A common finding in children with auditory processing disorder (APD) is a left-ear deficit on dichotic speech tasks. A 1998 study demonstrated significant improvement in binaural listening tasks and academic performance when dichotic training tasks were incorporated into the auditory training program.1 Pre-training, the listener demonstrated a unilateral deficit on dichotic digits tests and moderate bilateral deficits on compressed speech with reverberation tests.
The training technique for dichotic listening directs the stimuli to the stronger ear at a reduced level while maintaining a higher intensity level to the poorer ear. This allows good identification of the stimulus directed to the weaker ear. This auditory training procedure is called dichotic interaural intensity difference training or DIID, and is based on our split-brain research conducted in the late 1970s and the 1980s.
Split-brain subjects have no transfer from the right to left hemisphere due to sectioning of the corpus callosum. In dichotic listening there is suppression of the ipsilateral pathways, hence the auditory system becomes a crossed system. This means that (during dichotic listening) right-ear information goes to the left hemisphere and left-ear information to the right hemisphere. Since most people are language-dominant for their left hemisphere, they can easily repeat words, numbers, and sentences presented to their right ear. However, stimuli presented to the left ear must cross the corpus callosum to access the speech hemisphere and draw a verbal response. If the corpus callosum is severed, this cannot happen and a severe left-ear deficit on dichotic listening occurs.
Interestingly, shortly after testing several split-brain patients, we discovered that major left-ear deficits also occur in children, especially those with learning disabilities.2 This is probably due to delayed myelination of the corpus callosum. Many children we evaluated for APD demonstrated severe left-ear deficits on dichotic listening, similar to those of the split-brain patients.
AFTER TRAINING, PERFORMANCE IMPROVED
We found that if we presented dichotic stimuli with greater intensity to the left than the right ear, the scores for the left ear increased and scores for the right ear decreased for both split-brain and children with left-ear deficits.3 We also found that if we trained children on various kinds of dichotic stimuli using greater intensity to the left ear to maintain good performance in that ear, the intensity level of the stronger ear could be gradually raised over time and the poorer ear (usually the left) would maintain its high performance. Some children required more training than others, but a high percentage did improve their left-ear performance. Often after training, both ears would perform well on subsequent dichotic listening tests.
Though the mechanisms underlying this “dichotic improvement” are currently being studied, it appears that greater intensity to the left releases the left auditory pathway from right auditory pathway suppression. Our ongoing research, as well as studies by collaborators Deb Moncrieff and Diane Wertz, has been highly promising for improving left-ear deficits. A recent article on a patient with mild head trauma who demonstrated a left-ear deficit on dichotic digits showed marked improvement on dichotic listening after DIID training.4 Therefore, it seems possible that the effectiveness of the DIID procedure is not limited to children with learning difficulties. We continue to study the clinical feasibility of DIID training.
The testing and set up for DIID are straightforward. Various dichotic test materials and a two-channel audiometer are essentially all that is needed. We have even set up the training procedure for schools and at home using modifications of a stereo system and interchangeable earphones. This informal approach has worked, but bear in mind that the stimuli cannot be controlled as well as in the formal situation.4
This procedure can also be done by using temporal offsets that “lag” in the poorer ear. The lag allows the poorer ear to perform better. Then, by use of adaptive techniques, the offset differentials are reduced over multiple practice sessions. This allows the improved function of the good ear to stabilize back to normal and maintain the improvement of the weak ear.
Both binaural integration and separation tasks can be used in DIID training. The integration task requires the patient to respond to the stimuli in both ears. In the separation tasks, usually the patient is asked to ignore one ear and respond to stimuli presented in the other ear. If a child shows more difficulty with binaural integration than separation, DIID training should emphasize this aspect of dichotic listening and vice-versa. How much training is required depends on how well the child performs on day-to-day therapy. However, we generally suggest three or four training sessions a week of 20 to 30 minutes apiece. Further research may provide more direction in regard to this aspect of DIID training.
1. Musiek F, Schochat E: Auditory training and central auditory processing disorders. Sem Hear
2. Musiek FE, Gollegly K, Baran J: Myelination of the corpus callosum and auditory processing problems in children: Theoretical and clinical correlates. Sem Hear
3. Musiek F, Shinn J, Hare C: Plasticity, auditory training, and auditory processing disorders. Sem Hear
4. Musiek F, Baran J, Shinn J: Assessment and remediation of an auditory processing disorder associated with head trauma. JAAA