Aphasias

Aphasia is a neurological impairment of the human language system. Language includes several subsystems (speech sounds, grammatical structure, meaning) and is conveyed and understood through various input and output channels (spoken, written, gestured). In aphasia, each of these channels is affected to some degree, with difficulty ranging from minimal to severe. Minimal deficits include being slower to understand reading material, to get a joke in conversation, or to choose a precise word. Severe aphasia impairs even simple language tasks, such as matching a single printed word to the object it represents, or providing one's name and address.

Aphasia results from damage that is nearly always isolated to the left side of the brain. This damage interrupts the blood supply to the brain and starves the affected areas of the oxygen and glucose they need to survive. After several minutes, those brain regions die.

Aphasia is often divided into subtypes. These subtypes are distinguished roughly by relative strengths and weaknesses in language channels and subsystems. Different aphasia types also have been linked with different locations of damage within the left brain. Robert Wertz and colleagues summarize the controversy about the value and validity of these subtype distinctions. Specifically, within each aphasia type, there is a lot of individual variation in the presentation of strengths and [Page 60]weaknesses, and in the location of brain damage. Aphasia also has been described as nonfluent versus fluent, which refers to the average length of uninterrupted stretches of spoken language. Broca's, transcortical motor, and global aphasia are all nonfluent aphasias with relatively short stretches of uninterrupted output, and Wernicke's, conduction, anomic, and transcortical sensory are fluent, with essentially normal phrase length. Some practitioners refer to aphasia as expressive or receptive. It has been argued that these descriptors are neither valid nor informative because people with aphasia have both expressive and receptive components to their disorder, to some degree. Aphasia also is sometimes described as anterior versus posterior. These adjectives are not widely used because they describe a general location in the brain rather than the language characteristics of aphasia.

Aphasia is not a deficit of basic sensory systems—hearing, vision, touch—or of motor function. However, aphasia overwhelmingly affects older adults, so concurrent problems with hearing or vision are common. This entry discusses perception in aphasia, implications of research on perception in aphasia, and mirror neurons and perception of actions in aphasia.