Chapter 2. Prehension

230 THE PHASES OF PREHENSION
(see Chapter 8 on Constraints).
6.2.2 Holding and jiggling: discriminating weight
To appreciate and discriminate the weight of an object, some have
held the view that humans depend on centrally generated commands
(efference copy or sense of effort) more than peripheral afferent
information (e.g., Gandevia & McCloskey, 1977a,b,c). Others have
suggested that the sensory afferents from muscles and tendons are
critical (e.g., Roland & Ladegaard-Pedersen, 1977). Victor Raj, Ingty
and Devanandan (1985) examined healthy individuals and leprous
patients with chronic anesthesia of the hands, in their ability to
discriminate weights with passive skin compression, compared to
active flexion about the elbow or metacarpophalangeal joint. They
found that active movements of the digits were required for forces
smaller than 100 g to be appreciated. Although afferent information
was sufficient to discriminate weights greater than 100 g, active
movements of the digits signicantly increased the sensitivity of the
discrimination. Elbow movements were much like cutaneous
stimulation alone. Based on the assessment of leprous hands, and
comparisons with healthy hands, they suggest that in humans the
afferents from the extrinsic hand muscles cannot adequately sense the
smaller forces (less than 100 g), and such force appreciation must be
dependent on the intrinsic hand muscles, and digital joints as well as
cutaneous afferents.
Compared to holding, 'jiggling' an object has been shown to
improve performance on a forced-c hoice weight discrimination task
(Brodie & Ross, 1985). Grasping with pad opposition, they had
subjects lift cylinders to a marked height of 20 cm (described as using
the shoulder as a fulcrum for the lift), then holding or "jiggling", then
lowering the object. Jiggling consisted of repeated vertical movements
for 3 sec. They used cylinders (4.5 x 2.5 cm) weighing 52 to 64 g, in
2 g. intervals. They considered a number of options explaining the
advantage of jiggling over holding, including that jiggling provides
inertial cues to mass beyond the forces required to overcome gravity,
and that receptors may be functioning optimally, through a complex
interaction betwen efferent and afferent signals.
For the complexity of weight appreciation, Marsden, Rothwell,
and Traub (1979) emphasized the need to seek more experimental
evidence in order to evaluate the importance of different sources of
information.