Chapter 2. Prehension

Chapter 5 - Movement Before Contact 129
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Figure 5.6 Vector Integration to Endpoint model for generating
arm trajectories. The difference V between the desired muscle
length, T, and a model of the present muscle length, P, is
computed. On receiving the go signal, G, V is gated to the
internal model, integrating V through time. The arm follows the
evolving state of P (from Bullock & Grossberg, 1989; reprinted by
permission).
pointing movements, there is little variability within and between sub-
jects. There is little variability in the spatial path taken by the wrist.
The path is independent of speed, and inertial loads. Velocity profiles
tend to be bell-shaped, and superimposable when scaled in time or
amplitude, indicating perhaps a ‘ballistic’ controller driving the arm
without regard to sensory feedback. In general, goal-directed move-
ments have one accelerating phase and one decelerating phase if their
trajectory is approximately along a straight line. Asymmetries in tan-
gential velocity profiles may reflect the ‘precision demands’ of the
task, that is, the time spent in the deceleration phase of the movement.
For pointing tasks requiring more accuracy, movement time increases
and the velocity profile becomes more asymmetrical, with a longer de-
celeration tail. As well, longer deceleration phases are associated with
self-generated decelerations, when individuals are unable to use decel-
erative impact forces. Speed and accuracy are defined in a relationship
knows as Fitts’ Law which states that movement time is directly pro-