Intersegmental Coordination in the Kinematics of Prehension Movements of Macaques

Abstract

The most popular model to explain how prehensile movements are organized assumes that they comprise two "components", the reaching component encoding information regarding the object's spatial location and the grasping component encoding information on the object's intrinsic properties such as size and shape. Comparative kinematic studies on grasping behavior in the humans and in macaques have been carried out to investigate the similarities and differences existing across the two species. Although these studies seem to favor the hypothesis that macaques and humans share a number of kinematic features it remains unclear how the reaching and grasping components are coordinated during prehension movements in free-ranging macaque monkeys. Twelve hours of video footage was filmed of the monkeys as they snatched food items from one another (i.e., snatching) or collect them in the absence of competitors (i.e., unconstrained). The video samples were analyzed frame-by-frame using digitization techniques developed to perform two-dimensional post-hoc kinematic analyses of the two types of actions. The results indicate that only for the snatching condition when the reaching variability increased there was an increase in the amplitude of maximum grip aperture. Besides, the start of a break-point along the deceleration phase of the velocity profile correlated with the time at which maximum grip aperture occurred. These findings suggest that macaques can spatially and temporally couple the reaching and the grasping components when there is pressure to act quickly. They offer a substantial contribution to the debate about the nature of how prehensile actions are programmed.

Fig 1. Graphical representation of the monkeys' postural conditions.

Schematic drawing of the reach-to-grasp movement performed by the monkey (from right to left). Marker positions upon the hand are illustrated in the callout.

Fig 2. Superimposition of the velocity and grip profiles for the snatching and the unconstrained conditions.

Arrows indicate the correspondence between the time at which the maximum grip aperture and the beginning of the low velocity phase occur for an individual movement for the snatching condition. Dashed lines = velocity profiles. Solid Lines = grip profiles.

Fig 3. Superimposition of the velocity profiles for the snatching and the unconstrained conditions for the six individuals considered.

This figure well represents how stereotypical this type of movements was with specific reference to the break point occurring along the deceleration phase. The arrow indicates the beginning of the break point.