Medial septum lesions disrupt exploratory trip organization: Evidence for septohippocampal involvement in dead reckoning

Rats organize their open field behavior into a series of exploratory trips focused around a central location or home base. In addition, differences in movement kinematics have been used to fractionate the exploratory trip into tour (i.e., sequences of linear movement or progressions punctuated by stops) and homeward (i.e., single progression direct to the home base) segments. The observation of these characteristics independent of environmental familiarity and visual cue availability has suggested a role for self-movement information or dead reckoning in organizing exploratory behavior. Although previous work has implicated a role for the septoltippocampal system in dead reckoning based navigation, as of yet, no studies have investigated the contribution of the medial septum to dead reckoning. First, the present study examined the organization of exploratory behavior under dark and light conditions in control rats and rats receiving either electrolytic or sham medial septum lesions. Medial septum lesions produced a significant increase in homeward segment path circuity and variability of temporal pacing of linear speeds. Second, as an independent assessment of the effectiveness of the medial septum lesions, rats were trained to locate a hidden platform in the standard water maze procedure. Consistent with previous research, medial septum lesions attenuated learning the location of the hidden platform. These results demonstrate a role for the medial septum in organizing exploratory behavior and provide further support for the role of the septohippocampal system in dead reckoning based navigation. (c) 2006 Elsevier Inc. All rights reserved.