Do Rats with Fimbria-Fornix Lesions Ever Return Home? S.L.Bennett 1* ; D.A.Hamilton 2 ; D.G.Wallace 1 1. Dept Psychology, Northern Illinois Univ., DeKalb, IL, USA 2. Dept Psychology, Univ. of New Mexico, Albuquerque, NM, USA Introduction Rats organize their exploration of novel and familiar environments into a series of trips, independent of light or dark testing conditions. Kinematic and topographic analysis of these trips suggests that they are composed of two components. The outward exploratory component is a circuitous set of progressions punctuated by several stops. The homeward component is a single, direct progression occurring subsequent to the last stop which terminates with the animal arriving at a home base. Damage to the hippocampal formation disrupts exploratory trip organization. Although it has been suggested that impairments in self-movement based navigation contribute to the disruptions observed in exploratory trip organization, this conclusion is based on direct comparisons between outward and homeward trips. Because the outward component includes both progressions and stops, direct comparisons between the outward and homeward components may omit more subtle differences between the two components. Methods Female Long-Evans rats with fimbria-fornix transections (FF) and unoperated controls explored a circular tabletop for one hour under novel dark and light conditions. The first eight exploratory trips with homeward components at least half the diameter of the table were selected for analysis. Exploratory trips were divided into short, medium, and long outward components and a homeward component. 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: Coronal sections stained for AchE were taken from representative control (left) and fimbria-fornix (Right) rats. Note that the decrease in AchE observed in the fimbira-fornix rat is restricted to the dentate gyrus and Ammon’s horn. Figure 3: Mean proportion of time control and fimbria-fornix rats spent in the home base are plotted for dark (left) and light (right) conditions. Figure 4: Topographic and kinematic (line thickness) characteristics of a single exploratory trip are plotted in the left panel. Linear and angular moment-to-moment speeds from the same trip are plotted in the right panel. Figure 6: Mean progression distance ratio (top), progression distance (middle), and progression maximum speed (bottom) are plotted for both groups of rats under dark (left) and light (right) conditions. Figure 5: Mean correlation between linear and angular speeds are plotted for exploratory trips under dark (left) and light (right) conditions. Figure 7: The distribution of the peak speed locations are plotted for short, medium, and, long outward trip progressions (black dots) and the homeward trip progressions (gray dots), from a representative control rat. Figure 8: Mean standard deviation of the peak speed location is plotted for short, medium, and, long outward trip progressions and the homeward trip progressions for both groups under dark (left) and light (right) conditions. Conclusions Both control and fimbria-fornix rats adopt a home base under novel dark conditions. Exploratory trips are a concatenation of linear and angular movement primitives. Control rat’s homeward progressions are a unique component of the exploratory trip. Transection of the fimbria-fornix eliminates the unique characteristics of the homeward progression. Correspondence: Logan Bennett: Dr. Derek Hamilton: Dr. Douglas Wallace: