Pamela Jackson-Smith

Continuous recognition of spatial and nonspatial stimuli in hippocampal-lesioned rats

The present experiments compared the performance of hippocampal-lesioned rats to control rats on a spatial continuous recognition task and an analogous nonspatial task with similar processing demands. Daily sessions for Experiment 1 involved sequential presentation of individual arms on a 12-arm radial maze. Each arm contained a Froot Loop reinforcement the first time it was presented, and latency to traverse the arm was measured. A subset of the arms were repeated, but did not contain reinforcement. Repeated arms were presented with lags ranging from 0 to 6 (0 to 6 different arm presentations occurred between the first and the repeated presentation). Difference scores were computed by subtracting the latency on first presentations from the latency on repeated presentations, and these scores were high in all rats prior to surgery, with a decreasing function across lag. There were no differences in performance following cortical control or sham surgery. However, there was a total deficit in performance following large electrolytic lesions of the hippocampus. The second experiment employed the same continuous recognition memory procedure, but used three-dimensional visual objects (toys, junk items, etc., in various shapes, sizes, and textures) as stimuli on a flat runway. As in Experiment 1, the stimuli were presented successively and latency to run to and move the object was measured. Objects were repeated with lags ranging from 0 to 4. Performance on this task following surgery did not differ from performance prior to surgery for either the control group or the hippocampal lesion group. These results provide support for Kesners attribute model of hippocampal function in that the hippocampus is assumed to mediate data-based memory for spatial locations, but not three-dimensional visual objects.

Impaired place learning and unimpaired cue learning in hippocampal-lesioned pigeons

Hippocampal processing is often crucial for normal spatial learning and memory in both birds and mammals, suggesting a general similarity in avian and mammalian hippocampal function. However, few studies using birds have examined the effect of hippocampal lesions on spatial tasks analogous to those typically used with mammals. Therefore, we examined how hippocampal lesions would affect the performance of pigeons in a dry version of the water maze. Experiment 1 showed that hippocampal-lesioned birds were impaired in acquiring the location of hidden food in the maze. Experiment 2 showed that hippocampal-lesioned birds were not impaired when a single cue indicated the location of hidden food. These results support the notion that avian and mammalian hippocampal functions are quite similar, in terms of the tasks for which their processing is crucial and the tasks for which it is not.

Interaction of sample dimension and sample-comparison mapping on pigeons’ performance of delayed conditional discriminations

Coding strategies developed in the acquisition of delayed conditional discriminations can be assessed by independently manipulating sample and comparison memory load. Two stimulus dimensions that can affect memory load were examined: Number of stimuli in the sample and comparison sets (two vs. four) was manipulated between groups in a 2×2 design, and discriminability of sample and comparison stimuli (hues vs. lines) was manipulated between counterbalancing subgroups and within subjects. The results indicated large effects of sample discriminability but not of comparison discriminability, evidence for retrospective coding. There was also a significant effect of number of stimuli in the comparison set (although only with hard-to-discriminate samples) but not of number of stimuli in the sample set, evidence for prospective coding. These findings suggest evidence for retrospective coding with easy-to-discriminate samples, independently of number of stimuli in the comparison set, and evidence for prospective coding with hard-to-discriminate samples.

Memory codes in pigeon short-term memory: Effects of varying the number of sample and comparison stimuli ☆

Abstract In a delayed conditional discrimination task, pigeons can remember either some aspect of the conditional stimulus (i.e., they can code retrospectively) or some aspect of the stimulus to which they will respond at the end of the delay (i.e., they can code prospectively). To determine the nature of the memory code, we varied the number of possible sample stimuli (two or four) and the number of possible comparison stimuli (two or four) factorially across groups. Birds in all four groups were initially trained on a zero-delay, conditional discrimination with lines (vertical and horizontal) and/or shapes (circle and triangle), and were then tested with longer delays between sample offset and comparison onset. Acquisition of the conditional discrimination was affected by both the number of sample and comparison stimuli: birds were slower to reach criterion the greater the number in either stimulus set. During delay testing, however, only the number of comparisons affected performance. Overall, retention was poorer with four comparisons than with two. These data provide evidence for prospective coding in pigeon short-term memory.

Mediational use of internal representations of food and no-food events by pigeons☆

Abstract Two experiments were conducted with pigeons to determine whether internal representations of food and no-food events can act as mediators in the formation of new associations. Specifically we asked if the representation of an anticipated event can replace the event itself in an established conditional discrimination. In Phase 1 of both experiments pigeons were differentially autoshaped to peck hue stimuli, only one of which was followed by access to food. In Phase 2 they were trained on a symbolic 0-delay matching-to-sample (DMTS) task with food and no-food samples and line-orientation comparisons. In Phase 3 the birds were tested on a symbolic DMTS task in which the hue stimuli from Phase 1 were substituted for the Phase 2 food-event samples. For the Pos group, the hue followed by food in Phase 1 was substituted for the food sample and the hue associated with no-food was substituted for the no-food sample so that the resulting sample/comparison pairings were consistent with the food-event mediator. For the Neg group the pairings were reversed so that the sample/comparison pairings were inconsistent with the food-event mediators. In Experiment 1, when the no-food event was the absence of an event, acquisition of the transfer task was significantly more rapid in the Pos than in the Neg condition. In Experiment 2, when the no-food event was the presentation of an empty food hopper, the Pos group transferred at a significantly higher level than the Neg group. The two experiments provide evidence that in pigeons, representations involving event anticipations can be substituted for, and are thus similar to, the events themselves.

Hippocampal lesions impair rats' performance of a nonspatial matching-to-sample task

In the present study, we attempted to develop a nonspatial delayed match-to-sample task that would aid in assessing the role of the hippocampus in memory. It is difficult to compare directly the results of many past studies of hippocampal function because studies using different species often use very different tasks to assess the same theoretical constructs. Rodent studies typically have used spatial tasks, such as the radial arm maze, with or without delays, whereas primate studies typically have used match-to-sample (MTS) tasks. In the present study, we examined the performance of hippocampal-lesioned rats on nonspatial simultaneous matching-to-sample (SMTS) and delayed matching-to-sample (DMTS) in a cross maze. Postoperatively, the hippocampal-lesioned subjects were unable to reacquire the SMTS task they had learned preoperatively. The results suggest that this approach may provide a useful tool for probing the role of the hippocampus in memory, and that the configurai relationships between stimuli and events or the conditional nature of a learning/memory task may be especially important aspects for determining the impact of hippocampal damage.

Effects of ibogaine on sensory-motor function, activity, and spatial learning in rats

Ibogaine, a naturally occurring alkaloid, has been show to reduce naloxone-precipitated withdrawal symptoms from morphine. Given the clinical possibilities, it is important to determine ibogaines effects on sensory-motor function, activity, learning, and memory. Long-Evans rats injected with doses of 20-60 mg/kg of ibogaine displayed slower response times on sensory and sensory-motor tests and were impaired in performing specific motor reflexes at doses of 40-60 mg/kg. Furthermore, these rats showed a marked reduction in locomotor and nonlocomotor activity, as well as emotionality at doses ranging from 10-40 mg/kg. At the higher doses the rats appeared to be virtually inactive. There were also deficits in learning a spatial location task (a dry-land version of the Morris water-maze). The deficits, however, were probably due to a reduction in locomotor activity and reduction in detection of sensory information. In a final experiment, a single injection of 40 mg/kg of ibogaine had marked deleterious effects on the acquisition of the spatial location task 1 but not 7 days after the injection, even though in this case there were no effects on sensory motor function 1 or 7 days after the injection. Thus, there are severe sensory-motor activity and learning problems while the animal is under the influence of ibogaine (acute effect) as well as long-term consequences on learning without concomitant changes in sensory-motor function.

Discriminative stimulus properties and schedule effects of fencamfamine in rats

The behavioral properties of fencamfamine, a sympathomimetic central stimulant recently identified in alleged cocaine samples, were studied in two different paradigms. In Experiment I, rats were trained to discriminate between injections of saline (0.1 ml/kg, IP) and cocaine (3.0 mg/kg, IP) in a two-lever drug discrimination task on a fixed-ratio (FR) 10 schedule of water presentation. Fencamfamine (0.3-3.0 mg/kg, IP) produced cocaine-appropriate choice behavior and was slightly more potent than cocaine in producing this effect. In Experiment II, rats responded under a multiple fixed-interval (FI) 300 sec, FR 20 schedule of water presentation. Fencamfamine (0.1-10.0 mg/kg, IP) and cocaine (0.1-30.0 mg/kg, IP) produced qualitatively similar effects on responding under this schedule. With increasing doses of either drug, FI responses rates first increased, then decreased; FR response rates were only decreased. Fencamfamine was approximately three times more potent than cocaine in producing these effects. The results of these two experiments indicate that fencamfamine and cocaine have similar behavioral properties.

Categorical shape and color coding by pigeons.

Categorical coding is the tendency to respond similarly to discriminated stimuli. Past research indicates that pigeons can categorize colors according to at least three spectral regions. Two present experiments assessed the categorical coding of shapes and the existence of a higher order color category (all colors). Pigeons were trained on two independent tasks (matching-to-sample, and oddity-from-sample). One task involved red and a plus sign, the other a circle and green. On test trials one of the two comparison stimuli from one task was replaced by one of the stimuli from the other task. Differential performance based on which of the two stimuli from the other task was introduced suggested categorical coding rules. In Experiment 1 evidence for the categorical coding of sample shapes was found. Categorical color coding was also found; however, it was the comparison stimuli rather than the samples that were categorically coded. Experiment 2 replicated the categorical shape sample effect and ruled out the possibility that the particular colors used were responsible for the categorical coding of comparison stimuli. Overall, the results indicate that pigeons can develop categorical rules involving shapes and colors and that the color categories can be hierarchical.

Rats do show primacy and recency effects in memory for lists of spatial locations: A reply to Gaffan

It has been demonstrated that rats display both primacy and recency effects in list-learning experiments for both item- and order-recognition memory for spatial location information. Contrary to a suggestion made by Gaffan (1992) that previous demonstrations of primacy effects in rats are not valid due to statistical and methodological problems, we show that (1) it is not likely that our results are due to experimenter bias and/or nonindependence of trials, (2) primacy and recency effects are obtained whether or not one uses a training criterion, and (3) when many of the published reports from our laboratory are combined, the number of subjects and trials are sufficient to evaluate the validity of primacy and recency effects in list-learning experiments.