Evolutionary consideration of serial order processing by the normal rodent brain

The granular mid-dorsolateral prefrontal cortex of the primate brain is the final stage in the evolutionary development from the phylogenetically older agranular frontal cortex that lies above the anterior part of the corpus callosum, i.e., the anterior part of the cingulate gyrus. In the rodent brain, the prefrontal cortex is limited to this earlier agranular stage and is often referred to as the "medial prefrontal cortex." The primate mid-dorsolateral prefrontal cortex is critical for the monitoring of information in working memory, including the monitoring of the precise serial order of flexible randomly varying sequences of stimuli. Thus, a fundamental question arises: what are the functional limits of serial order processing in the rodent brain which, anatomically, possesses only the earlier stage in the evolution of the prefrontal cortex? The present study examined the ability of normal rats to monitor the serial order of sequences of spatial stimuli (locations). The normal rodent brain can code accurately the order of directional left-to-right or right-to-left sequences of locations. However, the serial order of nondirectional randomly varying sequences can be processed only to a limited extent: when performance relies considerably on the salient memory of the most recent stimulus to code an approximate order of the earlier stimuli within working memory. Thus, the development of the granular prefrontal cortex in the primate brain provides an additional higher level of executive control permitting precise coding of the order of stimuli in working memory that is essential to manipulate information to plan action.