Chapter 2. Prehension

120 THE PHASES OF PREHENSION
tance as well (Keele 8z Ells, 1972; Laabs, 1973; Marteniuk, 1973).
More recently, neural evidence has been found for the notion that
distance and direction are encoded separately (Georgopoulos, Ketmer,
8z Schwartz, 1988; Kettner, Schwartz, 8z Georgopoulos, 1988;
Schwartz, Kettner, & Georgopoulos, 1988). In these studies, rhesus
monkeys were trained to reach forward and press buttons in a three-
dimensional workspace. The placement of the targets allowed free
motions from the resting position (center and in front of the animal’s
body) to 8 periperal targets. Recording from motor cortex, Schwartz
et al. (1988) found an orderly change in frequency of discharge of a
majority of the cells with direction of the reaching. A single cell’s
activity varied approximately as a cosine function of the movement
direction, centered on one preferred direction that varies from cell to
cell. Each cell emitted a graded signal for a broad range of movements,
and each movement evoked a complex pattern of graded activation that
varied with movement direction. Georgopoulos et al. (1988)
suggested that motor cortical neurons are signaling a vectorial ‘vote’
with a direction in the cell’s preferred direction and a magnitude pro-
portional to the change in the cell’s discharge rate associated with the
particular direction of movement. The vector sum of these contribu-
tions, the neuronal population vector, is the outcome of the population
code and points in the direction of movement in space. In addition, in
comparing a time series of the population vector to the instantaneous
velocity of the movement, the direction of the vector points in the di-
rection of the movement, and even is an accurate prediction of the up-
coming movement. Georgopoulos et al. (1988) showed an example
where the population vector formed 160 ms before the movement
began. Kettner et al. (1988) showed that steady state activity of motor
cortical cells is modulated in orderly fashion with active maintenance
of hand position in space.
In terms of specific neural areas, area 5 of the parietal association
area (Kalaska, 1988) and the dorsal premotor cortex have several
response properties similar to motor cortex (see above), as well as
activity of the cerebellum (Kalaska et al., 1983; Kalaska, 1988;
Fortier, Kalaska 8z Smith, 1989). It is known that parietal regions
directly and cerebellum indirectly (via thalamus) connect with motor
cortex (Strick 8z Kim, 1978; Caminiti et al., 1985). For monkeys
making reaching movements, Kalaska and Crammond (1992) showed
overlapping and sequential activity in the following cortical areas,
prior to movement initiation: premotor cortex (Area 6), primary motor
cortex (Area 4), posterior parietal cortex (Area 5), then somatosensory
parietal cortex (Area 2). The firing patterns in Area 5 in particular are