Fig. 2

Band-pass filter and channel optimization and simple physiology experiment. a Single frame from color-magnified video of anesthetized axolotl applying a wide band-pass filter (0.5–1.2 Hz) (Additional file 2, first part). b Vertical scan line from magnified video over time. c Gill and background signal (yellow and gray rings in a) development over time. d–f Single frame (d), vertical scan line (e), and gill and background signal development over time (f) applying a narrow band-pass filter (0.7–0.8 Hz) (Additional file 2, second part). Notice less background noise in e than in b. Both the combined, red, green, and blue channel signals are displayed in f showing noticeable differences in signal amplitudes of the different channels. g Same frame as in d showing only the green channel. h Same vertical scan line as in e for the green channel. i Average signal amplitudes for the combined, red, green, and blue channel in the narrow band-pass filter magnified video. For this video, the green channel has the highest signal amplitude. j Fourier-transformed green channel signal in f in the frequency domain revealing an average heart frequency of 0.70 Hz (42.20 BPS). k–m Single frame (k), vertical scan line (l), and gill and background signal development over time (m) in resting axolotl (Additional file 3, first part). n–p Single frame (n), vertical scan line (o), and gill and background signal development over time (p) in exercised axolotl (Additional file 3, second part). q–s Single frame (q), vertical scan line (r), and gill and background signal development over time (s) in anesthetized axolotl (Additional file 3, third part). t–v Green channel signals from m, p, s in the frequency domain. x Bar chart (mean ± 95% confidence interval) of heart rate in rest, exercise, and anesthesia measured with Eulerian video magnification (EVM) and ultrasound (US) respectively. Letters (A and B) indicate statistically significant differences between groups