Partial and total-order planning: Evidence from normal and prefrontally damaged populations

This paper examines human planning abilities, using as its inspiration planning techniques developed in artificial intelligence. AI research has shown that in certain problems partial-order planners, which manipulate partial plans while not committing to a particular ordering of those partial plans, are more efficient than total-order planners, which represent all partial plans as totally ordered. This research asks whether total-order planning and/or partial-order planning are accurate descriptions of human planning, and if different populations use different planning techniques. Using a simple planning task modeled after tasks designed in artificial intelligence we tested 7-8 year-old children, 11-13 year-old children, adult controls, and adults with damage to the prefrontal cortex. We found that adults and older children exhibited performance on planning tasks of varying complexity which matched that of artificial partial-order planners, and that this pattern of performance did not vary with multiple presentations of the planning task. In contrast, young children and adults with damage to the prefrontal cortex exhibited performance matching that of artificial total-order planners. This pattern of performance did vary, however, with multiple presentations of the planning task, with the young children and adults with cortical damage displaying aspects of total-order planning. In a further study we found that adolescents who had sustained damage to the prefrontal cortex as children displayed two different patterns of performance; when measures of reaction time were analyzed they revealed a pattern of performance suggestive of partial-order plan representations. However, analyses of the adolescents' protocols revealed a pattern of performance suggestive of total-order plan representations. The significance of these results to psychology, neuroscience, and artificial intelligence are discussed.