Christopher K. Randall

Release from latent inhibition with delayed testing

A conditioned emotional response procedure was used to study the interactive effects of stimulus preexposure and retention interval in rats. In Experiment 1, the subjects were conditioned by presenting a light CS paired with mild footshock as the US. Half of the subjects were given nonreinforced preexposure to the CS, and the others were not. Separate preexposed and nonpreexposed groups were then tested 1,7, or 21 days after conditioning. Suppression of ongoing activity was used to assess the degree of conditioned fear. Latent inhibition was found at the 1-day retention interval; the preexposed subjects displayed less conditioned fear than did the nonpreexposed subjects. In contrast, equally strong conditioned fear was expressed by the preexposed and the nonpreexposed groups tested after the 7- and the 21-day retention intervals. These results indicate a release from latent inhibition similar to that obtained with conditioned taste aversions (Kraemer & Roberts, 1984). The results of Experiment 2 suggest that retention-interval-induced increases in sensitization, pseudoconditioning, or neophobia cannot account for the release from latent inhibition effect obtained in Experiment 1. The implications of these findings for a retrievaloriented view of latent inhibition are discussed.

Spatial learning in preweanling rats trained in a Morris water maze

In three experiments, spatial learning in preweanling rats was examined, using the Morris water maze (MWM). Contrary to previous findings, Experiment 1 revealed that rat pups younger than 20 days of age showed good spatial learning. Experiment 2 revealed that spatial learning in preweanlings was influenced by the nature of the training procedure; a spaced distribution of trials resulted in good spatial learning, but a massed distribution of trials did not. Despite evidence that preweanlings have the capacity for spatial learning, Experiment 3 indicated that they are not as proficient as adults. These results are discussed in terms of procedural variables that may influence MWM performance in immature rats.

Signal intensity and duration estimation in rats.

Rats were trained on a short (2-second) versus long (10-second) duration discrimination. The duration of an overhead light signaled which of two lever-press responses, left or right, would produce food reinforcement. After the rats had acquired the discrimination, probe tests were presented in which the light varied in intensity (bright or dim) and duration (values between 2 and 10 seconds). The results indicated that rats judged a bright light to be longer than a dim light of equal duration. Signal intensity has been shown to affect timing in humans and pigeons. The present results therefore extend the comparative generality of the effect to include rats as well.

Impact of d-Amphetamine on Temporal Estimation in Pigeons Tested with a Production Procedure

The influence of d-amphetamine on timing in pigeons was examined with a production procedure. Birds were trained with a fixed time schedule in which food reinforcement was contingent on the first response made after a duration signal had appeared for 30 s. Probe tests involved trials in which the duration signal was extended to 90 s and reinforcement was omitted. In Experiment 1, 2.0 mg/kg d-amphetamine shifted peak responding to a duration shorter than that found with saline. In Experiment 2, the dose-response function for this drug effect was examined. A 0.3-mg/kg dose of d-amphetamine had no impact on performance, but a 1.0-mg/kg dose shifted the peak duration significantly relative to saline; a 2.0-mg/kg dose shifted the function even more. These results complement previous findings with rats tested with the peak procedure and pigeons tested with a discrimination procedure.

Morphine-Induced Conditioned Place Preference in Preweanling and Adult Rats

The ability of morphine to support a conditioned place preference (CPP) in preweanling (18-22-day-old) and adult (70-90-day-old) rats was assessed. Prior to a 15-min compartment preference test, subjects received a saline-paired, 30-min exposure to a distinct compartment 2 h prior to receiving an injection of 1 or 5 mg/kg of morphine or saline, paired with a 30-min exposure to an alternate compartment for 4 consecutive days. Although overall activity levels differed substantially across age, preweanling and adult rats displayed similar patterns of activity during conditioning. Moreover, only adults exhibited a significant sex difference; females were more active than were males following an injection of 5 mg/kg of morphine. Both doses of morphine supported a comparable CPP in preweanlings and adults, and both ages exhibited relatively low activity levels while in the morphine-paired compartment. These similarities across age suggest that the CPP procedure may prove to be useful in elucidating the ontogeny of learning, memory, and stimulus selection in rats.

Common coding in pigeons: Partial versus total reversals of one-to-many conditional discriminations

Common coding in pigeons was examined using a delayed conditional discrimination in which each sample stimulus was associated with two different comparison stimuli (one-to-many mapping). In Experiment 1, pigeons matched circle and dot samples to red and green hues and vertical and horizontal line orientations. In Experiment 2, the samples were red and green and the comparisons were vertical and horizontal spatial positions (up vs. down and left vs. right). Following acquisition to high levels of accuracy in each experiment, the associations between the samples and either both sets or only one set of comparisons were reversed. Pigeons learned the total reversals faster than the partial reversals. These results suggest that when different comparisons are associated with a common sample, they may become functionally equivalent.

Win-stay/lose-shift and win-shift/lose-stay learning by pigeons in the absence of overt response mediation.

Win-stay/lose-shift and win-shift/lose-stay behavior in pigeons was compared using a two-alternative conditional discrimination for which the number of trials involving each of the task components could be precisely controlled. One group was rewarded for pecking the location just pecked if those pecks were followed by food and for pecking the other location if those pecks were not followed by food (win-stay/lose-shift). Another group was rewarded for pecking the location just pecked if those pecks were not followed by food and for pecking the other location if those pecks were followed by food (win-shift/lose-stay). With increasing delay to comparison choice, pigeons were more accurate on trials when initial pecking was followed by the absence of food than by food (Experiment 1). However, when hypothesized overt response mediation was discouraged (Experiment 2), a win-stay superiority effect emerged with increasing delay to comparison choice. Thus, unlike rats, pigeons may be somewhat predisposed to repeat a response to a location to which responses have been previously rewarded.

The influence of stimulus attributes on duration matching-to-sample in pigeons

A duration matching-to-sample procedure was used to examine the influence of signal properties on temporal estimation and working memory. The results indicated that pigeons responded to durations of a light as if they were longer than equal durations of the absence of the same light, but delay performances did not differ between the two sample types. Similarly, pigeons responded to durations of a bright white light as if they were longer than equal durations of a dim red light, but again, delay performances did not differ between the two sample types. These results are discussed in terms of theoretical issues pertinent to timing, attention, and working memory.

Olfactory cues and morphine-induced conditioned analgesia in rats

In a Pavlovian conditioning procedure, rats were exposed to an odor conditioned stimulus (CS) and then were given morphine with its effect serving as the unconditioned stimulus (US). After four CS-US pairings, the CS was tested alone to assess the presence of an analgesic conditioned response (CR) using a hot-plate test. In Experiment 1a, two groups were conditioned by pairing either 10 mg/kg morphine or saline with an odor CS. In Experiment 1b, two groups were given an odor CS paired or unpaired with 10 mg/kg morphine. These results established that an odor cue can support a morphine-induced analgesic CR. Experiment 2 characterized the dose-effect curve (0, 3, 10, and 30 mg/kg morphine) using an odor conditioning procedure. The dose-effect curve showed an inverted U-shaped function, with the 10 mg/kg morphine group having significantly longer paw-lick latencies compared to all other groups. This finding contrasts with the monotonically ascending dose-effect curve for the analgesic unconditioned response (UR) to morphine.

Latent inhibition in preweanling rats

Two experiments describe a latent inhibition effect in preweanling rats. Subjects were trained with a light-tone compound conditioned stimulus (CS) paired with a footshock unconditioned stimulus. Experiment 1 tested 17- and 21-day-old rats and Experiment 2 tested 18-day-old rats. In each experiment, rat pups given nonreinforced preexposure to the CS expressed less conditioned fear, as indexed by a suppression in general activity during the CS, than did same-age control rats given no preexposure. The results are discussed in terms of behavioral development and the role of the functional maturity of the hippocampus in latent inhibition.