![]() ![]() ![]() Some tests are strong indicators of the contralateral effect but the exceptions are indeed noteworthy. Though the contralateral effect is probably the most common finding in testing patients with unilateral brain damage, it is not always the case. It is likely all three of these factors and a variety of others contribute to the contralateral effect. 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. This in turn requires more processing – hence more fibers and therefore more fibers are damaged for a given lesion. Also, because the dominant system is the one that has more fibers it is likely it is handling more complex stimuli. 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. So in lesion situations there should be a opposite hemisphere effect -though albeit not as clear cut as in the other systems. ![]() It is estimated that there is a 5 to 1 ratio of contralateral to ipsilateral fibers in the auditory system (Musiek and Baran, 2007). Key to the contralateral effect in the auditory system was the fact that the majority of input fibers to the cortex were contralateral. 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. Unlike other systems the auditory system is not exclusively a crossed system, it has both contralateral and ipsilateral inputs to the cortex. So why wouldn’t this be true of the auditory system ? -Well it is -kind of……. 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 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. This finding has been well documented since. 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. 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. They then proceeded to develop more complex auditory tests that did seem to provide insight to the integrity of the higher (central) auditory system. They had learned that the pure tone audiogram was not useful in reflecting deficits of the central auditory nervous system (CANS). 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.
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