The Use of Sensory Information 329a total loss of sensitivity to touch, vibration, pressure, and
kinesthesia as well as an absence of tendon reflexes, although
the motor nerve conduction velocities were normal. With vi-
sion, the writing of “Il fait tiède” seems rather normal, but
without vision the placement of words, letters, and parts of
letters is severely impaired, while individual letters remain
largely intact. Similarly, in drawing ellipses with eyes closed,
single ellipses appeared rather normal, but successive el-
lipses were displaced in space. Thus, absence of sensory in-
formation affects different aspects of the skill differently, and
impairments are less severe when proprioception can serve as
a substitute for absent vision.
Target Information
Vision and proprioception serve at least two different func-
tions in motor control, which are not always clearly distin-
guished. First, they provide information about the desired
movement or target information, and, second, they provide
information about the actual movement or feedback informa-
tion. In the typical case, target information is provided by vi-
sion only, and feedback information both by proprioception
and by vision. Thus, vision provides both kinds of informa-
tion, and the effects of absent vision can be attributed to
either of them. The obvious question of whether target
information or feedback information is more important for
movement accuracy, as straightforward as it appears, cannot
unequivocally be answered. In the literature, contrasting find-
ings have been reported. For example, Carlton (1981) found
vision of the hand to be more important, whereas Elliott and
Madalena (1987) found vision of the target to be crucial for
high levels of accuracy. Perhaps the results depend on subtle
task characteristics. However, for throwing-like tasks, vision
of the target seems to be critical in general (e.g., Whiting &
Cockerill, 1974), and dissociating the direction of gaze from
the direction of the throw or shot seems to be a critical ele-
ment of successful penalties.
Specification of Spatial Targets
Targets for voluntary movements are typically defined in ex-
trinsic or extrapersonal space, whereas movements are pro-
duced and proprioceptively sensed in personal space. Both
kinds of space must be related to each other; they must be cal-
ibrated so that positions in extrinsic space can be assigned to
positions in personal space and vice versa. When we move
around, the calibration must be updated because personal
space is shifted relative to extrinsic space. Even when we do
not move around, the calibration tends to be labile. This
lability can be evidenced from the examples of handwriting
in Figure 12.9: With the writer’s eyes closed, calibration gets
lost with the passage of time, so positions of letters or parts of
them exhibit drift or random variation. This effect is much
stronger when no proprioception is available.
An interesting example of failures that are at least partly
caused by miscalibrations of extrinsic and personal space are
unintended accelerations (cf. Schmidt, 1989). These occur in
automatic-transmission cars when the transmission selector
is shifted to the drive or reverse position, typically when the
driver has just entered the car; when he or she is not familiar
with the car, this is an additional risk factor. In manual-
transmission cars, incidents of unintended acceleration are
essentially absent. According to all that is known, unintended
accelerations are caused by a misplacement of the right foot
on the accelerator pedal rather than on the brake pedal with-
out the driver’s being aware of this. Thus, when the car starts
to move, he or she will press harder, which then has the un-
expected effect of accelerating the car.
The position of the brake pedal is defined in the extrinsic
space of the car, whereas the foot placement is defined in the
personal space of the driver. In particular upon entering a car,
and more so when it is an unfamiliar car, there is the risk of
initial miscalibration. Thus, when extrinsic and personal
space are not properly aligned, the correct placement of the
foot in personal space might reach the wrong pedal in extrin-
sic space. Manual-transmission cars, in contrast, have a kind
of built-in safeguard against such an initial miscalibration,
because shifting gears requires that the clutch be operated
beforehand. Thus, before the car is set into motion, the proper
relation between foot placements and pedal positions is
established.
Calibration, in principle, requires that objects, the loca-
tions of which are defined in world coordinates, be simulta-
neously located in personal space. Mostly it is vision that
serves this purpose. However, personal space embraces not
only vision: In addition to visual space, there are also a pro-
prioceptive and a motor space, and these different spaces
must be properly aligned with each other. For example, in
order for us to reach to a visually located target, its location
must be transformed into motor space, that is, into the appro-
priate parameters of a motor control structure. In addition, its
location must be transformed into proprioceptive space, so
that we can see and feel the limb in the same position. In a
later section I shall discuss the plasticity of these relations;
here I shall focus on the question of how a visually located
spatial target is transformed into motor space.
An object can be localized visually both with respect to an
observer (egocentrically) and with respect to another object
(allocentrically or exocentrically; cf. the chapter by Proffitt &
Caudek in this volume). Geometrically the location of the