J Neurophysiol 101: 2907-2923, 2009.
First published March 18, 2009; doi:10.1152/jn.91045.2008
0022-3077/09 $8.00
Dynamics of Primate Oculomotor Plant Revealed by Effects of Abducens Microstimulation
Sean R. Anderson1,
John Porrill1,
Sokratis Sklavos1,2,
Neeraj J. Gandhi3,
David L. Sparks4 and
Paul Dean1
1Department of Psychology, Sheffield University, Sheffield, United Kingdom; 2Department of Physiology, School of Nursing, University of Athens, Athens, Greece; 3Department of Otolaryngology, University of Pittsburgh, Pittsburgh, Pennsylvania; and 4Division of Neuroscience, Baylor College of Medicine, Houston, Texas
Submitted 17 September 2008;
accepted in final form 12 March 2009
Despite their importance for deciphering oculomotor commands, the mechanics of the extraocular muscles and orbital tissues (oculomotor plant) are poorly understood. In particular, the significance of plant nonlinearities is uncertain. Here primate plant dynamics were investigated by measuring the eye movements produced by stimulating the abducens nucleus with brief pulse trains of varying frequency. Statistical analysis of these movements indicated that the effects of stimulation lasted about 40 ms after the final pulse, after which the eye returned passively toward its position before stimulation. Behavior during the passive phase could be approximated by a linear plant model, corresponding to Voigt elements in series, with properties independent of initial eye position. In contrast, behavior during the stimulation phase revealed a sigmoidal relation between stimulation frequency and estimated steady-state tetanic tension, together with a frequency-dependent rate of tension increase, that appeared very similar to the nonlinearities previously found for isometric-force production in primate lateral rectus muscle. These results suggest that the dynamics of the oculomotor plant have an approximately linear component related to steady-state viscoelasticity and a nonlinear component related to changes in muscle activation. The latter may in part account for the nonlinear relations observed between eye-movement parameters and single-unit firing patterns in the abducens nucleus. These findings point to the importance of recruitment as a simplifying factor for motor control with nonlinear plants.
Address for reprint requests and other correspondence: P. Dean, Department of Psychology, University of Sheffield, Western Bank, Sheffield S10 2TP, UK (E-mail: p.dean{at}sheffield.ac.uk)
Copyright © 2009 by the The American Physiological Society.
