Back in the 1950’s Ettore Bocca and his Italian colleagues (Bocca, Calearo, Cassinari, 1954) were the first to develop and make significant clinical use of central auditory tests. They had learned that the pure tone audiogram was not useful in reflecting deficits of the central auditory nervous system (CANS). They then proceeded to develop more complex auditory tests that did seem to provide insight to the integrity of the higher (central) auditory system. It was well known and indeed logical at that time to recognize that the pure tone audiogram reflected loss of hearing sensitivity for the involved ear or an “ipsilateral” effect. Counter to this, Bocca and colleagues realized after testing patients with lesions limited to one hemisphere that the ear contralateral to the involved hemisphere was the one that usually demonstrated the deficit on their central auditory tests. This finding has been well documented since. So let us take a closer (but still introductory) view of this now well known contralateral effect in reference to mechanisms, test findings and some related issues.
Why a contralateral ear effect ?
So why do we see the contralateral ear effect (contralateral effect) on central auditory tests? It is well known in in neurology that if damage is incurred in the motor, somatosensory, visual and olfactory systems on one side of the brain, the effect is on the opposite or contralateral side. So why wouldn’t this be true of the auditory system ? —Well it is —kind of……. Unlike other systems the auditory system is not exclusively a crossed system, it has both contralateral and ipsilateral inputs to the cortex. This fact made many investigators and clinicians in the early days ponder (and possibly doubt) about the possibility of a contralateral effect —until Bocca’s work. Key to the contralateral effect in the auditory system was the fact that the majority of input fibers to the cortex were contralateral. It is estimated that there is a 5 to 1 ratio of contralateral to ipsilateral fibers in the auditory system (Musiek and Baran, 2007). So in lesion situations there should be a opposite hemisphere effect —though albeit not as clear cut as in the other systems.
The rationale for the contralateral effect exists because there are more fibers in the contra rather than ipsilateral system, therefore a lesion at a given location and size, on the contralateral side should yield more of an effect. Also, because the dominant system is the one that has more fibers it is likely it is handling more complex stimuli. This in turn requires more processing – hence more fibers and therefore more fibers are damaged for a given lesion. Another notion is that in the contralateral system there is likely a denser array of fibers for a given area—hence a lesion creates more damage per area contralaterally than ipsilaterally. It is likely all three of these factors and a variety of others contribute to the contralateral effect. Though the contralateral effect is probably the most common finding in testing patients with unilateral brain damage, it is not always the case. Some tests are strong indicators of the contralateral effect but the exceptions are indeed noteworthy.
Central Auditory Tests
Probably the procedure that yields the strongest contralateral effect in patients with compromise of one auditory hemisphere is dichotic listening (Musiek and Pinheiro, 1985; Musiek and Chermak, 2014)). In general, these procedures are highly sensitive to lesions and related dysfunction of the auditory cortex. This is probably why large deficits for right versus left ears have often been reported. However, when the corpus callosum is involved the mechanism of the contralateral effect can be altered (discussed later). Low redundancy speech tests, localization and lateralization procedures all show the contralateral effect in patients with unilateral CANS damage but in our view, the dichotic procedures seem most sensitive. Perhaps the most notable test(s) that don’t show the contralateral effect is the frequency and duration pattern tests. It seems most of the time that even though only one cortex is damaged, pattern tests yield bilateral deficits rather than only a contralateral effect. Our previous research has demonstrated mean data that is essentially the same for each ear in auditory cortex lesions limited to one side (Musiek, Pinheiro, Baran, 1987). However, this same research showed some cases where there was a contralateral effect. Why this is we are not sure and theories about it, though interesting and well founded is beyond the scope of this paper (see Musiek, Pinheiro and Wilson, 1980). In terms of electrophysiologic tests middle and late auditory evoked potentials (AEPs) can show a contralateral ear effect but it is a weak finding. In our experience contralateral effects in the middle and late AEPs occur about as often as they don’t occur ! However, middle and late AEPs can demonstrate a difference in amplitude if one compares the results from the electrode over the lesioned hemisphere to the non lesioned hemisphere (see Musiek, F., Charette, L., Kelly, T., Lee, W. & Musiek, E.,1999).
Of interest, are a number of patients that we have seen over the years that report hearing difficulty in the ear contralateral to the neurologically compromised hemisphere. Though they report this hearing deficit, the pure tone audiogram is many of these cases is normal and symmetrical. This clinical situation was best illustrated a case we reported recently of a post auditory cortex stroke (Musiek, Guenette, & Fitzgerald, 2013).
Many other factors can serve to negate or lessen the contralateral effect but we can only mention a few. Most one sided, low brainstem lesions will demonstrate an ipsilateral rather than contralateral effect on central auditory tests (Baran and Musiek, 1999). This trend is related to most ascending fibers at the low brainstem have not yet crossed to the other side, hence a large portion of these fibers are ipsilateral not contralateral. Another factor is whether or not the corpus callosum is involved (Musiek and Pinheiro, 1985. If the corpus callosum is involved, dichotic listening tests will show a left ear deficit but most other tests will be unaffected except patterns which will reflect a bilateral deficit. The callosal fibers course well into the left hemisphere therefore the lesion can be in the left hemisphere yet yield an ipsilateral left ear deficit for dichotic tests. This is related to left ear information, which is projected to the right hemisphere cannot reach the left hemisphere for a verbal response due to callosal damage. This creates a marked left (ipsilateral) ear deficit – even though the lesion is localized in the left hemisphere. This is termed the “paradoxical left ear deficit”. Another factor, that may alter the contralateral effect is long standing damage to one hemisphere or brainstem pathway – especially from early childhood. This may influence the 5 : 1 contra to ipsilateral nerve fiber ratio which in turn can alter the contralateral effect.
Contralateral deficits on central auditory tests in patients with compromise of the CANS are a long standing and valuable marker of central auditory dysfunction. It’s basis is an anatomical one which places it on solid footing, however much remains unknown about the details of this effect. Not all central auditory tests yield a contralateral effect and certain factors can also mitigate this effect and these are important to understand.
Grasping the fundamentals of the contralateral effect will lead to better interpretation of tests results and the planning of intervention techniques.
- Bocca, E., Calearo, C., Cassinari, V. (1954) A new method for testing hearing in temporal lobe tumors; A preliminary report. Acta Oto-laryngol. 44: 219
- Musiek, F.E., Pinheiro, M.L. & Wilson, D. (1980). Auditory Pattern Perception in Split-Brain Patients. Arch Otolaryngol. Head and Neck Surg., 106, 610-612, October.
- Musiek, F.E. & Pinheiro, M.L. (1985). Dichotic Speech Tests in the Detection of Central Auditory dysfunction. In M.L. Pinheiro and F.E. Musiek (Eds.), Assessment of Central Auditory Dysfunction: Foundations and Clinical Correlates, (pp. 201-218). Baltimore, MD: Williams and Wilkins.
- Musiek, F.E. & Pinheiro, M.L. (1987). Frequency Patterns in Cochlear, Brainstem, and Cerebral Lesions. Audiology, 26, 79-88.
- Baran, J. & Musiek, F. (1999). Behavioral Assessment of the Central Auditory System. In F. Musiek and W. Rintelmann (Eds.), Contemporary Perspectives on Hearing Assessment, (pp. 375-415). Boston, MA: Allyn & Bacon.
- Musiek, F., Charette, L., Kelly, T., Lee, W. & Musiek, E. (1999). Hit and False-Positive Rates for the Middle Latency Response in Patients with Central Nervous System Involvement. Journal Amer. Acad. Audiology, 10(3), 124-132.
- Musiek, F.E. & Baran, J.A. (2007). The Auditory System, Anatomy, Physiology and Clinical Correlates. Boston, MA: Allyn & Bacon.
- Musiek, F., & Chermak, G. (2014) 2nd Edition Handbook of Central Auditory Processing Disorder, Vol. I: Auditory Neuroscience and Diagnosis. San Diego, CA. Plural Publishing.
- Musiek, F., Guenette, L., & Fitzgerald, K. (2013). Lateralized Auditory Symptoms in Central Neuroaudiology Disorder. Journal of the American Academy of Audiology, 24(7), 556-563.