Age-related differences in the temporal modulation transfer function with pure-tone carriers.

Detection of amplitude modulation (AM) in 500 and 4000 Hz tonal carriers was measured as a function of modulation frequency from younger and older adults with normal hearing through 4000 Hz. The modulation frequency above which sensitivity to AM increased ("transition frequency") was similar for both groups. Temporal modulation transfer function shapes showed significant age-related differences. For younger subjects, AM detection thresholds were generally constant for low modulation frequencies. For a higher carrier frequency, AM detection thresholds then increased as modulation frequency further increased until the transition frequency. In contrast, AM detection for older subjects continuously increased with increasing modulation frequency, indicating an age-related decline in temporal resolution for faster envelope fluctuations. Significant age-related differences were observed whenever AM detection was dependent on temporal cues. For modulation frequencies above the transition frequency, age-related differences were larger for the lower frequency carrier (where both temporal and spectral cues were available) than for the higher frequency carrier (where AM detection was primarily dependent on spectral cues). These results are consistent with a general age-related decline in the synchronization of neural responses to both the carrier waveform and envelope fluctuation.