Chapter 2. Prehension

Chapter 3 - Serial Order in Prehension 59
transport and grasping components. Research that restricts visual in-
formation to the central or peripheral visual fields shows differential
effects on the transport and grasping components, thereby extending
the distinctions made in Paillard’s conceptual model.
Significant limitations to these conceptual models relate to the un-
derlying question of the sequencing of phases. Jeannerod’s analyses
were strictly limited to movement before contact with the object, yet he
also argues for the separation of this movement into two distinct
phases. As Paillard (1982b) anticipated, and others have experimen-
tally demonstrated, there are obvious differences in the movement be-
fore and after contact with the object. For example, multiple phases of
force application have been identified after contact with the object. As
well, much has been written in the robotics literature about stably
grasping and manipulating objects. A more comprehensive model
must be developed to reflect current knowledge of the entire complex-
ity of grasping movement.
For the modeller, the conceptual model does not explain exactly
what information is being transferred or controlled. What are the in-
puts and outputs? For example, what is the size information being
passed to the ‘finger adjustment’ schema, and what exactly does it
mean to adjust the fingers? Many researchers, such as Jeannerod,
have looked at the aperture between the thumb and index finger as a
metric of hand shaping. Is this a valid metric, or are there more
revealing, complete methods for quantifying hand shaping? In light of
the prehensile classification schemes outlined in Chapter 2, the size of
the aperture may be a reasonable measure for pad opposition, but is it
valid as a measure for other grasps involving hand surfaces other than
the fingers pads?
Two main messages can be seen in Jeannerod’s results and the
conceptual models of Arbib, Greene, and Paillard. First, the system is
a distributed one, involving parallel activation and coordinated control
of several components or subsystems. In addition to the transport and
grasping components, parallel activation and control of head and eye
movement occurs in order to foveate objects. Likewise, there are cor-
responding postural adjustments to optimize interaction with the object
and maintain stability or balance. While all these subsystems are im-
portant, this book concentrates primarily on the transport and grasping
components. The second main message is that there are different
phases as the unified act of grasping unfolds. The problem of serial
order in behavior has long been of interest (see Lashley, 1951).
Detailing the phases of prehension at a conceptual level, based on ex-