Conclusions Both groups set up a home base under novel-dark and light conditions and display a similar topographical organization of exploratory trips. Medial septum lesions impair the temporal pacing observed on the homeward progression under novel-dark conditions. These results support a role for the medial septum in distance estimation during dead reckoning based navigation. Medial Septum Lesions Impair Rats' Estimate of Distance but Not Direction During Exploration M.M. Martin*; K.L. Horn; D.G. Wallace Dept Psychology, Northern Illinois Univ., DeKalb, IL, USA Introduction When rats are exposed to a novel environment, they set up a home base and organize their exploratory trips around this location. Exploratory trips can be further decomposed into searching and returning components. Although the searching component is a circuitous path composed of a series of slow linear progressions, the returning component is a single rapid progression that directly returns the rat to the home base. That the homeward progression is direct and temporally paced suggests at least two critically distinct components of homeward progressions – direction and distance. These characteristics of exploratory behavior are observed independent of environmental novelty or visual cue availability; thereby suggesting a role for dead reckoning based navigation in organizing behavior. Hippocampal lesions and fimbria-fornix transections have been shown to disrupt both the directional and distance components of the homeward progression. Although previous work has demonstrated that electrolytic medial septum lesions impair performance in the T maze and water maze, the navigational strategy that is compromised remains unclear. Methods Control rats and rats that received electrolytic lesions of the medial septum (MS) were provided with a refuge cage that permitted access to a large circular table. Topographic and kinematic characteristics of movements were analyzed for the first eight exploratory trips that extended at least halfway across the table under novel-dark and light conditions. Figure 2: The upper left panel is a photograph of the testing room and apparatus under normal light conditions. Topographic (top right) and kinematic (bottom) characteristics of a single exploratory trip are plotted for a control rat. Figure 1: Photographs of coronal sections stained for AChE are presented from a representative control (left) and MS (right) rat. Note the atrophy as a result of the lesion and the associated decrease in AChE restricted to the hippocampus. Figure 3: The photograph of the apparatus represents the division of the table into quadrants. Percent time spent in each quadrant is plotted for a representative control and MS rat (upper bar graphs). Using a modified Brown’s mean search difference score, the mean proportion of time spent in the home base quadrant did not differ between groups (bottom). Figure 4: Mean distance traveled did not differ between groups. Figure 6: Linear speed with path curvature is plotted for a representative control and MS rat under novel-dark conditions (left). Mean linear speed / path curvature correlations did not differ between groups or conditions (right). Figure 8: Temporal pacing for normalized long outward and homeward progressions are plotted for a control (top) and MS rat (bottom) under both conditions. Note differences in homeward progression temporal pacing observed between control and MS rats under novel-dark conditions. Figure 9: Mean standard deviation of the peak speed location is plotted for outward and homeward trip progressions under novel-dark (left) and light (right) conditions. MS lesions produced a selective deficit in temporal pacing of the homeward progression under novel-dark conditions A B C D Control MS Homeward Outward Homeward Outward Control MS Outward Homeward Distance (m) Distance (m) Figure 7: Exploratory trip progression max speeds are plotted by progression distance for a control and MS rat under novel-dark conditions (left). The average correlations between max speed and distance were stronger for control rats under novel-dark conditions (right). Figure 5: Left side: Outward and homeward paths from a representative control and MS rat. Right side: Mean distance traveled for each progression did not differ between groups (top). Under light conditions, homeward progressions for MS rats were slightly more circuitous than controls (middle). Mean speed of each progression did not differ between groups (bottom). Control MS Correspondence: M. Martin D. Wallace Web: (A-B)+(A-C)+(A-D) 3 Control Medial septum