Chapter 2. Prehension

Chapter 8 - Constraints on Human Prehension 315
relevant to prehension shows altered muscle spindle sensitivity (i.e.,
dynamic changes in la afferent firing patterns) with flexion and
extension of the hand (Edin, 1988). This could possibly play an
important role in hand preshaping.
Constraints exist on the speedtiming of sensorimotor integration.
Sensory information appears to adjust ongoing movements in less
time than previous estimates of visual reaction time, estimates in the
range of 190-260 ms (Keele, 1968) or 120-200 ms (see Schmidt,
1988). Cole and Abbs (1988) have shown responses in grip force of
the index and thumb having a latency of 60-90 msec after onset of an
increase or decrease in load force. The changes were dependent on the
size and velocity of the perturbation force and independent of the
original grip force level. The latency is longer than a monosynaptic
reflex but shorter than the reaction time estimates, reflecting a rapid,
automatic mechanism responding specifically to unexpected load force
changes and not maintaining a preferred safety margin. They showed
that this depends on cutaneous stimulation such as increased shear
forces at the digital pulps due to object slips. One possible explanation
is that sensory information is being used here for triggering a
response, instead of as feedback for modulating a response.
As well, availability of a sensory modality can affect movement.
Movements occur differently depending on the availablity of the
sensory information. An example is seen in a patient having a lesion
in the parietal area of the cerebral cortex who could not preshape her
hand while reaching to grasp an object, because she lacked
somatosensory information (Jeannerod, 1986). Once the hand was
within view, the patient using visual guidance could open her hand
and grasp the object, although she was still unable to apply the correct
grasping forces. Peripheral nerve damage can influence behavior as
well (Rothwell et al., 1982). The patient in this study could initiate
grasping tasks, but a stable grasp could not be maintained. He could
not perform automatic reflex corrections in voluntary movements, nor
could he sustain levels of muscle contraction without the use of vision.
Johansson and Westling (1987) have shown that as human subjects
pick up objects, microslips occur beneath the conscious awareness of
the subject; that is, the object slips slightly in the grasp as the object is
lifted. Recording from the median nerve, Johansson and Westling
demonstrate that sensory information about the object’s state is being
transmitted into the nervous system. The microslips are reduced to
zero as the force level is adjusted and balanced to the frictional
conditions, stabilizing the object in the grip. The Rothwell et al.
patient, having lost access to microslip information, cannot make low