Chapter 2. Prehension

162 THE PHASES OF PREHENSION
position. On 20% perturbed trials, the nearest sphere was f it illu-
minated, then at movement onset one of the more distant spheres be-
came illuminated. They found two clear submovements on perturbed
trials, with the second movement aiming to the more distant target oc-
curring at about 370 ms. Following Paulignan, MacKenzie,
Marteniuk, and Jeannerod (1990, 1991), evidence for the first change
in the transport kinematics was that peak acceleration occurred earlier
in the perturbed conditions (about 150 ms) compared to the control
conditions (186 ms). The values of peak acceleration did not differ
between the two perturbed conditions, thus they concluded that the
first movement was interrupted by the visual perturbation. The
beginning of the second grip occurred significantly later for trials per-
turbed to 40 cm than for trials perturbed to 27.5 cm. Like Paulignan et
al., the manipulation of an extrinsic object property (in this case dis-
tance) affected both transport and grasping components. They ac-
knowledge that the two targets presented in succession may require
two separate motor plans, hence two submovements and two grip
apertures. They note the importance of their finding that finger closure
time remained invariant over distance and experimental perturbation
conditions.
Perturbing obiect orientation and position, S telmach, Castiello,
and Jeannerod (1993) found that if the perturbation required addition
of a pronation component to orient the hand, there was a
corresponding increase in movement time, and the time devoted to
deceleration of the hand was lengthened. These findings were
interpreted as indicating the necessity for a kinematic rearrangement
with the addition of a pronation component due to perturbation; in
contrast, the pronation component can emerge as part of the motor
plan in natural, unperturbed prehension.
Introducing the visual perturbation paradigm to study intrinsic
properties in motor interactions with objects, Jeannerod (198 1,
Experiment 2) used an ellipsoid object (axes: 7 cm and 4 cm) placed
above a mirror. To perturb obiect shape, rotation with a small motor
made the object appear spherical (4 cm). Each trial started with pre-
sentation of a sphere. At the onset of movement, on perturbed trials,
with rotation of the object, the mirrored reflection of the sphere ap-
peared to expand suddenly to an elliptical object. For perturbed and
unperturbed trials, the physical object of the corresponding target
shape was placed at the expected location on the table, so that the
shape of the object actually grasped always corresponded to the virtual
image present in the mirror at the same time. Jeannerod reported that
the shape and shape change of the target object had no effect on the