Hwang EJ, Donchin O, Smith MA, Shadmehr R (2003) A Gain-Field
Encoding of Limb Position and Velocity in the Internal Model of Arm Dynamics. Public
Library of Science Biology, 1:209-220.
Abstract: Adaptability of reaching movements
depends on a computation in the brain that transforms sensory cues, such as
those that indicate the position and velocity of the arm, into motor commands.
Theoretical consideration shows that the encoding properties of neural elements
implementing this transformation dictate how errors should generalize from one
limb position and velocity to another. To estimate how sensory cues are encoded
by these neural elements, we designed experiments that quantified spatial
generalization in environments where forces depended on both position and
velocity of the limb. The patterns of error generalization suggest that the
neural elements that compute the transformation encode limb position and
velocity in intrinsic coordinates via a gain-field; i.e., the elements have
directionally dependent tuning that is modulated monotonically with limb
position. The gain-field encoding makes the counterintuitive prediction of
hypergeneralization: there should be growing extrapolation beyond the trained
workspace. Furthermore, nonmonotonic force patterns should be more difficult to
learn than monotonic ones. We confirmed these predictions experimentally.
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