Eye movements reveal planning in humans: A comparison with Scarf and Colombo's (2009) monkeys

On sequential response tasks, a long pause preceding the first response is thought to reflect participants taking time to plan a sequence of responses. By tracking the eye movements of two monkeys (Macaca fascicularis), Scarf and Colombo (2009, Eye Movements During List Execution Reveal No Planning in Monkeys [Macaca fascicularis], Journal of Experimental Psychology: Animal Behavior Processes, Vol. 35, pp. 587-592) demonstrated that, at least with respect to monkeys, the long pause preceding the first response is not necessarily the product of planning. In the present experiment, we tracked the eye movements of adult humans using the paradigm employed by Scarf and Colombo and found that, in contrast to monkeys, the pause preceding the first item is indicative of planning in humans. These findings highlight the fact that similar response time profiles, displayed by human and nonhuman animals, do not necessarily reflect similar underlying cognitive operations.