Age-related auditory deficits in temporal processing in F-344 rats.

Older human listeners demonstrate perceptual deficits in temporal processing even when audibility has been controlled. These age-related auditory deficits in temporal processing are thought to originate in the central auditory pathway. Precise temporal processing is necessary to detect and discriminate auditory cues such as modulation frequency, modulation depth and envelope shape which are critical for perception of speech and environmental sounds. This study aims to further understanding of temporal processing in aging using non-invasive electrophysiological measurements. Amplitude modulation following responses (AMFRs) and frequency modulation following responses (FMFRs) were recorded from aged (92-95-weeks old) and young (9-12-weeks old) Fischer-344 (F-344) rats for sinusoidally amplitude modulated (sAM) tones, sinusoidally frequency modulated (sFM) tones and ramped and damped amplitude modulation (AM) stimuli which differ in their envelope shapes. The modulation depth for the sAM and sFM stimuli and envelope shape for the ramped and damped stimuli were systematically varied. There was a monotonic decrease in AMFR and FMFR amplitudes with decreases in modulation depth across age for sAM and sFM stimuli. There was no significant difference between the response amplitudes of the young and aged animals for the largest modulation depths. However, a reduction in modulation depth resulted in a significant decrease in the response amplitudes and higher modulation detection thresholds for sAM and sFM stimuli with age. The aged animals showed significantly lower response amplitudes for ramped stimuli but not for damped stimuli. Cross correlating the responses with the ramped, symmetric, or damped stimulus envelopes revealed a decreased fidelity in encoding envelope shapes with age. These results indicate that age related temporal processing deficits become apparent only with reduced modulation depths or when discriminating envelope shapes. This has implications for psychophysical or diagnostic testing as well as for constraining potential cellular and network mechanisms responsible for these deficits.