Chapter 2. Prehension

Chapter 5 - Movement Before Contact 183
they continued to decelerate at a constant rate until tactile contact was
achieved. These were not submovements indicated by local changes
in acceleration and deceleration, but a constant deceleration.
Subsequent matching tests revealed that subjects could not estimate
distance of the dowel accurately, thinking it to be closer than it was.
Subjects did not adapt after practice. They had no difficulty determin-
ing the direction for the movement. Movement time was increased
using only central vision following peak deceleration of the transport
component. The acceleration profile of the transport seems to show
two deceleration phases. In terms of the grasping component, in
marked and reliable contrast to the effects on the transport component,
the evolution of grip aperture was completely unaffected by restricting
information to only central vision compared to normal vision. Central
vision was also as sensitive to object size as normal vision, in terms of
aperture profiles for grasping with pad opposition. Thus peripheral
vision does not appear to be necessary for planning and control of the
grasp component (Sivak & MacKenzie, 1990).
The major difference between the central and normal visual condi-
tions was the small field of view with only central vision. The small
field size reduced the amount or quality of dowel location information,
and on line visual feedback information about the moving limb.
Although it has been suggested that peripheral vision is used to pro-
vide information necessary for directing the limb to the proper location
of an object (Bard, Hay & Fleury, 1985; Paillard, 1982b; Paillard &
Amblard, 1985), subjects in the present experiment had no difficulty
with the direction, only the distance of the movement in the absence of
peripheral vision17. In the absence of peripheral vision, eye and head
position (and the motor control process of foveation) may provide
sufficient information about the direction of an object to be grasped.
Replicating others, the results from this study suggest that direction
and distance are separable control parameters for location of an object
(Sivak, 1989; Sivak & MacKenzie 1990).
The roles of central and peripheral vision in visuomotor integration
processes are suggested to be as follows for grasping under normal
viewing conditions. Peripheral vision provides important information
about the distance of an object (Sivak & MacKenzie, 1990), direction
(Bard, Hay & Fleury, 1985), and information about object or limb
motion (Bard et al., 1985; Paillard, 1982b). As suggested by Paillard
l7 Since this experiment required only one direction to move, i.e., in the midline
plane, it is not clear whether peripheral vision was necessary to assess the direction
of movement.