Antipointing adherence to Fitts' Law is eccentricity-dependent


Goal-directed reaches involving direct stimulus and response spatial relations (i.e., propointing) are supported by dedicated visuomotor networks residing in the posterior parietal cortex of the dorsal visual pathway that provide metrically precise visual information for movement planning and control (Goodale 2011). Notably, such actions adhere to lawful speed-accuracy relations as defined by Fitts' equation (1954); that is, movement time (MT) is predicted by the log/linear relationship between movement amplitude and target width. It is, however, unknown whether reaches involving dissociable stimulus and response spatial relations similarly adhere to Fitts' equation because they are mediated by visual inputs distinct from their propointing counterparts (Heath et al. 2014). To that end, we examined whether antipointing (i.e., reaching mirror-symmetrical to a target stimulus) adheres to Fitts' equation in line with their propointing counterparts. Participants (N=14) performed pro- and antipointing reaches under conditions with and without vision of the reaching limb to differently sized targets located at two target eccentricities (i.e., proximal and distal). The target widths were selected to produce equivalent index of difficulty (IDFitts) values across the proximal and distal target eccentricities (i.e., 3.0, 3.5, 4.3 and 6.3 bits of information). Results showed that propointing to proximal and distal target eccentricities, as well as antipointing to the proximal eccentricity, produced a linear increase in MT with increasing IDFitts, whereas antipointing MTs to the distal eccentricity did not reliably vary with IDFitts. Accordingly, propointing elicits lawful MT/ID relations as predicted by Fitts' equation, whereas adherence in an antipointing task is eccentricity-dependent.