Chapter 2. Prehension

Chapter 3 - Serial Order in Prehension 53
and shape) and extrinsic object properties (or egocentric spatial
properties such as distance, orientation, direction and velocity of
object motion with respect to the body). He suggested that the two
types of properties are likely to be detected through different structures
or channels. Specifically, he suggested that for grasping an object,
seDarate visuomotor channels are activated in parallel by a specific
visual input and controlling; a sDecific part of the limb musculature.
For example, extrinsic spatial properties of an object activate proximal
muscles (e.g., shoulder joint) for the transport component, and
intrinsic properties activate distal segments (e.g., fingers) for the
grasping component.
3.1 Conceptual Models of Prehension
Arbib’s conceptual model (198 1) of a coordinated control program
for grasping captured the data of Jeannerod. In the 1985 version, seen
in Figure 3.3, perceptual schemas in the upper half of the figure ex-
tract relevant task related information from the environment. The per-
ceptual schemas serve as identification algorithms (procedures) to de-
termine parameters for motor schemas, seen in the lower half of the
figure.
The perceptual schemas include a visual location schema which,
when activated, in turn activates size recognition and object orientation
recognition schemas. The motor schemas control some aspect of the
movement. Perceptual and motor units all working together, not un-
der one central controller, but under distributed control. On the left
half of the figure are depicted motor schemas controlling the arm for
the reaching, or transport, component of the movement. Visual in-
formation about object location provides data to the hand reaching
schemas. Jeannerod’s high velocity phase has been labelled as a
‘ballistic movement’. The low velocity phase after peak deceleration
has been called ‘adjustment’ by Arbib. Motor schemas in the right
half control the grasping component. Visual information about object
size and orientation provide data for the grasping schemas. The fin-
gers change their posture, making adjustments or preshaping into
some suitable shape. The hand is rotated into a suitable orientation as
well. Activation of the motor schemas simultaneously initiates ballistic
movement to the target and a preshaping of the hand. The fingers are
adjusted to the size of the object and the hand is rotated to the appro-
priate orientation. Ending the ballistic movement of the wrist gives
rise to activation of the ‘actual grasp’ schema whereby the fingers are