Chapter 2. Prehension

178 THE PHASES OF PREHENSION
books and papers: Goodwin and Darian-Smith (1985); Humphrey
(1979); Hyvarinen (1982); and Phillips (1986).
5.4.5 Restricted sensory conditions
One way to gather information to answer the coupling question is
to vary the available amount of sensory information, and see what the
effects are on the various movement components. To recall, Wing et
al. (1986) had subjects reach to grasp a vertically standing dowel un-
der conditions where visual feedback and speed of movement varied.
Focussing on the effects of the sensory manipulations, the peak grip
aperture was larger in movements faster than normal and in move-
ments where subjects had their eyes closed. There was more variabil-
ity at peak aperture than at contact. At the frame just before contact
(pre-contact), they noted more variability in the blind and fast condi-
tions than in control conditions. This suggests the possibility of a
very conservative Reshape controller, opening the hand wider when
reliable visual information is not available. Recalling Jeannerod' s ex-
periments (1981, 1984), he noted that transport movements fell short
of the target under conditions in which subjects saw only the objects,
not the hand; in contrast, movements with vision had a longer MT and
low-velocity phase. However, he did not observe a difference in
maximum grip aperture and precontact grip aperture for the various vi-
sual manipulations.
While the importance of visual information has been demonstrated,
the type and resolution of visual information can also have an effect.
Normal vision is a combination of central vision (the central lo" of the
visual field) and peripheral vision. Research has shown that central
vision is functionally specialized for responding to spatial patterns,
while peripheral vision responds to movement stimuli. In the Paillard
(1982b) model seen in Chapter 3, it was suggested that movement and
location cues from peripheral vision were used for transporting the
arm, and size and shape cues from central vision were used for form-
ing the hand. Interestingly, if the eyes are focused on the object, then
at the end of the reach, both the object and the hand are in central vi-
sion.
Sivak (1989; Sivak 8z MacKenzie, 1990, 1992) performed ma-
nipulations of visual information in grasping tasks, using pad opposi-
tion. Infrared markers were placed on the wrist, index finger, and
thumb in order to study the kinematics of the grasping and transport
components using a WATSMART system. In the first experiment, a
subject sat with the head stationary in a chin rest and was instructed to