Chapter 2. Prehension

Chapter 5 - Movement Before Contact 119
Jeannerod, 1988). The reaching movements are inaccurate and much
slower, having lower peak velocities and longer deceleration phases.
Thus, the posterior parietal cortex is implicated in disruption of the
convergence of visual and proprioceptive maps for visually guided
reaching.
Other evidence has been observed in behavioral pointing studies
for a mapping between extrinsic and intrinsic coordinate frames (step
2). In Soechting and Terzuolo (1990), subjects were asked to remem-
ber a target’s location, and then asked to point to that remembered spot
in the dark. The distance and direction of the target were the extrinsic
measures while intrinsic measures were the shoulder and elbow joint
angles in terms of angular elevation and yaw. Subjects were able to
point in the correct direction but there were errors in the radial distance
from the shoulder, ranging from 6.7 to 11.7 cm average root mean
square total error in each subject. Soechting and Terzuolo argued that
the errors were due to the transformation between extrinsic and
intrinsic coordinates, and suggested that the problem is solved only
approximately, thus leading to predictable distortions. Soechting and
Flanders (1991) showed that for pointing tasks, a spatial location is
first transformed from head centered to shoulder centered spatial
coordinates. The location is then redefined in an intrinsic kinematic
framework by two separate channels that convert target azimuth
(horizontal direction) into intended yaw angles of arm in sagittal plane
when the hand is at the target, and target elevation and distance into
arm segment elevation angles. The difference between current and
intended arm angles is then transformed into a motor command
signaling direction of hand movement. The error patterns suggest that
the CNS uses linear approximations of the exact nonlinear relation
between limb segment angles and target location7.
Separate encoding of distance and direction was obtained in motor
memory research in the 1970s, using linear or lever positioning tasks.
In such tasks, blindfolded humans grasped a handle using palm, pad
or side opposition, and moved to a preselected or experimenter-
defined endpoint. Then, either immediately or after some filled or
unfilled retention interval, the individuals were asked to reproduce the
distance or location, from a new or different starting location.
Evidence supported a qualitatively different memory for location than
distance (subjects are better at reproducing locations than distances),
and that there were different patterns of forgetting of location and dis-
7Separate planning channels may also exist for direction, amplitude and force
(Favilla et al., 1989; Riehle & Requin, 1989; Rosenbaum, 1980).