Darlene M. Skinner

The effects of hippocampal lesions on response, direction, and place learning in rats.

Rats with hippocampal or sham lesions were trained to find food on a T maze located at 2 positions. Response rats were required to make a right or left turn. Direction rats were required to go in a consistent direction (east or west). Place rats were required to go to a consistent location, relative to room cues. One place group had distinguishable start points at the 2 maze positions, whereas another place group had start points facing the same side of the room. Controls took longer to solve a place problem than the response and direction problems when the start points were not distinguishable. Rats with hippocampal lesions were not different than controls on the response problem but were impaired on the direction and place problems.

Acquisition of conditional discriminations in hippocampal lesioned and decorticated rats: evidence for learning that is separate from both simple classical conditioning and configural learning.

This study examined whether hippocampal or neocortical lesions would impair acquisition of a discrimination task using taste aversions. Rats were injected with a drug 15 min before a flavored solution-lithium chloride pairing. On alternate days, vehicle injections preceded and followed access to the same flavored solution. Rats learned to consume significantly more of the flavored solution after vehicle injections than after drug injections. Rats with hippocampal lesions or neonatal decortication performed as well as controls. Rats with hippocampal lesions also learned a similar task in which visual and textural cues predicted whether access to a flavored solution would be followed by an injection of lithium chloride or vehicle. However, these hippocampal lesions did impair performance in the Morris water task. Occasion setting may involve a type of learning dissociated from both simple classical conditioning and configural learning.

Acquisition and retention of negative patterning in hippocampal-lesioned rats

In this series of experiments, we examined whether animals with aspiration lesions of the hippocampus could retain a negative patterning task learned preoperatively (Experiment 1) or acquire a negative patterning task postoperatively (Experiment 2). In a negative patterning task, each of two single cues is followed by an unconditioned stimulus (A+, B+), while the two cues presented together are not followed by the unconditioned stimulus (AB−). Rats were exposed to a novel context and allowed to drink a saccharin solution for 15 min on AB− trials. On A+ trials, the rats were exposed to saccharin alone, and on B+ trials, the rats were exposed to the novel context with a water bottle and were given an injection of LiCl in both cases. The rats showed aversions to the context and saccharin alone but consumed large amounts of the saccharin when the two cues were presented together. While hippocampal-lesioned rats showed some deficits on retention tests, they showed good acquisition of the negative patterning task.

Drug Discrimination Learning Using a Taste Aversion Paradigm: An Assessment of the Role of Safety Cues

In drug discriminations using conditioned taste aversions one drug signals a taste—toxin pairing while a second drug, or vehicle, signals the same taste in the absence of the toxin. Here we examine the role of safety signals, the cues that predict the absence of the toxin and further characterize the properties of drug states as danger cues, the cues that predict the toxin. Drugs, as safety cues, result in slightly faster acquisition and better discriminative performance than does conditioning using a saline vehicle or no explicit stimulus as the safety signal. Saline injections, as safety cues, provided no additional cueing information when a drug state serves as a danger cue. Drug states, as safety cues, were treated like conditioned inhibitors when saline served as the danger cue. Interposing a naloxone-induced trace between the morphine-induced state and the presentation of the flavor revealed that overlap of the drug and the flavor was necessary when morphine was a safe cue but not when morphine was a danger cue. These data suggest that morphine as a danger cue acts as an occasion setter but that morphine as a safety cue acts as a conditioned inhibitor.

A Two-Platform Task Reveals a Deficit in the Ability of Rats to Return to the Start Location in the Water Maze

The ability of rats to return to the start location was examined with a 4-arm radial water maze. The task required rats to find 2 hidden platforms in sequence. Rats were released from 1 of 3 arms and there was a platform located in the fourth arm. Once a rat found this platform, a 2nd platform was raised in another location, which was either the start location, for 1 group, or another fixed location, for a control group. Across 3 experiments, all rats learned the location of the 1st fixed platform in 80 to 120 trials. However, rats had difficulty finding a 2nd platform if it was at the start location. Control groups revealed that rats could learn 2 platform locations and that the difficulty in learning to return to the start location did not seem to be attributable to its aversive nature. In separate groups, exposure to the start location was increased by starting the rats from an initially stable platform. Rats still did not readily learn to return to the start location. The authors suggest that start location, when varied, cannot readily be used to define the location of a hidden platform.

Amygdala lesions impair context aversions but not the ability of contexts to serve as occasion setters

Rats with large amygdala lesions (Experiment 1) were compared with sham controls on a conditional discrimination task that allowed assessment of occasion-setting learning. A saccharin solution was paired with lithium chloride in one context, but with saline in a second context. All the groups learned to suppress fluid consumption in the first, relative to the second, context. Sham lesioned rats, but not amygdala-lesioned rats, also showed a large aversion to the first context on a choice test. Shamlesioned Pavlovian control groups, given direct pairings of Context 1 with lithium chloride and of Context 2 with saline, showed large aversions to Context 1, whereas similarly trained amygdala-lesioned rats did not avoid the context associated with lithium chloride. Rats with discrete lesions of either the central nucleus or the basolateral nucleus (Experiment 2) of the amygdala were not impaired on the conditional discrimination task but did show deficits on the place choice test. The data from amygdala-lesioned rats in the present study support previous behavioral data in suggesting that the aversive properties of contextual cues, as acquired through Pavlovian conditioning, are neither necessary nor sufficient for occasion-setting learning.

Lost in time: rats are unable to return to a start location that varies

Path integration provides guidance based on cues generated by a point of reference (usually start location) and subsequent self-movement. This well-established mechanism suggests start location may have special significance and might provide a useful window into memory in rats. In an earlier study rats did not learn to return to a start location in a four-arm radial water maze when the start location varied across trials. Here we examine return to start location in appetitive tasks. Initially, rats were released from one of three arms with the food located in the fourth arm. Once a rat found the food, a second arm was baited; either the start arm, for one group, or another fixed location, for a control group. Rats had difficulty finding the second food reward in the start arm, but not in another fixed location. Performance was similar when rats were trained with a three-arm maze. It was also observed that rats learned the initial fixed location more slowly if they were required to learn a variable second location. This suggested the nature of the journey affected the rate of problem solution. One explanation for the failure of rats to return to the start location is that the path integrator is reset upon reaching the first correct arm. In a final experiment, a foraging task was used where resetting of the path integrator should not occur. Again, rats failed to return to the start location when it varied across trials. These findings suggest rats did not time tag start locations, which indicates that there may be constraints on the occurrence of episodic-like memory in rats.

The moving fire hydrant experiment : Movement of objects to a new location reelicits marking in rats

Three experiments were carried out to investigate the influence of object relocation on object marking in an open field by hooded and albino rats. Object marking was reelicited when an object was moved to a new location in the second half of an open field test. Control conditions revealed that an object briefly moved and returned to the original location elicited no more marking than a stationary object. The higher level of marking of the relocated object suggests that object marking may provide an index of spatial knowledge. The implication of spatial knowledge in controlling marking behavior is congruent with observations that rats with hippocampal damage show increased marking.

The effects of feature type in operant serial feature-positive discriminations ☆

Abstract In two experiments, rats were trained on two operant serial feature positive discriminations in which one feature was a flavored solution and the second feature was a visual or auditory cue. As in a previous study ( Goddard & Holland, 1996 ), transfer of a feature’s control to the target of the other discrimination was not observed when the flavor feature and the reinforcer were flavored sucrose solutions (Experiment 1). The performance of comparison groups showed that this lack of transfer was not due to confounded differences in the event contingencies resulting from having similar stimuli serve as feature and reinforcer. By contrast, in Experiment 2, transfer was observed between visual and flavor features when the flavor feature was unsweetened and the reinforcer was plain sucrose. These results suggest that the lack of transfer in Experiment 1 and in Goddard and Holland’s (1996) study were related to the biological significance or hedonic properties of the sucrose feature.