Techniques for antidromic identification and example recordings. (A) Diagram for a collision test circuit. The gain and polarity of the amplifier are manually adjusted online depending on the amplitude and polarity of the spike so that they can be detected by crossing the threshold of the inverter. The timer chip LM555 is used in the monostable mode to generate a one-shot pulse to trigger the pulse generator, which triggers the stimulus isolator. It also acts as a hold-off to prevent multiple stimulations faster than 1 Hz. (B) Schematic of the bipolar stimulating electrode and its holder. (C) Schematic of the song system in the zebra finch indicating the locations of bipolar stimulating electrodes in RA and Area X, and a recording electrode in HVC. (D) An example of a multiunit “hash” recorded in HVC following the electrical stimulation in RA (current amplitude 200 µA). Overlay of 20 trials aligned to the onset of the stimulation. (E) An example of a simultaneous single-unit recording of two X-projecting HVC neurons during singing, recorded in an adult male zebra finch. (Top) Spectrogram of the song with syllable labels. (Bottom) Raw voltage traces from two different electrodes. (Right) Enlarged bursts showing individual action potentials. (F) Collision tests for the neurons shown in (E). Voltage traces are aligned to the onset of the electrical stimulation of Area X. Antidromically evoked spikes (black traces) are absent when the stimulation is triggered 2 msec after spontaneous spikes (red traces).