Simultaneous temporal processing

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

Timing for the absence of a stimulus: the gap paradigm reversed

Contrary to data showing sensitivity to nontemporal properties of timed signals, current theories of interval timing assume that animals can use the presence or absence of a signal as equally valid cues as long as duration is the most predictive feature. Consequently, the authors examined rats' behavior when timing the absence of a visual or auditory stimulus in trace conditioning and in a "reversed" gap procedure. Memory for timing was tested by presenting the stimulus as a reversed gap into its timed absence. Results suggest that in trace conditioning (Experiment 1), rats time for the absence of a stimulus by using its offset as a time marker. As in the standard gap procedure, the insertion of a reversed gap was expected to "stop" rats' internal clock. In contrast, a reversed gap of 1-, 5-, or 15-s duration "reset" the timing process in both trace conditioning (Experiment 2) and the reversed gap procedure (Experiment 3). A direct comparison of the standard and reversed gap procedures (Experiment 4) supported these findings. Results suggest that attentional mechanisms involving the salience or content of the gap might contribute to the response rule adopted in a gap procedure.     

Role of the hippocampus in blocking and conditioned inhibition of the rabbit's nictitating membrane response.

Bilateral aspiration of the dorsal hippocampus produced a disrupttion of blocking of the rabbit's nictitating membrane response in Kamin's two-stage paradigm (Experiment 1) but had no effect on the formation of a Pavlovian conditioned inhibitor (Experiment 2). The results of Experiment 1 indicated that normal animals and those with cortical lesions given conditioning to a light-plus-tone conditioned stimulus (CS) gave conditioned responses (CRs) to both the light and the tone during nonreinforced presentations of each (test phase). If, however, compound conditioning was preceded by tone acquisition, only the tone elicited a CR during testing; that is, blocking was observed. In rabbits with hippocampal lesions, however, CRs were given to both the light and the tone during testing whether or not compound conditioning was preceded by tone acquisition. The data from Experiment 2 showed that rabbits with hippocampal lesions could discriminate as well as normal rabbits and those with cortical lesions between a light (CS+) and light plus tone (CS-). In addition, when the inhibitory tone was subsequently paired with the unconditioned stimulus in retardation testing, animals in all three lesion conditions acquired the CR at the same rate. Thus, it appears that hippocampal lesions do not disrupt conditioned inhibition. The results of these experiments were taken as support for the view that the hippocampus is responsible for "tuning out" stimuli that have no adaptive value to the organism.     

Timing of Conditioned Responses Utilizing Electrical Stimulation in the Region of the Interpositus Nucleus as a CS

A large body of evidence indicates that the cerebellum is essential for the acquisition, retention, and expression of the standard delay conditioned eyeblink response and that the basic memory trace appears to be established in the anterior interpositus nucleus (IP). Adaptive timing of the conditioned response (CR) is a prominent feature of classical conditioning-the CR peaks at the time of onset of the unconditioned stimulus (US) over a wide range of CS-US interstimulus intervals (ISI). A key issue is whether this timing is established by the cerebellar circuitry or prior to the cerebellum. In this study timing of conditioned eyeblink responses established via electrical stimulation of the interpositus nucleus as a conditioned stimulus (CS) was analyzed prior to and following modification of the CS-US interval in well-trained rabbits. Consistent with previous results, learning under these conditions is very rapid and robust. The CR peak eyeblink latencies are initially timed to the US onset and adjust accordingly to lengthening or shortening of the CS-US interval, just as with peripheral CSs. The acquisition of conditioned eyeblink responses by direct electrical stimulation of the IP as a CS thus retains temporal flexibility following shifts in the CS-US delay, as found in standard classical eyeblink conditioning procedures.     

Timing of conditioned responses utilizing electrical stimulation in the region of the interpositus nucleus as a CS

A large body of evidence indicates that the cerebellum is essential for the acquisition, retention, and expression of the standard delay conditioned eyeblink response and that the basic memory trace appears to be established in the anterior interpositus nucleus (IP). Adaptive timing of the conditioned response (CR) is a prominent feature of classical conditioning—the CR peaks at the time of onset of the unconditioned stimulus (US) over a wide range of CS-US interstimulus intervals (ISI). A key issue is whether this timing is established by the cerebellar circuitry or prior to the cerebellum. In this study timing of conditioned eyeblink responses established via electrical stimulation of the interpositus nucleus as a conditioned stimulus (CS) was analyzed prior to and following modification of the CS-US interval in well-trained rabbits. Consistent with previous results, learning under these conditions is very rapid and robust. The CR peak eyeblink latencies are initially timed to the US onset and adjust accordingly to lengthening or shortening of the CS-US interval, just as with peripheral CSs. The acquisition of conditioned eyeblink responses by direct electrical stimulation of the IP as a CS thus retains temporal flexibility following shifts in the CS-US delay, as found in standard classical eyeblink conditioning procedures.     

The role of response-reward correlation in stimulus-response overshadowing

Rats pressed levers for food reward which was delivered, when appropriate, 0·4 s after the response. For one group, the delay interval was filled by a light cue; for the other group, the same number of lights was given but they were not correlated with food delivery. In Experiment I, all lever presses were reinforced and there were no differences in response rate between groups. In Experiments II and III, lever pressing was rewarded according to a VI and VR schedule respectively. Group differences were observed in Experiment II but they disappeared in Experiment III. The results of Experiments I and II show that a reward-related stimulus does not overshadow a lever response unless the stimulus is a better predictor of reward. Differences in salience or competition from sign-tracking behaviors were ruled out as causes of this phenomenon. Experiment III demonstrated, however, that a weak response-reward correlation is not a sufficient condition for the overshadowing effect. A fourth experiment replicated the results of Experiment III using naive animals. The results of these last two experiments are not consistent with an information theory approach unless (a) a response-units concept is adopted or (b) the cue involved in overshadowing is not the pre-food light but the end of a temporal interval, whose salience is enhanced by the light.     

The effects of spatial stimulus-response compatibility on choice time production accuracy and variability

Five experiments examined the relations between timing and attention using a choice time production task in which the latency of a spatial choice response is matched to a target interval (3 or 5 s). Experiments 1 and 2 indicated that spatial stimulus-response incompatibility increased nonscalar timing variability without affecting timing accuracy and that choice reaction time practice reduced choice time production variability. These data support a "temporal discounting" model in which response choice and timing occur in series, but the interval timed is shortened to account for nontemporal processing. In Experiment 3, feedback and anticipation task demands improved choice time production accuracy. In Experiments 4 and 5, the delay between the start-timing and choice-decision signals interacted with choice difficulty to affect choice time production accuracy and variability when timing a 3- but not a 5-s interval, suggesting that attention mediates timing before and after an interruption in timing.     

Short-Term Retention of “Surprising” Events By Pigeons

Terry and Wagner (1975) have suggested that the short-term retention of information about an event is enhanced if the occurrence of the event is made unexpected or surprising. Three experiments tested this idea using delayed conditional position (Experiment I) and colour discriminations (Experiments II and III). The subjects were pigeons and the presentation of food was the target event to be remembered. Choice of one of two simultaneously presented stimuli was reinforced if the retention interval had been initiated by presentation of target food. Contrary to Terry and Wagner's claim, retention was superior on probe test trials in Experiments I and II if the presentation of the target food was preceded by a previously established signal for food (CS+) rather than by a stimulus which had not been paired with food (CS-). Experiment III systematically manipulated the signalling conditions used during initial discrimination training. Retention was better following a CS+ presentation if the target food had been signalled during initial discrimination training but worse if the food presentation had been unsignalled. These results do not favour the idea that retention of an event is directly affected by whether or not the target event is surprising.     

Generalization and response latency

The present experiments investigated generalization in a reaction time situation where the generalization stimulus, a tone, preceded the reaction time signal, a light. The hypotheses under investigation were that the duration of the cue stimulus would determine the degree of generalization (Experiment I) and that the response latency independent of the stimulus duration would be related to the amount of generalization (Experiment II). A particular generalization test stimulus (a tone of 40, 45, 50, 60, 65, or 70 dB) was presented only once always following two bar-pressing responses to training stimulus (tone of 55 dB) under each of two conditions of stimulus duration in Experiment I and under each of two conditions of response latency in Experiment II. It was found that under the condition of short response latency generalization was broader.     

Second-order contrast based on the expectation of effort and reinforcement

Pigeons prefer signals for reinforcement that require greater effort (or time) to obtain over those that require less effort to obtain (T. S. Clement, J. Feltus, D. H. Kaiser, & T. R. Zentall, 2000). Preference was attributed to contrast (or to the relatively greater improvement in conditions) produced by the appearance of the signal when it was preceded by greater effort. In Experiment 1, the authors of the present study demonstrated that the expectation of greater effort was sufficient to produce such a preference (a second-order contrast effect). In Experiments 2 and 3, low versus high probability of reinforcement was substituted for high versus low effort, respectively, with similar results. In Experiment 3, the authors found that the stimulus preference could be attributed to positive contrast (when the discriminative stimuli represented an improvement in the probability of reinforcement) and perhaps also negative contrast (when the discriminative stimuli represented reduction in the probability of reinforcement).     

Stimulus motion and retrospective time judgments

The effect of stimulus motion on retrospective time judgments was investigated in four experiments. Subjects reproduced the duration of a 32-s interval which was filled by either a stationary or moving visual element presented on a computer monitor. In Experiments 1 and 4, the element moved horizontally back and forth, and in Experiments 2 and 3 it traced a circular pathway. In Experiments 1 and 2, the element moved at speeds of either 5 or 20 cm/s. In Experiment 3, it moved at a constant speed, alternating direction between clockwise and anti-clockwise rotation once every 1, 4, 8 or 16 s. In Experiment 4 the element moved at linear speeds of 1, 2, 4, 8 or 16 cm/s back and forth along a 16 cm horizontal path thereby alternating between left- and rightward motion-directions once every 16, 8, 4, 2 and 1 s, respectively. Temporal reproductions were not systematically influenced by stimulus speed. Rather, the pattern of results indicated a nonmonotonic relationship between remembered duration and the frequency of motion-direction changes; whereas remembered duration was unaffected by either infrequent or very frequent rates of changes, moderate rates of motion-changes lengthens remembered duration. These findings are discussed in relation to the change models of retrospective timing, and the claim that stimulus speed, as distinct from changes in the direction of stimulus motion, is not an important determinant of retrospective timing.     

Generalization and response latency

The present experiments investigated generalization in a reactio? time situation where the generalization stimulus, a tone, preceded the reaction time signal, a light. The hypotheses under investigation were that the duration of the cue stimulus would determine the degree of generalization (Experiment I) and that the response latency independent of the stimulus duration would be related to the amount of generalization (Experiment II). A particular generalization test stimulus (a tone of 40, 45, 50, 60, 65, or 70 dB) was presented only once always following two bar-pressing responses to training stimulus (tone of 55 dB) under each of two conditions of stimulus duration in Experiment I and under each of two conditions of response latency in Experiment II. It was found that under the condition of short response latency generalization was broader.     

Effects of signaling on temporal control of behavior in response‐initiated fixed intervals

Behavior and events distributed in time can serve as markers that signal delays to future events. The majority of timing research has focused on how behavior changes as the time to some event, usually food availability, decreases. The primary objective of the two experiments presented here was to assess how behavior changes as time passes between two time markers when the first time marker was manipulated but the second, food delivery, was held constant. Pigeons were exposed to fixed‐interval, response‐initiated fixed‐interval, and signaled response‐initiated fixed‐interval 15‐ and 30‐s schedules of reinforcement. In Experiment 1, first‐response latencies were systematically shorter in the signaled response‐initiated schedules than response‐initiated schedules, suggesting that the first response was a more effective time marker when it was signaled. In Experiment 2, responding in no‐food (i.e. “peak”) trials indicated that timing accuracy was equivalent in the three schedule types. Compared to fixed interval schedules, timing precision was reduced in the signaled response‐initiated schedules and was lowest in response‐initiated schedules. Results from Experiments 1 and 2 coupled with previous research suggest that the overall “informativeness” of a time marker relative to other events and behaviors in the environment may determine its efficacy.     

Circadian time perception

The variability of anticipating a meal was investigated. Sprague-Dawley rats earned food by inspecting a food source during a 3-hr interval. Food was not available at other times. In Experiment 1, the meal started 3 or 7 hr after light offset in a 12-hr light-dark cycle. Experiment 2 was conducted in constant darkness with 14-, 22-, 22.5-, 24-, 25.5-, 26-, or 34-hr intermeal intervals. Inspections increased before the meal. Rats timed intervals in the circadian range (22-26 hr) with lower variability than that for intervals outside this range (3-14 and 34 hr). Higher precision in timing selected intervals violates the scalar property. Proximity to a circadian oscillator improves timing precision. Variability may be used to identify oscillators with noncircadian periods.     

Obligatory “expectations” of expressive timing induced by perception of musical structure

Previous research has shown that, in a task requiring the detection of local deviations from mechanically precise timing in music, the relative detectability of deviations in different positions is closely related to the typical expressive timing pattern musicians produce when playing the music. This result suggests that listeners expect to hear music expressively timed and compensate for the absence of expressive timing. Three new detection experiments shed additional light on the nature of these timing expectations in musically trained listeners. Experiment 1 shows that repeated exposure to an atypically (but not unmusically) timed performance leaves listeners' timing expectations unaffected. Experiment 2 demonstrates that the expectations do not manifest themselves when listeners merely imagine the music in synchrony with a click track. Experiment 3, however, shows that the timing expectations are fully operational when the click track is superimposed on the music. These results reveal timing “expectations” to be an obligatory consequence of the ongoing auditory perception of musical structure. Received: 5 November 1996 / Accepted: 26 February 1997     

Enhancement of food-rewarded instrumental responding by an appetitive conditioned stimulus

Four experiments are reported in which a stimulus (with a minimum duration of 60 s) signalling the delivery of “free” food was presented to rats lever-pressing for food available on a variable interval schedule. It was found that responding was enhanced in the presence of the stimulus when the baseline schedule of reinforcement was lean (Experiment I) and that the enhancement was dependent upon the pairing of the stimulus with free food (Experiments II and III). Experiment IV showed that an enhancement could be found after initial training in which stimulus-food pairings were given to subjects that were not concurrently lever pressing for food. It is argued that these results are consistent with the suggestion that an appetitive conditioned stimulus can energise appetitive instrumental behaviour.     

The Effect of Nontemporal Information Processing on Time Estimation in Pigeons

Pigeons' ability to time a stimulus while simultaneously engaged in another information-processing task was examined in three experiments using a matching-to-sample procedure. Pigeons were trained to match temporal samples of 2 and 10 s and were tested with durations of 2, 3, 4.5, 6.7, or 10 s while simultaneously processing information from another dimension. Experiment 1 revealed that the psychophysical curve for long judgments taken when pigeons also had to evaluate line orientation was shifted to the right of a control curve taken from trials on which only time had to be judged. In Experiment 2, results from probe duration tests showed that processing the spatial location of a stimulus while attending to duration caused a more general loss of timing ability across the probe durations (shorter durations were judged as longer and longer durations were judged as shorter). In Experiment 3, a distracter light was illuminated on some probe trials to determine how such a perceptual distraction would affect time judgments on the probe trials. Results showed a general loss of timing ability similar to that found in Experiment 2. It is proposed that the data might best be explained by a divided-attention effect rather than by a systematic effect on the timing mechanism.     

Effect of classical conditioning on an internal clock

Six experiments with rats used a psychophysical choice procedure to study the internal clock used to discriminate duration. They investigated if the clock is sensitive to the signal value (associative strength) of a stimulus. The experiments involved two types of trials. On choice trials, a stimulus lasted a short (e.g., 3 s) or long (e.g., 12 s) duration; then the rats chose between two levers. The rewarded choice depended on the duration of the stimulus. On conditioning trials, the stimulus used on choice trials was presented, but it ended without food (extinction trials) or with food (pairing trials) regardless of what the rat did. The main stimulus minus accuracy with the long stimulus. Experiment 1 showed that extinction trials increased short bias relative to training without conditioning trials or to training with pairing trials. The rest of the experiments tested explanations of these results. The same results were found when extinction trials were the same duration as the short stimulus (Experiment 2), when extinction trials were a random duration (Experiment 5), and when the signal value of the conditioned stimulus was changed in another way (Experiment 6). The effect of conditioning trials was modality specific (Experiments 3 and 4). Of the explanations considered, the best one--the only one not contradicted at least once--is that changing the signal value of a stimulus changes how the clock times the stimulus. Reducing signal value reduces the measured duration.     

Detecting deviations from metronomic timing in music: Effects of perceptual structure on the mental timekeeper

The detectability of a deviation from metronomic timing—of a small local increment in interonset interval (IOI) duration—in a musical excerpt is subject to positional biases, or “timing expectations,” that are closely related to the expressive timing (sequence of IOI durations) typically produced by musicians in performance (Repp, 1992b, 1998c, 1998d). Experiment 1 replicated this finding with some changes in procedure and showed that the perception-performance correlation is not the result of formal musical training or availability of a musical score. Experiments 2 and 3 used a synchronization task to examine the hypothesis that participants’ perceptual timing expectations are due to systematic modulations in the period of a mental timekeeper that also controls perceptual-motor coordination. Indeed, there was systematic variation in the asynchronies between taps and metronomically timed musical event onsets, and this variation was correlated both with the variations in IOI increment detectability (Experiment 1) and with the typical expressive timing pattern in performance. When the music contained local IOI increments (Experiment 2), they were almost perfectly compensated for on the next tap, regardless of their detectability in Experiment 1, which suggests a perceptual-motor feed-back mechanism that is sensitive to subthreshold timing deviations. Overall, the results suggest that aspects of perceived musical structure influence the predictions of mental timekeeping mechanisms, thereby creating a subliminal warping of experienced time.     

Timing light and tone signals in pigeons

Pigeons' ability to time light and tone stimuli was examined in four experiments. In Experiment 1, two groups of pigeons were trained to discriminate between 2- and 8-s durations of lights or tones and then were transferred to reversal or nonreversal discriminations in the alternate modality. Pigeons learned the light discrimination faster than the tone discrimination and showed immediate positive intermodal transfer from tone to light but not from light to tone. In Experiments 2-4, the peak procedure was used to study birds' timing of 15- and 30-s fixed-interval light and tone signals. Peak times on empty trials under baseline conditions closely approximated the length of fixed-interval signals. When pigeons were tested with time-outs and intermodal switches introduced midway through an empty trial, they tended to reset the timing mechanism and begin timing again from 0 s. With both estimation and production procedures, pigeons were less accurate when timing the tone stimuli than when timing the light stimuli. A comparison of these data with data from timing experiments with rats suggests several possible differences in timing processes between pigeons and rats.