Studies investigating the use of prism adaptation to treat symptoms of spatial neglect have used goggles inducing a rightward shift of 6 to 15°. However, the impact of prism power (i.e., the magnitude of visual shift) on after-effects (i.e., leftward shift in pointing movements after prism exposure), and effects on more functional tasks (e.g., a throwing task) are unknown. The purpose of this study was to clarify the relationship between these variables in a sample of healthy young adults as a first step towards the validation of a new home-friendly prism adaptation procedure (Peg-the-Mole/PTM).

Sixty participants were randomly assigned to one of four conditions and used PTM with 5° (condition 1), 10° (condition 2), 5°/10°/15° deviating goggles (gradual shift – condition 3) or with sham/non-deviating goggles (condition 4). Pointing tasks and a throwing task were administered before and after PTM to assess a prism-induced leftward shift in pointing or throwing movements.

The main effect of prism power on after-effects and throwing effects was significant (P<0.05). After-effects were larger in the gradual and 10° group when compared to the 5° and sham groups (gradual=10°>5°>sham). For the throwing task, only the effects in the gradual condition differed from the sham condition (only participants in the gradual condition threw significantly more to the left after PTM). Direct effects (pointing errors during PTM) predicted after-effects (P<0.05) but not throwing effects. Furthermore, after-effects did not correlate with throwing (functional) effects.

These findings suggest that larger prism power induces larger after-effects and a gradual transition from increasingly larger prism power may facilitate the generalization of after-effects to functional tasks. The lack of relationship between after-effects and throwing effects should be investigated further as it may explain the mixed findings regarding the generalization of prism effects to functional activities.