Chapter 2. Prehension

192 THE PHASES OF PREHENSION
shape, size, texture, etc20.
Another way to distinguish a preshaping phase from an enclosing
phase is to look at what occurs at the muscle level. Smith et al. (1983)
used electromyographic (EMG) data, to study muscle activation in
primate pinching movements between thumb and index finger in side
opposition21. As seen in Figure 5.31, the extensor digitorum
communis (EDC) muscle activates at around 300 ms before the onset
of force, extending the fingers as the hand opens prior to grasp. The
abductor pollicis longus (ABPL) acts next, at -220 ms, extending the
thumb, while the lumbricals (LUMB) at -200 ms aid in extending the
fingers. The enclosing phase begins with the thumb muscles, the
opponens at -100 ms (OPP), then the flexor pollicis brevis at -90 ms
(FPB) and abductor pollicis brevis at -40 ms (ABPB) to flex the
thumb, and adductor pollicis at -30 ms (ADP ). The extensor pollicis
longus at -20 ms (EPL ) extends the thumb toward the radial side of
the hand for the side opposition. The fingers are adjusted by action of
the first dorsal interosseus, which also acts on the thumb, at -20 ms
(INTD 1). The flexor digitorum profundus activates at 0 ms (FDP) to
apply the actual force. The onset of contraction of both adductor
pollicis and flexor pollis brevis are time locked to the onset of force,
suggesting that these two muscles function as prime movers in this
isometric precision grip. The extrinsic (FDP, FDS, FPB, FPL) and
intrinsic flexors (INTD 1, LUMB, ABPB) are activated at force onset,
where FDS is the flexor digitorum superficialis and FPL is the flexor
pollicis longus.
In order to grasp an object, it is necessary to set up the relevant
force-producing muscles for the anticipated interaction. Smith et al.
(1983, p. 376) argued that the early onset of the EDC may function to
20Supporting data has been found in the hand representation of somatosensory
cortex. Iwamura and Tanaka (1978) found neurons responding in Area 2 to
particular object features, such as cylinders or flat objects. Hyvarinen and Poranen
(1978) found other neurons in the same area responding to direction of movement
across the palm. Other neural evidence for the difference can be Seen in the ablation
of area 5 causing tactile avoidance, and ablation of SMA causing tactile following
(the release of instinctive grasp reflex). Perhaps SMA inhibits the enclosing until
the preshape has been formed.
2lWhile the muscles of other primates and their action are slightly different from
ours, the reader is encouraged to see Appendix A for a list of muscles.