328 Motor Control
Figure 12.9 Examples of handwriting with (upper example) and without
(lower example) vision in (a) a deafferented patient (from Teasdale et al.,
1993) and (b) a healthy girl.two target lines in the pace of a metronome. With the partici-
pants’ eyes closed, accuracy was only little affected by
frequency, but with the participants’ eyes open, accuracy in-
creased relative to that found with closed eyes as soon as less
than about two movements per second were produced. A next
major step was a study by Keele and Posner (1968) with dis-
crete movements. Movement times were instructed, and the
movements were performed with full vision or in the dark,
with the room light being switched off at the start of the
movements. Except for the shortest movement time of about
190 ms, the percentage of movements that hit the target
was larger with than without vision. Subsequent studies
showed that the minimal duration at which accuracy gains
from the availability of vision becomes shorter—about
100 ms—when conditions with and without vision are
blocked rather than randomized (Elliott & Allard, 1985;
Zelaznik, Hawkins, & Kisselburgh, 1983). This minimal du-
ration reflects processing delays, but it also reflects the time it
takes until a change of the pattern of muscular activity has an
effect on the movement.
Woodworth (1899) distinguished between two phases of a
rapid aimed movement, an “initial adjustment” and a second
phase of “current control.” This distinction seems to imply
that accuracy should profit mainly when vision becomes
available toward the end of aimed movements. However,
even early vision can increase accuracy (Paillard, 1982), and
accuracy increases when both initial and terminal periods of
vision increase in duration (Spijkers, 1993). Thus, the view
that vision is important only in the late parts of an aimed
movement seems to be overly simplified.
From the basic findings it is clear that, in general, vision is
not really necessary for the production of movements, but
that it serves to improve accuracy. The same kind of general-
ization holds for the second important type of sensory infor-
mation for motor control, proprioception. (For tasks that
involve head movements, including stance and locomotion,
the sensors of the inner ear also become important, although
I shall neglect them here.) Regarding the role of propriocep-
tion for motor control, classic observations date back to
Lashley (1917). Due to a spinal-cord lesion, the left knee
joint of his patient was largely anesthetic and without cuta-
neous and tendon reflexes. In particular, the patient did not
experience passive movements of the joint, nor could he re-
produce them; only fairly rapid movements were noted, but
the experienced direction of movement appeared random.
However, when the patient was asked to move his foot by a
certain distance specified in inches, the movements were
surprisingly accurate, as were the reproductions of active
movements; the latter reached the accuracy of a control
subject. The basic finding that aimed movements are possible
without proprioception (and, of course, without vision also)
has been confirmed both in monkeys (e.g., Polit & Bizzi,
1979; Taub, Goldberg, & Taub, 1975) and—with local tran-
sient anesthesia—in humans (e.g., Kelso & Holt, 1980),
although, of course, without proprioception there tends to be
a reduction of accuracy.
The very fact that movements are possible without vision
and proprioception proves that motor control is not just a
closed-loop process but involves autonomous processes that
do not depend on afferent information. The very fact that
accuracy is generally increased when sensory information
becomes available proves that motor-control structures also
integrate this type of information. Beyond these basic gener-
alizations, however, the use of sensory information becomes
a highly complicated research issue because sensory infor-
mation can be of various types and serves different purposes
in motor control.
As a first example of some complexities, consider a task
like writing or drawing. Normally we have no problems writ-
ing with our eyes closed, except that the positioning of the
letters and words tends to become somewhat irregular in both
dimensions of the plane. This is illustrated in Figure 12.9b.
Figure 12.9a shows the writing of a deafferented patient both
with and without vision (Teasdale et al., 1993). The patient
had suffered a permanent loss of myelinated sensory fibers
following episodes of sensory neuropathy, which resulted in