334 Motor Control
the position of the car on the road that is controlled but,
rather, parameters like the curvature of the path.
In tasks like tracking there is not only visual feedback, but
also proprioceptive feedback, and both types of feedback are re-
lated to different objects: proprioceptive feedback to one’s own
movements, but visual feedback to the motions of a controlled
object. The relation between both types of feedback depends on
the transformation implemented by the controlled machine.
Only when the transformation is simple or well-learned, or both,
can proprioceptive feedback replace visual feedback, but in
general such intermodal matching is associated with some loss
of accuracy as compared with intramodal matching of target and
feedback information (e.g., Legge, 1965). On the other hand,
when visual and proprioceptive feedback are different but
nevertheless refer to the same object, as in the classical task of
mirror drawing, the absence of proprioceptive feedback can ac-
tually enhance performance (Lajoie et al., 1992).
Even when visual and proprioceptive feedback refer to
the same object, they are not necessarily redundant. For ex-
ample, when we use a knife, both vision and proprioception
provide information about its current position with respect to
the object to be cut. Of course, visual information is more
accurate in this respect, and as far as the spatiotemporal
characteristics of the movement are concerned, propriocep-
tive information is not really needed. However, it provides
information that is not available visually, in particular about
the resistance of the cut object. Thus, although vision is crit-
ical for registering spatiotemporal characteristics, proprio-
ception is critical for registering force characteristics. The
lack of this latter kind of information is a problem in remote
control and other tasks that followed from recent technolog-
ical developments (cf. the chapter by Klatzky & Lederman
in this volume).
An example is minimally invasive surgery (cf. Tendick,
Jennings, Tharp, & Stark, 1993). Such operations are per-
formed by means of an endoscope and instruments that are
pivoted roughly at their place of insertion into the tissue.
Although the facts that movements of the hand result in
movements of the tip of the instrument in the opposite direc-
tion and that the gain of lateral movements depends on trans-
lational movements seem not to pose severe practical
problems, the lack of appropriate force feedback seems to be
more critical. In particular, there is only poor proprioceptive
information about reactive forces at the tip of the instrument,
so there is the risk of damage to the tissue operated on.
Sensory Information for Motor Control and Perception
Much of the sensory information that is involved in the con-
trol of movements apparently has no access to consciousness.
Folklore knows that one just has to do it without attending too
much to how it is done. In fact, Wulff, Höß, and Prinz (1998)
found better learning of gross motor skills when the attention
of the learners was focused on the effects of the movements
rather than on the movements themselves, for example on a
stabilometer platform rather than on the feet (for review, see
Wulf & Prinz, 2001). It is not only that we do not perceive our
movements in all details—for example, in skills like the long
jump we do not normally perceive the details of the move-
ments of our extremities (Voigt, 1933)—but, in addition, our
movements can be more precise than would be expected
from the limits of our perceptual skills. This was not only one
of the major claims of a motor branch of the so-called
Ganzheitspsychologie (Klemm, 1938), but it has also been
emphasized in more recent times. For example, McLeod,
McLaughlin, and Nimmo-Smith (1985) ascribed the very
small temporal variability in batting of only a few millisec-
onds to the functioning of a dedicated special-purpose mech-
anism. In any case, hitting a falling ball at a certain position
of its path is more precise than pressing a key when the ball
reaches the same position (Bootsma, 1989).
Clinical cases illustrate that humans can reach to visual
targets that they do not perceive, provided that the blind areas
of the visual field (scotoma) are caused by certain lesions
(e.g., Campion, Latto, & Smith, 1983; Perenin & Jeannerod,
1978). This phenomenon has become known as blindsight. In
addition, clinical data give evidence of double dissociations.
For example, some patients can identify and describe objects,
but they cannot use the information about size, form, and
orientation of the objects to grasp them; other patients, in
contrast, cannot perceive these features of objects, but never-
theless can grasp them (Goodale & Milner, 1992).
The dissociability of visual information for perception and
for motor control supports a theoretical distinction that has
received much attention during the last 20 years. Basing their
idea mainly on lesion studies, Ungerleider and Mishkin
(1982) proposed the distinction between two cortical visual
systems, one involving the inferotemporal cortex and the
other the posterior parietal cortex (ventral streamanddorsal
stream,respectively). In functional terms, these two systems
have been characterized as the whatsystem and the where
system, the former serving object identification and the latter
space perception. Alternatively, they are characterized func-
tionally as the whatsystem and the howsystem, the former
being in the service of perception and the latter in the service
of motor control (e.g., Goodale & Humphrey, 1998; Goodale
& Milner, 1992). This latter functional characterization does
largely coincide with a distinction between a cognitiveand a
sensorimotorsystem (Bridgeman, Kirch, & Sperling, 1981;
Bridgeman, Lewis, Heit, & Nagle, 1979).