324 Motor Control
the subjects had not only to grasp the bar, but also to place it
in one of eight target positions with the pointer toward the
LED that signaled the target position in each trial; thus, there
were differences in final orientation. For targets 6–8 and 1–2,
holding the bar in the final orientation with the thumb toward
the pointer is more or less comfortable, but for targets 3–5,
holding the bar with the thumb away from the pointer is more
comfortable. In Figure 12.6b the relative frequency of grasp-
ing the bar in its initial position with the thumb toward the
pointer is shown. These data reveal not only an effect of the
initial orientation of the bar, but also a clear effect of the final
orientation. Thus, the effect of end-state comfort can be evi-
denced at the very start of the action, and it clearly indicates
that motor preparation embraces anticipation.
The anticipatory nature of motor preparation implies that
there is some kind of representation of the forthcoming
movement before it begins. The existence of such a represen-
tation also implies that open-loop processes of motor control
are of a particular nature in that they are predictive. In fact,
the answer to the question of what goes on during motor
preparation in functional terms may be largely that this kind
of internal representation of the forthcoming movement is set
up, which then allows for a more or less autonomous control.
Motor-Control Structures
There are different ways to conceptualize autonomous
processes of motor control. In psychology it had been
common to designate the anticipatory representation of a
forthcoming movement as a motor program(and the process
of setting it up as programming). However, this term has be-
come associated with a particular conceptualization. There-
fore, as a broader and more neutral term, Cruse et al. (1990)
have suggested motor-control structures. There seem to be
basically two different ways of modeling them, either in
terms of prototypical functions or in terms of generative
structures (Heuer, 1991).
Prototypical Functions
Movements vary qualitatively as well as quantitatively. One
of the attempts to capture this basic observation is the notion
of a generalized motor program, most explicitly introduced
by Schmidt (1975). A generalized motor program is thought
to control a set of movements that have certain characteristics
in common. The specifics of each particular movement are
thought to be determined by the program’s parameters. Thus,
for a certain type of movement there should be invariant char-
acteristics, which represent the signature of the program, and
variable characteristics, which reflect the variable settings of
its parameters. Of course, such a concept requires that the
invariant characteristics of movements of a certain type be
identified.
The theoretical problem of identifying invariant character-
istics met with observations of an invariance of relative tim-
ing in different motor skills (see Gentner, 1987, for a review),
which led Schmidt (1980, 1985) to propose that the relative
timing is an invariant feature of movements that are con-
trolled by a single generalized motor program. In addition,
relative force was hypothesized to be a second invariant char-
acteristic. With these assumptions, a generalized motor pro-
gram can be described by way of a prototypical force-time
function( ), which can be scaled in time by a rate parame-
ter and in amplitude by a force parameter.
The notion of a prototypical force-time function, which
can be scaled in time as well as in amplitude, is reminiscent
of the way we use coordinate systems to represent force-time
curves. Thus one might suspect that the concept is related
more to how we plot force as a function of time than to how
the brain controls movement. Nevertheless, the notion is
not biologically implausible. One can think of a spatially
organized representation that is read at a certain rate and
thus transformed into a temporally organized movement
(cf. Lashley, 1951). The speed of reading would correspond
to the rate parameter. Similarly, as the read signal is chan-
neled to the muscles, it could be amplified to variable degrees
(cf. von Holst, 1939). Thus, in principle, the notion of proto-
typical functions implies a certain degree of independence of
temporal control and force control.
The most detailed application of prototypical force-time
functions has been in models of the speed-accuracy trade-
off in rapid aimed movements. These so-called impulse-
variability models account for the trade-off in terms of
noise in the motor system (Meyer, Smith, & Wright, 1982;
Schmidt, Sherwood, Zelaznik, & Leikind, 1985; Schmidt
et al., 1979). However, it is not really necessary that proto-
typical curves specify forces; instead, they can also be
thought of as specifying kinematic characteristics (e.g.,
Heuer, Schmidt, & Ghodsian, 1995; Kalveram, 1991). In
fact, formal models of the autonomous processes of motor
control are generally somewhat diverse or even indetermi-
nate with respect to their output variables.
The motor transformation involves a number of different
variables, and in principle any of these can be taken as output
variable for models of motor-control structures. Ultimately,
of course, muscles must be activated. In fact, the concept of a
motor program has often been associated with a prestructured
sequence of muscle commands (Keele, 1968). At the other
extreme, motor-control structures can be modeled with the
trajectory of the end-effector as the output. In the first case,