Assessment of executive functions in mice with the Fixed-Ratio schedule of reinforcement

Abstract

The simplest operant schedule of reinforcement is the Fixed Ratio (FR). In this task, animals must provide a determined number of responses (i.e., a ratio of responses) to obtain each food reward. In FR1, a single response is required, 2 responses in a FR2, 5 in a FR5 and so on. The FR is often used only to shape the operant response before to go further with another task, but of course, it can be used with more interesting goals. Classically, measures such as inter-responses time (IRT), latency to collect the delivered food reward, response or reward rate, but also post reinforcement pause are collected. Together, they allow to obtain information about motor ability, learning and motivation. However, there is a lack of interest in the executive aspect potentially implied by the FR3, especially in situations where the ratio is held constant across multiple training sessions. Indeed, in such situations, as training progress the subject can learn that a certain quantity of responses is required before the reward is available. And hence, checking the food tray to see if the reward is available should be more probable as the animal progress in the ratio. With this operant procedure, it seems therefore possible to investigate the animal’s executive function. In this study, we thus focused on the premature head entries into the empty food tray (PHE, i.e., when the reward was not delivered yet) and their evolution across sessions of FR30. The analysis of the distribution of PHE could offer a way to analyze the subject’s ability to optimize its operant behavior. We also compared this aspect of the operant behavior between young and old mice as the age is factor affecting the executive function