Chapter 2. Prehension

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Chapter 5 - Movement Before Contact 181
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4 Peripheral Vision
0 Normal Vision
1.0 2.5 5.5
Dowel size (crn)
Figure 5.26 Maximum aperture as a function of dowel size (1.0,
2.5, and 5.5 cm in diameter) when grasping with pad opposition,
using normal vision or with vision restricted to the peripheral
regions of the retina using specially designed contact lenses. In
all cases, maximum aperture is greater when using peripheral
vision than with normal vision. Note the increased sensitivity of
normal vision to object size, compared to grasping with peripheral
vision only (from Sivak, 1989; reprinted by permission from
Sivak & MacKenzie, 1992).
ponent as part of this control strategy, that is, there is a communication
link between the visuomotor channels controlling grip formation and
transport of the hand (Sivak & MacKenzie, 1990).
Sivak also compared central vision to normal vision (Sivak, 1989;
Sivak & MacKenzie, 1992). Subjects wore goggles to allow vision
in only the central lo" of the visual field. Using central vision alone,
the parameters of the transport component were organized to suggest a
nearer target than in reality. Subjects spent a greater proportion of
time after peak deceleration with only central vision (52%), compared
to normal vision (39%). Two distinct phases can be seen in the accel-
eration profile in Figure 5.27, suggesting subjects planned to stop the
movement earlier than they did. When they did not contact the target,
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