Reference memory and allocentric spatial localization deficits after unilateral cortical brain injury in the rat

Abstract

Traumatic brain injury (TBI) produces learning and memory impairments in humans. This study investigated the effects of TBI on memory and spatial localization strategies in rats. Prior to TBI, separate groups of rats were trained in an 8-arm radial maze with either all 8 arms baited (Expt. 1) or only 4 of the 8 arms baited (Expt. 2). TBI was produced by a controlled pneumatic impactor striking the entire right sensorimotor cortex of the anesthetized rat. Rats used in Expt. 1 were selected because they did not use a stereotypic response strategy (going to adjacent arms) in performing the maze before injury. After TBI the rats were not different from control rats in the number of working memory (WM) errors made. They did, however, display a distinct propensity to go to adjacent arms, i.e., exhibit stereotypic behavior, with a right-handed (ipsiversive) bias ( P<0.005). After TBI, rats which were trained with only 4 of 8 arms baited committed more reference memory (RM) errors than control rats ( P<0.05). They did not differ from controls on WM errors. Injured rats took longer to re-attain criteria than controls ( P<0.0001). Injured rats also initially displayed a propensity to enter the adjacent arm sequentially before re-attaining criteria. Further analysis indicated that injured rats re-learned the maze with a right-hand bias ( P<0.0001). The results of both experiments suggest that after TBI, rats shifted from an allocentric to an egocentric strategy to re-learn the maze. It was suggested that damage to the parietal cortex may have been responsible for both RM errors and the shift away from an allocentric strategy to an egocentric strategy. Possibly, the ipsiversive (right-hand) bias may be the result of a behaviorally or injury-induced neurochemical asymmetry within the motor system.