The Use of Sensory Information 327such trials from the direction of the target (90°) to the direc-
tion of the movement (180°). The population vector is com-
puted from the activity of directionally tuned neurons of the
motor cortex and generally points in the direction of move-
ment. Basically it is a weighted mean of the preferred direc-
tions of a sufficiently large sample of cortical units, with the
weights being derived from the spike frequencies. The rota-
tion of the population vector starts with a certain delay and
proceeds with an almost constant slope until the target direc-
tion is reached. In human subjects this kind of rotation pre-
sumably gives rise to a systematic increase of reaction time
when the angle between target and required direction of
movement is increased (Georgopoulos & Massey, 1987).
The timed-response procedure allows one to trace the
gradual specification of movement parameters from behav-
ioral data. The method has been introduced for the study of
the speed-accuracy trade-off in choice reaction time experi-
ments (Schouten & Becker, 1967), and it has been adapted to
the study of the advance specification of characteristics
of isometric contractions and movements by Ghez and
coworkers (Ghez et al., 1997; Hening, Favilla, & Ghez,
1988). Basically the method specifies a moment for the start
of the movement; typically the movement has to be initiated
in synchrony with the last of four tones which are presented
in regular intervals. At a variable time before the last tone the
target is presented, so the time available for motor specifica-
tions can be varied. The method is only suited for rapid
movements or isometric contractions with short durations, so
that the movement characteristics are largely determined in
advance and little changed during execution.
Ghez and coworkers demonstrated the gradual specifica-
tion of peak forces of isometric contractions as well as
amplitudes and directions of movements with a time course
similar to that of the neuronal population vector (cf. Figure
12.8). In addition, they showed that the gradual specifications
break down when the differences between the alternative tar-
gets become too large (Ghez et al., 1997). When the difference
between target directions is about 90° or larger, or the ratio of
target amplitudes is about 12:1 or larger, the intermediate values
between the two targets are no longer observed, and the choice
between movement parameters becomes discrete. Thus, there
seem to be two qualitatively different modes of parameter spec-
ification, namely gradual adjustments and discrete choices.
While the timed-response procedure provides a window
into the gradual or discrete specification of movement char-
acteristics, it has not been used as extensively as chronomet-
ric procedures. The latter type of studies is largely based
on the movement precuing rationale of Rosenbaum (1980,
1983). Consider a set of four responses that differ on two di-
mensions like direction and amplitude. In a reaction time
task, before presentation of the response signal, there is thus
uncertainty with respect to both direction and amplitude, and
after presentation of the response signal–during the reaction-
time interval–both response characteristics have to be speci-
fied. When one of the dimensions is precued, it can be
specified in advance of the response signal, and only one
dimension remains to be specified after its presentation. Re-
action time should be reduced by the time it takes to specify
the precued dimension. When both dimensions are precued,
both can be specified in advance, and reaction time should be
reduced even more. In principle, if the rationale were fully
valid, the times needed to specify various movement charac-
teristics or combinations thereof could be estimated.
There are some broad conclusions that can be drawn from
the results obtained, but there are also a number of problems
that sometimes cast doubt on the general validity of the ratio-
nale (cf. Goodman & Kelso, 1980; Zelaznik, Shapiro, &
Carter, 1982). Among the broad conclusions were that move-
ment features are specified sequentially and in variable rather
than fixed order (Rosenbaum, 1983). The first of these two
broad conclusions can be doubted because the time needed
to specify two dimensions can be smaller than the sum of
the times needed to specify each of these dimensions (e.g.,
Lépine, Glencross, & Requin, 1989). In addition, timed-
response studies show essentially parallel specifications of
amplitude and direction, perhaps accompanied by some slow-
ing when two response characteristics are specified in parallel
(Favilla & De Cecco, 1996; Favilla, Hening, & Ghez, 1989).
Exceptions to the second broad conclusion seem to be rare.
Fixed order of specifications is indicated by a shortening of re-
action time when a movement dimension A is precued, which
can be observed only when movement dimension B is precued
as well, but not otherwise. This implies that the specification
of dimension B is a prerequisite for specifying A. Such a result
would be expected when dimension B embraces qualitatively
different movements, related to different motor-control struc-
tures rather than to different parameters of a single control
structure. Qualitative variations of movement characteristics,
however, have rarely been studied, but some results of Roth
(1988) indeed suggest that precuing the direction and the
force for throwing a ball does not result in systematic reaction
time benefits as long as the type of throw is not known.THE USE OF SENSORY INFORMATIONThe use of sensory information for the control of voluntary
movement was among the historically early questions
addressed by experimental psychology. Woodworth (1899)
asked his subjects to produce reciprocal movements between