Abstract
Patients with alien hand syndrome (AHS) experience making apparently deliberate and purposeful movements with their hand against their will. However, the mechanisms contributing to these involuntary actions remain poorly understood. Here, we describe two experimental investigations in a patient with corticobasal syndrome (CBS) with alien hand behaviour in her right hand. First, we show that responses with the alien hand are made significantly more quickly to images of objects which afford an action with that hand compared to objects which afford an action with the unaffected hand. This finding suggests that involuntary grasping behaviours in AHS might be due to exaggerated, automatic motor activation evoked by objects which afford actions with that limb. Second, using a backwards masked priming task, we found normal automatic inhibition of primed responses in the patient's unaffected hand, but importantly there was no evidence of such suppression in the alien limb. Taken together, these findings suggest that grasping behaviours in AHS may result from exaggerated object affordance effects, which might potentially arise from disrupted inhibition of automatically evoked responses.
Highlights
Most healthy adults feel that they have a great deal of control over their actions, some neurological patients do not
If grasping behaviour in alien hand syndrome (AHS) arises because of disruption of normal automatic suppression of afforded responses, one might predict that (i) object affordance effects are exaggerated in the alien hand compared to the non-alien hand; and (ii) automatic inhibition of automatically evoked responses is reduced in the alien limb
There is some evidence that the priming effect may have returned to positive again at the tail end of the distribution in bin 8, which is consistent with previous studies, but this effect may have been skewed by outliers in the tail end of the distribution, and did not reach statistical significance (Bonferroni-corrected p > .1)
Summary
Most healthy adults feel that they have a great deal of control over their actions, some neurological patients do not. Seeing a teapot with the handle to the right might automatically prime the observer to reach out with the right hand to grasp the handle Object affordance effects such as these have been extensively studied in healthy adults using stimulus-response compatibility paradigms (e.g., Cho and Proctor, 2010; Derbyshire et al, 2006; Iani et al, 2011; McBride et al, 2012b; Pellicano et al, 2010; Phillips and Ward, 2002; Tucker and Ellis, 1998, 2001). If grasping behaviour in AHS arises because of disruption of normal automatic suppression of afforded responses, one might predict that (i) object affordance effects are exaggerated in the alien hand compared to the non-alien hand (and relative to healthy controls); and (ii) automatic inhibition of automatically evoked responses is reduced in the alien limb
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