Working memory for temporal order is impaired after selective neonatal hippocampal lesions in adult rhesus macaques.

A previous study in this laboratory demonstrated, for the first time, that neonatal lesions of the hippocampus impair monitoring working memory, as measured by a self-order task, but spare recency memory, as measured by the session-unique delayed nonmatching task. To substantiate and extend this novel finding, we assessed working memory in these same animals using a serial order memory task. In humans and non-human primates the serial order memory task has been shown to be dependent upon the integrity of the dorsolateral prefrontal cortex. Additionally, the serial order task has the ability to examine the integrity of non-dorsolateral dependent working memory functions, providing specificity to conclusions drawn from this task. Thus, monkeys with neonatal lesions of the hippocampus and sham-operated control subjects were tested on two versions of the serial order memory task (3 and 4 objects). The results of this study demonstrated that neonatal hippocampal lesions did not impair performance on the 3-object version of the task, confirming our previous finding of intact non-dlPFC dependent working memory. In contrast, these same animals showed a significant impairment on the dlPFC dependent phase of the 4-object serial order task. This finding was further confirmed through a series of probe trials. These results, in combination with our earlier finding, suggest that early lesions of the hippocampus may have impacted the function of the dlPFC or its interactions with the hippocampus.