Motor Coordination 335The evidence for the functional separation of a cognitive
and a sensorimotor system is based on differences between
psychophysical judgments and motor responses to identical
stimuli. For example, Bridgeman, Peery, and Anand (1997)
exploited the long-known effect of asymmetric stimuli in the
visual field on the perceived direction of a target. They pre-
sented targets within a frame which was centered or shifted to
the left or to the right. The target could appear in five differ-
ent positions, and participants had to give their judgments by
pressing one of five keys immediately after the stimulus had
disappeared. For these perceptual judgments there was a
clear effect of the position of the frame: When the frame was
shifted to the left, judgments were shifted to the right, and
vice versa. In contrast, when participants had to rotate a
pointer so that it pointed to the target just presented, about
half the participants exhibited no effect of the position of the
frame. This was so although the response mode varied ran-
domly and was cued only after the target had disappeared.
When delays of a few seconds between the disappearance
of the target and the response were introduced, all partici-
pants showed effects of the frame position on pointing.
Bridgeman et al. (1997) took their findings to indicate that the
sensorimotor representation of the target is short-lived and
overridden by the cognitive representation when the delay
between disappearance of the target and response becomes
sufficiently long. In some subjects the sensorimotor represen-
tation might even be so short-lived that it hardly survives the
target presentation.
Although the whatversushowdistinction currently has a
dominant influence, it is most likely a simplification. Pro-
cessing of visual information is widely distributed across
the brain, and so is motor control. Thus, it is easy to conceive
of a set of systems that for different kinds of responses make
use of different combinations of the various neural represen-
tations of the visual world. From such a perspective, there
would be a multiplicity of perception-action systems, for
which there is indeed evidence in other primates than humans
(cf. Goodale & Humphrey, 1998).
MOTOR COORDINATION
In a general sense, coordination is a characteristic of almost
any skilled movement, in that skilled performance requires
fairly precise relations between various kinematic, kinetic,
and physiological variables. For example, in cranking (and
related tasks like pedaling), force pulses need to be precisely
timed to occur during a certain phase of the rotation of the
crank or pedal (cf. Glencross, 1970); in rapid finger tapping,
muscle activity of flexors and extensors must be timed to
occur at certain phases of the movement cycle (Heuer, 1998);
in reaching for an object, the opening of the fingers must be
related to the movement of the hand toward the object (e.g.,
Jeannerod, 1984); and so on. With this broad meaning, the
termcoordinationbecomes almost equivalent to motor con-
trol. However, for this section I use a narrower meaning in
that I focus on the coordinated movements of different effec-
tors, mainly the two arms (interlimb coordination).Task Constraints and Structural ConstraintsCoordinated movements of the two hands are largely deter-
mined by the task constraints. For example, the coordination
pattern for sweeping with a broom is different from that for
bathing a baby. This certainly is not a fact that deserves elab-
oration. However, there are more subtle consequences of task
constraints. Perhaps the most important of these is compen-
satory covariation.
As an example, consider the lip aperture in speaking. A
particular lip aperture can be achieved by various combina-
tions of the positions of the upper lip, the lower lip, and the
jaw. These positions exhibit compensatory covariation such
that, for example, a high position of the upper lip will be ac-
companied by higher positions of the lower lip and/or the
jaw, and a low position of the upper lip by lower positions of
the lower lip and/or the jaw (Abbs, Gracco, & Cole, 1984).
Compensatory covariation can be observed not only when lip
positions vary spontaneously, but also when they are per-
turbed by means of some mechanical device (Kelso, Tuller,
Vatikiotis-Bateson, & Fowler, 1984). A task similar to reach-
ing a certain lip aperture is that of grasping an object with a
precision grip, wherein there is compensatory covariation of
the positions of thumb and index finger (Darling, Cole, &
Abbs, 1988).
Compensatory covariation can be seen as a way to reduce
the degrees of freedom in motor control. In addition, com-
pensatory covariation contributes to solving the problem of
achieving a stable movement outcome in spite of variable
components. For example, with the appropriate covariation
of lip and jaw positions, lip aperture will hardly vary. In fact,
the principle of compensatory covariation seems to be a gen-
eral principle of stabilizing movement outcomes (Müller,
2001) which is not restricted to tasks in which different limbs
are involved.
A highly illustrative task for compensatory covariations is
throwing a ball a certain distance. For physical reasons, when
the initial flight angle varies, the initial velocity of the ball
has to covary to reach a certain target distance. In particular,
with an initial angle of 45° the initial velocity has to be small-
est, and it has to be increased as the initial flight angle