Comparison of age-related declines in behavioral auditory responses versus electrophysiological measures of amplitude modulation.

Amplitude and frequency modulations are important for speech intelligibility, especially in noise. Neurophysiological responses assessed by envelope following responses (EFRs) are smaller at faster amplitude modulation frequencies (AMF) in older subjects compared to younger subjects. A typical assumption is that a decline in EFRs necessarily results in corresponding perceptual deficits. To test this in an animal model, we investigated the behavioral AMF discrimination of young and aged Fischer-344 rats and compared those abilities to their EFRs. A modified version of prepulse inhibition of the acoustic startle reflex was used to measure behavior. When AMF differences and modulation depths were large, young and aged animals' behavioral performances were comparable. Aged animals' discrimination abilities declined as the difference between background and prepulse AMF decreased and as modulation depth decreased. These declines were larger than in younger animals, even compared to young rats with similar peripheral activation (ABR wave I amplitudes), whose EFR amplitudes were smaller than the aged animals. The results revealed larger age-related deficits in behavioral perception compared to EFRs, suggesting additional factors that affect perception in aging.