Frequency discrimination is a fundamental auditory process underlying more complex auditory tasks, such as speech comprehension and understanding. Also referred to occasionally as pitch perception, frequency discrimination is typically measured by comparing a stimulus tone to a reference tone in order to determine the minimum difference in Hz that the listener requires to differentiate the two tones.
This minimum difference is referred to as “a difference limen for frequency.” In normal-hearing individuals, difference limens vary by reference frequency; a listener may be able to distinguish between 1000 and 1004 Hz, but not 4000 and 4004 Hz. The Weber fraction is a ratio used to equate difference limens across reference frequencies, and may have a value as low as 0.002 for the mid-frequencies (600–2000 Hz) in normal listeners at moderate intensities.
Frequency discrimination has long been studied in normal-hearing individuals, as well as more recently in persons with peripheral hearing loss. Studies have consistently shown that peripheral auditory pathology may lead to increased frequency difference limens and abnormal values for the Weber fraction.1
Additionally, animal studies have been conducted to assess the influence of the central auditory nervous system on frequency discrimination. However, few studies have examined the effect of central auditory disorder on frequency discrimination in humans, and their findings have been somewhat equivocal. A review of the results of major human studies as well as discussion of possible explanations for the divergent results follows. (See Musiek, Weihing, and Shinn for a more complete review of the literature.2)
A REVIEW OF STUDY FINDINGS
Several studies have examined the effect that unilateral pathology of the central auditory nervous system has on frequency discrimination by comparing frequency discrimination scores or frequency difference limens of left and/or right hemisphere lesion patients to those of normal controls.
Results from two studies revealed significantly poorer frequency discrimination in patients with right hemisphere injury than in a control group or in a left hemisphere injury group. The scores of patients with left hemisphere injury were not significantly different from those of the control group. However, a similar study, conducted by Thompson and Abel, showed opposite results.3 The left hemisphere group showed abnormal scores that were different from those of the control and of the right hemisphere group, and the right hemisphere group scores were much closer to those of the controls. In addition, several published studies have shown no deficit in performance of frequency discrimination in patients with unilateral cortical lesions.
Other studies have examined frequency discrimination in persons with unilateral temporal lobe excisions, and found that performance seems to be weakest in subjects with excisions infringing on Heschl's gyrus in the right hemisphere. It should be noted, however, that the deficits in performance in these studies were noted on tasks of frequency discrimination that were more complex than a simple same/different choice.
Taken together, the above studies seem to show conflicting results. There may be several reasons for this. First, the studies used different tasks of frequency discrimination, and indeed some studies using tasks of multiple complexities demonstrated that deficits may become apparent only when the task is sufficiently complex. For example, Johnsrude, Penhune, and Zattore used two tasks to measure frequency discrimination in their neurological patient group, a task that required the patients to judge if two tones were the same or different, and one that required labeling the second tone as higher or lower than the first tone.4 Results from that study showed that patients with damage to the right Heschl's gyrus performed more poorly than controls on only the second, more complex task.
Second, examining frequency discrimination by ear varied among studies; that is, sound field or binaural presentation may yield different results from monaural stimulation of each ear. Third, peripheral hearing status was not always measured in these patients, and when it was measured may have revealed peripheral hearing loss. This is particularly significant given the discussion above of peripheral pathology and its effect on frequency discrimination. Overall, however, it does appear that pathology of the central auditory nervous system, especially cortical pathology, can affect frequency discrimination, particularly if the damage occurs in the right hemisphere.
1. Moore BCJ: Perceptual consequences of cochlear hearing loss and their implications for the design of hearing aids. Ear Hear
2. Musiek FE, Weihing JW, Shinn JB: Auditory neuroscience: Clinical trends relative to audiology. In Ingham R, ed., Neuroimaging in Communication Sciences and Disorders.
San Diego: Plural Publishing, 2008: 193–332.
3. Thompson M, Abel S: Indices of hearing in patients with central auditory pathology. Scand Audiol Suppl.
4. Johnsrude I, Penhune V, Zattore R: Functional specificity in the right human auditory cortex for perceiving pitch perception. Brain