Biol Cybern (2006) 95:555–566 DOI 10.1007/s00422-006-0119-z ORIGINAL PAPER Dynamic control of the central pattern generator for locomotion R. Jacob Vogelstein · Francesco Tenore · Ralph Etienne-Cummings · M. Anthony Lewis · Avis H. Cohen Received: 11 September 2006 / Accepted: 16 October 2006 / Published online: 18 November 2006 © Springer-Verlag 2006 Abstract We show that an ongoing locomotor pat- tern can be dynamically controlled by applying discrete pulses of electrical stimulation to the central pattern generator (CPG) for locomotion. Data are presented from a pair of experiments on biological (wetware) and electrical (hardware) models of the CPG demonstrating that stimulation causes brief deviations from the CPG’s limit cycle activity. The exact characteristics of the devia- tion depend strongly on the phase of stimulation. Appli- cations of this work are illustrated by examples showing how locomotion can be controlled by using a feedback loop to monitor CPG activity and applying stimuli at the appropriate times to modulate motor output. Even- tually, this approach could lead to development of a R. J. Vogelstein and F. Tenore contributed equally to this work. R. J. Vogelstein (B ) Department of Biomedical Engineering, Johns Hopkins University, Baltimore, MD 21218, USA e-mail: jvogelst@bme.jhu.edu F. Tenore · R. Etienne-Cummings Department of Electrical and Computer Engineering, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA e-mail: fra@jhu.edu R. Etienne-Cummings e-mail: retienne@jhu.edu M. A. Lewis Iguana Robotics, Inc., Urbana, IL 61803, USA e-mail: tlewis@iguana-robotics.com A. H. Cohen Department of Biology and Institute for Systems Research, University of Maryland, College Park, MD 20742, USA e-mail: avis@isr.umd.edu neuroprosthetic device for restoring locomotion after paralysis. List of Abbreviations CPG Central pattern generator DAC Digital-to-analog converter HCO Half-center oscillator LCO Limit cycle oscillator LHE Left hip extensor LHF Left hip flexor PCB Printed circuit board PDR Phase-dependent response RHE Right hip extensor RHF Right hip flexor SFA Spike-frequency adaptation ZPM Zero-phase marker 1 Introduction In all vertebrates studied to date, locomotion is con- trolled by specialized neural circuits in the spinal cord collectively called the central pattern generator (CPG) (Delcomyn 1980; Dimitrijevic et al. 1998). Under nor- mal conditions, the CPG integrates command signals from the brain with sensory feedback from the limbs and torso and produces rhythmic outputs to the motor neu- rons that effect locomotion. However, descending input is not required for the CPG’s basic functionality and it has been shown to operate in the complete absence of brain control if provided with an appropriate initiation signal (Cohen and Wallèn 1980; Dimitrijevic et al. 1998; Grillner and Zangger 1984). In humans, the CPG for locomotion is thought to be located in the lumbar spinal cord (Dimitrijevic et al. 1998). Spinal cord injuries above