Chapter 2. Prehension

Chapter 5 - Movement Before Contact 147
Table 5.1 Comparison of percentage of time in deceleration phase
for three task choices. Note that as the precision requirements in
the task increase, the percentage of time spent in the deceleration
phase also increases. Data from Marteniuk et al. (1987).
% Timein
Deceleratio
Pointto ' Grasp
disk
Task
Point
W A
2cm 2ocm
40
4 m
Phase
34% f10.2
34% f12.7
33%* 5.1
40 30% f11.2
2 Widths
2 Amplitudes
Grasp
2
4
u)
40
20
58% f 2.3
50% f 4.6
52%f 1.8
tennis ball
Grasp ' light bulb
0 Grasp
ball
6
40
30
51% f 2.6
48%f 3.8
Width fixed
Amplitude fixed
2 Obi& properties
Grasp 6 30 51% f 4.5
light bulb
throw Graspand 4 30 46% f 4.4
Grasp, then ( fit )
throw
Width fixed
Amplitude fixed
Object properties
fixed
2 Contexts
Graspand 4 30 52% f 5.4
fit
could be a reliable measure for the precision needed in a task.
Less variability between conditions was seen in the early or accel-
eration phase of the movement, and more variability during the decel-
eration phase. Increasing precision requirements of a task may induce
subjects to use more sensory information, particularly in the 'homing
in' part of the task. Marteniuk et al. (1987) argued that the early part
of the movement is more likely to be directly influenced by central
stereotyped movement planning or programming, while the later part
of the movement, during the deceleration phase, is using more senso-
rimotor adjustments for controlling the movement, causing more vari-