Handbook of Psychology, Volume 4: Experimental Psychology

326 Motor Control

Figure 12.8 Change of the direction of the population vector as a function

of the time since presentation of an imperative stimulus (after Georgopoulos

et al., 1989).

is different with the VITE model of Bullock and Grossberg
(1988). (VITE stands for vector-integration-to-endpoint.)
The formal structure of an element of the model is illustrated
in Figure 12.7. The variable Pis an internal representation of
the position of an effector, and Trepresents a target position.
The variable Vrepresents the (delayed) difference, and Gthe
Go signal. In principle, the structure of Figure 12.7 is thought
to be multiplied for different muscles that are involved in a
voluntary movement, with V≥0 for each particular muscle.
Without going into mathematical details, it is worth noting
that the difference Vin the case of aimed movements is again
governed by a second-order differential equation (provided
thatGis a constant). In spite of this similarity, there are sev-
eral basic differences from the model of Saltzman and Kelso
(1987), in addition to the differences with respect to the role
of physiological and psychological considerations in justify-
ing the mathematics. The structure of Figure 12.7 is a kind of
central closed-loop system. This system, however, is inoper-
ative as long as the Go signal is zero; it is energized by the Go
signal, which in addition can change across time so that the
system is no longer linear. Bullock and Grossberg (1988)
refer to a “factorization of pattern and energy.” Basically, the
Go signal allows a separation of movement planning
from movement initiation (cf. Gielen, van den Heuvel, &
van Gisbergen, 1984), which implies that processes of motor
preparation can be temporally separated from execution of
the movement, but also that movements can be initiated
before advance specification is finished.
Generative structures are not restricted to aimed move-
ments. In fact, models of generative structures for periodic
movements as they occur in locomotion are historically older.
Network models of central pattern generators had already
been proposed early in the twentieth century (Brown, 1911),
and more elaborate versions continue to be developed (e.g.,
Grossberg, Pribe, & Cohen, 1997). In more abstract models,

of course, point attractors can be replaced by limit-cycle at-

tractors which produce stable oscillations (e.g., Kay, Kelso,

Saltzman, & Schöner, 1987).

The Advance Specification of Movement Characteristics

During motor preparation an anticipatory representation of

the forthcoming movement is constructed. This representa-

tion can be described as a motor-control structure, which

allows (relatively) autonomous control of the movement in-

dependent of sensory feedback. In addition to being set up, the

structure must be specified, with the appropriate parameters.

This is a time-consuming process. Thus, variations in neces-

sary preparatory activities are reflected in reaction times. In

addition, when the available time is varied, it is possible to

trace the time course of the specification of movement char-

acteristics. Thus far, almost all studies on the advance speci-

fication of movement characteristics have employed aimed

movements or isometric contractions with different quantita-

tive characteristics, yet qualitatively different movements

have hardly been used. Therefore, little can be said about set-

ting up different motor-control structures, but more can be

said about the advance specification of parameters.

Figure 12.8 gives an example for the gradual specification

of movement direction, adapted from Georgopoulos, Lurito,

Petrides, Schwartz, and Massey (1989). These data are from

a monkey who had been trained to perform a movement to

one of eight potential targets arranged on a circle. When the

target was dimly illuminated, the monkey had to reach for it

directly, but when the luminance of the target was high, the

monkey had to perform a movement that was rotated by 90°

counterclockwise relative to the target. What is shown in

Figure 12.8 is the gradual rotation of the population vector in

T

V

P











 G

Figure 12.7 Variables of the VITE model of Bullock and Grossberg (1988)
and their interrelations.