Origin of the guinea pig cochlear action potential produced by a click

1. The sound pressure level required to produce a detectable gross action potential (N1) in the guinea pig was measured for a range of frequencies. The resulting electrocochleogram was compared with the click level require to produce a detectable N1. 2. Over most of the auditory range (1-30 kHz) the required click level was 5 dB higher than the level of the most sensitive part of the electrocochleogram. 3. The click evoked N1 therefore reflects the condition of neither the whole cochlea nor any fixed part of it, instead it is a measure of the most sensitive region of any particular cochlea. 4. A click which produces a threshold N1 fails to produce a single-unit change which is detectable by the usual criteria. At threshold, the click-evoked N1 is a result of partial synchrony of single units.