Insights into the neural control of locomotion from walking through doorways in Parkinson's disease

Recent evidence suggests that a network of brain areas may be involved in visually guided walking. Here we study patients with Parkinson's disease (PD) who experience ‘freezing’ behaviour to investigate the visual control of locomotion and the role of the basal ganglia in this system. We use a variable-width doorway to measure the scaling of motor output to visual input specifying door width. By measuring walking behaviour as participants passed through the doorway, we show that both PD and healthy control participants scaled their locomotor outputs to door width. Both groups reacted to narrower doors by walking more slowly with shorter strides. However, the changes were greater in the PD group, where walking speed dramatically decreased while approaching the doorway. Such a pattern could help explain why doorways cause freezing episodes in PD. Neither explicit perceptual judgements of door width, nor performance on motor tasks, predicted the door behaviour. On the basis of these findings, we propose that PD is associated with a visuomotor disturbance, such that responses to action-relevant visual information are exaggerated. In the PD group, dopaminergic medications improved many baseline gait variables but did not affect their sensitivity to door width, suggesting that this visuomotor effect is not mediated by the basal ganglia. This hypothesis provides a novel framework for interpreting a variety of results with PD patients.