Chapter 2. Prehension

Chapter 5 - Movement Before Contact 123
in the motor cortex, providing evidence that the motor cortex may be
viewed as a computational map. At the same time, some features of
limb movements may not be explicitly planned by supraspinal sys-
tems. It is possible that the CNS is using simplifying rules to generate
close but inexact approximations (or ballparks) of an intended mov-
ment. The hand path, for example, which tends to be in straight lines
and exhibit bell-shaped velocity profiles, may not be being planned,
but may emerge as a result of the CNS transformations from goal lo-
cation to intrinsic joint angles or muscle lengths and/or from the me-
chanical properties of the limb.
5.3.2 Symmetric vs asymmetric velocity profiles
A pointing study performed by Atkeson and Hollerbach (1985)
brought out important features about unrestrained arm movements.
Subjects started with their hand in one of six locations in space, and
were asked to move their arm within a plane to another location which
was lit with a light-emitting diode. In order to collect these data,
Atkeson and Hollerbach used a SELSPOT system, allowing them to
track the subject’s index finger tip, wrist, elbow, and shoulder move-
ments over time. Small, light-emitting diodes were attached to the
body part to be tracked, and solid state cameras captured the kinemat-
ics of the movement. In terms of the path of the wrist, some paths
were straight-line, some were curved, depending on the location of the
movement in the workspace. Subjects were required to carry a load in
their hands for some of the trials. For a given direction, there was
path invariance with speed and load. Atkeson and Hollerbach found
that practice did not influence the movements. They showed that the
tangential velocity profile, scaled both in time and amplitude, was
invariant for all subjects, speeds, loads and directions (shown in
Figure 5.4).
Other investigators have not always found symmetric velocity
profiles in pointing tasks. MacKenzie, Marteniuk, Dugas, Liske, and
Eickmeier (1987) replicated the conditions from Paul Fitts’ study on
discrete aiming (Fitts 1954, Experiment 2), and found systematic
effects of target size on the degree of asymmetry in the tangential
velocity profiles. Subjects were asked to point with a stylus ‘as
quickly and as accurately as possible’ to a target of varying size and at
varying distances. The question being asked concerned whether there
was a reliable kinematic measure of the precision requirements of the
task. In this case, Fitts’ Law (Fitts, 1954) was used, which states