Chapter 2. Prehension

Chapter 5 - Movement Before Contact 179
grasp and lift a vertically standing dowel (2.5 cm diameter, 11 cm
high) placed on a table 30 cm in front of the start position of the hand,
in the midline sagittal plane. In some of the trials, special contact
lenses were worn that occluded the central lo" of visual field, con-
firmed with static perimetry (see Sivak, Sivak & MacKenzie, 1985,
Sivak & MacKenzie, 1990). In these periuheral vision trials, the
maximum grip aperture between the index finger and thumb was larger
(116 mm) than with normal vision (86 mm), and occurred much ear-
lier (see Figure 5.25). Individuals moved slower with peripheral vi-
sion only than normal vision. With normalization in time, the velocity
profile of the wrist scaled similarly for both vision conditions, i.e., the
same proportion of time was spent prior to and after peak velocity.
However, a marked difference in the curves could be observed; the ac-
celeration profiles showed that with peripheral vision, subjects decel-
erated, then appeared to maintain a near zero acceleration. Subsequent
analyses revealed that, with peripheral vision, the forward movement
of the arm toward the dowel stopped when approximately 72% of the
movement duration was completed, in contrast to 91% for normal vi-
sion. When the wrist stopped moving forward, the aperture was
larger for peripheral than normal vision. In effect, with only periph-
eral vision, the transport component seemed to have completed before
the grasp component. Other kinematic analyses showed that with pe-
ripheral vision, subjects adopted a tactile control strategy whereby ei-
ther the thumb or index finger made initial contact over 80 ms before
lifting the dowel, perhaps to provide information and trigger the con-
tinued closing of the aperture.
To compare the sensitivity of central and peripheral vision for cali-
brating grasp aperture to object size, Sivak & MacKenzie (1992) had
subjects perform the same grasp and lift movement but with dowels of
1.0,2.5 or 5.5 cm in diameter. Seen in Figure 5.26, peripheral vision
shows less sensitivity than normal vision in providing size information
for calibrating the grip to object size. The obvious difference between
the peripheral and normal visual conditions is the lack of visual acuity
or high resolution vision with peripheral vision. Without high resolu-
tion information about intrinsic object characteristics (normally pro-
vided by central regions of the retina under natural viewing condi-
tions), the aperture is much larger, and subjects adopt a tactile control
strategy to acquire intrinsic object information, complete the aperture
closure and actually grasp the object. Thus central visual information
about object size obtained before and during the movement is critical
for the planning and control of the grasp component. We suggested
that the transport component was modified along with the grasp com-