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The Journal of Neuroscience, August 1, 1998, 18(15):5976-5987

Patterns of Arm Muscle Activation Involved in Octopus Reaching Movements

Yoram Gutfreund¹, Tamar Flash², Graziano Fiorito³, and Binyamin Hochner¹

¹ Department of Neurobiology and Center for Neuronal Computation, Institute of Life Sciences, Hebrew University, Jerusalem 91904, Israel, ² Department of Applied Mathematics, The Weizmann Institute of Sciences, Rehovot 76100, Israel, and ³ Department of Neurobiology, Stazione Zoologica "A. Dohrn," Naples 80121, Italy

The extreme flexibility of the octopus arm allows it to perform many different movements, yet octopuses reach toward a target in a stereotyped manner using a basic invariant motor structure: a bend traveling from the base of the arm toward the tip (). To study the neuronal control of these movements, arm muscle activation [electromyogram (EMG)] was measured together with the kinematics of reaching movements. The traveling bend is associated with a propagating wave of muscle activation, with maximal muscle activation slightly preceding the traveling bend. Tonic activation was occasionally maintained afterward. Correlation of the EMG signals with the kinematic variables (velocities and accelerations) reveals that a significant part of the kinematic variability can be explained by the level of muscle activation. Furthermore, the EMG level measured during the initial stages of movement predicts the peak velocity attained toward the end of the reaching movement. These results suggest that feed-forward motor commands play an important role in the control of movement velocity and that simple adjustment of the excitation levels at the initial stages of the movement can set the velocity profile of the whole movement. A simple model of octopus arm extension is proposed in which the driving force is set initially and is then decreased in proportion to arm diameter at the bend. The model qualitatively reproduces the typical velocity profiles of octopus reaching movements, suggesting a simple control mechanism for bend propagation in the octopus arm.

Key words: movement control; muscular-hydrostats; reaching movements; EMG; muscle activation; motor programs; octopus; cephalopods

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Copyright © 1998 Society for Neuroscience  0270-6474/98/18155976-12$05.00/0

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