Effects of age on the adult auditory middle latency response

The middle latency components of the auditory evoked response were obtained from a group of normal-hearing, healthy female subjects from 22 to 68 years of age. Recordings were made at several intensity levels to assess the level-dependence of any age-related effects. Cross-sectional analyses revealed that the amplitude of component Pa grows linearly with age, becoming significantly larger in older (50-68 years of age) compared to younger (22-37 years) subjects. The amplitude-intensity function is steeper in the older subjects by a factor of two. Correlational analyses suggested that at higher intensity levels age accounts for about 20% of the variance in the amplitude of Pa. A positive shift in response baseline was observed in the older subjects, and could contribute to the age-related increase in the absolute amplitude of Pa. However, a similar increase in the peak-to-peak and area measures of Pa suggests that some of the increase in the magnitude of Pa is independent of baseline shift. A confounding of age and hearing sensitivity in this study makes it difficult to interpret the age-related effects as strictly central in nature.