Effects of breaks in the interval cycle on temporal tracking in pigeons.

Two experiments examined the effects of inserting a break in a cyclic interval schedule on the temporal control of keypecking responses in pigeons. In Experiment 1, pigeons were exposed to intervals that changed from 45 to 15s and returned to 45 s. A break was inserted between the last 15-s and following 45-s interval and was in effect for either 0, 60, or 120 s. Either a blackout of lights in the test chamber or turning off the response key alone signaled breaks. In Experiment 2, we examined the effects of a wider range of breaks-0, 120, and 360 s. Post-reinforcement pause (PRP) tracked changes in the interval requirement across all conditions. However, breaks in the schedule, even one lasting 360 s, did not disrupt the overall time course of responding. The only effect that a break had on temporal performance was an elevation in the rate of responding and a shorter PRP in the interval following a break. The results suggest that breaks did not affect the birds' memory for short intervals, and that the momentary increase in responding may be related to the reinforcement omission effect.

Interval timing in Siamese fighting fish (Betta splendens).

The present study evaluated the temporal performance of Siamese fighting fish (Betta splendens) given short-term exposure to four fixed interval (FI) schedules of reinforcement, FI 30, 60, 120, and 240 s, during which a reinforcer (mirror image) was given for the first response (swimming through a hoop) after the interval requirement had elapsed. Response levels were generally low early in an interval and increased as the interval elapsed; wait times and break points in an interval increased with increases in the FI requirement. The results were similar to that obtained with other species and different types of responses and reinforcers, and demonstrate that the procedure is a feasible method for studying interval timing in fish.

Habituation, memory and the brain: the dynamics of interval timing.

Memory decay is rapid at first and slower later-a feature that accounts for Jost's memory law: that old memories gain on newer ones with lapse of time. The rate-sensitive property of habituation-that recovery after spaced stimuli may be slower than after massed-provides a clue to the dynamics of memory decay. Rate-sensitive habituation can be modeled by a cascade of thresholded integrator units that have a counterpart in human brain areas identified by magnetic source imaging (MSI). The memory trace component of the multiple-time-scale model for habituation can provide a 'clock' that has the properties necessary to account for both static and dynamic properties of interval timing: static proportional and Weber-law timing as well as dynamic tracking of progressive, 'impulse' and periodic interval sequences.

A tuned-trace theory of interval-timing dynamics.

Animals on interval schedules of reinforcement can rapidly adjust a temporal dependent variable, such as wait time, to changes in the prevailing interreinforcement interval. We describe data on the effects of impulse, step, sine-cyclic, and variable-interval schedules and show that they can be explained by a tuned-trace timing model with a one-back threshold-setting rule. The model can also explain steady-state timing properties such as proportional and Weber law timing and the effects of reinforcement magnitude. The model assumes that food reinforcers and other time markers have a decaying effect (trace) with properties that can be derived from the rate-sensitive property of habituation (the multiple-time-scale model). In timing experiments, response threshold is determined by the trace value at the time of the most recent reinforcement. The model provides a partial account for the learning of multiple intervals, but does not account for scalloping and other postpause features of responding on interval schedules and has some problems with square-wave schedules.

The choose-short effect and trace models of timing.

The tuned-trace multiple-time-scale (MTS) theory of timing can account both for the puzzling choose-short effect in time-discrimination experiments and for the complementary choose-long effect. But it cannot easily explain why the choose-short effect seems to disappear when the intertrial and recall intervals are signaled by different stimuli. Do differential stimuli actually abolish the effect, or merely improve memory? If the latter, there are ways in which an expanded MTS theory might explain differential-context effects in terms of reduced interference. If the former, there are observational and experimental ways to determine whether differential context favors prospective encoding or some other nontemporal discrimination.

Time and memory: towards a pacemaker-free theory of interval timing.

A popular view of interval timing in animals is that it is driven by a discrete pacemaker-accumulator mechanism that yields a linear scale for encoded time. But these mechanisms are fundamentally at odds with the Weber law property of interval timing, and experiments that support linear encoded time can be interpreted in other ways. We argue that the dominant pacemaker-accumulator theory, scalar expectancy theory (SET), fails to explain some basic properties of operant behavior on interval-timing procedures and can only accommodate a number of discrepancies by modifications and elaborations that raise questions about the entire theory. We propose an alternative that is based on principles of memory dynamics derived from the multiple-time-scale (MTS) model of habituation. The MTS timing model can account for data from a wide variety of time-related experiments: proportional and Weber law temporal discrimination, transient as well as persistent effects of reinforcement omission and reinforcement magnitude, bisection, the discrimination of relative as well as absolute duration, and the choose-short effect and its analogue in number-discrimination experiments. Resemblances between timing and counting are an automatic consequence of the model. We also argue that the transient and persistent effects of drugs on time estimates can be interpreted as well within MTS theory as in SET. Recent real-time physiological data conform in surprising detail to the assumptions of the MTS habituation model. Comparisons between the two views suggest a number of novel experiments.

Interval schedule performance in the goldfish Carassius auratus.

In experiment 1, five goldfish (Carassius auratus) paddle-pressed on fixed-interval (FI) and variable-interval (VI) schedules for food pellet reinforcement. The order of conditions was FI 60 s, FI 240 s, FI 30 s, FI 60 s, and VI 60 s. FI responding showed a scalloped pattern and response-rate break points were proportional to interval duration. Post-food wait times varied with interval duration, but were not proportional. Response rate on VI was constant. Experiment 2 studied the properties of food reinforcement as a time marker. The same five fish were presented an FI 60 s schedule of reinforcement with 25% of intervals ending in non-reinforcement (N). The fish responded faster and paused less following the omission stimulus (omission effect) and response rate was flat or declined through post-N intervals.

Temporal control in rats: analysis of nonlocalized effects from short interfood intervals.

The present experiment analyzed temporal control of postreinforcement pause duration during within-session changes in the criterion for reinforcement (interfood interval, IFI). Analysis of interval-by-interval changes in the pause revealed localized and nonlocalized effects from short intervals that caused specific changes in performance. In Phase 1, rats were presented with five consecutive 15-s IFIs intercalated into a series of 60-s IFIs. The 15-s set decreased the pause in adjacent and more remote 60-s intervals. In Phase 2, two sets of 15-s intervals were intercalated. The spacing between the two sets varied so that 0, 5, 10, or 15 60-s IFIs separated the sets. The postreinforcement pause tracked all changes in the IFI duration, and the localized effect from a short set extended beyond the next interval to the next few 60-s IFIs. Effects from one set, however, did not combine with a second set: Changes in the pause after two sets were the same regardless of the spacing between sets.

Dynamics of temporal control in rats: the effects of a brief transition in interval duration.

Five rats lever-pressed for liquid reinforcers delivered according to a fixed-interval (FI) reinforcement schedule, where the interval requirement changed at an unpredictable point within a session. In a short square wave (SSW) condition, eight 30-s intervals were intercalated in a series of 120-s intervals so that the intervals changed from 120 to 30 s then back to 120 s. In a long square wave (LSW) condition the intervals changed from 120 to 480 s then back to 120 s. We observed rapid temporal control of post-reinforcement wait time duration by the IFI duration in the SSW condition only: Wait times decreased significantly during a transition to shorter (30 s) intervals; whereas wait times did not reliably increase during a transition to longer (480 s) intervals. Furthermore, in the SSW condition, wait time in post-transition intervals was shorter than that observed during pre-transition intervals. The results show that rats' wait times are sensitive to moment-to-moment changes in interval duration and that the dynamics depend on the direction in which the intervals change.

Multiple time scales in simple habituation.

Habituation is the waning of a reflex response to repeated stimulation. Habituation to closely spaced stimuli is faster and more complete than to widely spaced stimuli, but recovery is also more rapid (rate sensitivity). We show that a 2-unit, cascaded-integrator dynamic model can explain in detail an extensive data set on rate-sensitive habituation in the nematode Caenorhabditis elegans. Many apparently complex properties of habituation and learning dynamics may reflect interactions among a small number of processes with different time scales.

Dynamics of time discrimination: II. The effects of multiple impulses.

According to a diffusion generalization model, time discrimination is determined by the frequency and recency of preceding intervals of time. A procedure for studying rapid timing was used to investigate whether pigeons' wait-time responses were sensitive to these factors. In Experiment 1 the number (two or eight) and spacing (consecutive or far apart) of 5-s interfood intervals (called impulses) intercalated in a series of 15-s interfood intervals (nonimpulses) were studied. Experiment 2 was identical to the first but the interfood intervals were increased by a factor of three. Overall, impulses shortened wait times in the next interfood interval. However, several impulses occurring in succession extended the localized effect of an impulse: Wait times following a set of eight-close impulses were slow to recover to preimpulse levels. The results show that linear waiting is only an approximation to the dynamic process, and a process that is sensitive to events in an animal's remote past, such as the diffusion generalization model, provides a better account of rapid timing effects.

Pigeons' wait-time responses to transitions in interfood-interval duration: Another look at cyclic schedule performance.

Recent developments reveal that animals can rapidly learn about intervals of time. We studied the nature of this fast-acting process in two experiments. In Experiment 1 pigeons were exposed to a modified fixed-time schedule, in which the time between food rewards (interfood interval) changed at an unpredictable point in each session, either decreasing from 15 to 5 s (step-down) or increasing from 15 to 45 s (step-up). The birds were able to track under both conditions by producing postreinforcement wait times proportional to the preceding interfood-interval duration. However, the time course of responding differed: Tracking was apparently more gradual in the step-up condition. Experiment 2 studied the effect of having both kinds of transitions within the same session by exposing pigeons to a repeating (cyclic) sequence of the interfood-interval values used in Experiment 1. Pigeons detected changes in the input sequence of interfood intervals, but only for a few sessions-discrimination worsened with further training. The dynamic effects we observed do not support a linear waiting process of time discrimination, but instead point to a timing mechanism based on the frequency and recency of prior interfood intervals and not the preceding interfood interval alone.

"Transitive inference" in multiple conditional discriminations.

We used multiple conditional discriminations to study the inferential abilities of pigeons. Using a five-term stimulus series, pigeons were trained to respond differentially to four overlapping pairs of concurrently presented stimuli: A+ B-, B+ C-, C+ D-, and D+ E-, where plus and minus indicate the stimulus associated with reinforcement and extinction, respectively. Transitive inference in such situations has been defined as a preference for Stimulus B over Stimulus D in a transfer test. We measured this and other untrained preferences (A vs. C, A vs. D, B vs. E, etc.) during nonreinforced test trials. In three experiments using a novel, rapid training procedure (termed autorun), we attempted to identify the necessary and sufficient conditions for transitive inference. We used two versions of autorun: response-based, in which the subject was repeatedly presented with the least well-discriminated stimulus pair; and time-based, in which the subject was repeatedly presented with the least-experienced stimulus pair. In Experiment 1, using response-based autorun, we showed that subjects learned the four stimulus pairs faster than, but at a level comparable to, a previous study on transitive inference in pigeons (Fersen, Wynne, Delius, & Staddon, 1991), but our animals failed to show transitive inference. Experiments 2 and 3 compared time- and response-based autorun. Discrimination performance was maintained, but transitive inference was observed only on the second exposure to the response-based procedure. These results show that inferential behavior in pigeons is not a reliable concomitant of good performance on a series of overlapping discriminations. The necessary and sufficient conditions for transitive inference in pigeons remain to be fully defined.

Dynamics of time discrimination.

Pigeons tracked sinusoidal sequences of interfood intervals (IFIs) by pausing in each interval for a time proportional to the preceding interval. Schedules with either long (30-90 s) or short (5-15 s) values, with variable numbers of cycles and starting phase each day, were tracked about equally well. Tracking was apparently immediate and did not improve across sessions. Experiment 2, in which long and short series were presented on alternate days, showed that tracking on long was more impaired than on short. Experiment 3 showed that occasional presentation of a short IFI in a series of fixed, longer IFIs caused a reduction in waiting time in the next IFI. These effects are evidence for a fast-acting timing mechanism in which waiting time in the IFI N + 1 is strongly determined by the preceding IFI, N. Earlier IFIs have some cumulative effect, but the details remain to be elucidated.

Discrete and continuous measures of dimensional stimulus control.

In two sets of experiments, we examined dimensional stimulus control of pigeons' responses to a visual flicker-rate continuum. In the first experiment, responses to a single key were reinforced periodically during stimuli from one half of the stimulus continuum, and responses during other stimuli were extinguished. In the second experiment, two response keys were simultaneously available, with reinforcement for each response alternative associated with different halves of the stimulus continuum. Conditions of the second experiment involved either free-operant or discrete-trial stimulus presentations. Results from these experiments show that positive dimensional contrast appeared in discrimination tasks with one or two response alternatives, but only with free-operant procedures. In addition, discrimination between stimulus classes established by differential reinforcement was assessed as accurately by continuous rate measures as by discrete response choice in the two-alternative situation. The general implication of these experiments is that response rate measures, when properly applied, may reveal sources of variation within stimulus classes, such as dimensional contrast, that are not evident with discrete measures.

Massed and spaced stimulus sequences in dimensional discrimination.

In two experiments, pigeons' responding to a visual flicker-rate continuum was established by a maintained generalization procedure. For both experiments, variable-interval reinforcement was available for responses during stimuli from one half of the stimulus continuum while responses during other stimuli were extinguished. The first experiment compared gradients of dimensional stimulus control produced by randomly presented positive and negative stimuli with gradients produced by massed positive and negative stimuli. The second experiment alternated the order of massed stimulus sequences. In both cases, massing of stimulus sequences diminished positive dimensional contrast effects, without seriously disturbing discrimination between positive and negative stimuli. The results indicate that stimulus sequences can have an important role in the production of dimensional contrast effects.

Behavioral contrast in rats when qualitatively different reinforcers are used.

Behavioral contrast was studied during multiple schedules which provided qualitatively different reinforcers in the two components. Five rats responded on a baseline schedule in which both components delivered food reinforcers (food-food), and then on a contrast schedule in which one component delivered food and the other delivered water (food-water). Following this, baseline was recovered. Five other rats responded on a baseline schedule in which both components delivered water reinforcers (water-water), then on a food-water schedule, and then on the baseline, water- water, schedule. Contrast was not observed when relatively low rates of reinforcement were use but it was sometimes observed when high rates of reinforcement were used. The rate of responding for a constant water reinforcer decreased when food replaced water in the other component. The rate of responding for a constant food reinforcer did not change when a water reinforcer replaced food in the other component. These results are similar to those reported by Ettinger and McSweeney (1981) when pigeons served as subjects.