Countermanding in Rats as an Animal Model of Inhibition of Action: Validation of the Race Model
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Executive function, the cognitive processes that allow the voluntary control of goal-directed behaviour, can be studied through the examination of inhibition of action. The countermanding paradigm has been shown to be a powerful tool to examine a subject’s ability to withhold responses to a go stimulus when a stop signal is presented occasionally. Logan and Cowan (1984) developed a race model to account for countermanding performance in humans, proposing that independent go and stop process initiated by the go and stop signals respectively, race toward a finish line whereby the first process to cross its finish line determines the behavioural outcome. The model allows estimation of the stop signal response time, a variable that is not directly observable. The race model has yet to be validated for countermanding performance in rats. Using a new rodent countermanding task inspired directly from human studies, male Wistar rats were trained to respond to a visual stimulus (go signal) by pressing a lever below an illuminated light for food reward, but to countermand lever the press (25% of trials) subsequent to an auditory tone (stop signal) presented after a variable delay. The ability to cancel a response decreased as stop signal delay increased. The stop signal response time for rats was estimated to be 157 ms, a value within the range of human estimates. Predictions of countermanding performance made by the race model were generally respected. Response times of movements that escape inhibition: 1) were shorter than those of movements made in the absence of a stop signal; 2) gradually lengthened with increasing stop signal delay; and most importantly, 3) were predicted by the race model. These findings demonstrate that the countermanding performance of rodents can be accounted for by a simple race model, which has been applied successfully in human studies and nonhuman primate models. This new animal model will permit complementary invasive investigations of brain mechanisms underlying inhibitory control and refine the existing rodent models of neurological disease and impulsivity.