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B
u
]
+
,
u
i
=[
G
(
V
i
−
V
j
)+
(10.3)
where
i
j
designated opponent neural commands and
B
u
was the baseline activity
of the phasic-MT area 4 cell activity.
The co-contractive vector (P) represented area's 4 phasic activity of bidirectional
neurons (i.e., neurons whose activity decreases or increases for both directions of
movement [19]), and it was organized for the co-contraction of antagonist muscles
(see columns 1 and 3 of Fig. 10.2). It was given by
,
B
P
]
+
.
P
=[
G
(
V
i
−
V
j
)+
(10.4)
Fig. 10.2: Comparison of peristimulus time histograms (PSTH) of reciprocally or-
ganized neurons (column 1; reproduced with permission from [19, Fig. 4A, p. 182],
Copyright Springer-Verlag) in area 4, simulated area's 4 reciprocally organized pha-
sic (DVV) cell activities (column 2), PSTH of area's 4 bidirectional neurons (col-
umn 3; reproduced with permission from [19, Fig. 4A, p. 182], Copyright Springer-
Verlag) and simulated area's 4 co-contractive (P) cells activities (column 4) for a
flexion (row 1) and extension (row 2) movements in normal monkey. The vertical
bars indicate the onset of movement. Note a clear triphasic AG1-ANT1-AG2 pattern
marked with arrows is evident in PSTH of reciprocally and bidirectionally organized
neurons. The second AG2 burst is absent in simulated DVV cell activities.
While the reciprocal pattern of muscle activation served to move the joint from
an initial to a final position, the antagonist co-contraction served to increase the
apparent mechanical stiffness of the joint, thus fixing its posture or stabilizing its
course of movement in the presence of external force perturbations. The Renshaw
population cell activity was modelled by
dR
i
dt
=
φ
(
λ
B
i
−
R
i
)
z
i
max
(
M
i
,
0
)
−
R
i
(
1
.
5
+
max
(
R
j
,
0
))
,
(10.5)
