Electrophysiological investigation of auditory recruitment by averaged electroencephalographic-evoked response

An objective study of auditory recruitment by the method of the slow vertex-evoked potentials was carried out on 18 subjects presenting recruitment at 4 000 Hz. Evoked potentials were induced by tone burst stimulation of the recruiting ear and recorded by means of an active electrode located on the vertex. For each subject a study was made: (1) of the input-output curves of the N1P2 amplitudes and of the latencies of N1 and P2 of the evoked potentials as a function of the intensity of the stimulation and (2) of the best estimation of the input-output curve of N1P2 to a power function and a logarithmic function by the least-squares regression line, after checking that it was statistically possible. The results of the 18 recruiting subjects, classified according to their audiometric thresholds, were statistically compared with those of 9 normal subjects. The relation between the amplitude of the auditory-evoked response and the intensity of the stimulation could be expressed nearly equally well by a logarithmic function and a power function. The studies revealed a very significant increase of the slope of the least-squares regression line in recruiting subjects compared with normal subjects. No significant difference was obtained by the analysis of the latencies.