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Getting lost in the dark: Disruption of directional but not distance estimates following mammillothalamic tract lesions
*S.S. Winter, S.J. Wagner, & D.G. Wallace Dept Psychology, Northern Illinois Univ., DeKalb, IL, USA
Disruptions in performance associated with mammillary body or mammillothalamic tract (MTT) lesions have been attributed to memory impairment. Considering that these deficits have been observed on traditional spatial tasks and that head direction cells have been discovered within the lateral portion of the mammillary bodies, it is possible that this system may have a more selective role in organizing spatial behavior. The current study examined the effects of MTT lesions on the ability of an animal to use self-movement cues to guide navigation in a spontaneous exploration paradigm.
Twenty female Long-Evans rats that were 90 days old were assigned to one of two groups, lesion (n=15) or sham (n=5). Rats in the lesion group were given bilateral electrolytic lesions of the mammillothalamic tract (MTT). Lesions were verified by visual inspection. Only a portion of the lesions resulted in bilateral destruction (bilateral; n=5), whereas the remaining lesion rats had either unilateral damage (unilateral; n=5), or bilateral sparing (miss; n=5). The miss and unilateral groups did not differ and were combined (miss/unilateral; n=10). All rats were tested under complete dark conditions in spontaneous exploration. The animals were placed on a large circular table in a refuge provided around which they organized their behavior. Exploratory trips were taken after the animal had established a home base, which was considered after the animal had done a full body groom in the refuge. Animals were allowed to freely explore the table for three hours at a time for up to five days.
Figure 3: Exploratory trips are plotted from a representative sham animal. Topographic (left) and kinematic (right) characteristics are plotted for three trips in which the homeward segment originated from varying distances from the refuge.
Figure 1: Photographs of coronal sections stained with cresyl violet at the level of the MTT (top) and mammillary bodies (center). Sections stained for acetylcholinesterase within the hippocampus (bottom) from representative animals from the miss (left), unilateral (center), and bilateral (right) groups.
Figure 5: Location of the peak speed is plotted for outward (left) and homeward (right) progressions. All groups’ peak in speed occurred in the center of a progression independent of distance.
• All groups were found to establish a home base in the refuge and organized their behavior around it under dark conditions.• No group differences were observed in the characteristics of the outward segments, indicating equivalent organization and levels of motivation to explore across groups.•Group differences were observed in the characteristics of the homeward segment, with the bilateral group having significant impairments relative to both the sham and miss/unilateral groups.
•Bilateral lesions did not disrupt measures of peak speed, peak speed location, or peak variability indicating a sparing of distance estimation.•Bilateral lesions disrupted measures of distance traveled, circuity, and angular error on homeward progressions indicating impairment in direction estimation
•Bilateral lesions of the MTT resulted in selective impairments in the ability to use internally generated cues to derive direction estimates while sparing distance estimates. These results provide support that the MTT and mammillary system is involved in on-line processing of spatial information (dead reckoning). Further work needs to be done to investigate how this may influence mnemonic function.
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Introduction
Correspondence: S.S. Winter [email protected]. Wallace [email protected] www.niu.edu/user/tj0dgw1
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Figure 2: The quadrant preference (left), and total distance traveled (right) are plotted for each group during a 45 minute period containing the most exploratory trips. Groups did not differ in these characteristics of exploratory behavior.
Distance Estimation
Figure 6: Variability in peak speed location is plotted for each groups’ outward (left) and homeward (middle) progressions. In addition, the absolute error in peak location is plotted for the homeward progressions (right). Groups did not differ in any measures. There was a significant linear trend of distance on outward progressions, indicating longer trips were more variable.
Figure 4: Peak speeds are plotted for outward (left) and homeward (right) progressions. Progressions were divided into varying distances (S, M, L), and distance was found to have a significant linear trend for outward and homeward progressions, indicating all groups modulated their peak speed dependent upon the distance to be traveled.
Figure 10: Path circuity (shortest distance/actual distance) is plotted for outward (left) and homeward (right) progressions. Groups did not differ, nor was there an effect of distance for outward progressions; however, the bilateral group had a significantly lower circuity score than either the sham or miss/unilateral groups for homeward progressions.Direction Estimation
Figure 9: Example angular error computation (left) with illustration of outward (blue) and homeward (red) progressions. Plot of angular error scores (right) for homeward progressions. All groups differed significantly from one another.
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Figure 7: Topographic characteristics are plotted for the outward (top) and homeward (bottom) segments from one representative animal from the sham (left), miss/unilateral (center), and bilateral (right) groups.
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Figure 8: Distance traveled is plotted for outward (left) and homeward (right) progressions. Groups did not differ on outward progressions; however, the bilateral group traveled significantly farther than both the sham and miss/unilateral groups for homeward progressions.
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