Handbook of Psychology, Volume 4: Experimental Psychology

Motor Coordination 339

single-hand dual-hand

same

(a) Lines

dual-hand

different

2.5 cm

2.5 cm

single-hand

(b) Circles

dual-hand

same

dual-hand

different

2.5 cm

2.5 cm

Figure 12.15 (a) Periodic lines drawn with one hand when with the other
nothing was done or lines and circles were drawn concurrently; (b) periodic
circles drawn with one hand when with the other hand nothing was done or
circles and lines were drawn concurrently (after Franz et al., 1991).

Spijkers and Heuer (1995; Heuer, Spijkers, Kleinsorge, &
Steglich, 2000) studied conditions in which one hand had to
produce constant-amplitude oscillations and the other hand
oscillations with alternating short and long amplitudes. They
found that the requirement to change the amplitude in the one
hand produced a contralateral effect in addition to the one ob-
served with constant-amplitude oscillations. Specifically,
after a change from a short to a long amplitude, the amplitude
of the contralateral hand was larger than when only long am-
plitudes were repeated, and after a change from a long to a
short amplitude the amplitude of the contralateral hand was
smaller than when only short amplitudes were repeated.
These findings indicate that cross-manual effects result not
only from concurrent execution of different amplitudes, but
perhaps also from cross-talk between processes of amplitude
specifications. This is also indicated by the observation
that contralateral involuntary amplitude modulations can be

produced not only by movements of alternating short and

long amplitudes, but also by the imagery of such movements

(Heuer, Spijkers, Kleinsorge, & van der Loo, 1998).

Discrete-movement studies give clear evidence of a tight

temporal coupling in that movements of different amplitude

and accuracy requirements tend to be of (almost) the same du-

ration (Kelso et al., 1979; Marteniuk, MacKenzie, & Baba,

1984). However, amplitude assimilation is only weak and

tends to be asymmetrical in that the amplitude of a shorter

movement is increased, while the amplitude of the concurrent

longer movement is only slightly or not at all reduced

(Heuer, Spijkers, Kleinsorge, van der Loo, & Steglich, 1998;

Marteniuk et al., 1984; Sherwood, 1991). The tighter temporal

than spatial coupling is also indicated by the typical finding that

movement durations are more strongly correlated across trials

than movement amplitudes (e.g., Sherwood, 1991).

Structural constraints on coordination are double-faced:

On the one hand, they support the performance of certain

tasks, like the production of strictly symmetric movements of

the upper limbs, and on the other hand they impede the per-

formance of other tasks, like the production of asymmetric

movements. Together with the basically soft nature of struc-

tural constraints, this suggests that their strength might be

modulated depending on task requirements. In particular,

their strength might be enhanced when this is appropriate for

the task at hand, and it might be reduced when this is appro-

priate. Such a task-related modulation of structural con-

straints on coordination does indeed exist.

Sherwood (1991) found higher intermanual correlations

between the amplitudes of discrete rapid reversal movements

when same amplitudes rather than different amplitudes were

required. The same result was reported by Heuer, Spijkers,

Kleinsorge, van der Loo, and Steglich (1998), who in addi-

tion found aftereffects such that the intermanual amplitude

correlation was higher subsequent to same-amplitude move-

ments than following different-amplitude movements, and by

Steglich, Heuer, Spijkers, and Kleinsorge (1999) for peak

forces of isometric contractions with same and different tar-

get forces for the two hands.

Rinkenauer, Ulrich, and Wing (2001) showed for isomet-

ric contractions that the requirement to produce different

peak forces is associated not only with a reduction of the in-

termanual correlation between peak forces, but also with a re-

duction of the intermanual correlation between rise times.

Similarly, when different rise times were to be produced in

another experiment, both the correlations between rise times

and between amplitudes were reduced. These findings indi-

cate that the decoupling, which can be observed when differ-

ent movements or isometric contractions are to be produced