Evidence for two routes to expertise in mental rotation of images

One's ability to imagine a spatial transformation of an object (e.g. "mental rotation") can be improved with practice. Some researchers propose that practice speeds the rate of mental rotation; others suggest that performance improvements arise through the adoption of an alternative, memory-based cognitive strategy. Thus far, it has not been possible to adjudicate between these alternatives with behavioural evidence alone. We measured behavioural responses and brain activity in two experiments designed to assess the evidence for these two pathways to improvement in mental rotation; one with a small set of stimuli to encourage direct memory retrieval strategies and another with a much larger set designed to discourage memory-based strategies. Before practice participants displayed two well-established signatures of mental rotation: both response time and EEG negativity increased linearly with rotation angle. After extensive practice with a small stimulus set, both signatures of mental rotation had all but disappeared. In contrast, after the same amount of practice with the large stimulus set, both signatures remained even though performance improved markedly. Taken together these results constitute compelling evidence for the existence of two routes to expertise in mental rotation.