Annette Hurley, PhD., CCC-A, Dept. of Communication Disorders LSU Health Sciences Center, [email protected]
Dichotic listening tests are one of the most frequently used behavioral tests included in the behavioral Auditory Processing disorder (APD) test battery (Emanuel et al, 2011). Dichotic tests are sensitive to the maturation and development of the central auditory nervous system (CANS), underlying lesions of the CANS, and inter-hemispheric transfer of auditory information (Musiek, 1983; Musiek et al, 1984). Some abnormal unilateral findings or specific ear deficits or weaknesses are associated with various language- based learning disorders (Moncrief & Musiek, 2002).
The Dichotic Digits Test (Musiek, 1983) is a widely used test in the APD test battery. During this test, two numbers are presented to the right ear at the same time as two numbers are presented to the left ear. The listener is instructed to repeat all four numbers in any order. This test assesses the ability of the auditory system to integrate information from the right and left cerebral hemispheres.
Interpretation of this test is most often based upon the percentage correct for each individual ear in comparison to normative data. This normative data is provided in the Dichotic Digits Test Manual (Musiek, 1983) and reflects age cut-off scores, which is the mean age minus 2 standard deviations (SD). These cut-off values are shown in Table 1. Notice the increase in the performance of each ear as a function of age.
Table 1. Age cut-off scores for the Dichotic Digits Test
Another less common way to interpret a dichotic listening test is the Dichotic Difference Score (DDS) (Musiek, 2018). The DDS is a simple calculation of the individual right ear score minus the left ear score. This computation may be useful in assessing unusual auditory weaknesses or asymmetries in one ear, not explained by age. This calculation may be referred to as the right or left ear advantage or the Standard Integration Ratio (Katz, 2015).
The age-predicted DDS calculated from the normative data provided by the Dichotic Digits Test are shown in Table 2. Rosenburg (2011) reported a Mean Ear Difference Score, or DDS, in her published normative data from 200 children between the ages of referred to the Sarasota Florida School District for an audiological evaluation. Data for each age group are also provided in Table 2. These values are very similar to those from the Dichotic Digits Test and may allow audiologists to have greater confidence in interpretation of ear differences on the Dichotic Digits Test.
Table 2. DDS values from the Dichotic Digits Test as provided in the manual and from normative data from Rosenburg (2011)
|Age||Dichotic Digits Test
An abnormal DDS may occur when there is a unilateral weakness. For example, a 7 year old may score 44% and 78% for the left and right ears, respectively. A left ear weakness is noted, after comparing the individual ear scores to normative data (Table 1). However, there is an abnormal DDS of 34% DDS, greater than the predicted 15%, as evidenced in Table 2.
An abnormal DDS may also occur when one ear performs much better than expected. For example, a 7 year old may score 60% and 98% for the left and right ears, respectively. Each individual ear is within normal limits; however, the DDS is a 32% and would be interpreted as abnormal. An abnormality of the DDS is auditory driven and is not related to high-order processes of cognition, such as memory and attention. These would affect scores from both ears, and not be ear-specific.
Unfortunately, there is little published normative data for interaural differences and sometimes the published papers will report, “…an abnormally large asymmetry or significant ear deficit,” but sometimes fail to define the criterion. Moncrief & Wertz (2008) did note “A significant asymmetry was defined as a difference of greater than 20% for children younger than eight years, 15% for children eight to nine years and greater than 10% for children ten years and older (p 86).” Moncrief has used the DDS to make a diagnosis of amblyaudia.
We were curious if the using the DDS would provide additional information from our former patients who were referred for auditory processing disorder (APD) assessment.
We reviewed the APD test battery results of over 200 patients, age 7-15, who were referred to our clinic for an APD assessment. The LSUHSC Institutional Review Board approved this retrospective chart review. We excluded patients who reported they were left-handed and any patient with a known brain lesion such as cerebral vascular accident, Landau Kleffner Syndrome, hemispherectormy, encephalitis, or a diagnosis of Autism Spectrum Disorder. The total number of participant test results was 192. The number of test results reviewed for each age group is shown in Table 3.
An initial analysis compared individual ear results to published normative data, comparing the individual ear scores. This resulted in 106 ‘normal’ and 86 ‘abnormal’ patients.
Second, we then utilized the DDS to review the 106 results. We set an abnormal DDS criterion to > 15% for age 7 years and >10% for children 8 years and older. The use of the DDS resulted in an additional 15 test results interpreted as “abnormal.” Overall, there was a total of 91 “normal” and 101 “abnormal” test results on the Dichotic Digits Test.
Table 3. Number of participants that were normal, abnormal using only the percentage correct and abnormal after employing the DDS.
|Age||Number of Participants||Initial
Individual Ear Score
|2ndanalysis: Individual Ear & DDs
Clinicians and investigators are always encouraged to collect their own normative data to ensure data this reflects the population they are working. Our mean- 2 SD for each age group of our “normal” results are shown in Table 4. Also included in this table are the cut-off scores from the from the Dichotic Digits Test. Our normative data are in very close agreement to the normative data accompanying the Dichotic Digits Test.
Table 4. Normative cut-off scores from this investigation compared to the Dichotic Digit Test.
|Normative Data||Hurley et al||Normative Data||Hurley et al|
The use of the DDS may help provide additional information about children at risk for CAPD and may be a risk factor for a language-based learning disability. Fifteen test results, initially classified as “normal” were re-classified as abnormal after employing the DDS as a criterion for interpretation. We reviewed other chart information and other results from the APD battery. We found that fourteen patients were currently enrolled in speech-language therapy, so we know there is a language disorder. Six of these fourteen had a diagnosis of dyslexia. Six of the 15 patients were identified as having APD by additional tests in the behavioral APD battery. We strongly encourage the use of the DDS as an additional method to interpret dichotic listening tests.
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- Rosenberg, G.G. (2011). Development of local child norms for the dichotic digits test. Journal of Educational Audiology, 17, 6-10.