340 Motor Control
with the two hands, is a generalized phenomenon which is
not restricted to the movement characteristic, that is actually
different. In addition, coupling with respect to peak forces
can be more easily modulated than temporal coupling; in
fact, peak forces can be fully decoupled, but timing cannot.
In right-handed people, the right and left hands take
different roles in bimanual actions: The typical function as-
signed to the left hand is holding, while the right hand per-
forms manipulations relative to the left one. Generalizing this
typical assignment of functions, Guiard (1987) characterized
the left and right hand as macrometricandmicrometric,
respectively. Whereas the left hand is specialized for large-
amplitude and low-frequency movements, the right hand is
specialized for accurate small-amplitude and high-frequency
movements. This characterization of the two hands, together
with the typical functions assigned to them in bimanual tasks,
suggests that structural constraints on coordination may be
asymmetric. Although the results on lateral asymmetries in
bimanual tasks tend to be somewhat unreliable, there seem to
be at least two consistent findings.
The first finding is that bimanual tasks are easier when
lower-frequency movements are assigned to the left hand and
higher-frequency movements to the right hand than with the
opposite assignment of movements to hands. This is true for
oscillatory movements (Gunkel, 1962) and discrete finger
taps (Ibbotson & Morton, 1981; Peters, 1981). An example is
tapping a steady beat with the left hand and a certain rhythm
with the right hand. With the opposite assignment the task is
harder, and when performance breaks down, it is typically in
the right hand, which also starts to produce the rhythm as-
signed to the left hand. Thus, conforming to Guiard’s (1987)
notion of macrometric and micrometric functional specializa-
tions of the two hands, task assignments that conform to these
specializations are easier than task assignments that violate
them. Also conforming to Guiard’s notion, Spijkers and
Heuer (1995) observed stronger assimilations of movement
amplitudes when large-amplitude oscillations were produced
with the right hand and small-amplitude oscillations with the
left hand than with the opposite assignment of amplitudes to
hands.
A second rather consistent finding is a lead of the right
hand in bimanual tasks like circle drawing (Stucchi &
Viviani, 1993; Swinnen, Jardin, & Meulenbroek, 1996).
Stucchi and Viviani (1993) hypothesized that in particular the
timing of bimanual movements might originate from the
hemisphere contralateral to the dominant hand. This hypoth-
esis has received some support from a PET study (Viviani,
Perani, Grassi, Bettinardi, & Fazio, 1998), and it is consistent
with the evidence for tight temporal coupling (or unitary
timing mechanisms) in bimanual tasks.
Levels of CouplingStructural constraints on coordination can largely be under-
stood as resulting from cross-talk between signals involved
in motor control, specifically as a product of coupling, so that
movements become more similar than intended. Formally,
coupling terms are basic ingredients of dynamic models like
the well-known model of Haken et al. (1985). However, these
models collapse different kinds of coupling that may exist at
different levels of motor control. Such levels can be distin-
guished both in functional and in anatomical terms. Here I
shall focus on functionally defined levels. However, it may
be worth mentioning that in anatomical terms there is some
evidence for different origins of temporal and spatial cou-
pling. For example, split-brain patients give no indication of
a relaxed temporal coupling; if anything, temporal coupling
becomes tighter (Preilowski, 1972; Tuller & Kelso, 1989). In
contrast, spatial coupling seems to be relaxed in split-brain
patients (Franz, Eliassen, Ivry, & Gazzaniga, 1996).
In functional terms, Marteniuk and MacKenzie (1980;
Marteniuk et al., 1984) suggested a distinction between an
execution level and a programming level, with cross-talk ef-
fects originating at both levels. There can be little doubt
about the existence of cross-talk at the execution level. This
kind of cross-talk reveals itself in the form of associated or
mirror movements, that is, involuntary movements that ac-
company voluntary movements of the other hand. These can
be observed in healthy adults (e.g., Durwen & Herzog, 1989,
1992; Todor & Lazarus, 1986), but they tend to be more con-
spicuous under a variety of neurological conditions or in chil-
dren when inhibitory mechanisms, which serve to focus the
basic bilateral innervation unilaterally, are impaired or not
yet fully developed (McDowell & Wolff, 1997; Schott &
Wyke, 1981). Models that rely on cross-talk at the execution
level can account for several observations on, for example,
bimanual circle drawing (Cattaert, Semjen, & Summers,
1999). Nevertheless, there are some results that strongly
favor the notion of coupling during motor programming (or
parametric cross-talk) in addition to cross-talk at the execu-
tion level.
Figure 12.16 shows some data obtained with the timed-
response procedure (Heuer, Spijkers, Kleinsorge, van der
Loo, & Steglich, 1998). The task of the participants was to
produce bimanual reversal movements with short or long am-
plitudes. Movements were to be initiated in synchrony with
the last of four pacing tones, and cues were presented at vari-
able cuing intervals before the last tone. The cues indicated
the amplitudes of the movements, which could be short-
short, long-long, short-long, and long-short. Participants had
been instructed to prepare for movements with intermediate