A complex associative structure formed in the mammalian brain during acquisition of a simple visual discrimination task: dorsolateral striatum, amygdala, and hippocampus.

A review of empirical evidence supporting the multiple memory systems view of the organization of learning and memory in the mammalian brain is presented as a powerful component of a broader foundation of basic scientific information necessary for understanding human behavior. However, it is argued that there are significant gaps in our knowledge about these different learning and memory systems, how they interact with one another, and how they interact with the rest of the brain. To demonstrate how little we know about these complex processes, this article reviews recent evidence showing the complexity of associative structure formed during the acquisition of a simple visual discrimination task. The results show that the dorsolateral striatum is necessary for the acquisition of this task but that both the amygdala and hippocampus incidentally acquire and store information during this training period. A new experiment is also presented showing that rats with complete or partial (dorsal vs. ventral) hippocampal lesions show a retrograde amnesic effect on the simple visual discrimination task despite the fact that these same lesions produce no impairment in the anterograde direction. Evidence is presented in support of one interpretation of this effect suggesting that the retrograde amnesia occurs, at least in part, because the hippocampus acquires a context-specific inhibitory association during original training. Although this representation is not required for acquisition of the task in the anterograde direction, removal of this representation has a disruptive effect on expression of the task. (c) 2007 Wiley-Liss, Inc.