Hemispheric asymmetry is the lack of functional balance, or symmetry, between the two brain hemispheres, which means that one is dominant, or is highly specialized with regard to speech production and comprehension as well as motor functions. The faculty of speech is unique to humans. Internal speech is the prerequisite for thought, just as the spoken word is the basis for communication and writing is the information transmitted over thousands of years. In the individual person, speech depends on the integrity of specific cortical areas that usually lie only in one hemisphere. This hemisphere is called the dominant hemisphere and is normally the left one in right-handed persons. In left-handed persons, it may be the right or the left hemisphere, or the faculty may be represented in both hemispheres. Thus, handedness is not a reliable indication for dominance of the contralateral hemisphere.
In the posterior region of the superior temporal gyrus of the dominant hemisphere lies Wernicke’s speech center. Injury to this area results in disturbed word comprehension (receptive aphasia, or sensory aphasia). It is an integration area that is indispensable for the continuous availability of learned word patterns and for the interpretation of heard or spoken words. Patients with sensory aphasia utter a senseless word salad (schizophasia), and the speech of other persons sounds to them like an incomprehensible foreign language. Injury to the angular gyrus, which borders on the supramarginal gyrus, results in the loss of the abilities to write (agraphia) and to read (alexia). Stimulation of adjacent areas, especially of the middle temporal gyrus, causes disturbance in spontaneous speech or writing. Broca ’s area for motor speech coordination lies in the inferior frontal gyrus.
Injury to the right (non-dominant) hemisphere may cause disturbance of the visual and spatial orientation or the appreciation of music (amusia). Although speech is preserved, the melody of the language and the emotional timbre of speech are affected. Different ways of thinking have often been assigned to one of the two hemispheres: the left, dominant hemisphere is thought to work in a logical, rational, and analytical way, while the right hemisphere is supposed to be integrative, synthetic, and intuitive. These generalizations are largely speculative.