Fig. 4. Two control experiments. (A) Verification of induced vibrations on the scanning laser Doppler vibrometer (SLDV) and the extension arm. The noise level was measured with the accelerometer at the tip of the extension arm (dotted line). During application of a vertical stimulus (frequency, 10-600 Hz, velocity, 1x10^-4 m s^-1), the skull velocity was measured with the SLDV (dashed line) while the accelerometer simultaneously monitored the motion of the extension arm (solid line). (B) Relative plot of the mounting block response in three orthogonal directions for lateral and (C) vertical stimulation. The colors refer to those in Fig. 3A. The driving direction (intended excitation direction) of the vibration exciter is represented by a flat line at 0 dB (green line for lateral, and blue line for vertical stimulation, respectively).