Both fornix and anterior thalamic, but not mammillary, lesions disrupt delayed non-matching-to-position memory in rats.

Rats with radiofrequency lesions of the fimbria/fornix, or neurotoxic lesions of the mammillary bodies or the anterior thalamic nuclei were tested on their ability to perform a delayed non-matching-to-position task that had been learnt before surgery. In this task rats had to respond to a sample lever in an operant chamber and, after a variable delay (during which they were required to respond at the magazine tray), press the other lever when both were presented. Extensive mammillary body lesions had no effect on performance. In contrast, lesions in either the anterior thalamic nuclei or the fimbria/fornix produced marked deficits, the pattern of these deficits being consistent with a mnemonic impairment. It is argued that the anterior thalamic nuclei represent an important hippocampal output for spatial problems, but that the mammillary bodies are only necessary for certain types of mnemonic task.