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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