Chapter 2. Prehension

Chapter 5 - Movement Before Contact 185
calibrate the hand opening at contact to successfully grasp the object.
Under both binocular and monocular viewing conditions, the objects
could be seen in foveal vision. Recall that with only central vision,
Sivak also found that subjects underestimated the distance to objects,
but calibrated the grip to the size of the object, in exactly the same way
as with normal, binocular vision (Sivak, 1989; Sivak & MacKenzie,
1992).
Evidence indicates that some sort of proprioceptive information is
also crucial to the preshaping of the hand. For patients having no sen-
sory information from the periphery (from tactile, muscle, joint and
tendon receptors) or having lesions in the parietal cortex, preshaping
of the hand could only be accomplished with vision (Jeannerod,
1986). Jeannerod studied a patient who lost all sensation on her right
side due to the blood flow being blocked to her left anterior parietal
cortex (patient 6, R.S. in Jeannerod, 1986). As seen on the left of
Figure 5.28, she could preshape her normal left hand as she reached
out to grasp the object under visual control. The center column of the
figure shows her affected right hand doing the same task, much
slower, and with more corrections under visually guided control. She
could preshape the hand, but only when the hand was within her eye-
sight. On the right side of the figure, it is shown that she had no
ability to preshape the hand without visual feedback. It appears that
with interruption of the somatosensory pathways, subjects need visual
information in order to configure the hand. Without visual
infomation, the affected hand cannot be preshaped, and the transport
component is also affected.
In addition to lesions in central pathways, disruptions in peripheral
pathways can lead to impaired motor performance. In Rothwell et al.
(1982), a patient known as G.O. lost light touch, vibration and
proprioceptive sensation due to a sensory peripheral neuropathylg. He
was able to perform a variety of tasks, such as thumb positioning and
drawing figures in air with his index finger. Kinematic measures of
his performance in thumb positioning tasks requiring fast and accurate
movements were similar to those of normal subjects. On the slower
movements, he exhibited more variability and less accuracy than
normal subjects, and clearly demonstrated his reliance on vision.
Interestingly, while the subject was able to perform well in laboratory
tests, his need to constantly monitor his movements by vision severely
limited his hand usage in his daily living. Without proprioceptive
information for sustained contractions of muscles, he had lost the
19A disorder in the peripheral nerves.