The Origin of N 2 of the Cochlear Whole-Nerve Action Potential

Several studies exist concerning the nature of the cochlear whole-nerve responses to acoustic transients. All focus on the properties of the first negative deflection N1, but none offers a satisfactory explanation for N2. A number of lines of evidence are here presented to support the concept of separate loci of generation of N1 and N2: (1) Changes in relative position of recording electrodes produce differential changes in amplitude of N1 and N2; (2) paired-click studies permit electrical summation of an N2 response with a succeeding N1 response; (3) high-frequency noise masking attenuates N1 to a greater degree than it does N2; (4) certain intensities of low-frequency masking attenuate N2 to a greater degree than they do N1; (5) a narrow-band noise-induced temporary threshold shift between 6 and 14 kHz, in behaviorally trained monkeys with chronic electrical implants, causes a marked reduction in N1 amplitude but no significant change in N2; (6) selective cooling of the apex of the guinea pig cochlea results in a relatively greater reduction in N2 than in N1. These and other lines of evidence suggest that the N2 potential may be arising from a more apical region of the cochlea than N1. [This work was supported by the John A. Hartford Foundation and by NIH.]