Effect of speed manipulation on the control of aperture closure during reach-to-grasp movements

Abstract

This study investigates coordination between hand transport and grasp movement components by examining a hypothesis that the hand location, relative to the object, in which aperture closure is initiated remains relatively constant under a wide range of transport speed. Subjects made reach-tograsp movements to a dowel under four speed conditions: slow, comfortable, fast but comfortable, and maximum (i.e., as fast as possible). The distance traveled by the wrist after aperture reached its maximum (aperture closure distance) increased with an increase of transport speed across the speed conditions. This finding rejected the hypothesis and suggests that the speed of hand transport is taken into account in aperture closure initiation. Within each speed condition, however, the closure distance exhibited relatively small variability across trials, even though the total distance traveled by the wrist during the entire transport movement varied from trial to trial. The observed stability in aperture closure distance across trials implies that the hand distance to the object plays an important role in the control law governing the initiation of aperture closure. Further analysis showed that the aperture closure distance depended on the amplitude of peak aperture as well as hand velocity and acceleration. To clarify the form of the above control law, we analyzed four different mathematical models, in which a decision to initiate grasp closure is made as soon as a specific movement parameter (wrist distance to target or transport time) crosses a threshold that is either a constant value or a function of the above-mentioned other movement-related parameters. Statistical analysis performed across all movement conditions revealed that the control law model (according to which grasp initiation is made when hand distance to target becomes less than a certain linear function of aperture amplitude, hand velocity, and hand acceleration) produced significantly smaller residual errors than the other three models. The findings support the notion that transport-grasp coordination and grasp initiation is based predominantly on spatial characteristics of the arm movement, rather than movement timing.

Getting hold of approaching objects: In search of a common control of hand-closure initiation in catching and grasping

“…In line with previous studies, which had shown that closing time (Laurent et al, 1994;Rand & Stelmach, 2005;Rand et al, 2006Rand et al, , 2008Zaal & Bootsma, 2004) and closing distance (e.g., Carnahan & McFadyen, 1996;Rand & Stelmach, 2005;Rand et al, 2006;Wang & Stelmach, 2001;Watson & Jakobson, 1997;Zaal & Bootsma, 2004;Zaal et al, 1998) varied with factors such as object velocity, object orientation, object width, object size, reaching amplitude, and reaching velocity, we found a large effect of the velocity with which the object approached on these two variables. The effects of object velocity on these two factors that came out of our experiment corroborated the conclusion that closing time or distance are not being kept constant, and, thus, do not act as control variables in the coordination of reaching and grasping or in the timing of catching in the situation that the hand is not moving.…”

“…In Figs. 1C and D,hand aperture is plotted as a function of actual distance to contact (i.e., hand-object distance), a less familiar representation, although given in a subset of prehension studies that stress the role of distance rather than time (e.g., see Haggard & Wing, 1998;Rand, Squire, & Stelmach, 2006;Wallace, Stevenson, Spear, & Weeks, 1994;Wing & Fraser, 1983;Zaal & Bootsma, 2000). Finally, Figs.…”

“…Several empirical studies tested this hypothesis by manipulating reaching amplitude (Zaal et al, 1998), object width (Zaal & Bootsma, 2004), object size (Wang & Stelmach, 2001), object orientation (Rand & Stelmach, 2005), reach or object velocity (Carnahan & McFadyen, 1996;Rand, Squire, & Stelmach, 2006;Zaal et al, 1998), and the direction of the object's approach (Watson & Jakobson, 1997), which, taken together, demonstrated that in prehension the initiation of hand closure is not time-invariant over a range of conditions.…”

“…Wang & Stelmach, 1998, 2001. Recently, Stelmach and colleagues have amended this original proposal (Rand et al, 2006;Rand, Shimansky, Hossain, & Stelmach, 2008), after finding that closing distance varied with object orientation (Rand & Stelmach, 2005) and reach velocity (Rand et al, 2006). They proposed that the initiation of hand closure occurs at a closing distance that is a function of, among other variables, reaching amplitude and velocity.…”

Prehension is really reaching and grasping

“…Most often, when the coordination of prehension has been addressed, hypotheses regarding the moment of peak hand aperture, the moment that hand opening goes into hand closing, have been put forward (for an overview, see Zaal and Bootsma 2004). For instance, it has been proposed that peak hand aperture would occur at the moment of peak deceleration of the reaching movement (Jeannerod 1984), at a Wxed time (Gentilucci et al 1992) or distance (Rand and Stelmach 2005;Rand et al 2006;Stelmach 1998, 2001) before hand-object contact, or that coordination is based on time-to-contact information (Bootsma and Van Wieringen 1992;Zaal and Bootsma 2004;Zaal et al 1998). In our opinion, the latter hypothesis is the most promising of the ones currently available, but it certainly still needs a critical test.…”