332 Motor Control
Figure 12.12 (a) Spatial layout of a simple interceptive task. (b) Variables
in the analysis of the task; time zero is defined by the target’s reaching the
intersection.
as the delay becomes longer. With impaired tactile feedback,
sensory consequences should also be delayed centrally, and
negative asynchrony is increased (Aschersleben, Gehrke, &
Prinz, 2001).
Synchronization of movements with discrete tones is
necessarily anticipatory, provided that the interval between
successive tones is sufficiently short (Engström, Kelso, &
Holroyd, 1996). This is different in interceptive tasks. For ex-
ample, when an object is approaching and one has to perform
a frontoparallel movement that reaches the intersection of the
object path and the movement path at the same time as the
object does (cf. Figure 12.12a), it is possible in principle to
continuously adjust the distance of the hand from the inter-
section to the distance of the object. In fact, this may actually
happen if both the target object and the hand move slowly. At
least, it is true that slower movements are adjusted more ex-
tensively to the approaching target after their start than rapid
movements.
Let the start time be the time interval between the start of
the interceptive movement and the time the target object
reaches the intersection, and the temporal error be the time
between the hand’s and the target object’s reaching the inter-
section (Figure 12.12b). Then, when the movement is started
and runs off without further adjustments of its timing, the
start time should be highly correlated with the temporal error.
This strategy, in which the start time is selected according to
the expected duration of a pre-selected movement pattern, is
sometimes called operational timing(Tyldesley & Whiting,
1975). However, with temporal adjustments the correlation
between start time and error should be reduced (Schmidt,
1972). This happens when the instructed movement duration
is increased (Schmidt & Russell, 1972). Thus it seems that on
the one hand the interceptive movement can be triggered by a
particular state of the approaching object and then run off
without further adjustments, and that on the other hand the
time course of the interceptive movement can be guided by
the approaching object, with mixtures of these two modes
being possible.
In the simple task considered thus far the position of the
intersection of object path and hand path is given. This is dif-
ferent for more natural tasks. Consider hitting a target that
moves on a straight path in a frontoparallel plane like a spider
on the wall. In principle, spiders can be hit in arbitrary places,
but nevertheless the direction of the hitting movement has to
be adjusted to an anticipated position of the moving target. A
robust strategy is to adjust the lateral position of the hand to
continuously updated estimates of the target position at the
time the hand will reach the target plane; this requires an esti-
mate of the time that remains until the hand reaches the plane
and an estimate of the target’s velocity, which, however, need
not really be correct (Smeets & Brenner, 1995).
The situation is somewhat different when balls have to be
intercepted in a lateral position, either for catching them or
for hitting them. According to Peper, Bootsma, Mestre, and
Bakker (1994), the hand will be in the correct position in the
plane of interception at the right time when its lateral veloc-
ity is continuously adjusted to the current difference between
the lateral position of the hand and the approaching target, di-
vided by the time that remains until the target reaches the
plane of intersection. Proper lateral adjustments, which imply
temporal adjustments as well, are evident even in high-speed
skills like table tennis, although the relevant information is
less clear (Bootsma & van Wieringen, 1990).
What is the basis for anticipations of temporal targets? For
example, when we view an approaching ball, what allows us
to predict when it will be in some position where we can
intercept it (cf. the chapter by Proffitt & Caudek in this
volume)? The time it takes until a moving object reaches a
certain position is given by the distance of the object divided
by its velocity. This ratio has time as unit, and it specifies time
to contact with the position, provided the object moves on a
straight path with constant velocity. As noted by Lee (1976),
the information required to determine time to contact with an
approaching object, or with an object the observer is ap-
proaching, is available even without determining distance and
velocity, namely by the ratio of the size of the retinal image of
the object and its rate of change. This variable, called , has