The Dichotic Digits Test (DDT) is a central auditory test that assesses binaural integration skills. This is the ability of the listener to process different information being presented to each ear at the same time. Individuals with difficulties in binaural integration often show a significant ear deficit, usually the left ear. These persons may have difficulty hearing in background noise or when more than one person is speaking.
The DDT is a dichotic speech test with a relatively light linguistic load using a non-directed response method. That is, the patient repeats what is heard in both ears. It is commonly administered as part of an auditory processing test battery, and is also used to assess these skills in the elderly population, which can provide valuable information in fitting hearing aids. The test requires relatively little time to administer and score and is easy for the patient to understand and complete.
Administering the DDT requires a two-channel audiometer. The channels must be calibrated separately, and the clinician must be sure the designation of channel to ear is accurate. One can do this by turning down the monitor for one channel and checking that the channel being monitored is directed to the appropriate ear. Mark the score sheet to indicate the ear being tested.
The test is usually presented at 50 dB SL in reference to the spondee threshold. The reference can also be the pure-tone average or the 1000-Hz threshold, but the spondee threshold is preferred. For people with hearing loss, the clinician may have to present at the most comfortable loudness level. The test has been found to be relatively resistant to at least mild peripheral cochlear hearing losses, especially high-frequency losses. In cases of peripheral hearing loss, it may be impossible to present the test at 50 dB SL. The DDT reaches maximum performance around 20 dB SL in regard to hearing thresholds at 1000 Hz. We have shown no difference in performance of normals at either 20 dB SL or 50 dB SL. Hence it is not critical to present the test at 50 dB SL. The important thing to remember is that the SL must be the same in each ear.
HOW IT'S CONDUCTED
The patient should be instructed as follows: “You will be hearing two numbers in each of your ears. Listen carefully in both ears and repeat all the numbers you hear. The order doesn't matter. If you are unsure of the numbers, please guess. The first few items will be for practice.” (You may also provide oral examples of the numbers, i.e., in the right ear you may hear 2, 7, and in the left ear 5, 3.)
Give the patient ample time to respond, even though that may require pausing the CD occasionally. The established norms allowed patients as much time as they wished to respond. If a patient cannot respond orally, he/she can write the digits or point to them on a response board.
If the patient is unable to repeat four digits, a version of the test is available that presents only one digit to each ear. This can be used as practice before completing the DDT, or it may be administered to determine if a significant ear difference is present. Remember, however, the norms are based on the double-digit version (two numbers to each ear).
The first three sets of digits in the test are for practice and are not included in the scoring. The test consists of 20 stimulus presentations, or 80 total digits (40 per ear). To score, count the number of correctly repeated digits for each ear separately, and divide by the number presented to each ear (40 items to each ear). Another way is to count the correct number repeated and multiply that by 2.5.
I strongly recommend that you collect your own norms in your own area. Here are the norms we use presented as a guide:
ACD is available with four randomized versions of the DDT. These tests can be used for research, additional testing, to determine the crossover point (the dB levels required for the same performance in each ear), and for follow-up testing following any formal or informal therapy.
The DDT may also be useful in amplification assessment. A study by Strousse Carter et al., suggested including the DDT in cases of symmetric hearing loss, since it has been found that listeners with an auditory-based deficit in dichotic listening may function better with a monaural hearing aid fitting or with the use of an ALD device such as an FM system.
Musiek F: Assessment of central auditory dysfunction: The Dichotic Digit Test revisited. Ear Hear
Musiek F, Gollegly K, Kibbe K, Verkest-Lenz S: Proposed screening test for central auditory disorders: Follow-up on the Dichotic Digits Test. Am J Otol
Strousse Carter A, Noe C, Wilson R: Listeners who prefer monaural to binaural hearing aids. JAAA 2001;12(5):261–272.