Lecture 10: Aphasia

Asymmetry of Brain Representation

The brain is not symmetrical: in approximately 97% of people language functions are located in the left hemisphere. A number of sources of information led to this view: the correlation between left hemisphere damage and aphasia, and the evidence from studies of "split-brain" patients. The psychologists Roger Sperry and Michael Gazzaniga extensively studied the cognitive abilities of patients whose corpus callosum was severed to control otherwise intractable epilepsy. Due to the course of the visual pathways in the brain it is possible to visual information to only one hemisphere at a time (see diagram below). If a word is flashed to the right hemisphere and the subject is asked to report what she saw, she answers "nothing", but if she is asked to use her left hand to feel under a cloth for the object seen, she can pick out the object described by the word. In contrast, when a word is flashed to the left hemisphere, a split brain patient can recognize, comprehend, and repeat it.
Information in the right visual field goes to the left hemisphere and information in the left visual field to the right hemisphere.

The 19th Century Conception of Brain Representation of Language

The findings of Paul Broca in 1864 and Carl Wernicke in 1874 led to an eponymous classification of aphasias and a characterization of a simple brain circuit for the processing of language. Broca's area is an area no greater than 1-inch across that lies below the part of motor cortex that represents the mouth and tongue. Put your finger to your head just above your left temple and you will be touching the most probable site of your Broca's area. The effects of damage to this region, as observed by Broca in his patient "tan", are slow, laborious speech that lacks ordinary rhythm and intonation. The patient's speech is agrammatical: little grammatical words like the to and be are dropped (although he or she can produce homophones such as bee and too), grammatical markings indicating tense and number are dropped from the ends of words, and hardly any verbs are used. Here's an example of a Broca's aphasic, B.L., descibing what is happening in a picture of three people in a kitchen.
B.L.: Wife is dry dishes. Water down! Oh boy! Okay Awright. Okay ...Cookie is down...fall, and girl, okay, girl...boy...um...
Examiner: What is the boy doing?
B.L.:Cookie is...um...catch
Examiner: Who is getting the cookies?
B.L.: Girl, girl
Examiner: Who is about to fall down?
B.L.: Boy...fall down!
The "cookie theft" picture from the Boston Diagnostic Aphasia Examination
What is said usually makes sense, but it is very compressed or telegraphic in style. Speech comprehension is usually good, unless the sentences are grammatically complex. Broca's aphasics are often painfully aware that they are having language difficulties. This level of self awareness means that they often respond well to treatment.

Wernicke's area is located in the temporal lobe behind the primary auditory cortex. Put your finger just above and slightly behind your left ear to touch the probable location of your Wernicke's area. Patients with Wernicke's or fluent aphasia produce rapid fluent speech with normal rhythm and intonations. Most ordinary grammatical structure is intact. The speech of a Wernicke's aphasic is nonsensical, consisting of short sequences that may make sense in isolation, strung together in a meaningless, repetitive way, broken up by nonsense words. The following is a sample from a fluent, anomic aphasic asked to describe the same cookie theft picture shown above.

H.W.:First of all this is falling down, just about, and is gonna fall down and they're both getting something to eat...but the trouble is this is gonna let go and they're both gonna fall down...but already then...I can't see well enough but I believe that either she or will have some food that's not good for you and she's to get some for her too...and that you get it and you shouldn't get it there because they shouldn't go up there and get it unless you tell them that they could have it. and so this is falling down and for sure there's one they're going to have for food and, and didn't come out right, the uh, the stuff that's uh, good for, it's not good for you but it, but you love it, um mum mum (smacks lips)...and that so they've...see that, I can't see whether it's in there or not.
Examiner:Yes, that's not real clear. What do you think she's doing?
H.W.:But, oh, I know. She's waiting for this!
Examiner:No, I meant right here with her hand, right where you can't figure out what she's doing with that hand.
H.W.:Oh, I think she's saying I want two or three, I want one, I think, I think so, and so, so she's gonna get this one for sure it's gonna fall down there or whatever, she's gonna get that one and, and there, he's gonna get one himself or more, it all depends with this when they fall down...and when it falls down there's no problem, all they got to do is fix it and go right back up and get some more.
Wernicke's aphasics have severe difficulties with comprehension yet often remain unaware that they are having difficulties and become annoyed or frustrated when others can't seem to understand them. As a consequence of this lack of self awareness, Wernicke's aphasics rarely respond to treatment.
The Wernicke-Geschwind Model of Language Processing
conduction aphasia
EarPrimary
Auditory
Cortex		 Wernicke's
Area



via
arcuate
fasiculus		Broca's Area Motor Cortex

Current Modifications to the Wernicke-Geschwind Model

Sign Aphasia

ASL exhibits properties for which each of the hemispheres of hearing people shows specialization: it incorporates both complex language structure and complex spatial relations. Yet, sign language users are affected by brain damage in the same way as people using an ear/mouth rather than an eye/hand language. Damage to Broca's area results in slow signing where most grammatical inflections are omitted. In contrast, damage to Wernicke's area leaves signers signing fluently and rapidly, but making little sense, and experiencing great difficulty understanding others' signs.

What is the role of the right hemisphere in language?

Danmage to the right hemisphere can lead to a loss of the ability to express emotions, therefore speech is flat, lifeless, and mechanical. In contrast, a victim of severe left hemisphere damage who has totally lost the faculty of speech can still sing songs, sometimes very well, and even learn new songs.

When a signer suffers right hemisphere damage which destroys the ability to register ordinary facial expression, she can still make the facial expressions which form part of the grammar of sign language. Further, signers with left neglect due to right parietal lobe damage can still use the left side of the visual field normally when signing.

Aphasia for Specific Categories of Words - verbs/nouns living/non-living

The Damasios on Language

Antonio & Hanna Damasio believe that the brain processes language
by means of three interacting sets of structures:
sensory and motor representationsachromatopsia: damage to the occipital region containing areas V2 and V4 - loss of the ability to perceive AND imagine colors.
phoneme representations, combinations & syntactic rules for combining words (primarily left hemisphere)lesions to left posterior temporal and inferior parietal cortex - impaired ability to produce proper word morphology, e.g., "buh" for "blue" (This is a general impairment: not particular to color.)
mediation between concepts and the production of word-forms Color anomia: Damage to the temporal segment of the left lingual gyrus. Experience color normally and can match and discriminate colors nonverbally, but cannot name colors.
The Damasios' Diagram of Brain Systems for Language

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