Fig. 5. Relative amplitudes for the five frequency intervals within the 0.0095- to 1.6-Hz interval of the laser Doppler perfusion signal during iontophoresis with acetylcholine (ACh) and sodium nitroprusside (SNP). Data are shown as box plots. (A ) Before brachial plexus block. (B ) After brachial plexus block. (C ) Control group, probe 1, registrations 1 and 2. (D ) Control group, probe 2, registrations 1 and 2.