Fig. 3. Comparison of two submersions, the fourth of the naive session and the third of the trained session, for seal S. Arrows indicate the start and end of submersions. A spontaneous apnea is shown before submersion during the trained session and is indicated by the solid line. Note that heart rate and muscle blood flow (MBF) are higher and the change in muscle oxygenation (oxygenated hemoglobin and myoglobin, HbO+MbO) is lower during trained submersions. Units for Niroscope data are milli-vanders (mvd), which represent the change in concentration within the observed volume of tissue (see Materials and methods).