Methods

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Conclusions

Mammillothalamic tract lesions disrupt self-movementcue processing in a food hoarding task.

*S.S. Winter; J.L. McMillin, 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 use of environmental and self-movement cues subsequent to MTT lesions in the food hoarding paradigm.

Female Long-Evans rats were assigned to one of two groups, lesion and sham. Rats in the lesion group were given bilateral electrolytic lesions of the mammillothalamic tract (MTT). Lesions were verified by visual inspection of coronal segments stained for cresyl violet. Only a small number lesions resulted in bilateral destruction (bilateral; n=2), whereas the remaining lesion rats had either unilateral damage to one tract (unilateral; n=5) or bilateral sparing (miss; n=7). No significant differences were found between the sham and miss groups in any measure; therefore, the two groups were combined (sham/miss; n=16). All rats were trained in a food hoarding task on a large circular table (200 cm diameter) and provided with a visible refuge (17 x 26 x 12 cm). They were trained to search the table for a 1 g food pellet and return to the refuge for consumption. The final day of training was recorded as the cued probe. Next, the uncued probe was administered in which the refuge was moved underneath the table so that it could not be seen while the animal was on the table. The dark probe was identical to the uncued probe, but it was conducted under infrared light. Finally, the new location probe was administered using the uncued refuge placed 180° from the training location.

Figure 1: Photographs of the apparatus (200 cm diameter) used for each probe; cued, uncued, dark, and new with the refuge (17 x 26 x 12 cm; grey box) and former refuge location (striped box on new location probe).

Cued Uncued

Dark New

Figure 3: Topographic plots of the outward (top) and homeward (bottom) progressions of rats from the sham/miss (left), unilateral (center), and bilateral (right) groups for the cued probe.

Figure 2: Photographs of coronal sections stained with cresyl violet of the AP levels where the MTT region was lesioned (top) and mammillary bodies (center), and Acetylcholinesterase within the hippocampus (bottom) of representative animals from missed lesion (left), unilateral (center), and bilateral (right) animals.

Figure 4: Path circuity scores did not differ under cued conditions for outward or homeward progressions among animals in the sham/miss (□), unilateral (■), or bilateral (■) groups.

Figure 7: Topographic plots of the outward (top) and homeward (bottom) progressions of rats from the sham/miss (left), unilateral (center), and bilateral (right) groups for the dark probe.

Figure 5: Topographic plots of the outward (top) and homeward (bottom) progressions of rats from the sham/miss (left), unilateral (center), and bilateral (right) groups for the uncued probe.

Figure 8: Path circuity scores did not differ under dark conditions for outward progressions; however, the bilateral (■) group had significantly more circuitous paths than sham/miss (□) or unilateral (■) groups for homeward progressions.

Figure 6: Path circuity scores did not differ under uncued conditions for outward or homeward progressions among animals in the sham/miss (□), unilateral (■), or bilateral (■) groups.

Figure 10: Path circuity scores did not differ under new conditions for outward progressions; however, the bilateral (■) group had significantly more circuitous paths than sham/miss (□) or unilateral (■) groups for homeward progressions.

Figure 9: Topographic plots of the outward (top) and homeward (bottom) progressions of rats from the sham/miss (left), unilateral (center), and bilateral (right) groups for the new location probe.

• There were no disruptions in bilateral MTT lesion animals’ ability to accurately carry food to a former location when environmental and self-movement cues were available.

•There was a significant disruption in the bilateral MTT lesion animals’ ability to accurately carry food to a former location when restricted to self-movement cues or when self-movement and environmental cues were in conflict.

•There were no disruptions in unilateral MTT lesion animals’ ability to accurately carry food to a former location when either environmental and self-movement cues were available or when they were placed in conflict.

•These results are consistent with a role for the MTT in processing information related to self-movement cues used to guide navigation rather than a role in mnemonic processing.

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Correspondence:

Shawn Winter SWinter@niu.edu

Doug Wallace DWallace@niu.edu

Web www.niu.edu/user/tj0dgw1

Acknowledgements:

Jeana Jones

Steve Wagner

Bethany Barnes

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