Stimulus-response correspondence across peripersonal space is unaffected by chronic unilateral limb loss.

Previous findings show an advantage in response speed when stimulus and response correspond spatially (i.e., the Simon effect). Chronic unilateral amputees show altered spatial perception near their affected hand, providing an opportunity to investigate whether experience also affects the visuomotor stimulus-response (S-R) mapping that underlies the Simon effect. We used a two-alternative, forced-choice paradigm to probe the spatial correspondence between visual cues and responses, in 14 unilateral upper limb amputees and 14 matched controls. We presented visual stimuli in 5 different locations within peripersonal space, including the midline, and found a smooth gradient of S-R correspondence effects. This is consistent with the hypothesis that S-R correspondence is represented along a spatial gradient. Unilateral amputees performed indistinguishably from matched controls, regardless of whether stimuli appeared in the hemispace ipsi- or contralateral to their missing limbs. This is inconsistent with the hypothesis that experience-dependent visual distortions entail changes in the S-R mapping; alternatively, it could reflect a complete experience independence of the Simon effect. We propose that the affordance competition hypothesis (Cisek in Philos Trans R Soc B Biol Sci 362:1585-1599, 2007) explains the Simon effect and the underlying gradient of S-R correspondence.