Section of the descending columns of the fornix produces delay- and interference-dependent working memory deficits.

Rats were allocated to one of three surgical treatment groups, and given either sham operations (SHAM), transection of the descending columns of the fornix (DCF), or transection of fibres at the base of the lateral septal nuclei (SC). They were trained on a rewarded alternation task, run as a spatial working memory task on an elevated T-maze, with a minimal intratrial interval. There were no differences between the SHAM and the SC rats. The DCF rats showed an initial impairment in acquisition, but their performance gradually improved to control levels. The imposition of a 20-s intratrial interval impaired choice accuracy in all groups, but the extent of the impairment was significantly greater in the DCF group. Addition of an extra, and conflicting, 'information run', to provide a source of proactive interference, also impaired performance in all groups, but the impairment was again greatest in the DCF group, suggesting that this lesion increases intrusion errors. Subsequent testing in the acquisition of 2-way active avoidance revealed that the DCF lesion facilitated learning with respect to the other two groups. The results indicate that there are clear functional differences between the different outputs from the subiculum, and that the descending subicular output plays a significant role in normal memory.