Why do the eyes prefer the index finger? Simultaneous recording of eye and hand movements during precision grasping

Previous research investigating eye movements whengrasping objects with precision grip has shown that wetend to fixate close to the contact position of the indexfinger on the object. It has been hypothesized that thisbehavior is related to the fact that the index fingerusually describes a more variable trajectory than thethumb and therefore requires a higher amount of visualmonitoring. We wished to directly test this prediction bycreating a grasping task in which either the index fingeror the thumb described a more variable trajectory.Experiment 1 showed that the trajectory variability ofthe digits can be manipulated by altering the directionfrom which the hand approaches the object. If the startposition is located in front of the object (hand-before),the index finger produces a more variable trajectory. Incontrast, when the hand approaches the object from astarting position located behind it (hand-behind), thethumb produces a more variable movement path. InExperiment 2, we tested whether the fixation patternduring grasping is altered in conditions in which thetrajectory variability of the two digits is reversed. Resultssuggest that regardless of the trajectory variability, thegaze was always directed toward the contact position ofthe index finger. Notably, we observed that regardless ofour starting position manipulation, the index finger wasthe firstdigit to make contact with the object. Hence, weargue that time to contact (and not movementvariability) is the crucial parameter which determineswhere we look during grasping.