342 Motor Control
also for isometric contractions with same and different forces
(Steglich et al., 1999).
The data of Figure 12.16a reflect the gradual specification
of movement amplitudes, and they reveal a transient para-
metric coupling, that is, a coupling that is gradually relaxed
when required by the task. This is also indicated by the inter-
manual correlations shown in Figure 12.16b. Parametric cou-
pling does apply to concurrent specifications of movement
parameters, but not to the time-varying force signals or other
execution-related signals during actual performance of the
movements. Thus it should also show up in reaction time,
that is, before bimanual movements are actually initiated, and
it should show up even in unimanual tasks, provided that
parametric specifications for the movement executed have
temporal overlap with parametric specifications for a move-
ment with the other hand that is not produced concurrently.
There is indeed some evidence for such effects with move-
ments of the two hands with same and different amplitudes
(Spijkers, Heuer, Kleinsorge, & van der Loo, 1997; Spijkers,
Heuer, Kleinsorge, & Steglich, 2000).
Although parametric coupling seems to be transient as far
as amplitude and peak-force specifications are concerned,
this is different for temporal specifications. As reviewed
above, for different target durations correlations between
movement durations do not decline as strongly as correla-
tions between peak forces do. Thus, there is a stronger static
component to the parametric coupling, which accounts for
the fact that it is extremely hard or even impossible to
produce different temporal patterns with the two hands con-
currently. If this is indeed the case, one would expect that
reaction time for the choice between a left-hand and a right-
hand movement is longer when movements with different
rather than same temporal characteristics are assigned to the
two hands. The reason is that same temporal characteristics
can be prepared concurrently in advance of the response sig-
nal (or perhaps immediately after presentation as long as the
choice of the correct response is not yet finished), whereas
this is impossible for different temporal characteristics. Such
reaction-time differences do exist (see Heuer, 1990, for a re-
view; Heuer, 1995).
FLEXIBILITY OF MOTOR CONTROL
The motor transformation, the relation between motor com-
mands and resulting movements, is variable. On a short time
scale, variations arise when we handle objects, tools, and ma-
chines, and when we move in different directions relative to
gravity. On a longer time scale, variations result from body
growth and other bodily changes. As a consequence of such
variations, the internal model of the motor transformation,
which captures the relations between motor commands, pro-
prioceptive information, and visual information, must be
flexible. To study this flexibility experimentally, the relations
can be modified by way of transforming the normal visual
input; in addition, they can be modified by way of adding ex-
ternal forces. I shall consider the flexibility of motor control
in both respects in turn.Adapting and Adjusting to New
Visuo-Motor TransformationsVarious kinds of optical transformations can be used to
change the usual relation between movements (motor com-
mands and proprioceptive movement information) and their
visual effects (cf. Welch, 1978, for an overview). The history
of such research dates back to the late nineteenth century,
when Stratton (1896, 1897a, 1897b) used spectacles that
served to turn the visual world upside down. Later Kohler
(e.g., 1964) pursued this line of research with various sorts of
distorting spectacles. All in all, the perceptual consequences
of such severe transformations of the visual world are ex-
tremely complex and difficult, if not impossible, to under-
stand. However, as far as motor behavior is concerned, this
generally comes to appear fairly normal, even if adaptation
can take several days.
A somewhat different and simpler type of transformation
of the visual world was introduced by Helmholtz (1867),
namely the use of wedge prisms, which serve to shift the vi-
sual world laterally. When no visual background (or only a
homogeneous one) is available, the distorting effects of
wedge prisms can be neglected and the consequence of the
shifted egocentric visual direction can be studied. A typical
lateral displacement is 11°. Thus, when participants are in-
structed to point to a target that is visually displaced to the
right, their movements will end to the right of the physical
target. When they receive feedback on the pointing errors,
these will gradually disappear in the course of a series of
movements. In principle the disappearance of the systematic
pointing errors could be due to strategic corrections, that is,
to simply pointing to the left of the perceived target. Alterna-
tively, it can be due to a change of the internal model of the
visuo-motor transformation. More revealing than the disap-
pearance of the pointing error in the exposure phase is the
negative aftereffect that can be observed after removal of the
wedge prisms. Now, without visual feedback, subjects tend to
point in the opposite direction, that is, to the left of the target
when it had been visually displaced to the right. Negative
aftereffects can also be observed when the prism strength is
gradually increased in the exposure period with concurrent