Accumulation of c-fos mRNA in rat hippocampus during acquisition of a brightness discrimination

Training rats to attain a foot-shock-motivated brightness discrimination in a Y-maze results in an early and transient increase of hippocampal c-fos mRNA levels. Maximal accumulation was observed immediately after training, returning to basal levels during the following 2 h. A similar increase was obtained when rats were subjected to a pseudotraining with an equal number of runs, but with random pairing of the choice of bright and dark alleys with foot shock. It is suggested that induction of hippocampal c-fos mRNA expression is a necessary, but not sufficient, prerequisite for the formation of long-term memory trace. This early gene expression seems rather to correspond to an initial stage induced by complex stimulus presentation of both the training and the pseudotraining procedure. The subsequent late synthesis or processing of target proteins finally contributing to the formation of a permanent trace requires the action of further convergent signals to principal cells, probably mediating reward or emotional influences.     

Novel popout and familiar popout in a brightness discrimination task

A single novel word among several familiarized words may be localized more effectively than the familiarized words (novel popout). Early demonstrations of novel popout attributed the effect to the capture of attentional resources by novel stimuli. Christie and Klein (1995, 1996) argued that differential recollection of novel versus familiar words could alternatively account for the popout effect. In the present experiments, participants judged which of four locations contained a physically brighter word. A bright novel word was localized significantly better than a bright familiar word in one-novel/three-familiar arrays, inconsistent with a retrievability account of novel popout. However, a bright familiar word was also localized better than a bright novel word in three-novel/one-familiar arrays, inconsistent with the mismatch theory proposed by Johnston and Hawley (1994). The results suggest that familiarity and novelty provide a perceptual segregation of the odd item; superior brightness discrimination at that location may be due either to attentional capture or to locational ambiguity within the larger group.     

Effect of apomorphine on retention of a brightness discrimination in dopamine supersensitive rats

Apomorphine is known to improve the retention performance of a brightness discrimination task after post-training intrahippocampal application to rats. The present study shows that the retention performance of dopamine supersensitive rats was improved by a low dose of apomorphine which was still ineffective in control animals. Dopamine supersensitivity by itself had no effect on acquisition and retention parameters of brightness discrimination. The result further supports the assumption that dopamine receptors in rat hippocampus can modulate the formation of memory.     

Value transfer contributes to ambiguous-cue discrimination learning

Pigeons learned two concurrent simultaneous discriminations in which the S2 for one served as the S1 for the other. When all correct choices were reinforced, accuracy on the former (positive vs. ambiguous-cue or PA) discrimination was lower than on the latter (negative vs. ambiguous-cue or NA) discrimination. When correct choices on the PA discrimination were intermittently reinforced, however, pigeons chose the S2 more often than the S1 on those trials. By contrast, intermittently reinforcing correct choices on the NA discrimination did not affect NA-trial accuracy but yielded higher PA-trial accuracy relative to continuous reinforcement. Together with a separate preference assessment, these results indicate that value transfer, in which some of the positive value accrued by an S1 transfers to its companion S2, contributes to ambiguous-cue performances.