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As part of an ongoing study of auditory aging, detection of sinusoidal and quasitrapezoidal frequency modulation (FM) was measured with a 5-Hz modulation frequency and 500- and 4000-Hz carriers in two experiments. In Experiment 1, psychometric functions for FM detection were measured with several modulation waveform time patterns in younger adults with normal hearing. Detection of a three-cycle modulated signal improved when its duration was extended by a preceding unmodulated cycle, an effect similar to adding a modulated cycle. In Experiment 2, FM detection was measured for younger and older adults with normal hearing using two psychophysical methods. Similar to frequency discrimination, FM detection was poorer in older than younger subjects and age-related differences were larger at 500 Hz than at 4000 Hz, suggesting that FM detection with low modulation frequencies and frequency discrimination may share common underlying mechanisms. One mechanism is likely related to temporal information coded by neural phase locking which is strong at low frequencies and decreases with increasing frequency, as observed in animals. The frequency-dependent aging effect suggests that this temporal mechanism may be affected by age. The effect of psychophysical method was sizable and frequency dependent, whereas the effect of modulation waveform was minimal.

Psychometric functions for gap detection of temporal gaps in wideband noise were measured in a "yes/no" paradigm from normal-hearing young and aged subjects with closely matched audiograms. The effects of noise-burst duration, gap location, and uncertainty of gap location were tested. A typical psychometric function obtained in this study featured a steep slope, which was independent of most experimental conditions as well as age. However, gap thresholds were generally improved with increasing duration of the noise burst for both young and aged subjects. Gap location and uncertainty had no significant effects on the thresholds for the young subjects. For the aged subjects, whenever the gap was sufficiently away from the onset or offset of the noise burst, detectability was robust despite uncertainty about the gap location. Significant differences between young and aged subjects could be observed only when the gap was very close to the signal onset and offset.

Although a psychometric function describing a subject's responses to some physical stimuli is of considerable value, characterizing such functions is time consuming and, hence, is not carried out routinely in psychophysical experiments. A principal reason for the lack of efficiency in characterizing a psychometric function is the use of sampling methods that either converge on a single point on the psychometric function, such as the PEST method, or which distribute observations uniformly over a wide range, such as the constant stimuli method. As an alternative, a multimodal four-point sampling method has been proposed [C. F. Lam, J. H. Mills, and J. R. Dubno, J. Acoust. Soc. Am. 99, 3689-3693 (1996)]. A psychometric function is then fitted to the four points (each with several trials) to estimate the threshold and slope parameters of the psychometric function. Adaptive methods, such as the up-down methods [H. Levitt, J. Acoust. Soc. Am. 49, 467-477 (1971)], can be used to provide good initial estimates of the threshold and spread parameters of a psychometric function described by a logistic function. In ongoing studies of age-related changes in auditory masking and discrimination, this new four-point sampling method has been applied to determine psychometric functions for absolute thresholds as a function of duration, thresholds in simultaneous and forward masking, frequency discrimination, and intensity discrimination in both young and aged human subjects. Results indicate that a reduction in data collection time of about 50% with no increase in variance can be achieved. This increase in efficiency applies to simple detection tasks by normal hearing subjects as well as to complex discrimination tasks by older subjects with hearing loss.

The fine frequency structure of the 2f1-f2 acoustic distortion product (ADP fine structure) was examined in ten human subjects with normal hearing. Primary frequencies (f1 and f2) were incremented in steps of 1/32 octave with an f2/f1 ratio of 1.2. The primary levels were kept equal to each other and varied from 45 to 65 dB SPL in 2.5-dB steps. The results show that the ADP fine structure is characterized by a series of peaks and valleys across frequency, with a peak-to-peak frequency spacing of about 3/32 octave and a peak-to-valley amplitude ratio of up to 20 dB. At frequencies below 4000 Hz, as primary level increases, the sharpness of the ADP fine structure is not significantly reduced and the pattern gradually shifts to lower frequencies. At frequencies above 4000 Hz, a flattening of the pattern is sometimes observed at high levels. A consequence of the underlying process responsible for the fine structure is that ADP input/output (I/O) functions can be highly variable in shape. Dramatic shape changes can occur for ADP I/O functions obtained with primary frequency changes of as little as 1/32nd of an octave. The outward cause of I/O function variability is the behavior of the ADP fine structure with level; i.e., it remains robust at high levels and systematically shifts to lower frequencies with level. As a result, ADP peaks can shift to valleys with increasing level and vice versa. Thus, small shifts in primary frequency can result in significant changes in the shape of the ADP I/O function in humans.