Dual Dimensionality Reduction Reveals Independent Encoding of Motor Features in a Muscle Synergy for Insect Flight Control
Fig 1
Encoding of torque in muscle activation.
Each dorsolongitudinal muscle (DLM) is composed of five separately innervated subunits. The subunits fire simultaneously with a single muscle action potential acting as a combined motor unit [23]. Contraction of the DLMs produces the downstroke through deformation of the thorax, which is restored by the upstroke dorsoventral muscles (DVMs) (A—only main flight muscle shown; sub-figure derived from [14]). There are three major groups of steering muscles (axillary, basalar, and subalar) which are not shown for clarity, but lie just lateral to the DLMs and DVMs, near the wing hinge (see [24]). DLM recordings during tethered flight (B) are parameterized by the spike timing variables (tL, tR). Activation varies in the timing of the spike, not the number of spikes. Changes in timing can produce variation in yaw torque (C). In PLS feature analysis (D-H), we relate these motor signals (the U matrix) to the turning torque (the M matrix) (D). The raw torque signal is calibrated to extract the within-stroke torque (E). We then spike-trigger (dots in e; each of the first four extracted torque chunks is colored) and align the torque (D), find a reduced dimensional feature space via the partial least squares (PLS) method (F), test if synergy models can account for this feature space (G), and reconstruct the torque waveform from the timing variables (H).