Chapter 2. Prehension

Chapter 5 - Movement Before Contact 175
and held after contact is examined in Chapter 6.
In premotor cortex, Area 6, a specific population of neurons (in
subarea F5) showed discharge patterns specific to the shaping of the
hand by monkeys in grasping (Rizzolatti, Camarda, Fogassi,
Gentilucci, Luppino & Matelli, 1988). They identified whole hand
prehension neurons (palm opposition), precision grip neurons (pad
opposition) and finger prehension neurons. In an adjacent area
(subarea F4), they describe populations of neurons that fiied when
monkeys were reaching to grasp visual objects. These two subareas
were suggested to be related to the hand posturing and hand endpoint
respectively.
An excellent review of cortical control of visually directed reaching
and grasping is provided by Humphrey (1979). Figure 5.24 shows
pathways of activity flow during visually and proprioceptively guided
movements of the hand. Compared to control of the arm, the system
controlling reaching and positioning of the hand includes other brain
structures such as Brodmann’s Areas 2 and 5 of the parietal cortex
(Kalaska, Caminiti & Georgopoulos, 1983; Kalaska, 1988), and the
cerebellum (Fortier, Kalaska & Smith, 1989). Movements of the
hands and independent movements of the digits are effected chiefly
through the descending corticospinal pathways from the motor cortex .
Wrist and collective finger movements may also be produced through
corticospinal projections from Area 5, or via corticorubral projections
from Areas 4 and 5. Humphrey suggests that Area 4 effects hand
movements mainly through three inputs:
1) central commands for manipulative hand movements coming in
part from the intraparietal sulcus;
2) primary and association visual and somatosensory areas; and
3) a gating or modulating inDut from the SMA, in the presence of
excitatory tactile or visual inputs, until amropriate points in time
during a movement sequence.
Proprioceptive and tactile inputs to the motor cortex may be through
the postcentral gyrus, and also through Area 5. The loop from Area 4
to cord to hand and back to Area 4 via postcentral gyrus or direct tha-
lamocortical projections is identified as intimately involved in orienting
grasping responses elicited by tactile stimulation of the arm or hand.
Visual inputs to the motor cortex from visual association areas are by
way of Areas 7,8, posterior 9 and 6, and likely from Area 7 via Area
5 of the parietal cortex.