Chapter 2. Prehension

Chapter 4 - Planning of Prehension 77
measured while the posture was maintained for a few seconds. In this
pad opposition task, mean grip size correlated positively with object
size. Error in matching was computed for each subject as the
difference between the grip size used during normal grasping and the
person’s measured grip size. While object size estimation was shown
to be linearly related to the actual object size, errors ranged between
f1.5 cm with very little consistency (of the six subjects, two subjects
always underestimated while the rest tended to overestimated). This
experiment demonstrates the ability of the CNS to plan an opposition
space, but with qualifications. The visual perception of object
properties is an estimation (visual scaling) which is shown to be
highly accurate. Transforming this visual information into motor
commands (visuomotor scaling) causes errors, which Jeannerod and
Decety argue could stem from the change from one coordinate system
to another. Visual objects are encoded in retinal coordinates, whereas
hand configurations are encoded in some other frame. Retinal cues
related to object size are not sufficient for determining grip size,
particularly in a precision task using pad opposition. They suggest
that for accuracy, visual feedback is needed to improve the planned
hand posture for correcting visuomotor biases and reducing motor
variability.
In a similar experiment, Chan, Carello, and Turvey (1990) asked
subjects to use their other hand to evaluate the size of the object. An
object was placed in front of the subject, who then used the thumb and
index finger of the left hand to indicate the size of the object. Subjects
were able to do this accurately in a linear fashion for objects below a
10 cm width. Above that, there were non-linearities due to a ceiling
effect; that is, there were biomechanical constraints on the hand.
Using the thumb and middle finger removed these non-linearities.
The speed with which object properties are perceived is important.
Klatzky et al. (1987, 1990) analyzed the two-dimensional perception
of objects, arguing that the projection of a three dimensional object on
the retina can be used for making decisions about interacting with the
object. By varying the area and depth of the two-dimensional
projection of an object, they showed an effect on the chosen grasp.
As seen in Figure 4.6, subjects pinched small things and clenched
larger things. Since object size is a critical planning parameter, it is
noteworthy that a simple two-dimensional representation of the object
can be used for choosing a grasp posture. This representation is
available at the level of the retina, thus quickly accessible to the CNS.
Intrinsic object properties can be perturbed at movement onset in
order to observe how long it takes to perceive object properties, or at