Aging Degrades the Neural Encoding of Simple and Complex Sounds in the Human Brainstem

Background: Older adults, with or without normal peripheral hearing sensitivity, have difficulty understanding speech. This impaired speech perception may, in part, be due to desynchronization affecting the neural representation of acoustic features. Here we determine if phase-locked neural activity generating the brainstem frequency-following response (FFR) exhibits age-related desynchronization and how this degradation affects the neural representation of simple and complex sounds. Purpose: The objectives of this study were to (1) characterize the effects of age on the neural representation of simple tones and complex consonant-vowel stimuli, (2) determine if sustained and transient components of the FFR are differentially affected by age, and (3) determine if the inability to encode a simple signal predicts degradation in representation for complex speech signals. Research Design: Correlational Study Sample: Thirty four adults (aged 22-77 yr) with hearing thresholds falling within normal limits. Data Collection and Analysis: Stimuli used to evoke FFRs were 1000 Hz tone bursts as well as a consonant-vowel /da/ sound. Results: The neural representation of simple (tone) and complex (/da/) stimuli declines with advancing age. Tone-FFR phase coherence decreased as chronological age increased. For the consonant-vowel FFRs, transient onset and offset response amplitudes were smaller, and offset responses were delayed with age. Sustained responses at the onset of vowel periodicity were prolonged in latency and smaller in amplitude as age increased. FFT amplitude of the consonant-vowel FFR fundamental frequency did not significantly decline with increasing age. The ability to encode a simple signal was related to degradation in the neural representation of a complex, speechlike sound. Tone-FFR phase coherence was significantly related to the later vowel response components but not the earlier vowel components. Conclusions: FFR components representing the tone and consonant-vowel /da/ stimulus were negatively affected by age, showing age-related reductions in response synchrony and amplitude, as well as prolonged latencies. These aging effects were evident in middle age, even in the absence of significant hearing loss.