The timing of early aperture shaping metrics evince feedback- and feedfoward-based corrections

Abstract

Jeannerod's (1984: J Mot Behav) seminal work provided an understanding of the timing of several transport and aperture shaping metrics underlying goal-directed grasping. Most notably, Jeannerod's work is recognized for defining the invariant timing of peak grip aperture (75.7% of grasping time). It is, however, important to recognize that Jeannerod's work, as well as much subsequent research, has not focused on the spatiotemporal properties of early aperture shaping. Indeed, the importance of such an examination is recognized by the results of earlier work by our group showing that distinct visual metrics support early and late aperture shaping (e.g., Holmes et al. 2011: Vis Res). To that end, the present investigation examined the spatiotemporal properties of an early aperture shaping metric (i.e. peak aperture velocity: PA_V) in a large corpus of participants (N = 123) when grasping differently sized target objects. Results showed that the magnitude and size of grip aperture at PA_V increased linearly as a function of increasing object size. Notably, however, the time to PA_V exhibited a non-normal distribution that was exemplified by two subgroups. In particular, one group achieved PA_V early in (i.e., at 20% of,) the response whereas a second group achieved PA_V later in (i.e., at > ~50% of,) the response. Such findings indicate that PA_V is a temporally variant metric and may therefore reflect feedback- or feedforward-based modifications to an unfolding aperture trajectory.

Acknowledgments: Supported by an NSERC Discovery Grant and an NSERC USRA