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We investigated age-related differences in the time course of two-tone frequency discrimination. Healthy young and elderly adults with normal hearing acuity in the 500-2000 Hz range performed a two-alternative forced choice frequency discrimination task. The stimuli were short tones separated by either a 250-ms (short), 850-ms (medium), or 3000-ms (long) silent inter-interstimulus interval (ISI). Frequency discrimination thresholds were estimated using an adaptive staircase procedure. Although young listeners performed better than the elderly at all ISIs, the latter showed a dramatic elevation of discrimination thresholds at 250 ms, while the thresholds of the young subjects increased significantly at 3000 ms. These results suggest that the elderly may be especially vulnerable to the effects of masking produced by the second tone at the short interval, whereas the young listeners tend to be differentially affected by the trace decay at long ISI. The results are discussed in the framework of stimulus persistence hypothesis.

In Experiment 1, frequency-discrimination thresholds were estimated in a 2-interval, forced-choice, backward masking procedure with a masker acoustically dissimilar to the targets. Young subjects were more efficient in escaping the effects of masking than were their elderly counterparts. In Experiment 2, young and elderly subjects performed the same task, with a masker acoustically similar to the targets and with a target-dissimilar masker. Under target-similar masking and at short target-masker intervals, the elderly demonstrated significant improvement, reaching the level of performance of the young, whereas under the target-dissimilar masker, the age-related differences were restored. Both age-related slowing of information processing and increase in stimulus persistence can account for the results of Experiment 1, but only increased stimulus persistence explains the results of Experiment 2.

We investigated the relationship between age, structural properties of selected cerebral regions, and cognitive performance in healthy adults, 18 to 78 years old. Spin-lattice relaxation time (Tl), measured by nuclear magnetic resonance, was used to describe the structural composition ofthe brain tissue. Temporal lobe white-matter Tl showed age-related prolongation best described by a quadratic polynomial. There was a significant cubic trend in the association of hippocampal (gray-matter) Tl with age. In the examined regions of the medial temporal lobes, normally observed differentiation between gray-and white-matter Tl diminished linearly with age and disappeared almost completely in the elderly. Age and the ratio of gray-to white-matter Tl accounted for 53% ofthe variance in a measure offluid intelligence (Cattell Culture Fair Test); the unique contributions of age and of gray-white-matter Tl ratio were 23% and 3%, respectively. The largest share of the variance in fluid intelligence (27%) was explained by the common influence of age and gray-white-matter Tl ratio. The same set of variables explained no significant proportion ofthe variance in crystallized intelligence, The possible mechanisms underlying age-related changes in gray-white-matter differentiation, their relationship to age-related seleetive deterioration of cognitive functions, and the implications ofthe findings for research on biological markers of aging are discussed.