Abstract
A 'vectorial hypothesis' was proposed by which motor cortical cells
possessing directional preference and directional spread could, as
populations, generate movements in particular directions. The main
assumptions of this hypothesis are that (a) cells exert a directional
influence (vector) along the axis of their preferred direction, (b)
this directional influence is on the same axis for all directions
of movement, but (c) it is exerted toward the cell's preferred direction
when there is an increase above the average discharge level, or toward
the opposite direction when there is a decrease in discharge; and
(d) for a particular movement direction, the vectorial components
of individual cells sum linearly. Given these assumptions, the population
vectorial sum was determined for each of the 8 direction of movement
tested using experimentally observed data from 241 directionally
tuned motor cortical cells. The direction of this population vector
was within 11&Deg; from the straight-line movement direction. This
was within the range of the directions of the movement trajectories
produced by well trained animals. Possible relations between motor
cortical cells and individual muscles or muscles synergies were discussed
with respect to the hypothesis above. It was argued that the hypothesis
would hold under either assumption, i.e. whether motor cortical cells
are thought as controlling muscles or muscle synergies, assuming
that no changes in the position of parts of the limb occur that would
alter fundamentally the directions in which muscles exert their actions.
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