Characterizing patterns of damage to recognition, temporal order memory and spatial learning following mild traumatic brain injury

1.5 millions Americans sustain a traumatic brain injury (TBI) annually; 75% are mild TBI. Mild injury victims report persistent recognition, temporal order memory and spatial learning problems. Recognition memory is necessary for discriminating between familiar and new items, as well as remembering the items' order. Spatial learning is essential for new information about one's environment encoding and retrieval. Yet, little research targets recognition memory and learning performance after mild TBI. Spontaneous object recognition and temporal order memory behavioral pattern after mild TBI, at 6, 24, 48, & 72 hrs and 7, 14, 21 and 35 days post trauma were examined using validated spontaneous object recognition and temporal order memory tests. Animals were placed in a Y shape maze for 2 min/trial; on trial 1, the animals explored two identical objects (A A), on trials 2 & 3 (1 & 15 min delay) the animals explored one familiar object (A) and one novel object (B then C), and on trial 4, (60 min delay) the animals explored recently explored object (C) and oldest object (B). The time each animal spent exploring novel and familiar objects and discrimination ratios were calculated. The simvastatin and environmental enrichment effect on recognition and temporal order memory were compared. Also, mild and severe TBI effects' on spatial learning performance were tested using a modified Morris water maze task at 20 days post trauma. Escape latency and percentage of time traveled within each quadrant was calculated. Animals with mild TBI showed significant recognition and temporal memory impairment up to 35 days post injury, in addition to familiarity preference up to 72 hours following the trauma. Simvastatin and environmental enrichment treatment led to improved cognitive function at different time points. Mild and severe TBI animals spent significantly less time in the target quadrant searching for the escape platform than the sham animals and mild injured animals needed prolonged time to improve their escape latency.;We conclude that mild TBI leads to recognition and temporal order memory and spatial learning deficits. Further, simvastatin and environmental enrichment are promising therapies for memory deficits that follow mild TBI.