Chapter 2. Prehension

Chapter 5 - Movement Before Contact 157
bumped while reaching to grasp an object). As well we consider vi-
sual perturbation studies, which change the object characteristics in
some way (as when an object moves unexpectedly). These two ap-
proaches to perturbation share the view of probing the controller, ex-
amining how information is used in sensorimotor integration to
achieve the grasping goals in setting up opposition space. With me-
chanical perturbation studies, in addition to sensory information from
proprioceptors (cutaneous, joint, muscle and tendon receptors in the
entire limb), there are corresponding, complex alterations in limb dy-
namics which the controller takes into account. With visual perturba-
tions, mechanical responses in the limb to perturbation are not evident;
the visuomotor integration processes can be inferred from changes in
the kinematics of grasping.
Examining multiarticulate hand movements, in analogy to unex-
pected perturbations in speech and postural control, Cole, Gracco and
Abbs (1984) trained human subjects to generate rapid movements of
the thumb and index finger in pad opposition to produce a controlled
pinch contact force of 1 N. The hand was splinted to restrict available
motion to the interphalangeal joint of the thumb and the metacarpopha-
langeal joint of the index finger, so that the pads could be brought into
opposition. On 13% of trials, a force opuosing: thumb flexion was
delivered (1.9 N, rise time 15 ms, sustained throughout the move-
ment), within an interval 70 ms prior to activation of the flexor digito-
rum superficialis (the index finger flexor here). On perturbation trials,
the desired contact force was achieved by rapid compensatory adjust-
ments in both the thumb and the index finger. They reported that these
adjustments: had latencies of 60 - 90 ms; were manifest even with the
first perturbation and; were absent for loads opposing thumb move-
ment during a nonprehensile task. Cole et al. suggested that this con-
text dependency of the finger flexion responses extends the work of
Traub, Rothwell and Marsden (1980) who reported a ‘grab reflex’ or
‘sherry glass response’ whereby, regardless of loading or unloading
of the thumb flexor muscle (flexor pollicis longus) by mechanical
perturbations to the wrist, the functional response was to maintain
grasp, dependent and adapted to the task and the intent of the individ-
ual to maintain the digits in contact with the object.
In a followup study, Cole and Abbs (1987) showed that the sys-
tem’s response to an unexpected perturbation is to maintain the high
level goal of bringing the finger pads into contact to maintain the re-
quired force generation. If the thumb was perturbed, there was a short
latency response (about 60-90 ms) and the system, through a very dif-
ferent kinematic pattern, still achieved the goal of the criterion force