Fig. 1. Field stimulation. (A ) Electrolysis of bath solution by field stimulation (2 trains of 21 pulses, 1 ms each) generates gas bubbles at both electrodes. The edge of the electrode seems as a dark band at baseline conditions, but is obscured by bubbles after stimulation. (B ) Field stimulation (8 trains of 21 pulses, 1 ms each) produces pH of more than 11 and less than 4 on pH indicator paper inserted into the tissue bath. The two electrodes can be seen entering the top right and bottom right of the image and aligning with the edges of the pH-sensitive paper, which has right and left sections with different color ranges. The color code on the top corresponds to the right section of the pH indicator paper strip, and vice versa . (C ) Recording of the voltage in the bath during a stimulus train (21 pulses at 7 Hz, 1-ms pulses, SIU-102 stimulator, RC-21BRFS recording chamber; Warner Instruments, Hamden, CT) with a low-resistance (500 kΩ) recording electrode. Inset : recording of voltage at the stimulating electrodes. The field voltage accumulates during stimulator pulses and remains elevated several seconds after the last stimulus. (D ) Typical recording of cytosolic Ca2+transients from a control neuron using activation by field stimulation (21 pulses, 2 ms each) at four different frequencies. (E ) Summary data of recordings of that shown in D . Transient amplitude (left ) reaches a peak at stimulation frequencies greater than 20 Hz and then declines (P < 0.001, repeated-measures analysis of variance main effect, n = 58). Although the average decay time constant does not change with frequency (middle , n = 42), the individual line plots of decay constants (normalized to the average across frequencies, right ) show substantial variability and inconsistency across stimulation frequencies.