The Peak Interval Procedure in Rodents: A Tool for Studying the Neurobiological Basis of Interval Timing and Its Alterations in Models of Human Disease

Materials and Reagents

1. Subjects
   Inbred strains of mice or rats can be used as subjects to minimize the variation within the test group. Whatever the species or strain, experimenters should confirm that the animal can identify the discriminative stimulus. For instance, because older C57Bl/6 mice have age-related auditory impairments (Idrizbegovic et al., 2003); use a visual one instead If they are used in the context of aging.
2. Reinforcement
   Either liquid reinforcement such as condensed milk (e.g., Nestle Carnation^® condensed milk; IsosourceStandard, Nestle Health Science), a sucrose solution, or dry pellets (e.g., BioServ Dustless Precision Pellets F0023) can be used as reinforcement during testing. Reinforcements should be kept in cool storage. Condensed milk should be used on the day the can is opened –spoiled reinforcement is not reinforcing.
3. Animal Feed
   In the case of food restriction, subjects will usually require sufficient supplemental standard feeding after the test session (e.g., Bio-Serv, Rodent Diet). An industry standard is to keep their weights around 85% of the ad libitum weight. Growth charts can be obtained from the vendor, or from an in-house pilot study. With water restriction, the subjects will usually require sufficient supplemental water, too.
   

Equipment

1. Operant Boxes
   The Peak interval (PI) procedure is implemented in operant boxes (e.g., ENV-307W, Med Associates) with stainless steel grid floor (e.g., ENV-307W-GFW, Med Associates). Figure 1 illustrates one possible configuration of the operant box with two levers (e.g., ENV-312-2W, Med Associates), four receptacles that can be lit by LEDs for local stimuli (e.g., ENV-302RW, Med Associates). The head-entries to the receptacles can be detected by infrared detectors (e.g., ENV-313W, Med Associates). A speaker (ENV-324W, Med Associates) and a house light (e.g., ENV-315W, Med Associates) that can be used to generate global discriminative stimulus. For reinforcement, dry pellets can be delivered via pellet dispensers (e.g., ENV-203-20, Med Associates), liquid reinforcers can be delivered via dippers (e.g., ENV-302W-S, Med Associates).
   Note: There are other manufacturers of modular operant boxes, which can also be used to run this protocol (e.g., Harvard Apparatus, Modular Operant/Behavior Box (Panlab); Lafayette Instruments; Mouse Modular Test Chamber, Model 80015NS).
   
   
   Figure 1. Illustration of a possible configuration of an operant box from two different angles
   
   
2. Sound Attenuating Cubicles
   Ideally, operant boxes should be housed in sound attenuating cubicles (e.g., ENV-020M, Med Associates) to minimize the subject’s distraction by outside sources during testing (e.g., from other operant boxes, experimenter, etc.). Fans (e.g., ENV-375-F, Med Associates) in each cubicle can also help. They ventilate the cubicle, and their constant whirl and humming helps muffle outside noise.
3. Housing Cages
   Animals can be kept in standard shoebox cages (e.g., conventional polypropylene cages). If animals are individually housed, supplemental food and water can be given directly in the housing cage. If animals are group-housed, supplements can be given to the subjects prior to group-housing to prevent possible aggression between animals (although sometimes, animals will not fight).
   

Software

1. Operant boxes will be controlled either by commercially available software from the vendors of the operant boxes themselves (e.g., MedPC for Med-Associates or GraphicState for Coulbourne) or custom-built generic control software (e.g., Biopsychology Toolbox in Matlab Rose et al., 2008).
2. Data Analysis can be easily conducted either with custom code or the freely available TSLib (for Matlab: https://github.com/CRGallistel/Contingency; for R: https://github.com/freestone-lab/TSLibrary), and can find guidance in Freestone and Balcı (2018, https://github.com/freestone-lab/timing_tasks.git).
   

Procedure

1. Food or water deprivation of the subjects
   1. Get the animal’s ad libitum weight by weighing them over three subsequent days during which they have full access to chow and water. Use a growth chart from the animal’s vendor to estimate its growth over the course of the experiment; this can be important if the animals are younger, and still growing. If one cannot be obtained, create one with a pilot group of animals. No prior handling of the animals is required as testing takes place in the operant box.
      
      Restriction Protocols:
      Water restriction
      Three days prior to the start of the first training session, water is restricted while maintaining the animal at 85% of its ad libitum weight. In mice, this is achieved by providing full access to an average of 4 ml water through a mash. Subjects will also be receiving liquid during testing. During water restriction, subjects have full access to food in the home cages. If the weight of an animal falls too quickly, or falls far below the 85% criterion, put the animal on ad libitum water for at least a day before starting a less restrictive water schedule for that animal.
      
      Food restriction
      Three days prior to the start of the first training session, chow is restricted to maintain the animal at 85% of its ad libitum weight. This is achieved by providing chow once a day about an hour after the testing session. Subjects will also receive food during testing. During food restriction, subjects have full access to water in the home cages. If subjects are young, their normal growth curves should be factored into the updating of ad lib weight. These can be obtained from the animal’s vendor. If the weight of an animal falls too quickly, or falls far below the 85% criterion, put the animal on ad libitum food for at least a day before starting a less restrictive food schedule for that animal.
      
      It is often helpful to introduce the reinforcement before testing begins. Drop some of the reinforcement in their home cage a few days in a row before testing starts to minimize the effect of neophobia.
      
      Test the equipment before testing the animal
      Test each component of the operant box prior to bringing the animals in the test room. If there is any component that is malfunctioning, fix it or replace it prior to testing. Make sure that the pellet dispenser or the dipper (whichever is used) functions appropriately. It is recommended to have a number of additional units available in the lab to be able to replace the broken ones. This box test is repeated for each box every testing day. If pellet dispensers are used, make sure to collect the pellets dropped in the hopper during testing of the equipment before moving to testing the next subject. If there is a problem with one or more units that cannot be fixed or replaced on the same day, test the corresponding animal on the next day and record this event in the lab notebook.
        The most straightforward way to test an operant chamber is to carefully design and write a test protocol using the software that controls the boxes (e.g., MedPC). For example, write a protocol that turns on each output sequentially when some input is triggered. This way, one can distinguish faulty input from output.
   
   
2. Training Phase 1
   2. Place the animal in the operant box and close the doors. Before closing the door, make sure the animal’s tail is not near the latches to prevent injury to the animal. This step is repeated on each test day. At the beginning of testing, animals might be reluctant to get into the operant chamber when the cage is tilted toward the opening of the operant box. Carefully pick the animal up by its tail and place it in the operant chamber.
        Below, we provide two methods for training the animal on a Peak Interval procedure. The first is simpler and usually results in very clean data. The second is more involved, but more safely guards against those few animals who need more, or more careful, training.
        The goal of the simpler method is to start as close to the Peak Interval task as possible, minimizing both the number of training phases and the differences between phases.
        Training starts with Delay Conditioning: A long exponentially distributed intertrial interval begins the session. Longer intertrial intervals, relative to the trial duration, will lead to faster learning (Gibbon and Balsam, 1981; Ward et al., 2013). The trial starts with the onset of a stimulus, usually a light or a tone. The light or tone lasts for the FI used during the eventual peak interval task, then it turns off and a reward is immediately delivered. If the eventual FI is very long (say, several minutes), it may be a good idea to start with a shorter delay and adjust toward the long FI over a few days. Wherever possible, it is better to start with the final FI.
        Then, transition to the Peak Interval task (Step B8 below). Some may want to start with a few days of FI training without peak trials, but it is not strictly necessary. The benefit of starting with a Delay conditioning procedure is twofold: first, it teaches the animal the true FI interval right away, rather than asking the animal to learn it through trial-and-error. Second it allows the researcher to measure anticipatory entries into the food cup as a measure of acquisition.
      
      
      
   Fixed Time 60 s concurrent schedule
   The goal of the less simple method is to take smaller and more incremental steps toward the final peak task. In doing so, it ensures that each animal has completed each step before moving to the next one. This is helpful in a few cases in which the animal is slower to learn, or in cases where the researcher is uncertain about how slow acquisition will be, for example, if working with an unfamiliar strain of mice.
   
   Magazine training
   3. Deliver reward in the corresponding hopper every 60 s (called an FT60 procedure). If the reward is delivered via a dipper, then the dipper stays up for 60 s and the cup is refilled and presented again to the animal. If the liquid reward is presented through peristaltic pump or if it is in pellet form, the reward can accumulate over trials until it is consumed by the animal. This magazine training will last for at least one 20-minute-long session. Note that 0.7 ml of sucrose water indicated in Gür et al., 2019b; 2019c (that were used as the reference studies for this protocol) was the approximate total amount of water received during a Magazine Training session (it was mistakenly expressed as the amount delivered every 40 s).
        If reward is delivered via dipper, it should be presented long enough for the animal to notice and move to the dipper from the Lever (in subsequent the Lever Press Training, the animal will need to do this). If an animal does not spend time in the magazine where dipper is raised, the reinforcement presentation time can be extended. If there are infra-red beam detectors at the feeding hopper, the dipper can be kept available for a fixed duration after the detection of a nose poke.
        After the termination of the session, check for any pellets or liquid reinforcement accumulated in the feeding hopper and remove them (this is not an issue for reward deliveries via dipper). Then check how many times the animal has entered the feeding hopper.
   4. At the end of the session, use a cumulative record to chart the animal’s responses over the session, steeper slopes indicate higher response rates. It is often very clear if (and when) the animal acquired a response. After experience, the researcher can usually tell more quickly and easily by the total number of responses in the session (but a cumulative record is still important to catch inconsistencies in their behavior throughout the session). If needed, repeat the magazine training for that animal on the subsequent day.
      
      
      
   Lever Press Training
   5. The lever is inserted in the operant box and it is retracted either after 60 s or after it is pressed (detected by mechanical switches or break of infrared beam). When one of these events take place, the lever is retracted and the reward is delivered. If the reward is delivered via a dipper, then make sure to leave enough time for the animal to reach it (e.g., 3 s) or rely on the head entry to the feeding hopper as described in Step B3. Stationary levers can also be used, in which case it will be available at all times. The lever press training continues typically for 60 min.
        A crucial point is that lever press training should continue until the animal is reliably pressing the lever. This may be done by examining a raster plot or cumulative record; it may be done by a statistical test, say, on the difference in the number of responses per trial against the number of responses in the intertrial interval; or it may be done with a simple criteria: continue until the animal presses the lever for at least a number of times (e.g., 40 or 60), or obtains a certain amount of food, in an hour-long session for two subsequent sessions.
        Some subjects may take longer to train than others. There are a few simple techniques for helping them along. First, remove the FT component, and introduce a Fixed Ratio schedule in which every lever press is reinforced (FR1). If this doesn’t help, try shaping the lever press response by hand while the FR1 schedule is in effect. If this fails, it may be time to exclude the animal from the study; it may be an outlier. In this case, record the details in a laboratory notebook, and remember to report it in the paper. In our experience, subjects rarely need more than 2-3 sessions of FR training, and rarely need more than 2 sessions of shaping.
      
      
      
   Nose poke training
   6. The nose-poke hopper is lit and it is turned off either after 60 s or after the subject makes a nose-poke (detected by the break of infrared beam). When one of these events take place, the light is turned off and the reward is delivered. If the reward is delivered via a dipper, then make sure to leave enough time for the animal to reach it (e.g., 3 s) or rely on the head entry to the feeding hopper as described in Step B3. The nosepoke training continues typically for 60 min. The nose-poke training continues until the animal nosepokes a number of times (e.g., 40 or 60) in a session for two subsequent sessions. Either Step B5 or Step B6 will be realized based on the choice of the experimenter regarding the form of response.
      
   
   Fixed Interval (FI) Training
   7. A trial is initiated with the presentation of the discriminative stimulus. The stimulus stays for a minimum of the FI and it is turned off contingent upon the first response following FI. The first response following the FI is reinforced (some also require that the response occur earlier than 3xFI). The experimenter might choose to present the stimulus longer than 3xFI, but this typically not necessary. After each trial, an exponentially distributed inter-trial interval (ITI) is introduced. This is important to reduce how predictable the next trial is. This training phase, including only simple Fixed Interval trials, is usually conducted daily for at least 5 sessions before introducing peak trials. Use raster plots to track their acquisition. The data should be studied on a daily basis with simple visual or statistical summaries that showcase the most raw form of the data possible. If a scalloped responding is not observed in any of the subjects, FI training can be extended. Figure 2A illustrates the typical pattern of responding observed in FI trials.
      
      
      Figure 2. Illustration of fixed interval and peak interval trials and the typical form of responding that is observed in these two different trial types
      
      
      
   Peak Interval (PI) Testing
   8. Peak Interval (PI) trials are randomly mixed with FI trials. In PI trials, the stimulus lasts at least three times longer than the FI and the reinforcement is omitted, regardless of the animal’s behavior. The proportion of FI to PI trials can change between 1:1 to 1:3. A lower proportion of peak trials will facilitate the acquisition of peak interval responding but because we typically focus on PI trials in analysis, this will also result in less data per session (Kaiser, 2008 and 2009). The duration of the PI trials can also be varied between trials or sessions (but at least 3xFI). Figure 2B illustrates the typical pattern of responding observed in PI trials.
   9. After each session, gently remove the animal from the operant box and place it promptly either in the holding case or the home cage available nearby. Wipe the grid with a 70% ethanol solution to minimize olfactory cues between different sessions. Provide supplemental food or water about an hour after the test session.
   Figure 3 summarizes the key steps of the PI procedure in the form of a decision-tree to be used during the implementation of the protocol.
   
   
   Figure 3. An example illustration of the peak interval testing steps and contingencies
   Note: The criteria for switching between different states can be decided on by the experimenter. This figure will appear as an animation when viewed here through a web browser.
   

Data analysis

Analytical Approaches:
Analysis of the peak response curve

1. Peak response curves can be characterized by fitting unimodal distribution functions (such as Gaussian, Gamma, Inverse Gaussian, Weibull) to the averaged response curve. The choice of the function to be used to characterize the response curve can be based on measures of goodness of fit (e.g., omega squared; log likelihood), or based on theoretical considerations (Guilhardi et al., 2007). The mode of the function fit to the data can be used as an estimate of the peak time, whereas its coefficient of variation (CV) can be used as an estimate of temporal precision.
     The same estimates can be gathered using nonparametric techniques, that is, without fitting a function to the data. For example, one could choose to report the location of the amplitude of the response gradient as the peak time and the difference between the locations of a particular value prior and after the peak as the spread. Because the data is often a little noisy, one typically uses algorithms that smooth it out in order to find the peak or spread (e.g., Roberts, 1981; Balcı et al., 2009c). Some have averaged multiple points on the response gradient (e.g., 60%, 70%, 80% of the amplitude; Balcı et al., 2008; Figure 1). Make sure that the smoothing algorithm does not introduce bias in the estimates. Figure 4B shows an example response gradient from one mouse (averages are shown in blue and smoothened data are shown in red).
   
   
   

1. The primary aim of the individual trial analysis is to estimate the trial time at which animals shift from break to a run (start time) and the run back to a break (stop time). A number of approaches have been suggested in the literature. In this paper, we present one of the most widely used methods originally proposed by (Church et al., 1994).
     This method segments the single trial data into three periods: The period before the run–the first break–should have lower-than-average response rates; the run should have higher-than average response rates; and the second break should again show lower-than-average response rates. Any other pattern of behavior does not follow the traditional break-run-break pattern, although a small proportion of various other patterns do occur over the course of an experiment. Let r be the overall response rate in that trial, r₁ be the response rate of the (potential) first segment, and t₁ be the duration of the first segment. The quantity t₁ (r-r₁) measures how low the response rate was during the first break relative to the average, scaled by how long the break lasted. Graphically, it’s the area of a rectangle of width t₁ and height (r-r₁). Similar computations will be applied to the run period t₂ (r₂-r) and the second break period t₃ (r-r₃). Thus, maximizing t1(r – r1) + t2(r2 – r) + t3(r – r3) will generally provide a good estimate of the start and stop times. Figure 4A shows the raster plot of data from one mouse, where blue lines correspond to nose pokes, green circles show the start times and red circles show the stop (in terms of 1 s bins).
   
   
   Figure 4. Example mouse data. A. Example raster plots (nose pokes) in peak interval trials (steady state) from one mouse at steady state (data pooled across 10 sessions). B. Example response gradient (blue) from the same mouse with the smoothened response gradient (red) superposed on the average response gradient. Dotted vertical lines show the reward availability time (FI = 15s). Data come from a pilot study conducted at Dalhousie University (unpublished) with a 2 month of Nrxn1^+/- mouse on a B57BL/6J background. The actual experiment is ongoing. All procedures were approved by Dalhousie University Committee on Laboratory Animals (protocol #18-096).
   
   Another approach uses Maximum likelihood methods to find the start and stop times. Here, the researcher makes parametric assumptions about the way the animals respond in each segment, for example, that the interresponse times are always exponentially distributed, but shorter during a run than a break. The start and stop times correspond to the response times that maximize the likelihood of the data assuming three segments split at those start and stop points (rather than split somewhere else or not at all) (e.g., Balcı et al., 2009c). The main advantage of this approach over non-parametric ones is that it uses likelihood ratios to detect changes. This way, one can remove the trials without a clearly defined break-run-break pattern. The researcher is free to choose what constitutes a clear break-run-break pattern by choosing a looser or more strict likelihood ratio. This criterion should be justified and reported in the paper. Lastly, Bayesian methods can be used in place of maximum likelihood estimates. This gives the full posterior distribution for the start and stop times, which generally provides more information than point estimates alone.
     In order to avoid those trials in which animals did not show temporal control in their responding, and provided that they constitute only a fraction of the trials, researchers might choose to further exclude those trials in which the start time is longer than the FI or the stop time is shorter than the FI. It is important that this exclusion choice is made prior to data analysis. We find it useful to exclude trials where start-stop spread is smaller than 5% of the FI. The corresponding estimates gathered from single trial data and response curves have been shown to be correlated (e.g., Balcı et al., 2008), which should also be checked in studies that employ both methods of analysis.

Acknowledgments

Work that the current protocol was based on was supported by the Turkish Academy of Sciences (GEBİP 2015 award to Fuat Balcı), by the Natural Sciences and Engineering Research Council of Canada (Research grant 7441 to Richard E Brown). The Scientific and Technological Research Council of Turkey (TÜBİTAK BİDEB 2211E to Ezgi Gür).
  The protocol was derived from a number of previous studies (Balcı et al., 2008 and 2009c; Gür et al., 2019b and 2019c).

Competing interests

The authors declare that they have no conflict of interest.

Ethics

All procedures were approved by the Institutional Animal Care and Use Committee at PsychoGenics, Inc and Dalhousie University Committee on Laboratory Animals (protocol #15-097 & protocol #18-096).

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