Chapter 2. Prehension

Chapter 4 - Planning of Prehension 79
In summary, object size is veridically assessed and finely
calibrated by the CNS for prehension. Some object properties are
accessible more through vision than haptics. Due to the covarying
nature of object properties and our previous experience, we make
assumptions about object properties like weight, given observations
about size. With visual perturbations, replanning for intrinsic object
properties in prehension takes at least 300 ms.
4.3.2 Perceiving extrinsic object properties
Studying the nature of extrinsic object property perception can
also be done using perturbation studies. Paulignan, MacKenzie,
Marteniuk & Jeannerod (1991) performed an experiment where the
object’s location was perturbed. Subjects were asked to grasp a
vertically standing dowel between their thumb and index finger pad.
Made of translucent material, the dowel was made visible by means of
illuminating a light-emitting diode (LED) placed under the dowel. In
this way, Paulignan and colleagues could change the illuminated light,
making it appear as if the dowel moved from one location to another.
In comparing perturbed trials to control trials, they found that wrist
trajectories were modified about 250-290 ms after the perturbation,
and the first acceleration and velocity peaks occurred earlier in the
perturbed trials. The first detectable difference in wrist trajectories
was seen at 100 ms, because the time to the first peak acceleration for
control trials was about 130 ms, but for the perturbed ones it was
closer to 100 ms. This seems to correspond to the minimum delay
needed for sensory reafferents to affect ongoing movement4.
Corrections might occur by directly comparing target position and limb
position signals. Finally, they showed that perturbing the object loca-
tion influenced the grasping component as well as the transport com-
ponent. In showing the dissociation of transport and grasping com-
ponents, because the transport component was completed and the grip
size was readjusted, there may be different time constants for the two
components. These results are discussed further in Chapter 5.
The orientation of the object is also an extrinsic object property.
Jeannerod and Decety (1990) studied the accuracy of matching hand
orientations to visually presented object orientations. Subjects held a
plate between their thumb and palmar finger surfaces and were asked
to rotate the hand to match the seen orientation of the target bar.
4F0r proprioceptive processing, minimum delay is estimated at 70 to 100 msec
(Johansson & Westling, 1987).