Age differences in backward masking.

The present study was designed to assess the effects of age on the time course of backward masking. In experiment 1, thresholds for detecting a 10-ms, 500-Hz sinusoidal signal were measured as a function of the temporal separation between the signal and a 50-ms broadband masker. Subjects were younger (18-24) and older (over age 65) adults with normal hearing (thresholds less than 20 dB HL) for frequencies of 4 kHz and below. Younger subjects exhibited less overall masking and steeper recovery functions than did the older adults. Masked thresholds for younger participants approached unmasked thresholds for signal-masker delays greater than 6-8 ms. In contrast, older adults exhibited significant masking even at the longest delay tested (20 ms). In experiment 2, signal duration was decreased to 5 ms for a separate group of younger adults. Although overall thresholds were elevated for the shorter signal duration, the slope of the backward masking recovery function was not different from that observed for younger adults in experiment 1. The results suggest that age, independent of hearing loss, affects the temporal course of backward masking.

Auditory suppression and frequency selectivity in older and younger adults.

Age differences in auditory suppression were examined by comparing auditory-filter shapes obtained with simultaneous and forward masking at 2 kHz in young and elderly normal-hearing listeners. To compensate for the decay of forward masking, growth of masking functions were used to transform thresholds obtained with a notched-noise masker to the level of a continuous noise band that would give the same threshold values. Although both age groups exhibited smaller equivalent rectangular bandwidths (ERBs) when the filters derived from transformed thresholds were obtained with forward masking, the change from simultaneous to nonsimultaneous masking was significantly greater for young adults. Measures of the low- (Pl) and high- (Pu) frequency sides of the filters for young listeners indicated that the slopes of both sides increased following a change from simultaneous to forward masking but that the high-frequency side exhibited significantly greater sharpening. Filter slopes (both upper and lower) for older adults, in contrast, did not differ between the two masking procedures. The findings from the study are discussed as reflecting possible age differences in auditory suppression. However, it is also noted that conclusions regarding differences between filter shapes derived with simultaneous and forward masking are limited to filter parameters determined with transformed (as described above) thresholds.

Anisotropy of the transverse mode ultrasonic properties of fixed tendon and fixed myocardium.

This study investigates the influence of the fiber-reinforced nature of myocardium and tendon on the propagation of transverse mode ultrasonic waves. Formalin fixed specimens of normal human left ventricular cardiac muscle and bovine Achilles tendon were prepared for this study in such a way that transverse mode ultrasonic waves could be propagated perpendicular to the fiber axis of the tissue with the polarization oriented either parallel or perpendicular to the fiber axis. Measurements of velocity and attenuation were made at 3 MHz to assess the degree of anisotropy in these parameters for both tissues. Formalin fixed tendon exhibited a significant anisotropy whereas formalin fixed myocardium displayed a similar trend of more modest magnitude. Results of these measurements were used to compute two elastic stiffness coefficients for each tissue, yielding c44 = 37.2 MPa and c66 = 18.0 MPa for formalin fixed tendon, and c44 = 8.97 MPa and c66 = 8.45 MPa for formalin fixed myocardium. To validate this approach, additional studies were conducted to measure the transverse mode ultrasonic properties of silicone rubber and motor oil.