Individual Differences in Spatial Memory and Striatal ChAT Activity among Young and Aged Rats

Individual differences in spatial memory among young and aged rats were assessed using memory tasks related to integrity of the hippocampus and the neostriatum. Relationships were then examined between measures of spatial memory and regional choline acetyltransferase (ChAT) activity, a marker for cholinergic integrity. Twenty-four-month-old Long-Evans rats were impaired in comparisons with 6-month-old rats on measures of place learning, working memory, reference memory, and perseveration in water-maze tasks. Aged rats that were impaired on one measure of memory, however, were not necessarily impaired on other measures. ChAT activity in the ventromedial and dorsolateral neostriatum of aged rats was significantly reduced in comparisons with young rats whereas no difference was found in the hippocampus. Aged rats with the most ChAT activity in the anterior ventromedial neostriatum performed best on the place-learning and reference memory tasks but also made the most perseverative errors on the working memory task. In addition, young and aged rats with the most ChAT activity in the anterior dorsolateral neostriatum were those with the least accurate working memory. No relationships were found between ChAT activity in the hippocampus and spatial memory. Thus age-related memory impairment has components that can be segregated by measuring relationships between cholinergic integrity in subregions of the anterior neostriatum and memory tasks with different strategic requirements.     

The law of effect and CR contingent modification of the UCS

A persistent theoretical contention in the American conditioning literature is that Conditional Reflexes (CR) are reinforced through some hidden Thorndikian law of effect. In its most general form, law of effect theories hold that Conditional Reflex-Unconditional Stimulus (UCS) overlap modifies the sensory consequences of the UCS to provide the source of differential reinforcement of CRs. However, consideration of the historical development of law of effect theories reveals they have lacked testability due to the fact that the sources of reinforcement have been alleged to beintrinsically related to the occurrence and topography of CRs. Such a postulated intrinsic relationship has made it difficult to experimentally manipulate the amount of CR-correlated reinforcement, since the source of reinforcement is presumed to be under The Subject’s (S’s) control. Some experiments will be reported in which the relation between CRs and properties of the UCS are madeextrinsic and, accordingly, permit an experimental assessment of law of effect formulations.