Chapter 2. Prehension

54 THE PHASES OF PREHENSION
recognition
criteria visual
activation of
visual search
actioation
visual
Size
target Recognition
location
ow
Orientation
lvisual
size orien ta ta tion
FINGER
visual and
tactile input
grasping
Figure 3.3 Coordinated control program for reaching and grasping.
Perceptual schemas, on the top, extract relevant task information
from the visual scene. Motor schemas, on the bottom, perform
the transport component (on the left) and the grasping component
(on the right). Grasping involves adjusting the hand posture and
orienting the wrist. The differential effects of visual and tactile
inputs on motor schemas are unclear in this model. Solid lines
represent data lines, and dashed lines represent control lines (from
Arbib, 1985; adapted by permission).
enclosed around the object. On contact with the object, the actual
grasp occurs.
Based on the results of experimental investigations of the role of
vision in reaching and grasping (research on humans, split brain mon-
keys, and prismatic displacement), Paillard (1980, 1982b) also pre-
sented a schema model which segmented reaching behavior into sev-
eral temporal phases via two parallel visual channels performing object
identification and object location functions. Figure 3.4 indicates that
after the eyes and head are positioned for foveal grasping, there are
triggered (open loop) and guided (closed loop) phases for the arm and
hand. A location channel uses movement cues from peripheral vision
for transporting the arm (assisting the 'navigator') and in parallel, an