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PubMed1995

Hippocampal long-term potentiation does not affect either discrimination learning or reversal learning of the rabbit nictitating membrane response.

The theoretical premise that the acquisition and storage of information occurs through the strengthening of synaptic connections has contributed to the popularity of long-term potentiation (LTP) as a candidate neural mechanism for associative learning. However, whether experimentally induced LTP facilitates, disrupts, or has no effect on subsequent learning is a controversial issue. The present study examined the reported facilitative effect of LTP within hippocampal perforant path-dentate gyrus synapses on subsequent discriminative conditioning of the rabbit nictitating membrane response. In addition, the effect of LTP on subsequent reversal learning of the initial discrimination was examined. LTP did not significantly affect acquisition of the initial discriminative response or subsequent reversal learning. Furthermore, the magnitude of LTP could not be used to predict the rate of acquisition of either task. The failure to find an effect of LTP on classical conditioning of the rabbit nictitating membrane response mirrors the recent failures to replicate the disruptive effect of LTP on spatial learning in the rat. Thus, the potential contribution of an LTP-like mechanism to associative learning remains equivocal.

Animals↗
PubMed1991

Hippocampus and olfactory discrimination learning: effects of entorhinal cortex lesions on olfactory learning and memory in a successive-cue, go-no-go task.

Three experiments assessed the effect of entorhinal cortex lesions on olfactory learning and memory using a successive-cue olfactory discrimination paradigm. In contrast to the results of other studies that used a simultaneous-cue paradigm, lesions of the entorhinal cortex facilitated rats' acquisition of individual odor discrimination problems, with no impairment in memory for the individual odors across both short (24-hr) and long (65-day) retention intervals and despite limited training. When considered together with previous observations of facilitation or impairment in learning after damage to the hippocampal system, the present data suggest that the hippocampus is preferentially involved in encoding relations among multiple stimuli. By this account, facilitation of performance is due to an interaction between hippocampal system dysfunction and task conditions that hinder direct comparisons among cues.

Animals↗
PubMed2004

Opposing effects of D1 and D2 receptor activation on odor discrimination learning.

Dopaminergic modulation of cortical activity has been implicated in the formation of reward associations. There is abundant evidence for dopaminergic effects on olfactory processing. Using an olfactory discrimination task, the authors show that D1 and D2 dopamine receptors can regulate rats' olfactory discrimination capacities and that the effects of receptor activation functionally oppose one another. Injection of either the D1 agonist SKF 38393 (10 mg/kg) or the D2 antagonist spiperone (0.62 mg/kg) facilitated the discrimination of similar odorants but had no effect on the discrimination of dissimilar odorants, whereas both the D, antagonist SCH 23390 (0.025 mg/kg) and the D2 agonist quinpirole (0.2 mg/kg) significantly impaired rats' ability to discriminate similar and dissimilar odorants.

Animals↗
PubMed1981

Early postnatal x-irradiation of the hippocampus and discrimination learning in adult rats.

Rats with X-irradiation-produced degranulation of the hippocampal dentate gyrus were trained in the acquisition and reversal of simultaneous visual and tactile discriminations in a T-maze. These experiments employed the same treatment, apparatus, and procedure but varied in task difficulty. In the brightness and roughness discriminations, the irradiated rats were not handicapped in acquiring or reversing discriminations of low or low-moderate task difficulty. However, these rats were handicapped in acquiring and reversing discriminations of moderate and high task difficulty. In a Black/White discrimination, in which the stimuli were restricted to the goal-arm walls, the irradiated rats were handicapped in the acquisition (low task difficulty) and reversal (moderate task difficulty) phases of the task. These results suggest that the irradiated rats were not handicapped when the noticeability of the stimuli was high, irrespective of modality used, but were handicapped when the noticeability of the stimuli was low. In addition, these results are consistent with the hypothesis that rats with hippocampal damage are inattentive due to hyperactivity.

Animals↗
PubMed2006

Temporal discrimination learning by pigeons.

Memory for time by animals appears to undergo a systematic shortening. This so-called choose-short effect can be seen in a conditional temporal discrimination when a delay is inserted between the sample and comparison stimuli. We have proposed that this temporal shortening may result from a procedural artifact in which the delay appears similar to the intertrial interval and thus, produces an inadvertent ambiguity or 'instructional failure'. When this ambiguity is avoided by distinguishing the intertrial interval from the delay, as well as the samples from the delay, the temporal shortening effect and other asymmetries often disappear. By avoiding artifacts that can lead to a misinterpretation of results, we may understand better how animals represent time. An alternative procedure for studying temporal discriminations is with the psychophysical bisection procedure in which following conditional discrimination training, intermediate durations are presented and the point of subjective equality is determined. Research using the bisection procedure has shown that pigeons represent temporal durations not only as their absolute value but also relative to durations from which they must be discriminated. Using this procedure, we have also found that time passes subjectively slower when animals are required to respond to the to-be-timed stimulus.

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