Octopuses Use a Human-like Strategy to Control Precise Point-to-Point Arm Movements

One of the key problems in motor control is mastering or reducing the number of degrees of freedom (DOFs) through coordination [1]. This problem is especially prominent with hyper-redundant limbs such as the extremely flexible arm of the octopus [2]. Several strategies for simplifying these control problems have been suggested for human point-to-point arm movements [3-6]. Despite the evolutionary gap and morphological differences, humans and octopuses evolved similar strategies when fetching food to the mouth. To achieve this precise point-to-point-task, octopus arms generate a quasi-articulated structure based on three dynamic joints. A rotational movement around these joints brings the object to the mouth [7]. Here, we describe a peripheral neural mechanism-two waves of muscle activation propagate toward each other, and their collision point sets the medial-joint location. This is a remarkably simple mechanism for adjusting the length of the segments according to where the object is grasped. Furthermore, similar to certain human arm movements, kinematic invariants were observed at the joint level rather than at the end-effector level, suggesting intrinsic control coordination. The evolutionary convergence to similar geometrical and kinematic features suggests that a kinematically constrained articulated limb controlled at the level of joint space is the optimal solution for precise point-to-point movements.