There are several methods for studying cerebral lateralization; four methods in the forefront are the study of unilateral lesions, sodium amytal, dichotic listening, and functional brain imaging. The traditional method for studying cerebral lateralization is unilateral lesions study, which is the study of the location and effect lesions have on the brain and body functionality. In past, this was primarily done after the subject was deceased or through highly invasive surgery.
The sodium amytal test uses an injection of sodium amytal into a carotid artery on the side of the neck that will put the same side hemisphere to sleep for a few minutes allowing doctors to test the other hemisphere for primary language center.
The process is then repeated for the opposite side of the hemisphere. This method is invasive, but provides a fairly accurate method to identify the hemisphere that is the primary language center. The dichotic listening test uses earphones to speak a different sequence, such as numbers, in each ear; the sequence with the most digits reported indicates the dominant language hemisphere.
For most people, the left hemisphere is the dominant one, meaning the subject would have repeated more sequences heard in the right ear. The dichotic listening test is non-invasive, and it is just as accurate an indicator of primary language center hemisphere as the sodium amytal test method. Functional brain imaging uses positron emission tomography (PET) or functional MRI to measure brain activity while the subject is performing an activity such as reading; typically, functional brain imaging used on language tests reveals more activity in the left hemisphere.
Functional brain imaging is the method that shows more detail in brain activity.