Neurolinguistics: language and the brain



Neurolinguistics: language and the brain

Overview

What is neurolinguistics?

Innateness hypothesis

Evidence used in neurolinguistics

Lateralization

Localization

Lateralization---some variables

Summary

What is neurolinguistics?

Neurolinguists investigate:

How the brain processes language

Where the brain processes language

Who do neurolinguists study?

Normal subjects

E.g. via dichotic listening experiments: stimulus presented to different ears

Abnormal subjects

stroke patients

patients with brain injury

patients who have had brain surgery

Aphasia: language disorder associated with trauma to the brain

Lateralization

Contra-lateral control: a given hemisphere controls opposite side of body.

Lateralization and language (esp. for adult, male, left-handed, literate, monolingual subjects)

Left brain controls right side of body

Right brain controls left side of body

Right hemisphere specialties

Holistic 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---analytically---by musically sophisticated individuals)

mathematics

intellectual reasoning

language, speech sounds

Language processing as a left hemisphere task. Evidence from:

Split-brain patients (severe cases of epilepsy once treated by severing corpus callosum)

E.g., with left eye open (right brain) and right eye covered, task of naming object in left hand (right brain) much harder than naming object in left hand with right eye (left brain) open and left eye covered

Aphasia

Brain injury locations resulting in speech deficits are almost always in left hemisphere.

Dichotic listening experiments

Linguistic sounds: right ear (left brain) advantage

Environmental sounds: left ear (right brain) advantage

Tone

Thai speakers of Thai process tone with left hemisphere

English speakers process tone with right hemisphere

Lateralization and modality

Poizner, Howard, Edward Klima and Ursula Bellugi (1987) What the Hands Reveal about the Brain. MIT Press.

Studied aphasia and other problems in 6 ASL signers with brain damage

If left hemisphere damage, then sign language aphasia:

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 damage, then non-aphasic problems, such as use of left signing space.

Right-hemisphere damaged signer used left side of signing space better when such uses were linguistically required, but when describing (e.g.) furniture in a room, ‘furniture piled in helter-skelter fashion on the right, and the entire left side of signing space left bare...’

Cf. left hemisphere-damaged signer: many problems with syntax, but no spatial distortions in description of room layout

Localization

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

Lesions at specific areas

Broca's area

Broca's aphasia (a.k.a. expressive aphasia, motor aphasia). Basic message of meaning clear but:

speech is not fluent

phrases are telegraphic (absence of function words)

incorrect production of sounds

Wernicke's area

Wernicke's aphasia: Speech is fluent, but:

often nonsensical or circuitous

A self-monitoring problem? "I can't talk all of the things I do"

Arcuate fasciculus (subcortex nerve fibers connecting Broca’s, Wernicke’s areas)

Conductive/conduction aphasia: usually good comprehension, fluent speech but difficulty:

repeating

reading out loud

writing

(Dominant) angular gyrus

Anomia:

difficulty finding words, especially names

Electrical stimulation

Reaction: numbness, twitching, movement of contralateral body part

Stimulation at ‘language centers’: results in difficulty talking or some kind of vocalization

Different aphasic symptoms

Caramazza (John Hopkins):

spoken and written language are separately localized.

Evidence from 2 female aphasics (stroke patients):

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

Localization or phrenology?

Broca's aphasics: damage to Broca’s area results not only in language deficits, but also problems with:

motor control

cognitive and perceptual tasks

Challenge to role of cortex in higher cognitive functions like language

Linguistically normal hydrocephalic individuals

Other, non-localized neurological problems resulting in language deficits

Alzheimer’s disease

Effects on lateralization

Some variables which interact with left hemisphere specialization for language

Lateralization and handedness

General population:

90% predominantly right-handed

75% strongly 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 are 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 sex

In women, language may be bilateral more often:

left hemisphere damage 'has a lesser effect' or 'far 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 age

Young brains (before 10 years of age) can recover from trauma in a way that adults cannot:

If left hemisphere removed at birth, right hemisphere can take over language functions so that language use in later years is almost perfect.

(Older brains cannot do this.)

Lateralization and multilingualism

In multilingual individuals, languages may occupy unequal amounts of brain space.

More right hemisphere language dominance than in monolinguals.

If there is damage to the right hemisphere, multilingual individuals are 5x more likely to have aphasia than monolinguals.

Recovery from aphasia:

50% of time, recovery affects both languages equally; 25% of time, patients do not regain 1 or more languages

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

Lateralization, localization support Innateness Hypothesis: Humans are genetically programmed for language

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