Affiliations: University of Pennsylvania, Philadelphia, PA,
USA | Moss Rehabilitation Research Institute, Philadelphia,
PA, USA | Thomas Jefferson University, Philadelphia, PA,
USA | Cazenovia College, Cazenovia, NY, USA
Note: [] Corresponding author: H. Branch Coslett, Department of
Neurology, HUP, 3rd Floor Gates Bldg., 3400 Spruce St. Philadelphia, PA 19104,
USA. Tel.: +1 215 349 8275; Fax: +1 215 349 5579; E-mail:
hbc@mail.med.upenn.edu
Abstract: Converging behavioral findings support recent models of motor
control suggesting that estimates of the future positions of a limb as well as
the expected sensory consequences of a planned movement may be derived, in
part, from efference copies of motor commands. These estimates are referred to
as forward models. However, relatively little behavioral evidence has been
obtained for proposed forward models that provide on-line estimates of current
position. We report data from a patient (JD) who reached accurately to
visualized targets with and without vision of her hand despite substantial
proprioceptive loss. Additionally, we administered a double-start reaching test
to examine the possibility that efference copy information could be used to
estimate current limb position. JD reached accurately, without vision, to a
final target after actively reaching to a landmark, but exhibited severely
impaired reaching after passive movements to the landmark. This finding
suggests that forward modeling of efference copy signals may provide relatively
accurate estimates of current limb position for the purpose of motor planning.
The possibility that such estimates may also contribute to the awareness of
body position and to self-recognition is discussed.
Keywords: Forward model, parietal, proprioception, pointing, efference copy, sensory loss
