Chapter 2. Prehension

Chapter 4 - Planning of Prehension 103
The results of Rosenbaum et al. can be viewed in terms of
opposition space. In the third experiment, the experimenter’s placed a
restriction on the subjects, urging them to ‘grab the bar in the middle,
holding it like a tennis racket.’ In not specifying the orientation of the
wrist, subjects chose either a palm opposition or a pad opposition.
Using the latter, they were able to ‘twirl’ the bar into the required
orientation for the task. With the imposed restriction in the third
experiment, subjects used palm opposition. The direction of palm had
to be within the constraints of the task; i.e., it had to be rotatable
within the confines of the wrist’s range of mobility. In Figure 4.16,
the z-axis along which palm opposition occurs is shown, extending
from the palm.
As an aside, it is easy to observe ‘twirling’, i.e., reorienting objects
for a given task using degrees of freedom at the wrist or at the fingers.
It happens when people pick up pens and pencils to write. When the
point is directed away from a right-handed person, that person either
picks it up with the thumb on the left of the pencil’s body and twirls it
into position between the fingers, or else picks it up with the thumb on
the right of the pencil’s body and reorients the pencil with adduction of
the shoulder and supination of the forearm. We discuss twirling and
other object manipulations in Chapter 6.
Rosenbaum and colleagues termed the finding of initially awkward
postures for the sake of finally comfortable end postures, the end-state
comfort effect. In addition, they noted a thumb-toward bias effect, in
which the base of the thumb was oriented to the end of the bar to be
placed on the target. Reaction times to initiate reaches were longer
when the thumb was away from the pointer than when the thumb was
oriented towards the pointer; the thumb-towards bias was suggested
not to reflect visibility or perceptual factors, but attentional ones
(Rosenbaum, Vaughan, Barnes & Jorgensen, 1992). Further,
Rosenbaum and Jorgensen (1992) demonstrated a sequential or hys-
teresis effect, in that the probability of using an overhand grip was de-
pendent on previous grips used in a structured sequence, i.e., subjects
persisted in using a previous grip. They suggested that there is a com-
putational cost in selecting a new grasp.
The orientation of opposition space was addressed directly by
Stelmach, Castiello and Jeannerod (1993). They had subjects grasp a
prism 6 cm long, triangular in cross-section, by the ends, using pad
opposition between the thumb and index finger. When the object was
placed parallel to the subject’s midline, all subjects grasped with the
forearm in a semipronated position. There were six orientations of the
object, as shown in Figure 4.17. As the object was rotated clockwise