The impact of problem structure on planning: insights from the Tower of London task.

Despite the large number of behavioral and functional neuroimaging studies employing the Tower of London (ToL), the task's structural parameters and particularly their impact on planning have not been addressed so far. In this paper, we highlight the structural properties of ToL problems and provide evidence for their systematic and substantial effects on the cognitive processes involved in planning. In a problem set with three-move problems, the following structural parameters were experimentally manipulated: the ambiguity of goal hierarchy, the demand for subgoal generation, and the existence of suboptimal alternatives. Analysis of preplanning time as an indicator for the planning process revealed highly significant effects for all three parameters which seems to reflect differences in cognitive processing due to structural task properties. Therefore, we suggest that the common consideration of ToL problem difficulty solely in terms of the minimum number of moves is not sufficient. Moreover, the applied problem sets should be more carefully selected and their structural parameters explicitly noted.