Individual differences in auditory electric responses: comparisons of between-subject and within-subject variability. II. Amplitude of brainstem Vertex-positive peaks.

Recently, we (Lauter & Loomis, 1986) reported variability measures of the latency of five vertex-positive auditory brainstem response (ABR) peaks collected under a repeated-measures experimental design. Seven subjects were tested, each on eight separate sessions, for brainstem auditory evoked response to monaural right, monaural left, and binaural stimulus presentation. This paper presents variability measures for amplitudes of the same series of responses. Three types of variability measurement were made: 1) amplitude of each peak of the response to monaural right, monaural left, and binaural stimulation; 2) amplitude difference for each peak comparing binaural with right, and binaural with left; and 3) amplitude difference comparing binaural with the sum of the amplitudes of the two monaural responses. As in the previous report, between-subject variability and within-subject variability were expressed using a ratio of mean divided by standard deviation (this is the reciprocal of Pearson's Coefficient of Variation, and will here be referred to as the Coefficient of Stability, or Cs). For all amplitude comparisons, Cs profiles indicate that: 1) within-subject stability (i.e., consistency) is significantly greater than between-subject stability, 2) both within- and between-subject stability measures are sensitive to both peak and ear of presentation, and 3) stability profiles for individual subjects show individual differences and similarities, and are replicable over time. The variability measure also provides evidence of an ear asymmetry at peak III which has been noted in other ABR studies.