proof of principle for its applicability, we show data from head-fixed mice learning Pavlovian conditioning, operant conditioning, discrimination learning, as well as a timing task and a choice task. These can be run directly from a user-friendly graphical user interface (GUI) written in MATLAB that controls many independently running Arduino Mega microcontrollers in parallel (one per behavior box). In sum, B-CALM will enable researchers to execute a wide variety of associative learning and memory tasks in a scalable, accurate, and user-friendly manner.

Mice expressing P301S mutant human tau have deficits in interval timing

Interval timing is a key executive process that involves estimating the duration of an interval over several seconds or minutes. Patients with Alzheimer's disease (AD) have deficits in interval timing. Since temporal control of action is highly conserved across mammalian species, studying interval timing tasks in animal AD models may be relevant to human disease. Amyloid plaques and tau neurofibrillary tangles are hallmark features of AD. While rodent models of amyloid pathology are known to have interval timing impairments, to our knowledge, interval timing has not been studied in models of tauopathy. Here, we evaluate interval timing performance of P301S transgenic mice, a widely studied model of tauopathy that overexpresses human tau with the P301S mutation. We employed an interval timing task and found that P301S mice consistently underestimated temporal intervals compared to wild-type controls, responding early in anticipation of the target interval. Our study indicating timing deficits in a mouse tauopathy model could have relevance to human tauopathies such as AD.

Intra-dorsal striatal acetylcholine M1 but not dopaminergic D1 or glutamatergic NMDA receptor antagonists inhibit consolidation of duration memory in interval timing

The striatal beat frequency model assumes that striatal medium spiny neurons encode duration via synaptic plasticity. Muscarinic 1 (M1) cholinergic receptors as well as dopamine and glutamate receptors are important for neural plasticity in the dorsal striatum. Therefore, we investigated the effect of inhibiting these receptors on the formation of duration memory. After sufficient training in a peak interval (PI)-20-s procedure, rats were administered a single or mixed infusion of a selective antagonist for the dopamine D1 receptor (SCH23390, 0.5 µg per side), N-methyl-D-aspartic acid (NMDA)-type glutamate receptor (D-AP5, 3 µg), or M1 receptor (pirenzepine, 10 µg) bilaterally in the dorsal striatum, immediately before initiating a PI-40 s session (shift session). The next day, the rats were tested for new duration memory (40 s) in a session in which no lever presses were reinforced (test session). In the shift session, the performance was comparable irrespective of the drug injected. However, in the test session, the mean peak time (an index of duration memory) of the M1 + NMDA co-blockade group, but not of the D1 + NMDA co-blockade group, was lower than that of the control group (Experiments 1 and 2). In Experiment 3, the effect of the co-blockade of M1 and NMDA receptors was replicated. Moreover, sole blockade of M1 receptors induced the same effect as M1 and NMDA blockade. These results suggest that in the dorsal striatum, the M1 receptor, but not the D1 or NMDA receptors, is involved in the consolidation of duration memory.

Active intrinsic conductances in recurrent networks allow for long-lasting transients and sustained activity with realistic firing rates as well as robust plasticity

Recurrent neural networks of spiking neurons can exhibit long lasting and even persistent activity. Such networks are often not robust and exhibit spike and firing rate statistics that are inconsistent with experimental observations. In order to overcome this problem most previous models had to assume that recurrent connections are dominated by slower NMDA type excitatory receptors. Usually, the single neurons within these networks are very simple leaky integrate and fire neurons or other low dimensional model neurons. However real neurons are much more complex, and exhibit a plethora of active conductances which are recruited both at the sub and supra threshold regimes. Here we show that by including a small number of additional active conductances we can produce recurrent networks that are both more robust and exhibit firing-rate statistics that are more consistent with experimental results. We show that this holds both for bi-stable recurrent networks, which are thought to underlie working memory and for slowly decaying networks which might underlie the estimation of interval timing. We also show that by including these conductances, such networks can be trained to using a simple learning rule to predict temporal intervals that are an order of magnitude larger than those that can be trained in networks of leaky integrate and fire neurons.

Feedback-Based Learning of Timing in Attention-Deficit/Hyperactivity Disorder and Neurofibromatosis Type 1

OBJECTIVE

Patients with Neurofibromatosis Type 1 (NF1) frequently display symptoms resembling those of Attention Deficit/Hyperactivity Disorder (ADHD). Importantly, these disorders are characterised by distinct changes in the dopaminergic system, which plays an important role in timing performance and feedback-based adjustments in timing performance. In a transdiagnostic approach, we examine how far NF1 and ADHD show distinct or comparable profiles of timing performance and feedback-based adjustments in timing.

METHOD

We examined time estimation and learning processes in healthy control children (HC), children with ADHD with predominantly inattentive symptoms and those with NF1 using a feedback-based time estimation paradigm.

RESULTS

Healthy controls consistently responded closer to the correct time window than both patient groups, were less variable in their reaction times and displayed intact learning-based adjustments across time. The patient groups did not differ from each other regarding the number of in-time responses. In ADHD patients, the performance was rather unstable across time. No performance changes could be observed in patients with NF1 across the entire task.

CONCLUSIONS

Children with ADHD and NF1 differ in feedback learning-based adjustments of time estimation processes. ADHD is characterised by behavioural fluctuations during the learning process. These are likely to be associated with inefficiencies in the dopaminergic system. NF1 is characterised by impairments of feedback learning which could be due to various neurotransmitter alterations occurring in addition to deficits in dopamine synthesis. Results show that despite the strong overlap in clinical phenotype and neuropsychological deficits between NF1 and ADHD, the underlying cognitive mechanisms are different.

Human temporal learning with mixed signals

The influence of cue informativeness on human temporal discrimination was evaluated using a peak-interval (PI) procedure. A target moved across the computer monitor, reaching the center at 2 or 4 s. Key presses shot the center of the screen. Participants earned points when shots hit the target and lost points for misses. The target was masked during occasional, extended PI trials, allowing for measurement of temporal discrimination. During PI trials, the screen background color could exert stimulus control by providing information about target speed. Cue informativeness was represented as the correlation (φ) between light or dark green backgrounds and the 2- or 4-s target and was manipulated across 4 conditions: a multiple schedule (φ = 1), mixed signals (φ = 0.8, 0.4), and a mixed schedule (φ = 0). In Experiment 1, participants were randomly assigned to one of the 4 conditions. In Experiment 2, each participant experienced all 4 conditions. Participants learned to respond at both intervals in all conditions. Cue informativeness did not affect peak time or spread. For the most part, temporal distributions of responses for the two background colors suggested a cover-both-bases strategy in the presence of mixed signals. Participants incorporated probabilistic information from cues to allocate responding in time.

Cortical alpha-synuclein preformed fibrils do not affect interval timing in mice

One hallmark feature of Parkinson's disease (PD) is Lewy body pathology associated with misfolded alpha-synuclein. Previous studies have shown that striatal injection of alpha-synuclein preformed fibrils (PFF) can induce misfolding and aggregation of native alpha-synuclein in a prion-like manner, leading to cell death and motor dysfunction in mouse models. Here, we tested whether alpha-synuclein PFFs injected into the medial prefrontal cortex results in deficits in interval timing, a cognitive task which is disrupted in human PD patients and in rodent models of PD. We injected PFF or monomers of human alpha-synuclein into the medial prefrontal cortex of mice pre-injected with adeno-associated virus (AAV) coding for overexpression of human alpha-synuclein or control protein. Despite notable medial prefrontal cortical synucleinopathy, we did not observe consistent deficits in fixed-interval timing. These results suggest that cortical alpha-synuclein does not reliably disrupt fixed-interval timing.

Interval timing and midfrontal delta oscillations are impaired in Parkinson's disease patients with freezing of gait

Gait abnormalities and cognitive dysfunction are common in patients with Parkinson's disease (PD) and get worse with disease progression. Recent evidence has suggested a strong relationship between gait abnormalities and cognitive dysfunction in PD patients and impaired cognitive control could be one of the causes for abnormal gait patterns. However, the pathophysiological mechanisms of cognitive dysfunction in PD patients with gait problems are unclear. Here, we collected scalp electroencephalography (EEG) signals during a 7-s interval timing task to investigate the cortical mechanisms of cognitive dysfunction in PD patients with (PDFOG +, n = 34) and without (PDFOG-, n = 37) freezing of gait, as well as control subjects (n = 37). Results showed that the PDFOG +  group exhibited the lowest maximum response density at around 7 s compared to PDFOG- and control groups, and this response density peak correlated with gait abnormalities as measured by FOG scores. EEG data demonstrated that PDFOG +  had decreased midfrontal delta-band power at the onset of the target cue, which was also correlated with maximum response density and FOG scores. In addition, our classifier performed better at discriminating PDFOG + from PDFOG- and controls with an area under the curve of 0.93 when midfrontal delta power was chosen as a feature. These findings suggest that abnormal midfrontal activity in PDFOG + is related to cognitive dysfunction and describe the mechanistic relationship between cognitive and gait functions in PDFOG + . Overall, these results could advance the development of novel biosignatures and brain stimulation approaches for PDFOG + .

Similar attention and performance in female and male CD1 mice in the peak procedure

Inaccurate and distorted timing are associated with neurodegenerative disorders such as Alzheimer's disease and schizophrenia in humans, which generates interest in the discovery and understanding of the factors behind such timing difficulties. Timing research in mice has taken an important role, because the availability of genetically-altered strains allows establishing the causal role of specific genes on such neurodegenerative disorders. Nevertheless, few studies have considered mice's sex and some have found sex differences in timing, although results are not yet conclusive. We tested female and male CD1 mice, an outbred strain not yet studied in a peak procedure. By varying the percentage of peak trials and the presence of a gap and/or a distractor in the tests, we found no sex differences in accuracy, precision, or attention. Both females and males followed a stop-clock strategy after distractor and gap + distractor trials. This suggests that both male and female CD1 mice may be exposed to a peak procedure to study factors associated to neurotoxicology or neurogenesis.

Drosophila acquires seconds-scale rhythmic behavior

Detection of the temporal structure of stimuli is crucial for prediction. While perception of interval timing is relevant for immediate behavioral adaptations, it has scarcely been investigated, especially in invertebrates. Here, we examined whether the fruit fly, Drosophila melanogaster, can acquire rhythmic behavior in the range of seconds. To this end, we developed a novel temporal conditioning paradigm utilizing repeated electric shocks. Combined automatic behavioral annotation and time-frequency analysis revealed that behavioral rhythms continued after cessation of the shocks. Furthermore, we found that aging impaired interval timing. This study thus not only demonstrates the ability of insects to acquire behavioral rhythms of a few seconds, but highlights a life-course decline of temporal coordination, which is also common in mammals.

Dorsal striatum and the temporal expectancy of an aversive event in Pavlovian odor fear learning

Interval timing, the ability to encode and retrieve the memory of intervals from seconds to minutes, guides fundamental animal behaviors across the phylogenetic tree. In Pavlovian fear conditioning, an initially neutral stimulus (conditioned stimulus, CS) predicts the arrival of an aversive unconditioned stimulus (US, generally a mild foot-shock) at a fixed time interval. Although some studies showed that temporal relations between CS and US events are learned from the outset of conditioning, the question of the memory of time and its underlying neural network in fear conditioning is still poorly understood. The aim of the present study was to investigate the role of the dorsal striatum in timing intervals in odor fear conditioning in male rats. To assess the animal's interval timing ability in this paradigm, we used the respiratory frequency. This enabled us to detect the emergence of temporal patterns related to the odor-shock time interval from the early stage of learning, confirming that rats are able to encode the odor-shock time interval after few training trials. We carried out reversible inactivation of the dorsal striatum before the acquisition session and before a shift in the learned time interval, and measured the effects of this treatment on the temporal pattern of the respiratory rate. In addition, using intracerebral microdialysis, we monitored extracellular dopamine level in the dorsal striatum throughout odor-shock conditioning and in response to a shift of the odor-shock time interval. Contrary to our initial predictions based on the existing literature on interval timing, we found evidence suggesting that transient inactivation of the dorsal striatum may favor a more precocious buildup of the respiratory frequency's temporal pattern during the odor-shock interval in a manner that reflected the duration of the interval. Our data further suggest that the conditioning and the learning of a novel time interval were associated with a decrease in dopamine level in the dorsal striatum, but not in the nucleus accumbens. These findings prompt a reassessment of the role of the striatum and striatal dopamine in interval timing, at least when considering Pavlovian aversive conditioning.

Attention Does Not Affect the Speed of Subjective Time, but Whether Temporal Information Guides Performance: A Large-Scale Study of Intrinsically Motivated Timers in a Real-Time Strategy Game

Many prepared actions have to be withheld for a certain amount of time in order to have the most beneficial outcome. Therefore, keeping track of time accurately is vital to using temporal regularities in our environment. Traditional theories assume that time is tracked by means of a clock and an "attentional gate" (AG) that modulates subjective time if not enough attentional resources are directed toward the temporal process. According to the AG theory, the moment of distraction does not have an influence on the subjective modulation. Here, we show, based on an analysis of 28,354 datasets, that highly motivated players of the online multiplayer real-time strategy game StarCraft2 indeed respond later to timed events when they are distracted by other tasks during the interval. However, transient periods of distraction during the interval influence the response time to a lesser degree than distraction just before the required response. We extend the work of Taatgen, van Rijn, and Anderson (2007) and propose an alternative active check theory that postulates that distracted attention prevents people from checking their internal clock; we demonstrate that this account better predicts variance observed in response time. By analyzing StarCraft2 data, we assessed the role of attention in a naturalistic setting that more directly generalizes to real-world settings than typical laboratory studies.

Time perception and patience: individual differences in interval timing precision predict choice impulsivity in European starlings, Sturnus vulgaris

Impulsivity, in the sense of the extent rewards are devalued as the time until their realization increases, is linked to various negative outcomes in humans, yet understanding of the cognitive mechanisms underlying it is limited. Variation in the imprecision of interval timing is a possible contributor to variation in impulsivity. We use a numerical model to generate predictions concerning the effect of timing imprecision on impulsivity. We distinguish between fixed imprecision (the imprecision that applies even when timing the very shortest time intervals) and proportional imprecision (the rate at which imprecision increases as the interval becomes longer). The model predicts that impulsivity should increase with increasing fixed imprecision, but decrease with increasing proportional imprecision. We present data from a cohort of European starlings (Sturnus vulgaris, n = 28) in which impulsivity had previously been measured through an intertemporal choice paradigm. We tested interval timing imprecision in the same individuals using a tri-peak temporal reproduction procedure. We found repeatable individual differences in both fixed and proportional imprecision. As predicted, birds with greater proportional imprecision in interval timing made fewer impulsive choices, whilst those with greater fixed imprecision tended to make more. Contradictory observations in the literature regarding the direction of association between timing imprecision and impulsivity might be clarified by distinguishing between fixed and proportional components of imprecision.

Timing behavior in genetic murine models of neurological and psychiatric diseases

How timing behavior is altered in different neurodevelopmental and neurodegenerative disorders is a contemporary research question. Genetic murine models (GMM) that offer high construct validity also serve as useful tools to investigate this question. But the literature on timing behavior of different GMMs largely remains to be consolidated. The current paper addresses this gap by reviewing studies that have been conducted with GMMs of neurodevelopmental (e.g. ADHD, schizophrenia, autism spectrum disorder), neurodegenerative disorders (e.g., Alzheimer's disease, Huntington's disease) as well as circadian and other mutant lines. The review focuses on those studies that specifically utilized the peak interval procedure to improve the comparability of findings both within and between different disease models. The reviewed studies revealed timing deficits that are characteristic of different disorders. Specifically, Huntington's disease models had weaker temporal control over the termination of their anticipatory responses, Alzheimer's disease models had earlier timed responses, schizophrenia models had weaker temporal control, circadian mutants had shifted timed responses consistent with shifts in the circadian periods. The differences in timing behavior were less consistent for other conditions such as attention deficit and hyperactivity disorder and mutations related to intellectual disability. We discuss the implications of these findings for the neural basis of an internal stopwatch. Finally, we make methodological recommendations for future research for improving the comparability of the timing behavior across different murine models.

Medial prefrontal cortex and the temporal control of action

Across species, the medial prefrontal cortex guides actions in time. This process can be studied using behavioral paradigms such as simple reaction-time and interval-timing tasks. Temporal control of action can be influenced by prefrontal neurotransmitters such as dopamine and acetylcholine and is highly relevant to human diseases such as Parkinson's disease, schizophrenia, and attention-deficit hyperactivity disorder (ADHD). We review evidence that across species, medial prefrontal lesions impair the temporal control of action. We then consider neurophysiological correlates in humans, primates, and rodents that might encode temporal processing and relate to cognitive-control mechanisms. These data have informed brain-stimulation studies in rodents and humans that can compensate for timing deficits. This line of work illuminates basic mechanisms of temporal control of action in the medial prefrontal cortex, which underlies a range of high-level cognitive processing and could contribute to new biomarkers and therapies for human brain diseases.

Use of temporal and colour cueing in a symbolic delayed matching task by honey bees

Honey bees (Apis mellifera) are known for their capacity to learn arbitrary relationships between colours, odours and even numbers. However, it is not known whether bees can use temporal signals as cueing stimuli in a similar way during symbolic delayed matching-to-sample tasks. Honey bees potentially process temporal signals during foraging activities, but the extent to which they can use such information is unclear. Here, we investigated whether free-flying honey bees could use either illumination colour or illumination duration as potential context-setting cues to enable their subsequent decisions for a symbolic delayed matching-to-sample task. We found that bees could use the changing colour context of the illumination to complete the subsequent spatial vision task at a level significantly different from chance expectation, but could not use the duration of either a 1 or 3 s light as a cueing stimulus. These findings suggest that bees cannot use temporal information as a cueing stimulus as efficiently as other signals such as colour, and are consistent with previous field observations suggesting a limited interval timing capacity in honey bees.

Effects of variable durations of food availability on interval time-place learning with pigeons Columba Livia

We assessed the effects of variable durations of food availability on an interval time-place learning task. 3 pigeons were exposed to a task in which food could be obtained for responses in one of four feeders according to an RI 25 s during 3 min, after which, food could be obtained on a different feeder according to the same schedule. The correct feeder changed following a fixed sequence that was repeated four times throughout the session. After 50 training sessions, an Open Hopper Test was conducted, after which, the second training condition ensued. This condition was like the first one with the exception that the availability period could be either 1,2,3, or 6 min long. A second test was conducted after 50 sessions of this training. Another group of 3 birds experienced these conditions in the reverse order. Data suggest that birds solved the task via interval timing under the fixed duration condition, and via ordinal timing when faced with variable durations. Birds learned the fixed sequence involved in the task under both conditions. Although the present data agree with previous research exploring variability in TPL tasks, they do not necessarily support previous claims for an asymmetrical role of spatial and temporal information in these tasks.

Visual timing abilities of a harbour seal (Phoca vitulina) and a South African fur seal (Arctocephalus pusillus pusillus) for sub- and supra-second time intervals

Timing is an essential parameter influencing many behaviours. A previous study demonstrated a high sensitivity of a phocid, the harbour seal (Phoca vitulina), in discriminating time intervals. In the present study, we compared the harbour seal’s timing abilities with the timing abilities of an otariid, the South African fur seal (Arctocephalus pusillus pusillus). This comparison seemed essential as phocids and otariids differ in many respects and might, thus, also differ regarding their timing abilities. We determined time difference thresholds for sub- and suprasecond time intervals marked by a white circle on a black background displayed for a specific time interval on a monitor using a staircase method. Contrary to our expectation, the timing abilities of the fur seal and the harbour seal were comparable. Over a broad range of time intervals, 0.8–7 s in the fur seal and 0.8–30 s in the harbour seal, the difference thresholds followed Weber’s law. In this range, both animals could discriminate time intervals differing only by 12 % and 14 % on average. Timing might, thus be a fundamental cue for pinnipeds in general to be used in various contexts, thereby complementing information provided by classical sensory systems. Future studies will help to clarify if timing is indeed involved in foraging decisions or the estimation of travel speed or distance.

Direct and Indirect Timing Functions in Unilateral Hemispheric Lesions

Introduction

The neural substrates of temporal processing are not still fully known. The majority of interval timing studies have dealt with this subject in the context of "Explicit timing" (computing the time intervals explicitly). The hypothesis "Implicit timing" (implicitly using temporal processing to improve function) has also proposed. This lesion study addressed explicit and implicit timing paradigms simultaneously using identical experimental tasks.

Methods

In this case-control study, 15 patients with Right Hemisphere Damage (RHD) and 15 patients with Left Hemisphere Damage (LHD) and 15 age-matched normal subjects were included. Participants performed a temporal reproduction task (assessing explicit timing) and a temporal prediction task (assessing implicit timing) in two sub- and supra-second intervals.

Results

Our results showed that RHD can lead to significantly lower accuracy in the temporal reproduction task in sub-second (P=0.005) and supra-second (P=0.001) intervals, compared with the normal subjects. Also, LHD led to perturbation in temporal prediction task by an increase in reaction time (lower accuracy) in sub- (P=0.011) and supra-second (P=0.006) time intervals than the normal subjects.

Conclusion

Overall, our findings suggested that there is a right hemispheric bias in the neural substrate of explicit timing, in both sub- and supra-second intervals. Furthermore, for the first time in a lesion study, we showed the evidence of left-hemispheric bias in neural substrates of implicit timing.

A simple three layer excitatory-inhibitory neuronal network for temporal decision-making

Humans and animals do not only keep track of time intervals but they can also make decisions about durations. Temporal bisection is a psychophysical task that is widely used to assess the latter ability via categorization of durations as short or long. Many existing models of performance in temporal bisection primarily account for choice proportions and tend to overlook the associated response times. We propose a time-cell neural network that implements both interval timing and temporal categorization. The proposed model can keep track of time intervals based on lurching wave activity, it can learn the reference durations along with their association with different categorization responses, and finally, it can carry out the comparison of arbitrary intermediate durations to the reference durations. We compared the model's predictions about choice behavior and response times to the empirical data previously gathered from rats. We showed that this time-cell neural network can predict the canonical behavioral signatures of temporal bisection performance. Specifically, (a) the proposed model can account for the sigmoidal relationship between the probability of the long choices and the test durations, (b) the superposition of choice functions on a relative time scale, (c) the localization of the point of subjective equality at the geometric mean of the reference durations, and (d) the differential modulation of short and long categorization response times as a function of the test durations.

Subjective time estimation in Antarctica: The impact of extreme environments and isolation on a time production task

Interval timing measures time estimation in the seconds-to-minutes range. Antarctica provides a real-world context to study the effect of extreme photoperiods and isolation on time perception. The aim of this study was to explore interval timing as a cognitive measure in the crew of Belgrano II Argentine Antarctic Station. A total of 13 subjects were assessed for interval timing in short (3 s), intermediate (6 s) and long (12 s) duration stimuli. Measures were taken during the morning and evening, five times along the year. Significant variations were found for 3 s and 6 s during the morning and 6 s during the evening. Results suggest an impact of isolation on morning performances and an effect of the polar night on evening measures. These findings shed some light on the use of interval timing as a cognitive test to assess performance in extreme environments.

Interval timing deficits and their neurobiological correlates in aging mice

Age-related neurobiological and cognitive alterations suggest that interval timing (as a related function) is also altered in aging, which can, in turn, disrupt timing-dependent functions. We investigated alterations in interval timing with aging and accompanying neurobiological changes. We tested 4-6, 10-12, and 18-20 month-old mice on the dual peak interval procedure. Results revealed a specific deficit in the termination of timed responses (stop-times). The decision processes contributed more to timing variability (vs. clock/memory process) in the aged mice. We observed age-dependent reductions in the number of dopaminergic neurons in the VTA and SNc, cholinergic neurons in the medial septum/diagonal band (MS/DB) complex, and density of dopaminergic axon terminals in the DLS/DMS. Negative correlations were found between the number of dopaminergic neurons in the VTA and stop times, and the number of cholinergic neurons in MS/DB complex and the acquisition of stop times. Our results point at age-dependent changes in the decisional components of interval timing and the role of dopaminergic and cholinergic functions in these behavioral alterations.

The differential role of the dorsal hippocampus in initiating and terminating timed responses: A lesion study using the switch-timing task

This study investigated the role of the dorsal hippocampus (dHPC) in the temporal entrainment of behavior, while addressing limitations of previous evidence from peak procedure experiments. Rats were first trained on a switch-timing task in which food was obtained from one of two concurrently available levers; one lever was effective after 8 s and the other after 16  s. After performance stabilized, rats underwent either bilateral NMDA lesions of the dHPC or sham lesions. After recovery, switch-timing training resumed. In a subsequent condition, the switch-timing task was modified such that food was available after either 8 or 32 s. Although dHPC lesions had subtle and complex effects on when rats stopped seeking for food at the 8-s lever (departures), it more systematically reduced the time when rats started seeking for food at the 16-s and 32-s lever (switches). No systematic effect of dHPC lesions were observed on the coefficient of quartile variation (normalized dispersion) of latencies to switch. Within the context of the pacemaker-accumulator framework of interval timing, these findings suggest that partially or wholly independent mechanisms control the initiation and termination of timed responses, and that the dHPC is primarily involved in encoding the time to start responding.

Rats can learn a temporal task in a single session

Fixed interval, peak interval, and temporal bisection procedures have been used to assess cognitive functions and address questions such as how animals perceive, represent, and reproduce time intervals. They have also been extensively used to test the effects of drugs on behavior, and to describe the neural correlates of interval timing. However, those procedures usually require several weeks of training for behavior to stabilize. Here, we investigated a variation of the Differential Reinforcement of Response Duration (DRRD) task with a target time of 1.2 s. We compared three types of training protocols and reported a procedure in which performance by the end of the very first session nearly matches the performance of long-term training. We also showed that the initial distribution of the responses is uni-modal and, as training evolves (and rats improve their performance), a second peak emerges and progressively shifts toward longer times. This one-day training protocol can be used to investigate temporal learning and may be especially useful to electrophysiological and neuropharmacological studies.

Cerebellar D1DR-expressing neurons modulate the frontal cortex during timing tasks

Converging lines of evidence suggest that the cerebellum plays an integral role in cognitive function through its interactions with association cortices like the medial frontal cortex (MFC). It is unknown precisely how the cerebellum influences the frontal cortex and what type of information is reciprocally relayed between these two regions. A subset of neurons in the cerebellar dentate nuclei, or the homologous lateral cerebellar nuclei (LCN) in rodents, express D1 dopamine receptors (D1DRs) and may play a role in cognitive processes. We investigated how pharmacologically blocking LCN D1DRs influences performance in an interval timing task and impacts neuronal activity in the frontal cortex. Interval timing requires executive processes such as working memory, attention, and planning and is known to rely on both the frontal cortex and cerebellum. In our interval timing task, male rats indicated their estimates of the passage of a period of several seconds by making lever presses for a water reward. We have shown that a cue-evoked burst of low-frequency activity in the MFC initiates ramping activity (i.e., monotonic increases or decreases of firing rate over time) in single MFC neurons. These patterns of activity are associated with successful interval timing performance. Here we explored how blocking right LCN D1DRs with the D1DR antagonist SCH23390 influences timing performance and neural activity in the contralateral (left) MFC. Our results indicate that blocking LCN D1DRs impaired some measures of interval timing performance. Additionally, ramping activity of MFC single units was significantly attenuated. These data provide insight into how catecholamines in the LCN may drive MFC neuronal dynamics to influence cognitive function.

Eliciting contextual temporal calibration: The effect of bottom-up and top-down information in reproduction tasks

Bayesian integration assumes that a current observation is integrated with previous observations. An example in the temporal domain is the central tendency effect: when a range of durations is presented, a regression towards the mean is observed. Furthermore, a context effect emerges if a partially overlapping lower and a higher range of durations is presented in a blocked design, with the overlapping durations pulled towards the mean duration of the block. We determine under which conditions this context effect is observed, and whether explicit cues strengthen the effect. Each block contained either two or three durations, with one duration present in both blocks. We provided either no information at the start of each block about the nature of that block, provided written ("short" / "long" or "A" / "B") categorizations, or operationalized pitch (low vs high) to reflect the temporal context. We demonstrate that (1) the context effect emerges as long as sufficiently distinct durations are presented; (2) the effect is not modulated by explicit instructions or other cues; (3) just a single additional duration is sufficient to produce a context effect. Taken together, these results provide information on the most efficient operationalization to evoke the context effect, allowing for highly economical experimental designs, and highlights the automaticity by which priors are constructed.

Modality differences in timing and the filled-duration illusion: Testing the pacemaker rate explanation

Performance in temporal difference threshold and estimation tasks is markedly less accurate for visual than for auditory intervals. In addition, thresholds and estimates are likewise less accurate for empty than for filled intervals. In scalar timing theory, these differences have been explained as alterations in pacemaker rate, which is faster for auditory and filled intervals than for visual and empty intervals. We tested this explanation according to three research aims. First, we replicated the threshold and estimation tasks of Jones, Poliakoff, and Wells (Quarterly Journal of Experimental Psychology, 62, 2171–2186, 2009) and found the well-documented greater precision for auditory than visual intervals, and for filled than for empty intervals. Second, we considered inter-individual differences in these classic effects and found that up to 27% of participants exhibited opposite patterns. Finally, we examined intra-individual differences to investigate (i) whether thresholds and estimates correlate within each stimulus condition and (ii) whether the stimulus condition in which a participants’ pacemaker rate was highest was the same in both tasks. Here we found that if pacemaker rate is indeed a driving factor for thresholds and estimates, its effect may be greater for empty intervals, where the two tasks correlate, than for filled intervals, where they do not. In addition, it was more common for participants to perform best in different modalities in each task, though this was not true for ordinal intra-individual differences in the filled-duration illusion. Overall, this research presents several findings inconsistent with the pacemaker rate explanation.

The effects of microdose LSD on time perception: a randomised, double-blind, placebo-controlled trial

RATIONALE

Previous research demonstrating that lysergic acid diethylamide (LSD) produces alterations in time perception has implications for its impact on conscious states and a range of psychological functions that necessitate precise interval timing. However, interpretation of this research is hindered by methodological limitations and an inability to dissociate direct neurochemical effects on interval timing from indirect effects attributable to altered states of consciousness.

METHODS

We conducted a randomised, double-blind, placebo-controlled study contrasting oral administration of placebo with three microdoses of LSD (5, 10, and 20 μg) in older adults. Subjective drug effects were regularly recorded and interval timing was assessed using a temporal reproduction task spanning subsecond and suprasecond intervals.

RESULTS

LSD conditions were not associated with any robust changes in self-report indices of perception, mentation, or concentration. LSD reliably produced over-reproduction of temporal intervals of 2000 ms and longer with these effects most pronounced in the 10 μg dose condition. Hierarchical regression analyses indicated that LSD-mediated over-reproduction was independent of marginal differences in self-reported drug effects across conditions.

CONCLUSIONS

These results suggest that microdose LSD produces temporal dilation of suprasecond intervals in the absence of subjective alterations of consciousness.

Anticipation of a midsession reversal in humans

In a two-stimulus visual discrimination choice task with a reversal in reward contingencies midway through each session, pigeons produce a surprising number of anticipatory errors (i.e., responding to the second-correct stimulus before the reversal) based on failure to inhibit timing-based intrusion errors; limited prior research has suggested humans' performance is qualitatively different. Here we illustrate a partial replication of previous findings in humans, but suggest based on our results that humans process these tasks in a manner similar to pigeons. Humans made relatively few but consistent errors across both simultaneous- and successive-choice experiments. Anticipation errors were limited when the identity of the first-correct stimulus alternated between sessions, consistent with the behaviour of pigeons. Subsequent experiments found evidence for anticipation on a purely temporal simultaneous choice task, and fewer errors with symmetrical reinforcement and punishment of responses on a sequential choice task. Interval timing causes conflicts with decision-making processes on the midsession reversal task that are consistent, but differ in magnitude, across species.

Sex differences in the timing behavior performance of 3xTg-AD and wild-type mice in the peak interval procedure

We investigated interval timing behavior of 10-month-old male and female 3xTg-AD mice compared with their B6129F2/J wild type controls using the peak interval procedure with a 15 s target interval. Multiple parameters reflecting different aspects of timing performance were extracted from steady-state anticipatory nose-poking behavior using two different approaches: single trial analyses and average response curve analyses. These measures can dissociate the differences in performance due to distortions in the interval timing ability or to motivational decline (i.e. apathy); both of which have been reported in Alzheimer patients. We found that the interval timing ability of male and female 3xTg-AD mice did not differ from wild-type controls. However, in measures reflecting motivational state, we found significant sex differences regardless of genotype. Specifically, female mice initiated anticipatory responding later in the trial and had lower response amplitudes than males. Although our findings can also be interpreted in terms of differences in temporal control for response initiation, they more strongly suggest the effect of differential incentive motivation between sexes on timing behavior in these mice.

A divergence of sub- and supra-second timing abilities in childhood and its relation to academic achievement

Work with adult humans and nonhuman animals provides evidence that the processing of sub-second (<1 s) and supra-second (>1 s) durations are modulated via distinct cognitive and neural systems; however, few studies have explored the development of these separate systems. Moreover, recent research has identified a link between basic timing abilities and academic achievement, yet it is unclear whether sub-second and supra-second temporal processing may play independent roles in this relation. In the current study, we assessed the development of sub- and supra-second timing across middle childhood and examined how each ability may relate to academic achievement. Child participants (6- to 8-year-olds, n = 111) completed reading and math assessments and a temporal discrimination task that included comparisons in both the sub- and supra-second ranges. Results revealed that younger children performed comparably across the sub- and supra-second ranges, whereas 8-year-olds and adults (n = 72) were relatively better at discriminating durations in the supra-second range. Although discrimination performance in these distinct duration ranges did not uniquely predict math or reading achievement, overall timing abilities were related to math, but not reading, when controlling for age. Together, these data provide evidence for a divergence in timing abilities across sub- and supra-second durations emerging around 8 years of age; however, at least during this stage of development, the relation between children's timing and math achievement is unrelated to this divergence.

Striatal dopamine and the temporal control of behavior

Striatal dopamine strongly regulates how individuals use time to guide behavior. Dopamine acts on D1- and D2- dopamine receptors in the striatum. However, the relative role of these receptors in the temporal control of behavior is unclear. To assess this, we trained rats on a task in which they decided to start and stop a series of responses based on the passage of time and evaluated how blocking D1 or D2-dopamine receptors in the dorsomedial or dorsolateral striatum impacted performance. D2 blockade delayed the decision to start and stop responding in both regions, and this effect was larger in the dorsomedial striatum. By contrast, dorsomedial D1 blockade delayed stop times, without significantly delaying start times, whereas dorsolateral D1 blockade produced no detectable effects. These findings suggest that striatal dopamine may tune decision thresholds during timing tasks. Furthermore, our data indicate that the dorsomedial striatum plays a key role in temporal control, which may be useful for localizing neural circuits that mediate the temporal control of action.

Females in the forefront: time-based intervention effects on impulsive choice and interval timing in female rats

Impulsive choice has been implicated in substance abuse, gambling, obesity, and other maladaptive behaviors. Deficits in interval timing may increase impulsive choices, and therefore, could serve as an avenue through which suboptimal impulsive choices can be moderated. Temporal interventions have successfully attenuated impulsive choices in male rats, but the efficacy of a temporal intervention has yet to be assessed in female rats. As such, this experiment examined timing and choice behavior in female rats, and evaluated the ability of a temporal intervention to mitigate impulsive choice behavior. The temporal intervention administered in this study was successful in reducing impulsive choices compared to a control group. Results of a temporal bisection task indicated that the temporal intervention increased long responses at the shorter durations. Further, results from the peak trials within the choice task combined with the progressive interval task suggest that the intervention increased sensitivity to delay and enhanced timing confidence. Overall, these results indicate that a temporal intervention can be a successful avenue for reducing impulsive choice behavior in female rats, and could contribute to the development of behavioral interventions to prevent impulsive choice and maladaptive behaviors that can be applied to both sexes.

Basolateral amygdala inactivation eliminates fear-induced underestimation of time in a temporal bisection task

We examined interval timing - time perception in the seconds-to-minutes range - of the fear-inducing stimulus and the role of the amygdala in this phenomenon. Rats were initially trained to perform a temporal bisection task, in which their responses to levers A and B were reinforced following 2-s and 8-s tones, respectively. After acquisition, the rats were also presented with tones of intermediate durations and pressed one of the two levers to indicate whether the tone duration was closer to 2 or 8 s. Subsequently, the rats underwent differential fear conditioning, in which one frequency tone (conditioned stimulus; CS+) was paired with an electric foot shock, whereas another frequency tone (CS-) was presented alone. The rats were then infused with artificial cerebrospinal fluid (aCSF) or the GABA_A agonist muscimol into the bilateral basolateral amygdala (BLA) before performing the bisection task with CS+ and CS-. In rats infused with aCSF, the psychophysical function shifted rightward in CS+ relative to that in CS-. Moreover, the point of subjective equality of the CS+ was higher than that of CS-, suggesting that the duration of the fear -CS was perceived as shorter than that of the neutral CS. However, muscimol infusion into the BLA abolished this difference, suggesting that BLA inactivation suppresses the effect of the fear -CS. Our results demonstrate that normal BLA activity is essential for fear-induced underestimation of time.

Positive symptoms and time perception in schizophrenia: A meta-analysis

Positive symptoms of schizophrenia may be related to distortions in time perception. To examine this issue, we conducted a meta-analysis to determine whether positive symptoms are associated with deficits in time processing performance. MEDLINE and Web of Science were searched from January 1980 through March 2017, and all related articles and their references were scrutinized to find relevant studies. Studies were selected if they included participants with a diagnosis of schizophrenia, and reported data from behavioral measures of interval timing (e.g. duration discrimination and temporal order judgement). The results indicated that positive symptoms of schizophrenia are related with overestimation of interval timing (i.e., acceleration of the “internal clock”), and suggest that time perception may be associated with psychosis.

Insular cortex inactivation generalizes fear-induced underestimation of interval timing in a temporal bisection task

In this study, we investigated: (1) the effect of fear on interval timing-time perception in the seconds-to-minutes range-and (2) the role of the insular cortex in the modulation of this effect. Rats were first trained on a temporal bisection task in which their response to a lever A was reinforced following a 2.00-s tone, whereas their response to a lever B was reinforced following an 8.00-s tone. After acquisition, the rats were also presented with intermediate-duration tones and pressed one of two levers to indicate whether tone duration was closer to 2.00 or 8.00s. Subsequently, the rats underwent differential fear conditioning in which one pitch tone (conditioned stimulus; CS+) was paired with an electric foot shock, while the other pitch tone (CS-) was presented alone. Either artificial cerebrospinal fluid (aCSF) or the GABA_A agonist muscimol was then infused into the rats' bilateral insular cortex before the animals were tested on the bisection task using the CS+and CS- tones. We found that in the rats infused with aCSF, the point of subjective equality (PSE) of the CS+ was higher than that for CS-, suggesting that the duration for CS+ was perceived to be shorter than that of CS-. However, muscimol eliminated the difference in PSE between CS+ and CS- by generalizing of the effect from CS+to the CS-. Taken together, our results show that normal activity in the insular cortex is involved in fear-induced modulation of interval timing.

Oscillation patterns of local field potentials in the dorsal striatum and sensorimotor cortex during the encoding, maintenance, and decision stages for the ordinal comparison of sub- and supra-second signal durations

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.

Between-session memory degradation accounts for within-session changes in fixed-interval performance

A common assumption in the study of fixed-interval (FI) timing is that FI performance is largely stable within sessions, once it is stable between sessions. Within-session changes in FI performance were examined in published data (Daniels and Sanabria, 2017), wherein some rats were trained on a FI 30-s schedule of food reinforcement (FI30) and others on a FI 90-s schedule (FI90). Following stability, FI90 rats were pre-fed for five sessions. Response rates declined as a function of trial, due more to latency lengthening than to run-rate reduction. Latencies were best described by a dynamic gamma-exponential mixture distribution, in which latency lengthening was driven by the growth of the criterion pulse count for a response and not by a reduction in the speed of an endogenous clock. The speed of the clock was selectively sensitive to the length of the FI; the prevalence and length of exponentially-distributed latencies were selectively sensitive to pre-feeding. These findings reveal (a) that parameters governing FI latencies are selectively sensitive to a range of manipulations, (b) a potential degradation of the criterion pulse count between consecutive sessions, and (c) a subsequent recovery of the criterion pulse count within sessions.

Interval timing under a behavioral microscope: Dissociating motivational and timing processes in fixed-interval performance

The distribution of latencies and interresponse times (IRTs) of rats was compared between two fixed-interval (FI) schedules of food reinforcement (FI 30 s and FI 90 s), and between two levels of food deprivation. Computational modeling revealed that latencies and IRTs were well described by mixture probability distributions embodying two-state Markov chains. Analysis of these models revealed that only a subset of latencies is sensitive to the periodicity of reinforcement, and prefeeding only reduces the size of this subset. The distribution of IRTs suggests that behavior in FI schedules is organized in bouts that lengthen and ramp up in frequency with proximity to reinforcement. Prefeeding slowed down the lengthening of bouts and increased the time between bouts. When concatenated, latency and IRT models adequately reproduced sigmoidal FI response functions. These findings suggest that behavior in FI schedules fluctuates in and out of schedule control; an account of such fluctuation suggests that timing and motivation are dissociable components of FI performance. These mixture-distribution models also provide novel insights on the motivational, associative, and timing processes expressed in FI performance. These processes may be obscured, however, when performance in timing tasks is analyzed in terms of mean response rates.

Differential reinforcement of low rates differentially decreased timing precision

Timing processes have been implicated as potential mechanisms that underlie self-controlled choice. To investigate the impact of an intervention that has been shown to increase self-controlled choice on timing processes, accuracy and precision of temporal discrimination were assessed in an 18-s peak procedure (18-s fixed interval trials; 54-s peak trials). During an intervention phase, mice in three treatment groups experienced differential reinforcement of low rate (DRL) schedules of reinforcement of 27 s, 18 s, or 9 s. A fourth group received continued exposure to the peak procedure. After the DRL intervention, timing was reassessed using the peak procedure. In contrast to previous reports, the DRL intervention resulted in less precise timing as indicated by increased peak spread and disrupted single-trial measures of temporal control. These effects were only detected just after the DRL intervention suggesting a transient effect of DRL exposure on timing. The increase in peak spread in the present experiment suggests delay exposure via DRL schedules may produce a "dose-dependent" effect on temporal discrimination, which may also increase self-controlled choice.

Learning what to expect and when to expect it involves dissociable neural systems

Two experiments with Long-Evans rats examined the potential independence of learning about different features of food reward, namely, "what" reward is to be expected and "when" it will occur. This was examined by investigating the effects of selective reward devaluation upon responding in an instrumental peak timing task in Experiment 1 and by exploring the effects of pre-training lesions targeting the basolateral amygdala (BLA) upon the selective reward devaluation effect and interval timing in a Pavlovian peak timing task in Experiment 2. In both tasks, two stimuli, each 60 s long, signaled that qualitatively distinct rewards (different flavored food pellets) could occur after 20 s. Responding on non-rewarded probe trials displayed the characteristic peak timing function with mean responding gradually increasing and peaking at approximately 20 s before more gradually declining thereafter. One of the rewards was then independently paired repeatedly with LiCl injections in order to devalue it whereas the other reward was unpaired with these injections. In a final set of test sessions in which both stimuli were presented without rewards, it was observed that responding was selectively reduced in the presence of the stimulus signaling the devalued reward compared to the stimulus signaling the still valued reward. Moreover, the timing function was mostly unaltered by this devaluation manipulation. Experiment 2 showed that pre-training BLA lesions abolished this selective reward devaluation effect, but it had no impact on peak timing functions shown by the two stimuli. It appears from these data that learning about "what" and "when" features of reward may entail separate underlying neural systems.