How rats process spatiotemporal information in the face of distraction

How rats process spatiotemporal information in the face of distraction was assessed. Rats were trained on a time-place learning task in which the location of food availability depended on the amount of time elapsed since the beginning of the training session. In each training session each of four levers provided food pellets for 5 min on an intermittent schedule. In probe sessions interspersed with the final training sessions, the rats were presented with a second highly preferred food source-a piece of cheese-at various times into the session. Rats choose the correct lever after the cheese distraction, but it appeared that their internal clock had stopped during the cheese consumption period. Thus rats' internal clock, like that of pigeons, displays the properties of 'stop', 'reset', and 'restart'. Rat-pigeon differences in timing processes may be restricted to circadian or time of day timing. Present results also suggest that rats process spatial and temporal information separately.     

Interval time-place learning by rats: varying reinforcement contingencies

Two experiments with rats were conducted to study interval time-place learning when the spatiotemporal contingencies of food availability were more similar to those likely to be encountered in natural environments, than those employed in prior research. In Experiment 1, food was always available on three levers on a variable ratio (VR) 35 schedule. A VR8 schedule was in effect on Lever 1 for 5 min, then on Lever 2 for 5 min, and so forth. While rats learned to restrict the majority of their responding to the lever that provided the highest density of reinforcement, they seemed to rely on a win-stay/lose-shift strategy rather than a timing strategy. In Experiment 2, the four levers provided food on variable ratios of 15, 8, 15, and 30, each for 3 min. As expected the rats learned these contingencies. A novel finding was that the rats had a spike in response rate immediately following a change from a higher to lower reinforcement density. It is concluded that rats exposed to spatiotemporal contingencies behave so as to maximize the rate of obtained reinforcement.     

Shifts in the psychophysical function in rats

The primary goal was to compare results from a free-operant procedure with pigeons [Machado, A., Guilhardi, P., 2000. Shifts in the psychometric function and their implications for models of timing. J. Exp. Anal. Behav. 74, 25-54, Experiment 2] with new results obtained with rats. The secondary goal was to compare the results of both experiments with dependent variables that were not used in the original publication. As in the original study with pigeons, rats were trained on a two-alternative free-operant psychophysical procedure in which left lever press responses were reinforced during the first and second quarters of a 60-s trial, and right lever press responses were reinforced during the third and fourth quarters of the trial. The quarters were reinforced according to four independent variable interval (VI) schedules of reinforcement. The VI duration was manipulated in each quarter, and shifts in the psychophysical functions that relate response rate with time since trial onset were measured. The results obtained with rats were consistent with those previously obtained with pigeons. In addition, results not originally reported were also consistent between rats and pigeons, and provided insights into the perception, memory, and decision processes in Scalar Expectancy Theory and Learning-to-Time Theory.     

Alterations in time-place learning induced by lesions to the rat medial prefrontal cortex

This experiment examined the effect of medial prefrontal lesions on time-place learning in the rat. During the first phase, prior to lesioning, rats received training on an interval time-place task. Food was available on each of four levers for 3 consecutive min of a 12-min session. The levers provided food in the same sequence on all trials. Rats restricted the majority of their presses on each lever to the time in each session when it provided food and were able to anticipate when a lever was going to provide food. During the second phase some rats received lesions that were restricted to the medial prefrontal cortex. Following these very restricted lesions, rats continued pressing a lever after it stopped providing food (i.e. perseverated, as if their internal clock was running slow). The third phase involved changing the order in which the levers provided food. Lesions had no discernable effect on the rats' ability to learn the correct sequence of food availability. However, this change made the rats' timing perseveration even more noticeable. Our results suggest the medial prefrontal cortex is not necessary for acquisition of time-place sequencing information. However, lesions do appear to produce perseveration on components of the sequence.     

60 Hz electric fields and incandescent light as aversive stimuli controlling the behavior of rats responding under concurrent schedules of reinforcement

Several reports have shown that animals will sometimes engage in behaviors that reduce their exposure to a 60 Hz electric field (E-field). The field, therefore, can function as an aversive stimulus. In other studies, the E-field at equivalent strengths failed to function as an aversive stimulus. The present experiment, using rats, demonstrates how factors other than field strength can influence whether a subject engages in behavior that reduces field exposure. The general design consisted of giving the rat a choice between two alternatives, one of which sometimes included an added stimulus. Each subject was trained to press each of two levers to obtain food. Pressing one lever was reinforced intermittently under a variable interval 2 min schedule (VI 2); pressing the other lever was reinforced by a second VI 2 schedule operating independently of the first. Under this concurrent schedule the rat spent 50% of the daily 50 min session responding to each of the levers, indicating that they were equally “valued.” Next, while the schedules remained in effect, the first response to one of the levers turned on a 100 kV/m E-field which remained on until the rat pressed the other lever. The time spent responding under the schedule associated with the field was reduced by about 5–10%. When the procedure was changed so that no lever presses produced food, i.e., extinction, but the added stimulus contingency remained, the rats spent even less time in the presence of the field. Similar outcomes were observed during both the concurrent food or extinction schedules when incandescent light was used. Thus, both an E-field and incandescent light functioned as aversive stimuli, but the magnitude of the aversiveness was small. Aversiveness depended not only on stimulus intensity, but also on behavioral factors. Bioelectromagnetics 19:210–221, 1998. © 1998 Wiley-Liss, Inc.     

Further evidence of joint time-place control of rats' behavior: results from an 'open hopper' test

Rats were trained on an interval time-place task. Food was intermittently available on each of four levers for 4 min in a 16-min session. After baseline training the rats received 'open hopper' sessions in which food was available on all levers for all of the 16-min sessions. Despite the absence of any contingencies for doing so, the rats continued to press the levers in the 'correct' sequence, for roughly the 'correct' amount of time. This confirms that the rat behavior was controlled, in part, by a representation of an elapsed interval of time. The rats responding was more variable in 'open hopper' sessions and error increased (in an exponential fashion) as the session proceeded. This finding suggests that the rats may have used shifts in the location of food availability to minimize the accumulation of error throughout baseline sessions.     

Recovery of Pavlovian sign-tracking (autoshaping) following the discontinuation of inter-trial interval food in rats

In pigeons, Pavlovian autoshaped keypecking produced by keylight-food pairings has been eliminated by introducing food during periods between CS presentations (i.e., during the inter-trial intervals). Keypecking eliminated in this manner reappears when the inter-trial USs are discontinued even though the CS is no longer paired with US. The present experiment investigated whether this recovery of responding produced by discontinuing unpaired inter-trial US presentations could be extended to another species, rats, within a Pavlovian sign-tracking paradigm. Rats were initially trained on a procedure where insertion of one retractable lever (CS(+)) was followed, response independently, with food, while insertion of another lever (CS(-)) was not paired with food. Rats quickly came to contact the CS(+) lever at high rates, but contacted the CS(-) lever infrequently. In the next phase, CS(+) was no longer followed by food. Explicitly unpaired food was presented only during the inter-trial intervals when both levers were absent. This treatment essentially eliminated the sign-tracking response. In the final phase, the unpaired inter-trial food presentations were discontinued while both CSs continued to be presented without food. This produced a significant recovery of the sign-tracking elicited by the CS(+) lever, extending the species generality of the Pavlovian resurgence phenomenon that has previously only been reported in pigeons, to rats.     

Rats Work for Food They then Reject: Support for the Information-primacy Approach to Learned Industriousness

There is good evidence that in many situations, animals prefer to work for food even when identical food is freely available. This phenomenon is called ‘learned industriousness’ or ‘contrafreeloading’ and has been found in several species. This study shows that wild rats will also work hard for contaminated food that they associate with sickness and subsequently reject, even when wholesome food is continuously available at no extra cost. Seven wild rats (Rattus novegicus) lived for 4 wk in cages containing two operant levers and associated pellet dispensers. The rats earned all their food by lever pressing for 45-mg food pellets on a variable-interval 10 s (VI10s) schedule in each dispenser. After 1 wk, pellets containing a sub-lethal dose of an acute rodenticide were available for 12 h (the test period) from the preferred dispenser, before being replaced by normal food pellets. This procedure was repeated 1 wk and 4 wk later. The number of lever presses, pellets delivered, and pellets consumed at each dispenser were recorded for 12 h prior to the test period, during the 12-h test period, and 12 h after the test period. The rats quickly learned to shift their feeding preference during the test period away from the dispenser that provided the rodenticide pellets. However, during the second half of the test period, lever pressing at this dispenser increased, even though the pellets thereby obtained were still rejected. This divergence between the lever pressing rate and the pellet consumption rate increased over the three trials. This ‘learned-industriousness’ behaviour is not easily explained by the self-reinforcing hypothesis, by the obligate species-specific response hypothesis, or as some artefact of the operant situation. It is suggested that the lever pressing towards the end of the trial, for pellets that were rejected, enabled the animals to gather information about a rare, but very important, event, namely, the presence of dangerous food at a previously preferred and normally safe feeding site. The data lend support to the information-primacy theory of learned industriousness.     

Passive immunization against nicotine attenuates nicotine discrimination

Ten rats were trained in a two lever operant chamber to press different levers after a nicotine injection (0.14 mg/kg s.c.) or a saline injection on an FR10 schedule. The rats were then injected i.p. with either 150 mg nicotine-specific IgG or the same amount of control IgG from non-immunized rabbits. On successive days, they were retested with both levers active after a saline injection, a full training dose of nicotine and a half dose of nicotine (0.07 mg/kg s.c.). After saline injection, both groups pressed the saline lever almost exclusively. After each of the nicotine doses, the immunized rats performed a significantly lower percentage of their lever presses on the nicotine lever than did non-immunized rats. The results suggest that passive immunization can interfere with the stimulus properties of nicotine.     

Discriminative stimulus properties of L-phenylisopropyl adenosine: blockade by caffeine and generalization to 2-chloroadenosine

Recent neurochemical data on the effects of activation and blockade of adenosine A1 receptors has suggested a direct role of adenosine in neurotransmission. The present research used a drug discrimination procedure to test the hypotheses that A1 adenosine receptor activation could serve as a discriminative stimulus and that caffeine, a drug believed to be an A1 receptor antagonist, could block the adenosine discrimination. Food-deprived rats were trained to press one of two levers on an FR 10 schedule of food-pellet delivery. Responses on one lever were reinforced following i.p. injection of N6 - (L-phenylisopropyl) adenosine (L-PIA); responses on the other lever were reinforced following i.p. injection of saline. L-PIA training dose was increased from 0.064 to 0.08 mg/kg L-PIA in the course of the study. Subjects required an average of 91 sessions to acquire this discrimination. Stimulus control by L-PIA was dose-dependent, with the ED-50 being approximately 0.03 mg/kg. 2-Chloroadenosine (2CA) generalized to L-PIA with a tenth the potency. Caffeine blocked L-PIA-induced lever selection. These results indicate that 1) rats can be trained to discriminate L-PIA from saline in a two-lever food-reinforced task and 2) the discriminative stimuli produced by L-PIA are based on its agonistic action at the adenosine A1 receptor.     

Rats' memory for event duration in delayed matching-to-sample with nonspatial comparison response alternatives

Previous research has suggested that using stationary and moving levers as nonspatial response alternatives can significantly enhance the speed of acquiring a temporal discrimination in rats. In Experiment 1, rats were trained to discriminate 2 and 8s of magazine light illumination by responding to either a stationary lever or a moving lever with a cue light illuminated above it. Rats learned to discriminate event durations at a high level of accuracy after 25 sessions of training. During subsequent delay tests, rats exhibited a strong choose-long bias, indicating that they were timing from the onset of the magazine light until the entry of levers into the chamber. This occurred regardless of whether intertrial intervals and delay intervals were dark or illuminated. On test trials in which the sample was omitted, rats responded as if the short sample had been presented. In Experiment 2, the rats received extensive training with dark and illuminated variable delay intervals (1-4 s). However, they continued to exhibit a tendency to time from the onset of the magazine light until entry of the levers into the chamber. Although the use of stationary/uncued and moving/cued levers as response alternatives enhanced the speed of acquisition of the event duration discrimination in rats, additional procedural modifications will be necessary to prevent rats from timing during the delay interval.     

Responding maintained by primary reinforcing visual stimuli is increased by nicotine administration in rats

Nicotine has been shown to increase responding maintained by turning off a houselight. To examine whether this effect extends to other primary reinforcing visual stimuli, the present study assessed whether nicotine would increase responding maintained by the illumination, and not just the darkening, of a visual stimulus. One group of rats (n = 4) was initially trained to press two levers, using food as a consequence, while a separate group of rats (n = 4) was initially trained to press one lever. After training, all rats pressed an active lever to turn on or turn off a houselight for 10 s, while presses on an inactive lever had no programmed consequences. A third group of rats (n = 4) were never trained to press either of the two levers and did not experience any programmed consequences for pressing. Although nicotine slightly increased lever pressing on both levers in the third group, nicotine resulted in much greater increases in responding maintained by the visual stimuli in the first two groups. Nicotine selectively increased responding maintained by visual stimuli, regardless of which levers were originally trained and regardless of whether those stimuli consisted of turning on or turning off a houselight, suggesting that nicotine enhances the value of primary reinforcing visual stimuli.     

Rats' memory for event duration: differential effects of delaying the discriminative choice cue as opposed to the opportunity to execute the choice response

Rats were trained to discriminate short or long durations of houselight illumination using a choice procedure. During the test phase of each trial, the left and right levers were presented with an auditory cue above one of them on (cued lever) while the other was off (uncued lever). The auditory cue was presented immediately after sample offset and the levers were inserted after the auditory cue had been presented for 2 s. For half of the rats, the correct response following the short sample was to press the cued lever, while following the long sample, it was to press the uncued lever. This was reversed for the remaining rats. Following acquisition of the discrimination, two different types of delay tests were administered. In the first set, the delay between offset of the sample and onset of the auditory cue was manipulated (Cue Delay Test). In the second set, the delay between onset of the auditory cue and entry of the levers into the chamber was manipulated (Response Delay Test). Cue Delay testing resulted in a choose-long bias at the longer delays. Response Delay testing did not result in a systematic response bias and there was little forgetting over the delay interval. These data suggest that the rats did not stop the internal clock when the nominal sample was offset, but allowed it to keep running until the auditory cue was presented. The data from the Response Delay Test indicate that either a response decision was made based on the clock reading as soon as the auditory cue was presented, or the clock reading itself was retained over the delay with no subjective shortening and little forgetting.     

Training history affects magnitude of spontaneous recovery from extinction of appetitive conditioned responding

Three experiments examined spontaneous recovery from extinction of appetitive conditioned responding (CR) as a function of training history. Rats first received reinforced and non-reinforced conditioning trials. Groups of rats were equated for total number of reinforced trials but differed in the number of non-reinforced trials, or in the order of reinforced and non-reinforced trials. CR was then extinguished in all groups. Subsequently, the extent of recovery of CR was assessed in a test session performed either 1 or 17 days after the last extinction session. After a 17-day delay, rats that had received all reinforced trials immediately prior to the first extinction session showed stronger recovery than did rats having received all reinforced trials at the beginning of training, or interspersed among non-reinforced trials. No significant spontaneous recovery was observed after a 1-day test delay. These results, which may be of clinical relevance with respect to relapse after therapy, are explained in terms of the training schedules generating differences in strength of inhibitory associations, and a relatively long, but not a short, test delay attenuating these associations.     

Timescale dependence in a conditional temporal discrimination procedure

A conditional temporal discrimination procedure was used to test the scope and generality of the principle of timescale invariance (TSI). Rats were trained to make different temporal discriminations following four different auditory stimuli. After shorter stimuli, rats were reinforced for pressing the left lever, and after longer stimuli, rats were reinforced for pressing the right lever. The four auditory stimuli (a pure tone, pulsed tone, click, and white noise) were each presented for different pairs of durations (ranging from 2 vs. 8s to 32 vs. 128 s) within a single session, but the ratio between the shorter and longer duration was maintained constant at 1:4. With the shortest pair of durations (2 vs. 8s), rats showed a clear violation of TSI in both overall and relative response rates. Rats started responding later and persisted relatively longer on the left lever when the shorter duration was only 2s. We interpret these results with regards to a framing hypothesis, whereby TSI does not apply to relatively short durations when animals are simultaneously trained with wide ranges of intervals.     

Preference for fixed vs self-selected durations of brain stimulation in rats administered d-amphetamine sulphate

Rats implanted with stimulating electrodes in the region of the lateral hypothalamus were allowed the opportunity to respond on (a) a fixed duration lever (b) a variable duration lever or (c) were given a choice between responding on either the lever that delivered the fixed duration of electrical brain stimulation or the lever that allowed the animal to control the duration of electrical brain stimulation. Amphetamine increased rates of responding in both the fixed and variable duration conditions but total duration of brain stimulation was comparable in the two conditions. When given a choice of levers, rats did not prefer either the fixed or variable duration lever. After treatment with d-amphetamine, however, a marked preference for the variable duration lever was observed, although total duration of brain stimulation received in the choice procedure was comparable to that received when either the fixed or variable levers were presented alone. These findings were discussed in terms of brain stimulation preference following amphetamine and its relationship to reward value in the rat.     

Serial conditional discrimination and temporal bisection in rats selectively lesioned in the dentate gyrus

In positive serial conditional discrimination, animals respond during a target stimulus when it is preceded by a feature stimulus, but they do not respond when the same target stimulus is presented alone. Moreover, the feature and target stimuli are separated from each other by an empty interval. The present work aimed to investigate if two durations (4 or 16 s) of the same feature stimulus (light) could modulate the operant responses of rats to different levers (A and B) during a 5-s target stimulus (tone). In the present study, lever A was associated with the 4-s light, and lever B was associated with the 16-s light. A 5-s empty interval was included between the light and the tone. In the same training procedure, the rats were also presented with the 5-s tone without the preceding light stimuli. In these trials, the responses were not reinforced. We evaluated the hippocampal involvement of these behavioral processes by selectively lesioning the dentate gyrus with colchicine. Once trained, the rats were submitted to a test using probe trials without reinforcement. They were presented with intermediate durations of the feature stimulus (light) to obtain a temporal bisection curve recorded during the exposure to the target stimuli. The rats from both groups learned to respond with high rates during tones preceded by light and with low rates during tones presented alone, which indicated acquisition of the serial conditional discrimination. The rats were able to discriminate between the 4- and 16-s lights by correctly choosing lever A or B. In the test, the temporal bisection curves from both experimental groups showed a bisection point at the arithmetic mean between 4 and 16 s. Such processes were not impaired by the dentate gyrus lesion. Thus, our results showed that different durations of a feature stimulus could result in conditional properties. However, this processing did not appear to depend on the dentate gyrus alone.     

Temporal generalization accounts for response resurgence in the peak procedure

The peak interval (PI) procedure is commonly used to evaluate animals' ability to produce timed intervals. It consists of presenting fixed interval (FI) schedules in which some of the trials are replaced by extended non-reinforced trials. Responding will often resume (resurge) at the end of the non-reinforced trials unless precautions are taken to prevent it. Response resurgence was replicated in rats and pigeons. Variation of the durations of the FI and the non-reinforced probe trials showed it to be dependent on the time when reinforcement is expected. Timing of both the normal time to reinforcement, and the subsequent time to reinforcement during the probe trials followed Weber's law. A quantitative model of resurgence is described, suggesting how animals respond to the signaling properties of reinforcement omission. Model results were simulated using a stochastic binary counter.     

Response acquisition with delayed reinforcement in Lewis and Fischer 344 rats

Previous work has shown neurochemical and behavioral differences between Lewis rats and Fischer 344 rats. Some of this work suggests that there might be differential sensitivity to delayed reinforcement between the two strains. To further explore this possibility, Lewis (n=8) and Fischer 344 (n=8) rats were exposed to a response-acquisition task with a non-resetting 20s delay to reinforcement. A tandem fixed-ratio 1, fixed-time 20s schedule of reinforcement was programmed for one of two levers; presses on the alternate lever had no programmed consequences. A greater number of Lewis rats (5/8) acquired lever pressing compared to the Fischer 344 rats (2/8). Future work with these strains may lead to a better understanding of the genetic and/or neurochemical factors involved in temporal control of behavior.     

Effect of repeated regrouping and relocation on behaviour of steers

To investigate the effect of repeated regrouping and relocation (R&R) on behaviour of steers, 72 Holstein–Friesian (14-month-old; 441 ± 3.2 kg) steers were assigned to either control (n = 30; C) or regrouped (n = 42; R) treatments and housed six per pen in 12 pens. The R steers were exposed to six R&R over 84 days. New pen cohorts were allowed to stabilise for 14 days and none of the R steers were allowed to share the same pen or pen mates, where or with whom, they were previously housed. Control steers were housed in the same pen with the same pen mates. Each steer was marked on its back with an individual identification code. Twelve cameras were used to observe and record behaviour for each pen allowing observation of all individual steers continuously for a week following each R&R. The following behaviours were recorded for each steer: lying, standing, eating, drinking, head-to-head contact with another animal while not eating, head contact with the body of another animal and bodily contact with none, one, two or three steers. Behaviour was observed by instantaneous scan sampling after each R&R, at 2 min intervals for 2 h on day 1; at further 20 min intervals on days 1 and 2; and at 120 min intervals from day 3 to 7. Where appropriate, the % of time spent in each behaviour was calculated from the data on total counts in each behavioural category. The total count data were analysed by χ²-statistics for all behavioural categories. Steers were weighed before each R&R. Average daily gain from day 0 to 84 was analysed by ANOVA. During the first 2 h observation period following mixing, R steers displayed a greater (P < 0.05) % of time standing (following the first to sixth R&R), eating (first to fourth and sixth R&R) and drinking (first, third and fourth R&R) than control steers. In the 20 min observational period, a greater % (P < 0.05) of time was spent standing, eating and drinking in R than in C steers following each R&R. In the 120 min observation period, R steers spent a greater (P < 0.05) % of time lying with less body contact behaviours than C steers, and these findings increased in the fourth, fifth and sixth R&R. These data suggest that there was partial adaptation to repeated R&R at the first two R&R followed by complete adaptation at the third and subsequent R&R, with no detrimental effect on animal performance.