Fig. 2. Head angle rotation during withdrawal responses. (A) Overlay of lines drawn through the stiff rostral section of the fish at 4-ms intervals through the course of a tail stimulus elicited withdrawal. Kinematic landmarks, located at the head, are denoted by black (start of response), grey (transition point) and white (end of withdrawal) circles. (B) Head angle measurements for the same trial as in A, normalized to a start angle of zero and plotted as angle over duration of response. A six-degree polynomial was used to fit the data to a line. In this trial, the transition point (double-headed arrow) is reached at 0.036 s, and the end of withdrawal (single-headed arrow) is at 0.06 s. (C) Comparison of head angle during the periods from initiation of response to the transition point and from the transition point to the end of withdrawal show significantly greater angle movement during the former period (P>0.0001) but no statistical difference between head and tail-elicited responses.