Neurolinguistics

LING 200

Spring 2003Reading: File 9.1

Neurolinguistics

• Neurolinguists investigate – How the brain processes language – Where the brain processes language

• Who neurolinguists study– Normal subjects – Abnormal subjects

• patients with brain injury (e.g. stroke)

• patients who have had brain surgery

Aphasia

• Language disorder associated with trauma to the brain

• ‘aphasic’ individuals

Why neurolinguistics of interest

• Brain exhibits specialization for language

• lateralization– localization

• Language as an innate, species-specific property

Brain hemispheres

right hemisphere

left hemisphere

Lateralization

• Contra-lateral control– a given hemisphere controls opposite side of

body• Left hemisphere controls right side of body

• Right hemisphere controls left side of body

• Other hemispheric specializations:

Right hemisphere specialties

• Holistic, spatial processing– pattern-matching (e.g. recognizing faces) – spatial relations – emotional reactions – music (processing by musically naive

individuals)

Left hemisphere specialties

• Sequential processing – rhythm – temporal relations – analytical thinking – music (processed by musically sophisticated

individuals)– mathematics – intellectual reasoning – language, speech sounds

• especially so for adult, male, right-handed, literate, monolingual subjects

• Evidence from dichotic listening experiments – stimulus presented to different ears – linguistic sounds: right ear (left brain)

advantage– environmental sounds: left ear (right brain)

advantage

Language processing as a left hemisphere task

Language processing as a left hemisphere task

• Evidence from dichotic listening experiments– Thai tonal contrasts

• [ná:] ‘aunt’ (high) [nâ:] ‘face’ (falling)• [nā:] ‘field’ (mid) [na:] ‘thick’ (rising)• [nà:] (nickname) (low)

– Thai speakers process tone with left hemisphere– English speakers presented with tonal contrasts

process tone with right hemisphere

Language processing as a left hemisphere task

• Evidence from aphasia – Brain injury locations resulting in speech

deficits are almost always in left hemisphere

Language processing as a left hemisphere task

• Evidence from split-brain patients – Severe cases of epilepsy treated by severing

corpus callosum– Task of naming object held in left hand

(right brain) • left eye open (right brain), right eye covered

much harder than

• right eye open (left brain), left eye covered

corpus callosum (connects hemispheres)

Effects on lateralization

• Lesser left hemisphere specialization for language if:– left-handed– female– illiterate– multilingual

Lateralization and handedness

• General population – 90% predominantly right-handed – 10% strongly left-handed or ambidextrous

• Lateralization in right-handed individuals– 90% left hemisphere specialization for

language – 10% right hemisphere specialization

• Lateralization in left-handed individuals – 65-70% have left hemisphere specialization for

language

– 30-35% have right hemisphere specialization or apparently bilateral

• Aphasia in left handed individuals – tends to be less severe, shorter in duration

– 8x more likely to get aphasia if right hemisphere is damaged than right handed individual

Lateralization and handedness

Lateralization and gender

• In women, language may be bilateral more often – if left hemisphere damage, milder aphasia or

less likely to result in aphasia – dichotic listening tests don't show right ear

advantage as often as for men

Lateralization and literacy

• Language more symmetrically located in illiterate speakers

• Aphasia just as likely with right-hemisphere injury

Lateralization and multilingualism

• More right hemisphere language dominance than in monolinguals

• If right hemisphere damage, multilingual individuals 5x more likely to develop aphasia

• Recovery from aphasia – 50% recover both languages to same extent– 25% do not regain 1 or more languages

Aphasic French-Arabic bilingual

• French-Arabic bilingual nun in Morocco became severely aphasic after moped accident

• initially lost speech altogether• 4 days after accident, could speak a few words of

Arabic, no french• 14 days after accident, could speak French fluently• 15 days after accident, could speak only Arabic

fluently

Lateralization and modality

• Sign languages use visual-spatial mode of transmission

• How is lateralization for language affected by modality?

• Results of a study of aphasia and other problems in 6 ASL signers with brain damage– 3 left brain damage, 3 right brain damage

If left hemisphere was damaged

• Sign language aphasia resulted– GD: ‘halting and effortful signing,’ reduced to

single sign utterances without syntactic and morphological marking

– KL: ‘selection errors’ in phonological structure of ASL signs, ‘sign comprehension loss’

– PD: fluent signing but grammatical/syntactic impairment

If right hemisphere was damaged

• Non-aphasic problems resulted (e.g. avoidance of left signing space)

• Right-hemisphere damaged signer – avoided left side of signing space

• describing furniture in a room: ‘furniture piled in helter-skelter fashion on the right, and the entire left side of signing space left bare...’

– but used left side of signing space better when such uses were linguistically required

Modality and lateralization

• No effect of language modality on lateralization for language

• Left hemisphere specialization for language even for signed languages

Localization for language

• Hypothesis: specific parts of brain control specific parts of body or bodily functions, including language

Some language centers (left hemisphere)

Broca’s

Wernicke’s

Arcuate fasciculus

• Broca's area lesions result in Broca's aphasia (a.k.a. expressive aphasia, motor aphasia)

• Characteristics of Broca’s aphasia– basic message of meaning clear but

– speech is not fluent

– phrases are telegraphic (absence of function words)

– incorrect production of sounds

• Cinderella, as told by a Broca’s aphasic– Cinderella...poor...um ‘dopted her...scrubbed floor, um,

tidy...poor, um...’dopted...si-sisters and mother...ball. Ball, prince um...shoe.

Evidence for localization: aphasia

• Wernicke’s area lesions• Characteristics of Wernicke’s aphasia

– speech is fluent, but

– often nonsensical or circuitous

• Description of a knife by a Wernicke’s aphasic– ‘That’s a resh. Sometimes I get one around here that I

can cut a couple regs. There’s no rugs around here and nothing cut right. But that’s a rug and I had some nice rekebz. I wish I had one now. Say how Wishi idaw, uh windy, look how windy. It’s really window isn’t it?’

Evidence for localization: aphasia

Evidence for localization: aphasia

• Lesions at arcuate fasciculus (subcortex nerve fibers connecting Broca’s, Wernicke’s areas) – Conductive/conduction aphasia – Characteristics

• usually good comprehension, fluent speech but• difficulty repeating• difficulty reading out loud• difficulty writing

• Lesions at angular gyrus – Anomia

• difficulty finding words, especially names

– Reading difficulties

Evidence for localization: aphasia

Angular gyrus

Evidence for localization

• Electrical stimulation of brain– Normal reaction: numbness, twitching,

movement of contralateral body part

• Electrical stimulation at ‘language centers’– Results in difficulty talking or some kind of

vocalization

Evidence for localization

• Spoken vs. written language may be separately localized

• Johns Hopkins study of 2 female aphasics – both found it easy to read, speak and write

nouns – one could speak verbs but not write them – one could write verbs but not speak them

More than language centers in the brain

• Broca's aphasics – damage to Broca’s area results in

• language deficits

• motor control problems

• problems with cognitive and perceptual tasks

• Alzheimer’s disease – non-localized neurological problems result in– language deficits (among other problems)

Neurolinguistics summary

• Hemispheres of brain have different specialties, including language (most clearly for right-handed (etc.) individuals)

• Lateralization is not affected by language modality• Language centers within the brain: Broca's,

Wernicke's areas especially important• Neurolinguistics provides evidence for human

specialization for language