Aphasia is an inability to comprehend and formulate language because of damage to specific brain regions. This damage is typically caused by a cerebral vascular accident (stroke), or head trauma, however these are not the only possible causes. To be diagnosed with aphasia, a person's speech or language must be significantly impaired in one (or several) of the four communication modalities following acquired brain injury or have significant decline over a short time period (progressive aphasia). The four communication modalities are auditory comprehension, verbal expression, reading and writing, and functional communication.