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The C-start escape response of Polypterus senegalus: bilateral muscle activity and variation during stage 1 and 2

Eric D. Tytell^* and George V. Lauder

Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA

View larger version (17K): [in a new window]   Fig. 5. Individuals did not become habituated to the stimulus. Maximum velocity (Ls-1) plotted against the experiment number in order over time. Different individuals are shown with different colors and symbol shapes. The non-significant regression is shown as a dashed line with a 95% confidence interval in gray.

View larger version (20K): [in a new window]   Fig. 1. (A) Diagram of the electromyographic apparatus used to record EMGs, showing longitudinal positions of the electrodes, one on each side of the body. (B) The filming arrangement. A high-speed digital camera filmed a ventral view of the escape responses.

View larger version (8K): [in a new window]   Fig. 2. Diagram of a section of a fish's body with a bending moment M induced by bilateral contraction, with ipsilateral force FI and contralateral force FC. Both forces are applied an equal distance a from the midline.

View larger version (45K): [in a new window]   Fig. 3. Typical EMG traces and video frames from strong and weak responses. (A) EMG traces. Traces from ipsilateral and contralateral sites are shown in orange and blue, respectively, and traces from anterior sites are darker than posterior. Periods of escape response muscle activity are shown in a brighter color and are indicated by a box, where the left (L) and right (R) sides signify onset and offset times, respectively, and the height of the box indicates average EMG amplitude. Vertical lines indicate the time of the numbered video frames, shown in B. (B) Video frames corresponding to the same kinematic events in each response. Events shown are (1) first visible motion, usually at the head, (2) maximum acceleration, (3) maximum velocity achieved during stage 1, (4) maximum curvature, (5) end of kinematic stage 1 and (6) maximum overall velocity. Note that the stage 2 muscle activity occurs before the beginning of the kinematic stage 2. Events 4 and 5 (maximum stage 1 velocity and the end of stage 1) for the weak response occur at different times during the same video frame, so that frame is repeated.

View larger version (18K): [in a new window]   Fig. 4. Principal components analysis (PCA) on kinematic variables. (A) Principal components 1 and 2 (PC1 and PC2) loadings. Loadings are scaled so that the variance of the transformed data is the same along all axes. (B) Transformed data scores on the PC1 and PC2 axes. According to the kinematic strength metric, based on the PCA, responses with scores in the bottom right are strong, while those in the upper left are weak. The black line separates the two regions.

View larger version (23K): [in a new window]   Fig. 6. Final velocity and trajectory angle are correlated with the stimulus. (A) Maximum overall velocity plotted with respect to the distance between the stimulus and the fish's center of mass. The regression line is shown in black with a 95% confidence interval in gray. (B) Angular histogram of the angle between the final trajectory and the stimulus. Angles are normalized as if all turns were to the left. The mean value of this angle is 169±6°, which is shown by the solid line. Numbers inside the circles are the frequencies of occurrence for different turn angles.

View larger version (22K): [in a new window]   Fig. 7. Stage 1 maximum velocity correlates with ipsilateral stage 1 EMG magnitude. The linear regression line is shown in black with a 95% confidence interval in gray.

View larger version (25K): [in a new window]   Fig. 8. Least-squares onset time, offset time and magnitude values for strong and weak escape responses, based on the MANOVA calculation (Table 3). In the left panel, mean activity is plotted against time. Different longitudinal electrode positions are displayed along the vertical axis, with anterior positions in dark colors at the top and posterior ones in light colors at the bottom. Ipsilateral and contralateral electrodes at the same longitudinal position are offset slightly from each other and are shown in orange and blue, respectively. The left and right side of the boxes are onset and offset times, respectively, and the height of each box represents the average EMG amplitude. The area of the box, thus, is proportional to the magnitude of muscle activity. The lines at either end show standard errors for the onset and offset time. The right panel shows the magnitude itself for each electrode, with standard error bars. Between the two strength responses are the means for all positions along the body of onset time, offset time and magnitude.

View larger version (28K): [in a new window]   Fig. 9. Contralateral activity is significantly related to ipsilateral activity. In both plots, the regression line is shown with a 95 % confidence interval in gray. (A) Regression of contralateral magnitude on ipsilateral magnitude. (B) Regression of contralateral duration on ipsilateral duration.

View larger version (23K): [in a new window]   Fig. 10. Overall maximum velocity is proportional to mean contralateral stage 2 muscle activity magnitude. The linear regression line is shown in black with a 95 % confidence interval in gray.