Chapter 2. Prehension

70 THE PHASES OF PREHENSION
(right side of Figure 3.3). The details, as well as the variations that
might occur with each grasping instance, are present within each
schema. The only invariances suggested in Arbib’s model are the
triggering of the schemas.
4.2.3 Neural task plans
In a typical experiment to study movement latencies, Jeannerod
and Biguer (1982) had subjects move as “fast and accurate” as
possible to reach and grasp an object placed in a specific location, in
response to a visual go signal. They measured arm and hand muscle
activation as subjects reached for and grasped an object. Recording
the activity level of electromyography (EMG) data, Jeannerod and
Biguer noted a 250 ms lag time from the ‘GO’ signal to a burst in
EMG activity in the biceps muscle of the arm (see Appendix A).
About 20 ms later, an EMG burst was seen in the extensor digitorum
communis, the muscle used to extend the fingers as the hand opened
from the initial resting posture of thumb and index finger pads
touching. Actual movement of the fingers occurred about 80 to 100
ms later, which is the electromechanical delay time for isometric
tension to be established in the muscle.
While Jeannerod and Biguer noted a 250 ms lag time, the time lag
that it takes for the subject to initiate movement is not fixed. This time
to react after a stimulus, or reaction time, can vary depending on a
myriad of factors including the difficulty of the task. More complex
tasks generally increase the reaction time of the subject. For simple
movements, the reaction time is shorter than for more complex
movements. For example, in a classic study done by Henry and
Rogers (1960), subjects were asked, in response to an auditory “go”
signal, to make either a simple movement (lift finger from key), a
complex movement (lift finger, reach and grasp suspended ball), or a
more complex movement (lift finger, reach forward and strike
suspended ball with back of hand, reverse direction to push button,
and then strike another suspended ball). For adult subjects, mean
reaction time increased as the movement complexity increased, from
158 ms for the simple finger lift task to 197 ms for grasping to 213 ms
for the more complex movement sequence requiring ball strikes and
changes in direction. More recently, reaction times to initiate grasps
have been unaffected by systematic manipulations of object texture
(Fikes, Klatzky & Lederman, 1993) or orientation (Stelmach,
Castiello & Jeannerod, 1993). Thus time to intiate a grasp depends on
the sensory modality for processing the go signal (auditory or visual),