SYMMETRY BREAK AND LEG SPECIFIC ROLES DURING CURVE WALKING IN <EM>DROSOPHILA</EM>

When walking animals change directions, the motor system must break symmetry and generate differences in stepping kinematics between left and right legs. To understand these, we analyzed videos of free-walking flies, and induced curve walking. We found three major strategies to produce directional changes: adjustments, arcs, and turns. Adjustments arise from fast kinematic local changes on one step of the inner hind leg (HL) of the curve. Arcs comprehend several steps at constant speed and angular velocity, while turns are generated with fewer steps, and before execution flies decrease their speed. We found common kinematic lateral asymmetries between arcs and turns, such as slightly shorter swings and longer stances for middle leg (ML) and HL, and shorter stances for all inner legs, that become stronger in turns. Curve walking not only requires breaking the lateral kinematic symmetry, but also leg specific strategies are recruited sequentially when angular velocity increases. During arcs, the (1) outer stance amplitudes increase while (2) insider decrease, (3) Posterior Extreme Positions from outer front leg (FL) move away from the body and towards it for inner FL and ML. In addition, for turns (4) inside FL shift their Anterior Extreme Positions away from the body, (5) inner HL shortens the steps or (6) skips some, serving as pivot point, and in extreme cases (7) FL and ML generate lateral steps, and (8) inner HL and ML generate backward steps. This indicates that curve walking kinematics do not just display side-specificity, but leg specificity neural control.