Motor Preparation 323Figure 12.6 (a) Response panel used by Rosenbaum et al. (1992). The bar
with pointer had to be grasped and to be placed on one of the eight targets
with the pointer toward the LED (black dots). (b) Relative frequency of
grasping the bar with the thumb toward the pointer as a function of the
final orientation (targets 1–8), shown separately for four different initial
orientations.The Anticipatory Nature of Motor Preparation
The psychophysiological data indicate the existence of motor
preparation, but they are more or less silent to the question of
what goes on in functional terms. What they tell, of course, is
that at least to some degree preparatory processes are specific
for the forthcoming movement in that the data reflect some of
its characteristics, like the hand used. Näätänen and Merisalo
(1977) suggested that the essence of motor preparation is that
everything is done in advance of the overt response that can
be done, which amounts to activating the response up to a
level close to the motor action limit. This characterization of
motor preparation may be appropriate for simple movements
like keypresses, but it falls short of capturing essential char-
acteristics of motor preparation preceding a more complex
movement.
Preparatory activities in general can anticipate the future
to varying degrees. For example, in preparing for a vacation,
one might book a hotel in advance for only the first night, or
one might book hotels in different places for several nights to
come. Activating a response close to the action limit means
preparing only for movement initiation (like booking a hotel
for the first night). However, motor preparation is also con-
cerned with the future of the response (like booking hotels for
several nights to come). There are at least three kinds of evi-
dence for this.
The first kind of evidence is from reaction-time experi-
ments. When the task is to perform a sequence of simple
movements, simple reaction time increases with the length of
the sequence. The seminal study was by Henry and Rogers
(1960), who found increasing reaction times for (a) lifting a
finger from a key, (b) lifting the finger from the key and
grasping a tennis ball at a certain distance, and (c) lifting the
finger from the key, touching the ball with the back of the
hand, pressing another key, and hitting a second tennis ball.
More systematic explorations of the sequence-length ef-
fect have been reported by Sternberg and coworkers (see
Monsell, 1986, for an overview). With more homogeneous
elements like keypresses, letter names, or words with a cer-
tain number of syllables, reaction time increases linearly with
the number of sequence elements. At some length of about
6–12 elements, the increase of reaction time levels off, earlier
for longer elements (like trisyllabic words) and later for
shorter elements (like monosyllabic words).
The second kind of evidence is from studies of anticipatory
postural adjustments (see Massion, 1992, for review). When a
forthcoming movement threatens balance, the voluntary ac-
tion is preceded by the appropriate postural adjustments. For
example, Cordo and Nashner (1982) observed EMG activity
of postural muscles in the leg of their standing subjects which
preceded by about 40 ms the activity of arm muscles involved
in the task of pulling a hand-held lever in response to an audi-
tory signal. In a control condition with a passive support, the
preparatory postural activity was absent, and arm-muscle ac-
tivity had a shorter latency. Thus, anticipatory postural adjust-
ments are not only specific with respect to the forthcoming
voluntary movement (e.g., Zattara & Bouisset, 1986), but also
with respect to context characteristics.
The third kind of evidence, finally, shows that earlier parts
of a motor pattern are adapted to later parts. Evidence for
this can be found in many skills (cf. Rosenbaum & Krist,
1996), but I shall focus here on a particularly basic kind of
observation, the effect of end-state comfort (Rosenbaum &
Jorgensen, 1992; Rosenbaum et al., 1992). Figure 12.6a illus-
trates the task of Rosenbaum et al. (1992, Exp. 1). The stand-
ing subject had to grasp a bar with a pointer, which had
different initial orientations, the pointer pointing upward,
downward, to the left or to the right. With the pointer upward
or to the left, it is quite comfortable to grasp the bar with the
thumb toward the pointer, but with the other two initial ori-
entations this is less comfortable. Under speed instructions