SELECTIVE COGNITIVE IMPAIRMENT FOLLOWING TRAUMATIC BRAIN INJURY IN RATS

Impairment of cognitive abilities is a frequent and significant sequelae of traumatic brain injury (TBI). The purpose of this experiment was to examine the generality of the cognitive deficits observed after TBI. The performance of three tasks was evaluated. Two of the tasks (passive avoidance and a constant-start version of the Morris water maze) were chosen because they do not depend on hippocampal processing. The third task examined was the standard version of the Morris water maze which is known to rely on hippocampal processing. Rats were either injured at a moderate level (2.1 atm) of fluid percussion brain injury or surgically prepared but not injured (sham-injured control group). Nine days after fluid percussion injury, injured (n = 9) and sham-injured rats (n = 8) were trained on the one-trial passive avoidance task with retention assessed 24 h later. On days 11-15 following injury, injured (n = 9) and sham-injured (n = 8) rats were trained on a constant-start Version of the Morris water maze that has the animals begin the maze from a fixed start position on each trial. Additional injured (n = 8) and sham-injured (n = 8) animals were trained on days 11-15 after injury on the standard (i.e. using variable start positions) version of the Morris water maze. The results of this experiment revealed that performance of the passive avoidance and the constant-start Version of the Morris water maze were not impaired by fluid percussion TBI. However, performance on a task that is usually disrupted by hippocampal damage (variable-start version of the Morris water maze) was significantly impaired by TBI. Thus, the hippocampus appears to be selectively damaged following TBI.