What we know of the physiology of language: through observing effects of brain lesion"s on people"s speech. Interruption of blood flow deprives a region of its blood which causes its cells to die. Can get info about language processes from normal patients. Most common category of speech disorder is a p h a s i a : difficulty in producing or comprehending speech not produced by deafness or a simple motor deficit; caused by brain damage. In order to be diagnosed, patient must have difficulty comprehending, repeating or producing speech. Patient must also be aware of what is happening in their environment and to understand that others are trying to communicate with them. Lateralized function: most language disturbances occur after damage to the left side of the brain. Knecht assessed relationship between lateralization of speech and handedness in people who didn"t have brain damage.