Does the relation between inherent variability and target size influence the central tendency of target-directed aiming?


Eminent theories of target-directed aiming concur with the view of limiting error caused by signal-dependent noise without conceding the speed that would allow landing within the target (Elliott et al., 2004; Meyer et al., 1988). Perhaps one key difference surrounds the central tendency of primary movement endpoints. The present study aimed to examine this characteristic feature as a function of the target size (nominal) in relation to the inherent variability (effective). It was predicted that a target made to be proportionally smaller than ones inherent variability would tend to be undershot, although one that is made to fit near the effective target width would tend to be hit near centre. Participants (n=9) completed 2 days of aiming using their upper-limb to hold a stylus, where they were initially assessed for their inherent variability (Day 1), and then aimed to targets that were scaled according to this variability (i.e., within-participant spatial variability multiplied by 1-4; 38%-95% of the distribution) (Day 2). There was a positive linear relation between target size and movement time. While the within-participant spatial variability of the primary movement endpoint revealed no such effect, there was a positive linear relation between the movement velocity and effective target width. The mean primary movement endpoint revealed no significant effect with attempts averaging near target-centre. These findings concur more with the view of landing near target-centre to optimize hits following the primary movement. We suggest any previous theoretical discrepancies may be explained by the apriori values attributed to speed and accuracy.