Variable Intertrial Interval Research Articles

Long-term changes in mineralocorticoid and glucocorticoid receptor occupancy following exposure to an acute stressor

Stressors produce rapid activation of the hypothalamic–pituitary–adrenal axis, which typically resolves within 60–90 min following termination of the stressor. In addition, some stressors such as inescapable tailshock (IS) also produce elevated basal levels of corticosterone (CORT), and reduced serum levels of corticosteroid binding globulin (CBG). The elevated basal levels of CORT produced by IS are only observed at the trough of the circadian rhythm of CORT secretion, and are sustained for 2–3 days following stressor termination. The goal of the following experiments was to determine the extent to which the elevated basal levels of CORT observed following IS exposure produced greater corticosteroid receptor occupancy in the brain and pituitary. To do so, rats ( n=8–10 per group) received either sham or bilateral adrenalectomy (with CORT replacement in their drinking water; 25 μg/ml) and were given 3 days to recover. Rats were then exposed to 100 ISs (1.6 mA, 5 s each) administered on a 60 s variable intertrial interval, or remained in their home cages. As seen previously, IS produced an increase in basal CORT (5 μg/dl) and a decrease in CBG (30% decrease). Rats were sacrificed 24 h following IS for trunk blood samples and brain dissections. IS exposure had very little effect on corticosteroid receptor protein expression as determined by mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) binding levels in ADX rats. In addition, no changes in whole cell GR levels (as detected by Western blot) were observed in sham rats exposed to IS. On the other hand, IS exposure led to greater occupancy of MR (ranging from 25%–50%) in hippocampus, hypothalamus, pituitary, and posterior cortex. IS also produced greater occupancy of GR (approximately 20%) in hypothalamus and posterior cortex. These long-term changes in corticosteroid receptor activation, evident 24 h after IS exposure, may be responsible for some of the long-term neural, behavioral and immune changes observed following this acute stress procedure.

Pigeons’ memory for empty time intervals marked by visual or auditory stimuli

In Experiment 1, pigeons were trained to discriminate the duration (2 or 8 sec) of an empty interval separated by two 1325-Hz tone markers by responding to red and green comparison stimuli. During delay testing, a choose-short bias occurred at 1 sec, but a robust choose-long bias occurred at 9 sec. Responding in the absence of tone markers indicated that the pigeons were attending to the markers and not simply timing the total trial duration. The birds were then trained to match short (2-sec) or long (8-sec) empty intervals marked by light to blue/yellow comparisons. For both visual and auditory markers, delay testing produced a choose-short bias at 1 sec and a choose-long bias at 9 sec. In Experiment 2, the pigeons were shifted from a fixed to variable intertrial intervals (ITI) within sessions. On trials with tone markers, the duration of both the empty interval and the preceding ITI affected choice responding. On trials with light markers, only the duration of the empty interval influenced choice responding. Subsequent delay testing in the context of variable ITIs replicated the memory biases previously obtained. In Experiment 3, performance was assessed at various delay intervals on trials in which either the first or the second marker was omitted. The data from these omission tests indicated that the first marker initiated timing but that the second marker sometimes initiated the timing of a new interval. Explanations of these effects in terms of the internal clock model of timing are discussed, and a simple quantitative model of the delay interval data is tested.

The Timing of Auditory and Visual Signals by Pigeons: Differential Sensitivity to Intertrial Interval Duration

In Experiment 1, pigeons were trained to discriminate short (2-s) and long (8-s) durations of a light to yellow/blue comparisons and the same durations of a tone to red/green comparisons. Following training with a constant intertrial interval (ITI) of 10 s, the ITI was varied within sessions (4, 8, 16, or 32 s). On trials with the light, signal duration alone influenced choice responding. The preceding ITI duration had no effect. On trials with the tone, both the signal duration and the preceding ITI duration affected choice responding. Subsequent delay testing in the context of variable ITIs generally replicated the different response biases previously reported with tone vs light signals. In Experiment 2, signal omission testing resulted in a bias to respond “short” regardless of whether the comparisons associated with light signals or the comparisons associated with tone signals were presented. However, on signal omission trials the preceding ITI duration continued to affect choice responding when comparisons associated with the tone signals were presented, but not when comparisons associated with the light signals were presented. These findings regarding stimulus properties and ITI effects can be explained by the information processing model of timing.

Memory of auditory lists by rhesus monkeys (Macaca mulatta).

Monkey auditory memory was tested with increasing list lengths of 4, 6, 8, and 10 sounds. Five-hundred and twenty environmental sounds of 3-s duration were used. In Experiment 1, the monkeys initiated each list by touching the center speaker. They touched 1 of 2 side speakers to indicate whether a single test sound (presented from both side speakers simultaneously) was or was not in the list. The serial-position functions showed prominent primacy effects (good first-item memory) and recency effects (good last-item memory). Experiment 2 repeated the procedure without the list-initiation response and with a variable intertrial interval. The results of both experiments were similar and are discussed in relation to theories and hypotheses of serial-position effects.

Dissociable effects of anterior and posterior cingulate cortex lesions on the acquisition of a conditional visual discrimination: Facilitation of early learning vs. impairment of late learning

Two experiments investigated the effects of quinolinic acid induced lesions of the anterior and posterior cingulate cortices on the acquisition and performance of a conditional visual discrimination (CVD) task, in which rats were required to learn a rule of the type: “If lights are flashing FAST, press the right lever; if SLOW press left”. In Experiment 1, animals with lesions of the anterior cingulate cortex (ANT group) demonstrated a significant enhancement in learning during the early stages of task acquisition. Conversely, animals with lesions of the posterior cingulate cortex (POS group) were impaired in learning during the later stages of acquisition. There were no significant differences between the ANT and POS groups on the performance of the task when either variable inter-trial intervals or reduced stimulus durations were imposed. In Experiment 2, the specificity of the lesion effects for processes operative during the early and late stages of learning was tested. Animals were trained to a criterion of 70% correct choices on two consecutive sessions prior to lesioning, and subsequently allowed to continue to acquire the task to the mean asymptotic performance level of 85% correct choices on two consecutive sessions. Animals of the POS group were impaired in learning during this later stage of task acquisition, thus replicating the pattern of results obtained in Experiment 1. The animals in Experiment 2 were then tested following a 30-day retention interval and during extinction (removal of sucrose from the magazine). The extinction test revealed an impairment in the ability of animals in the ANT group to omit lever responses in the absence of reinforcement. These results indicate that the anterior and posterior cingulate cortices are functionally dissociable, and suggest that they may form part of complementary, but competing, learning and memory systems.

Interruptions, Tradeoffs, and Temporal Discounting

Variance in amount of rewards has been the focus of many studies and models of risk sensitivity. However, the timing of rewards has received much less attention. Animals tend to prefer immediate rewards, even when this preference reduces the long term rate of gain. This implies future rewards are devalued, a phenomenon known as time discounting. The value of a reward can be represented by a function that is decreasing and concave-up with increasing delays until receipt of reward. When a forager is given the choice between an immediate small reward and a delayed larger reward, preference reverses from the smaller to the larger as the delays to both rewards increase. There are several descriptive time discounting models. The exponential and the hyperbolic models both result in the concave-up value function and hence risk sensitivity with respect to timing. However, in the traditional models, only the hyperbolic model predicts preference reversal. We present a model of discounting as a continuous time Markov process. In this model, the discounting rate is determined by the perceived likelihood that the forager will be interrupted from the foraging process. In this continuous interruption rate model, we find both the concave-up value function and preference reversal. In an expanded model that accounts for the variable inter-trial intervals often used in tests of preference, we find that the two portions of the inter-trial interval are weighted differently. Finally, we generate a generalized fitness model to examine the influence of the fitness consequences of a given interruption on foraging choices and find that the type of trade-off involved determines the fitness outcome.

Re-Evaluating the Effect of Unilateral Brain Damage on Simple Reaction Time to Auditory Stimulation

Results of a previous experiments designed to test if there is a right hemisphere superiority over the left hemisphere in the capacity to react quickly to a stimulus have not lead to an unambiguous conclusion, although the majority of studies indicate that right-brain-damaged pa tients are generally slower than left-brain-damaged patients. The present experiment re-examines the speed of reaction time to auditory stimulation in a population of brain-damaged patients. Each case was presented with 120 auditory events and variable inter-trial intervals (from 1 s to 15 s). To ensure homogeneity of the population tested, only patients with a cerebro-vascular accident were chosen. The results show that the right-brain-damaged patients were not generally slower than left-brain-damaged patients. However, a marked deterioration in performance over time was observed in the right-hemisphere group while a small amount of improvement was found in the normal and left-brain-damaged patients. The outcome is discussed in terms of a sustained attention deficit associated with right-hemisphere damage.

Control of responding by sounds of different quality: an evolutionary analysis.

Two experiments investigated the acquisition of discriminations between two acoustic stimuli of different quality (noise bursts vs. a 2-kHz pulsed signal) when features of the everyday environment were incorporated into the experiments. In Experiment 1, rats were trained, using food, to press a lever. Throughout all sessions, 5-s trials of noise bursts (the random stimulus) were presented, after variable intertrial intervals, through a remote speaker mounted outside the experimental enclosure. The noise burst occurred randomly with respect to reinforcement of lever pressing and had no programmed relationship to the animal's behavior. When lever pressing was established, the 2-kHz signal was presented through a speaker adjacent to the response lever according to a different set of variable intertrial intervals. A response in the presence of the 2-kHz signal terminated the trial and was reinforced. The 2-kHz signal acquired control of responding within the first few trials, whereas the random stimulus exerted no control of responding. In Experiment 2, rats were trained to press the lever in the presence of the 2-kHz signal presented through the adjacent speaker on a variable intertrial interval. After 14 sessions, 5-s trials of noise bursts (random stimulus) were presented through the remote speaker on the second variable intertrial interval. The random stimulus initially elicited exploratory behavior, which then rapidly declined. Subsequently, the random stimulus exerted no or weak control of responding. The introduction of the random stimulus had no effect on responding in the presence of the adjacent stimulus. In Experiments 3 and 4 the random stimulus was presented through the adjacent speaker, and the stimulus correlated with reinforcement was presented through the remote speaker. In both experiments, there was persistent control of responding by the random stimulus and slow development of control by the stimulus correlated with reinforcement. In Experiment 5, both stimuli were presented through the adjacent speaker. There was persistent control of responding by the random stimulus.

Acquisition and reversal of a discriminated autoshaped response in male and female rats: Effects of long or short and fixed or variable intertrial interval durations

Groups of male and female Wistar rats were exposed to autoshaping procedures in which the duration (50 vs 20 s) and the variablity (fixed vs variable) of the intertrial interval were manipulated. Pellet delivery occurred after one of the levers (CS+) had been presented for 10 s. Presentation of the other lever (CS−) was not associated with pellet delivery. CS+ contacts occurred more frequently and at higher rates when the intertrial interval was 50 s than when it was 20 s, while intertrial interval variability did not play an important role. Males contacted CS+ and CS− more frequently and at a higher rate than did females. Goal-directed activity (nose pokes in feeder tray) occurred more often when the intertrial interval was short than when it was long. Females showed higher goal-directed activity than did males, both during CS presentation and during the intertrial interval. When experimental contingencies were reversed, CS+ contacts decreased and CS− contacts increased for males and females, but the decrease in CS+ contacts occurred more slowly for males than for females. The results of these experiments add to a growing body of literature suggesting the existence of fundamental behavioral differences between the sexes. These differences are partly attributable to the influence of gonadal hormones during development and maturation and partly attributable to the behavioral requirements of the experimental procedures.

Slow and multiple unit potentials in trace and temporal conditioning controlled by electrical reward in the rat

Rats trained to expect medial forebrain bundle electrical reward every 17 sec acquire, in their visual cortex, a linear gradient of increasing negative potential over the prereinforcement half of the interval. The postreinforcement half of the interval is occupied by a less linear reversal of the negative shift. Integrated multiple unit activity from the same electrodes shows for the same half intervals, respectively, acquisition of progressive increase and sudden decrease. Behaviorally, naive subjects are most active following the reinforcement, becoming progressively less active in the second half of the interreinforcement interval. After training, the above pattern is reversed with sudden cessation of activity following reinforcement and progressive increase beginning about 5 sec later and continuing to the time of reinforcement. Slopes for the negative anticipatory potential gradient (APG) ranged from 8 to 28 μV/sec in different subjects on the 17 sec interreinforcement interval. Doubling the interval halved the slope, the maximum prereinforcement negative voltage remaining constant. The phenomenon thus appears as a relative or pacing dynamic rather than as an absolute or fixed microvolts/second function. Making the interreinforcement interval variable caused loss of both the anticipatory gradient and the integrated multiple unit increment. Trace conditioning with variable intertrial interval restored an anticipatory gradient and multiple unit increment. The specific dissociation of unit firing activity during the APG previously reported in the quietly expectant cat is not seen in this study owing to the excited expectancy in the rat anticipating MFB reward. Although the longer periods of negative potential gradient and unit action potential activity appear associated rather than dissociated, transient dissociations in patterns were observed during shorter duration shifts in the slow potential. Resolution of these variations must await more definitive study of unit activity and sustained potential genesis.

Central cholinergic involvement in working memory: Effects of scopolamine on continuous nonmatching and discrimination performance in the rat.

Rats were trained to stable baselines of lever pressing on a variable intertrial interval continuous nonmatching to sample schedule (CNM) or on an analogous discrimination schedule. Scopolamine reduced accuracy of CNM performance to a similar extent over the three intertrial (retention) intervals: 2.5, 5, and 10 s, results indicating that the drug did not affect the time-dependent process of retention in working memory. When baseline levels of performance accuracy were similar in the CNM and discrimination tasks (but stimulus discriminability was greater in the CNM task), scopolamine reduced accuracy equally in the two procedures. Effects of scopolamine on accuracy of noncorrection trial CNM performance were simulated by reducing stimulus discriminability; however, scopolamine disrupted CNM correction trial performance much more than did reductions in stimulus discriminability. It is concluded that scopolamine's effects on working memory are not limited to possible effects on stimulus discrimination: Scopolamine may also affect retrieval of response rules from reference memory.

BEHAVIORAL CONTRAST AND THE AUTOMAINTAINED KEY PECK

In two experiments behavioral contrast was demonstrated during discrimination training in a positive automaintenance procedure. During the baseline condition in each experiment, a key was transilluminated for eight seconds by one of two colors (CS) following a variable intertrial interval signaled by a dark key. Keylight transillumination terminated with a response-independent food presentation. In the first experiment, food was eliminated during one CS for up to fifty sessions. After reinstatement of food following each CS, the discrimination was reversed. Six of the eight subjects showed positive behavioral contrast, i.e., response rates increased during the CS associated with food as they decreased during the CS associated with no food. The effect was replicated in Experiment II, but it did not occur when both the food and its associated CS were eliminated. These results were comparable to those obtained with operant discrimination training procedures (behavioral contrast) and with Pavlovian discrimination training. The results suggest that additivity theories of behavioral contrast may be insufficient to account for these data.

Do 16‐week‐old infants anticipate stimulus offsets?

Twenty alert 16-week-old infants were presented with 20-sec tones at variable intertrial intervals for 10 trials while beat-by-beat heart rate responses were recorded to assess response at offset as well as onset of each stimulus. Consistent with past research, onsets elicited deceleratory responses which habituated and showed some evidence of dishabituation with a change in stimulus frequency on the last 2 trials. Offsets also elicited significant deceleratory responses overall, but inspection of pre-offset heart rate suggested that deceleration in anticipation of the offset event appeared after a few stimulus repetitions and increased in magnitude over trials. However, individual variability was considerable and although the anticipatory response was significant averaged over all trials, the apparent increase over trials did not reach statistical significance. The evidence clearly indicates infants quickly process and act upon temporal information in a stimulus.

Effects of scopolamine on variable intertrial interval spatial alternation and memory in the rat.

A repeated measures procedure, variable intertrial interval (ITI) spatial alternation, was used to assess scopolamine effects on memory, and to compare effects of the drug on discrimination processes with effects on storage. Rats learned in two stages to press left and right levers in alternation on discrete trials separated by 5 different ITI's ranging from 2.5 to 40 s and presented in random order during the experimental session. In the first stage, alternating discrimination, alternation was controlled by a light on over the correct lever at the time of the trial; in the second stage, variable ITI spatial alternation, a centrally located panel light signalled all trials and alternation was controlled by stimuli from prior trials ('memory'). Alternation response occurrence declined moderately (but significantly) with increasing ITI duration in both the alternating discrimination and variable ITI spatial alternating discrimination and variable ITI spatial alternation stages; response occurrence was also significantly decreased by scopolamine treatment in both stages. Accuracy of alternating discrimination performance was not significantly altered by either ITI duration or scopolamine treatment. Accuracy of variable ITI spatial alternation performance on a trial varied inversely with the duration of the ITI that preceded the trial. Scopolamine treatment significantly reduced accuracy of lever pressing in variable ITI spatial alternation but did not alter the slope of the curves relating accuracy to ITI duration. These effects indicate that the drug impaired discrimination processes but did not alter memory storage.

Probability of conditioned responses as a function of variable intertrial intervals.

Probability of conditioned responses as a function of variable intertrial intervals.

Conditioning of the nictitating membrane response of the rabbit (Oryctolagus cuniculus) as a function of length and degree of variation of intertrial interval

The study involved three experiments. The first, a parametric investigation of nictitating membrane conditioning with eight constant intertrial intervals (ITIs) between 5 and 120 sec, orthogonal to interstimulus intervals (ISIs) of 250 and 750 msec plus three temporal conditioning control groups, revealed that performance improved rapidly with increasing ITI but stabilized at relatively low ITI values. At 750-msec ISI, a decrement in performance was found at 60-sec ITI. Experiment II, using constant ITIs of 45–75 sec in 5-sec steps, at 750-msec ISI confirmed the trend toward a performance decrement around 60 sec, although the trend was weak and highly variable. Experiment III evaluated the differences in performance between constant and variable ITI, using three ITI values and three conditions of variation at each value. Findings were discussed in terms of differences in conditioning resulting from both length and degree of variation of ITI and some subtle effects which may emerge only when constant ITIs are used.

Conditioned vocalization in rats.

Attempts to condition vocalization in the rat have produced inconsistent results. These inconsistencies might be understood in terms of two crucial variables: the mobility of the subject during conditioning and the variability of the intertrial interval. New research is reported which directly examines the effects of these two variables and supports the importance of immobilizing the subject, but not of varying the intertrial interval during conditioning. It is suggested that vocal behavior in the rat may be conditioned only when no competing species-specific defense reaction (e.g., freezing) is available to the subject. The first attempt to condition shockelicited vocalization in rats was reported in 1932. During the ensuing 40 years, reports have appeared indicating that rat vocal behavior is both easily conditioned and impossible to condition. The present paper attempts to reconcile some of these contradictory results by reference to key procedural differences in these studies. In uncovering which variables are crucial to the vocal conditioning process, it is hoped that a more comprehensive view of vocal behavior in the rat will emerge. Warner (1932) reported sporadic instances of what looked like conditioned vocalizations to shock in one rat. Following this lead, Schlosberg (1934) exposed rats to signal-shock pairings in a Pavlovian conditioning procedure with light or buzzer serving as the conditioned stimulus (CS) and shock as the unconditioned stimulus (US). Variable intertrial intervals (ITIs) regulated stimulus presentations, and all subjects were immobilized in an inverted cage which fit over them like a lid. Schlosberg reported a reliable conditioned vocal response (CR) in his subjects. Almost a decade later, Cowles and Pennington (1943) used a conditioned vocal re1JThe authors are grateful to Annette Ehrlich, David Perrott, E. D. Bailey, Veronika Coltheart, and Harry M. B. Hurwitz for their critical assistance. This paper is dedicated to Juarez, a responsible mouse.

Orienting response and discrimination conditioning.

Abstract The effects of habituation or no habituation trials, alternating and random pattern of CS+ and CS− presentations, and constant and variable intertrial intervals were observed on both rate and amount of discrimination conditioning of both first and second interval galvanic skin responses in 96 students. Red and green lights of 0.5 sec duration served as CSs. Unconditioned stimulus was a 0.5 sec shock. CS-UCS interval was 5.0 sec. Both variation in orienting response magnitude and discrimination conditioning were demonstrated. However, experimentally induced variation in orienting response magnitude did not interact with either rate or amount of discrimination conditioning. CS habituation trials decreased magnitude of responding nondifferentially to both CS+ and CS− over early acquisition trials when compared with magnitude of responding of the group not receiving CS habituation trials. Similarly, the constant ITI group gave smaller responses to both CS+ and CS− than did the variable ITI group. No differences in magnitude of response between alternating and random stimulus presentation groups were found.