Vibrissae resonate. The average amplitude of vibrissa motion in response to a periodic ex vivo PZ stimulation of a C3 vibrissa from the left side of the rat face at 5 Hz intervals, 5-600 Hz. C3 displays an 11-fold amplification when driven at its FRF of 385 Hz (gray bar). The vibrissa motion and the PZ motion, when driven by a periodic input at 320, 385, and 450 Hz, are shown in the insets. Note the increase in the amplitude and the 90° phase shift at the FRF. A similar phase shift (≥25°) was observed within 10 Hz of the FRF in the majority of cases (n = 17 of 20 measurements of 10 vibrissae, in vivo and ex vivo). Gray, Stimulus wave form. Black, Vibrissa motion.