Grasping without vision: time normalizing grip aperture profiles yields spurious grip scaling to target size

The analysis of normalized movement trajectories is a popular and informative technique used in investigations of visuomotor control during goal-directed acts like reaching and grasping. This technique typically involves standardizing measures against the amplitude of some other variable - most typically time. Here, we show that this normalizing technique can lead to some surprising results. In the first of two experiments, we asked participants to grasp target objects without ever seeing them from trial to trial. In the second experiment, participants were given a brief preview of the target and were then cued 3s later to pick it up while vision was prevented. Critically, on some trials during the delay period and unbeknownst to the participants, the previewed target was swapped for a new unseen one. The results of both experiments show that time-normalized measures of grip aperture during the closing phase of the movement appear to be scaled to target size well before the fingers make contact with the target - even though participants had no idea what the size of the target was that they were grasping. In contrast, a classical measure of anticipatory grip scaling, maximum grip aperture, did not show scaling to target size. As we demonstrate, however, in both experiments, movement time was longer for the larger target than the smaller ones. Thus, the comparisons of time-normalized grip aperture, particularly during the closing phase of the movements, were made across different points in real time. Taken together, the results of these experiments highlight a need for caution when investigators interpret differences in time-normalized dependent measures - particularly when the effect of interest is correlated with the dependent measure and a third variable (e.g., movement time) that is used to standardize the dependent measure.