Psycholinguistics / Language and The Brain

• Psycholinguists use scientific experimentation to discover how the humans process linguistic information:
  • lexical priming tasks
  • sentence comprehension
  • phonetic discrimination
• These methods give us insight about the processes that are used in language comprehension and production
• Neurolinguistic studies aim to discover which parts of the brain are responsible for different language tasks
• There are two common methods for studying the brain and language:

  brain imaging (CAT, fMRI, PET, MEG)

  study of brain trauma/abnormalities

But first, your brain…..

• Roughly 10 billion neurons in the human brain
• The left side of your body is controlled by the right hemisphere, and vice versa (ipsalateral control)
• Damage to the left hemisphere motor control center will cause loss of motion on the right side

Where is language in the brain?

• Impossible to pin down an exact location
• Every person's brain is slightly different

A generalization:

• Language is centered in the temporal lobe(s)

Handedness

• In a majority of right-handed people, it is the left temporal lobe
• For left-handers, language dominance is switched around 50% of the time
• Left-handers are more likely to show equal dominance of both hemispheres
• These are just generalizations (not absolute)

How do we know about dominance?

• Damage to the left hemisphere is more likely to cause language problems
• The WADA technique
  • Each carotid artery provides blood to exactly one hemisphere of the cortex
  • A chemical which temporarily "numbs" the brain is injected into one side of the brain
  • Performance on language tasks is monitored to determine dominance
  • This is done sometimes as a preliminary to brain surgeries

Age differences

• Damage to the left hemisphere in young brains doesn't always cause language problems
• Neural plasticity: hemisphere lateralization is not fixed a birth
• Lateralization can be switched in order to adapt to loss or damage
• Hemispherectomy: removal of an entire hemisphere to control severe epilepsy
• Young children who udergo these types of surgery still can develop fully functional language capapbilities in the other hemisphere

Gender differences?

• There is evidence that females have less lateralization than males:
• Better chance of recovery after left hemisphere damage
• fMRI evidence that some linguistics tasks are less lateralized
• Results from phonological rhyming task: females show slightly more lateralization
• Does not mean that "men and women are fundamentally different" cognitively
• Females may have, on average, slightly more diffuse language lateralization (they may retain language capabilities in both hemispheres)

Language deficits: Aphasias

• Aphasia is a general term for any loss of linguistic ability due to damage
• Can result in problems with production or perception
• Occurs in varying degrees (mild -> severe)
• Usually due to actual trauma or vascular problems (strokes, loss of oxygen)

Broca's Aphasia

• Named for Paul Broca, French neurologist (1861)
• His patient, Tan, was very intelligent but had a severe speech deficit
• Autopsy showed very selective damage to the left hemisphere
• Damage around the juncture of the temporal, frontal, and parietal lobes (henceforth called Broca's Area; see picture above)

Symptoms

• Broca's aphasics often have severe deficits in production:

  "Yes ...Monday ...Dad, and Dad ... hospital, and ... Wednesday, Wednesday, nine o'clock and ... Thursday, ten o'clock ... doctors, two, two ... doctors and ... teeth, yeah. And a doctor ... girl and gums, and I."

  "Me ... build-ing ... chairs, no, no cab-in-ets. One, saw ... then, cutting wood ... working ..."

• Often missing function words, but preserve basic word order
• Speech is often forced or halting

Wernicke's Aphasia

• Carl Wernicke, German neurologist (1873)
• Patient had fairly fluent speech, but very little comprehension
• Autopsy showed selective damage to the left hemisphere, posterior parietal/temporal junction (Wernicke's Area; see picture)

Symptoms

• Problems with comprehension and production

  Examiner: What kind of work have you done?

  Patient: We, the kids, all of us, and I, we were working for a long time in the ... you know ... it's the kind of space, I mean rear to the spedawn ...

  Examiner: Excuse me, but I wanted to know what work you have been doing.

  Patient: If you had said that, we had said that, poomer, for the fortunate, porpunate, tamppoo, all around the fourth of martz. Oh, I get all confused.

• Speech production fairly fluent, but lacks any real semantic coherence

Traditional view of language and the brain

• Broca's Area: responsible for speech production (close to motor cortex)
• Wernicke's Area: responsible for comprehension (close to auditory area)

What happens if you damage the link between the two areas?

Conduction Aphasia

• Damage to the arcuate fasciculus (bundle of connecting fibers) that connect Wernicke's and Broca's area
• Patients often have good speech and comprehension, but have a severe problem with repetition
• They can understand and produce speech, but efficient transmission of information from reception to production is impaired

The division between Wernicke's (comprehension) and Broca's (production) is not so clear

• Broca's patients have problems with comprehension

Passive sentences

• Broca's patients will often give the reverse (incorrect) answer for a passive sentence

  The dog chased the cat

  The cat was chased by the dog

• Both sentences have the same basic meaning, but Broca's patients will point to the wrong picture for the second, passive sentence
• If shown a picture of a dog chasing a cat (correct) and a cat cahsing a dog (incorrect), they will choose the incorrect version
• Why? They may not be paying any attention tot he synatx of the sentence.
• If they choose the first noun they see as the subject, and the second noun as the object, they will get the wrong meaning.
• The problem may be amore general problem with syntax, not just production

• Wernicke's patients don't have just comprehension deficit
• Problems reported with simple semantic judgments

"Which one is the oddball?"

dog turnip cat

man woman trout

• Wernicke's patients are just as likely to choose "man" or "dog", instead of the obvious oddballs
• Comprehension deficit may be a more general semantic problem

Some notes on brain damage

• The areas defined for Broca's and Wernicke's aphasia are not absolute
• Individual differences in structure have consequences for studying the link between brain structure/performance
• Damage is usually not localized (it is diffuse)
• Hard to pin down exactly what causes the problem
• Language use draws on many other resources, so damage to other parts of the brain may result in loss of linguistic ability

Alexia without agraphia

• alexia = inability to read
• agraphia = inability to write
• Damage to the visual cortex looks like damage to the language system
• Patient could write but not read
• Couldn't read what he had just written!
• This patient did not have damage to their language system, but to the system which acts as a visual input to language