Minor Functional Deficits in Basic Response Patterns for Reinforcement after Frontal Traumatic Brain Injury in Rats

Traumatic brain injury (TBI) is a major contributor to numerous psychiatric conditions and chronic behavioral dysfunction. Recent studies in experimental brain injury have begun to adopt operant methodologies to assess these deficits, all of which rely on the process of reinforcement. No studies have directly examined how reinforced behaviors are affected by TBI, however. The current study assessed performance under the four most common schedules of reinforcement (fixed ratio, variable ratio, fixed interval, variable interval) and one higher order schedule assessing motivation (progressive ratio) after bilateral, pre-frontal controlled cortical impact injury. TBI-induced differences on the basic schedules were minor, with the exception of the variable ratio, where increased efficacy (more reinforcers, higher response rates, lower interresponse times) at higher requirements was observed as a result of brain injury. Performance on the progressive ratio schedule showed some gross differences between the groups, in that sham rats became more efficient under this schedule while injured rats perseverated in lever pressing. Further, injured rats were specifically impaired at lower response requirements on the progressive ratio. Taken together, these findings indicate that simple reinforced behaviors are mostly unaffected after TBI, except in the case of variable ratio schedules, but the altered performance on the higher-order progressive ratio schedule suggests changes involving motivation or potentially perseveration. These findings validate operant measures of more complex behaviors for brain injury, all of which rely on reinforcement and can be taken into consideration when adapting and developing novel functional assessments.