Look before you reach: Fixation-reach latencies predict reaching kinematics in toddlers

Research on infant and toddler reaching has shown evidence for motor planning after the initiation of the reaching action. However, the reach action sequence does not begin after the initiation of a reach but rather includes the initial visual fixations onto the target object occurring before the reach. We developed a paradigm that synchronizes head-mounted eye-tracking and motion capture to determine whether the latency between the first visual fixation on a target object and the first reaching movement toward the object predicts subsequent reaching behavior in toddlers. In a corpus of over one hundred reach sequences produced by 17 toddlers, we found that longer fixation-reach latencies during the pre-reach phase predicted slower reaches. If the slowness of an executed reach indicates reach difficulty, then the duration of pre-reach planning would be correlated with reach difficulty. However, no relation was found with pre-reach planning duration when reach difficulty was measured by usual factors and independent of reach duration. The findings raise important questions about the measurement of reach difficulty, models of motor control, and possible developmental changes in the relations between pre-planning and continuously unfolding motor plans throughout an action sequence.