The Use of Sensory Information 333become quite popular. There can be little doubt that it con-
tributes both to temporal judgments (e.g., Schiff & Detwiler,
1979) and precisely timed actions (e.g., Savelsbergh,
Whiting, & Bootsma, 1991). However, it is not the only rele-
vant information; other kinds of information, for example
binocular distance information, are used as well (Bennett, van
der Kamp, Savelsbergh, & Davids, 1999; Heuer, 1993a). In a
recent overview, Tresilian (1999) notes that the relation be-
tween rapid interceptive actions and the kind of information
used is rather flexible and in no way invariant. There is a
degree of task dependence that at present does not allow firm
generalizations about how rapid interceptive actions are
adjusted to their temporal targets.
Feedback Information
Although movements can be performed in the absence of af-
ferent information from the moving limb with an astonishing
degree of accuracy, the use of feedback information is
indicated by the effects of perturbations of feedback on per-
formance. For example, proprioceptive information can be
distorted by way of tendon vibration with a vibrator placed in
the proper position on the skin. The effect is a tonic excitation
of muscle spindles, which under normal conditions corre-
sponds to a longer muscle and correspondingly different joint
angle. If, for example, the biceps tendon is vibrated, the
elbow angle is registered as being too large. When the elbow
angle has to be matched to the elbow angle of the other arm,
the matched angle is too small, corresponding to the dis-
torted proprioceptive feedback on joint angles (Goodwin,
McCloskey, & Matthews, 1972).
Regarding the effects of distorted visual feedback, a par-
ticularly striking example has been reported by Nielsen
(1963). The participant’s task was to move one hand along a
vertical line, but the visible gloved hand was that of the ex-
perimenter and followed a curved path rather than a straight
one. Subjects attempted to correct the error so that they devi-
ated from the target line in the opposite direction. In spite of
the strong discrepancies between intended and felt movement
on the one hand and visual feedback on the other hand it took
several trials before participants came to realize that the
visible gloved hand could not be their own.
In simple movements, feedback information is function-
ally of little importance because autonomous processes of
motor control can operate on the basis of a sufficiently accu-
rate internal model of the motor transformation, so that only
little error remains for closed-loop control to operate on (ex-
cept, of course, when feedback information is distorted).
However, in tasks in which a sufficiently accurate internal
model is not available, the availability of visual feedback
gains critical importance. This is the case when we operate
sufficiently complex machines or tools which effectively add
to the normal motor transformation. Experimentally tracking
tasks are suited to exploring the role of visual feedback
(Poulton, 1957).
For example, when the movement of the hand is propor-
tional to the motion of the cursor on a screen, tracking
performance is rather robust against short periods of elimi-
nated visual feedback. However, with velocity control–with
which the position of the hand is proportional to the veloc-
ity of the cursor on the screen–even short periods of elimi-
nated feedback can bring performance down to an almost
chance level (e.g., Heuer, 1983, p. 54). Thus, visual feed-
back gains in importance the less accurate the internal
model of the transformation by a machine is. Internal mod-
els of sufficiently complex transformations seem not to be
developed, so that practice does not reduce the critical im-
portance of visual feedback (Davidson, Jones, Sirisena, &
Andreae, 2000).
Feedback information does not only serve to guide an on-
going movement; it is also required to learn and to maintain
an internal model of a transformation (cf. Jordan, 1996), pro-
vided it is not too complex. For example, Sangals (1997) had
his subjects practice a nonlinear relation between the ampli-
tude of the movement of a computer mouse and the amplitude
of the cursor movement. When visual feedback during each
movement of a sequence was eliminated and only terminal
feedback at the end of each movement was provided, the re-
lation between (visual) target amplitudes and movement am-
plitudes remained nonlinear. However, when visual feedback
was completely eliminated for a sequence of several move-
ments, the relation between (visual) target amplitudes and
movement amplitudes became linear, which is likely to be a
kind of default relation (cf. Koh & Meyer, 1991).
Feedback information can be processed at various levels
of control; that is, it can be integrated with autonomous con-
trol processes in different ways. Consider the simple task of
sine tracking: Subjects control the motion of a cursor on the
screen, with the target following a sinusoidal time course. In
principle, subjects could function like a position servo, mini-
mizing the deviation between the position of the cursor and
the position of the target. In fact, with a low frequency of tar-
get motion this may actually be the case. However, with
higher frequencies, which approach the range where perfor-
mance breaks down, human subjects produce a sinusoidal
movement and seem to adjust its frequency and phase
(Noble, Fitts, & Warren, 1955). Similar indications for the
processing of parametric feedback rather than positional
feedback have been reported by Pew (1966), but in general
the processing of parametric rather than positional feedback
has received very little attention. In everyday tasks like
driving a car it may be of critical importance; perhaps it is not