As audiologists, it is curious that we do not clinically utilize auditory discrimination measures. If you asked a group of psychoacousticians what are some of the key aspects of hearing, they would all agree that auditory discrimination is crucial to communication, second only to sound detection.
Some audiologists would say that speech discrimination tests are conducted every day in clinics across the country. The problem is that these are not really discrimination tests; they are speech recognition tests. While speech discrimination tests such as the Wepman are available and used by speech pathologists from time to time, they are seldom, if ever, used by audiologists. Without a doubt, a clinically viable test of auditory discrimination is and has been difficult to develop.
There is one major type of auditory discrimination that has always interested me—duration discrimination. Though more attention is directed to frequency and intensity discrimination in our field, the ability to discern differences in the duration of a sound is a rising point of interest in my educational curiosity.
ROLE OF AGE
One reason for this interest is that duration discrimination places a temporal demand on the auditory system. For practically any type of auditory discrimination, it is likely that both auditory and cognitive processes are involved.
Procedures to measure duration discrimination vary, but perhaps the most common approach is to require the listener to compare two successive stimuli, usually tones, based on whether they are the same or different in duration.
According to this approach, the difference limens (DL), which are similar to just-noticeable differences, were approximately 20-25 ms in young adults for 250-, 1,000-, and 4,000-Hz tones, each of which was 200 ms in duration ( J Acoust Soc Am 1987;81:427-438). Other studies have revealed slightly greater DLs, around 40-50 ms, for stimuli of similar length. Obviously, difference limens are longer when the baseline tone length is long and shorter when the baseline tone duration is short.
The Weber fraction, which simply divides the absolute DL by the baseline duration, allows for the comparison of difference limens for long and short stimuli. Very generally speaking, Weber fractions for normal-duration discrimination range from slightly over 0.1 to approximately 0.2. It should be noted that even in normal individuals there is notable variability for duration discrimination.
In children, duration discrimination improves with age ( Psychol Sci 1993;4:104-107). Comparisons between preschool-age children and adults reveal marked differences, and it appears that even 10-year-olds do not do quite as well as adults. Perhaps duration discrimination requires more time to reach adult values than either intensity or frequency discrimination does. The variability of difference limens for children is high even if they are the same age, probably reflecting variance in the maturation of the central auditory system in the young. As each year goes by, the DL for children becomes smaller by hundreds of milliseconds.
On the other hand, difference limens for duration discrimination are greater in elderly subjects compared with younger individuals ( J Speech Hear Res 1994;37:662-670), even reaching about a twofold difference in some cases.
Interestingly, in both elderly and young subjects, hearing loss seems to have little influence on duration discrimination performance. Therefore, it is not surprising that the poorer performance in older subjects was attributed to physiological compromise of the central auditory system. The possible relationship between high-order auditory dysfunction and the elderly's decreasing ability to discern small changes in acoustic duration may have clinical implications; the use of this discrimination test as a measure of central auditory integrity looms as a possibility.
LACK OF INFORMATION
As with most topics in duration discrimination, there is a paucity of information on this procedure as an index of central auditory function. Early studies used animals and behavioral conditioning approaches. Duration discrimination was measured prior to ablation of the auditory pathway.
Notably, in a number of these studies, the animals could not relearn the discrimination task, even though the lesions were isolated to the auditory pathway. This finding raises questions as to whether the animal truly was unable to learn the task or the auditory function for the task was so poor that the animal could not discriminate within the limits of the test procedure. The lack of follow-up of these early studies limits the extent to which we can answer this pertinent question.
In cases where the animals could relearn the task after ablation, they performed poorly with respect to duration discrimination ( J Neurophysiol 1965;28:673-681). The ability to relearn the task was, in some instances, related to the particular locus of the lesion and the kind of task for which duration was manipulated, again implicating the central auditory system as critical to the process.
While duration discrimination tasks have seldom been employed in humans who have central auditory pathology, there are notable exceptions, with a key study in the mid-1990s revealing some interesting findings ( Scand Audiol Suppl 1992;35:3-15). Adults who had left hemisphere lesions showed markedly increased difference limens of about 200 ms compared with normal controls, who had DLs of about 50 ms. Difference limens also were increased in those with right hemisphere involvement, but not as much; they were about 100 ms in this group. As the baseline duration of the tone decreased, the performance of patients and controls grew worse, but the effect was greater for those with brain lesions.
This study by Thompson and Abel highlights several important factors related to duration discrimination. Clearly, duration discrimination tasks are difficult for those with temporal lobe damage. It appears there also may be a lateralization aspect to duration discrimination that manifests in neurological conditions. Further research in this area is necessary to determine the diagnostic value of this rather simple procedure (S. White: “Duration Discrimination in Normal and Abnormal Systems”; unpublished review, 2002).
Certainly we all know the major influence duration discrimination has on so many auditory processes, especially those that are temporal in nature. Perhaps we should take a closer look at this fundamental auditory process and see how it may affect the future of audiology.
References on Tap
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