Object affordance and spatial-compatibility effects in Parkinson's disease.

Movement in Parkinson's disease (PD) is strongly influenced by sensory stimuli. Here, we investigated two features of visual stimuli known to affect response times in healthy individuals; the spatial location of an object (the spatial effect) and its action-relevance (the 'affordance' effect). Poliakoff et al. (2007) found that while PD patients show normal spatial effects, they do not show an additional affordance effect. Here we investigated whether these effects are driven by facilitation or inhibition, and whether the affordance effect emerges over a longer time-course in PD. Participants (24 PD and 24 controls) viewed either a lateralised door handle (affordance condition), a lateralised abstract stimulus (spatial condition), or a centrally presented baseline stimulus (baseline condition), and responded to a colour change in the stimulus occurring after 0 msec, 500 msec or 1000 msec. The colour change indicated whether to respond with the left or right hand, which were either spatially compatible or incompatible with the lateralised stimulus orientation in the affordance and spatial conditions. The baseline condition allowed us to assess whether compatibility effects were driven by facilitation of the compatible response or inhibition of the incompatible response. The results indicate that stimulus orientation elicited faster responses from the nearest hand. For controls, the affordance effect was stronger and driven by facilitation, whilst the spatial condition was driven by inhibition. In contrast, the affordance and spatial-compatibility effects did not differ between conditions in the PD group and both were driven by facilitation. This suggests that the PD group responded as if all stimuli were action-relevant, and may have implications for understanding the cueing of movement in PD.