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The Journal of Neurophysiology Vol. 84 No. 5 November 2000, pp. 2191-2203
Copyright ©2000 by the American Physiological Society
Department of Cognitive and Neural Systems and Center for Adaptive Systems, Boston University, Boston, Massachusetts 02215
Ajemian, Robert,
Daniel Bullock, and
Stephen Grossberg.
Kinematic Coordinates In Which Motor Cortical Cells Encode
Movement Direction. J. Neurophysiol. 84: 2191-2203, 2000. During goal-directed reaching in primates, a
sensorimotor transformation generates a dynamical pattern of muscle
activation. Within the context of this sensorimotor transformation, a
fundamental question concerns the coordinate systems in which
individual cells in the primary motor cortex (MI) encode movement
direction. This article develops a mathematical framework that
computes, as a function of the coordinate system in which an individual
cell is hypothesized to operate, the spatial preferred direction (pd) of that cell as the arm configuration and hand location vary. Three
coordinate systems are explicitly modeled: Cartesian spatial, shoulder-centered, and joint angle. The computed patterns of spatial pds are distinct for each of these three coordinate systems, and experimental approaches are described that can capitalize on these differences to compare the empirical adequacy of each coordinate hypothesis. One particular experiment involving curved motion was analyzed from this perspective. Out of the three coordinate systems
tested, the assumption of joint angle coordinates best explained the
observed cellular response properties. The mathematical framework
developed in this paper can also be used to design new experiments that
are capable of disambiguating between a given set of specified
coordinate hypotheses.
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