Aphasia and Brain Lesions

Aphasia and the Left Hemisphere

Historically the study of the biology of language has been informed through accidents of nature, cases of stroke or brain injury that result in aphasia. Aphasia is the loss of a person’s ability to produce or comprehend language following an acute brain injury. Aphasias do not affect a person’s ability to see, hear, or understand nonlinguistic information. They also do not affect a person’s ability to gesture: It is not uncommon for aphasic patients to use gestures in an attempt to communicate. It is therefore an interesting question regarding whether deaf signers who experience strokes or other brain injuries in comparable locations will lose the ability to use sign language while retaining the ability to gesture. Additionally, it is critical to understand whether the specific locations of brain injuries result in similar kinds of linguistic disturbances across sign and spoken languages.

Since the 1860s, it has been known that speech aphasias occur when left-hemisphere brain structures are damaged. Several reports have now documented cases in which, following damage to the left hemisphere, a deaf signer has completely or partially lost the ability to use sign language but has retained an ability to use pantomime and nonlinguistic gestures. To date, these studies have documented this pattern of dissociation in two distinct signed languages, American Sign Language (ASL) and British Sign Language (BSL). Patient “William,” a user of ASL with a large left-hemisphere lesion, often substituted pantomimes for signs even when communicating with his deaf-signing wife, who was clearly troubled by this sudden change in behavior. For example, William substituted a gesture of flying (arms held straight out with a rocking of the torso) instead of the one-handed ASL sign for airplane. Another patient with left-hemisphere injury, “Charles,” a user of BSL, showed a similar deficit. Charles was frequently unable to remember BSL signs for common objects, relying instead on gestures. For example, when asked to produce the BSL sign for bicycle, he pantomimed a bicycling motion. These case studies show that sign and gesture production can be highly dissociated, even when the signs and gestures in question are physically quite similar.

Cases of sign aphasia are highly comparable to the difficulties experienced by hearing persons. This indicates that, like speech use, sign language use is reliant on an intact left hemisphere and, moreover, is dissociable from visual perception and from the comprehension and production of nonlinguistic gestures of the arms and hands. This type of dissociation makes it clear that the left hemisphere is crucial in the mediation of complex language systems, whether spoken or signed, and provides additional evidence that signed languages are formal language systems that are distinct from more general nonlinguistic gestural abilities.

...