Lecture 10: Language and Lateralization

Abilities that Display Cerebral Lateralization of Function

Language Lateralization

• Language Skills: Lateralized more than any other brain process, Broca’s and Wernicke’s areas are both only on the left side (for right-handed people)

Broca's Area

• Function: Central to language processing, especially in producing speech.

• Aphasia: Damage results in Broca's Aphasia, characterized by:

  • Difficulty in speech production (expressive aphasia).

  • Good comprehension but with meaningful, awkward speech.

Wernicke's Area

• Function: Focused on language comprehension

• Aphasia: Damage leads to Wernicke's Aphasia, where:

  • Speech remains fluent but lacks meaning (receptive aphasia).

  • Poor comprehension; speech sounds normal, but has no meaning

Wernicke-Geschwind Model

• Historical Context: Describes the relationship between damage in Broca’s and Wernicke’s areas on speech production and comprehension.

• Symptoms:

  • Broca's: Normal comprehension, meaningful but awkward speech.

  • Wernicke's: Speech sounds normal, but lacks meaning.

Advances in Language Research

• Techniques: Transcranial Magnetic Stimulation (TMS) and cognitive neuroscience techniques provide insights into brain activity during language tasks.

• Connectionist Model: Explains interaction between different brain regions in processing language tasks.

Connectionist Model Processes

1. Visual Cortex: Analyzes written word images.

2. Angular Gyrus: Decodes visual information into recognized words, linking them with spoken forms in Wernicke's area.

3. Arcuate Fasciculus: Transmits word information to Broca's area.

4. Broca's Area: Forms a motor plan to say the word.

5. Motor Cortex: Executes the speech plan.

Langauge Lesions and Outcomes

• Types of Aphasia:

  • Nonfluent (Broca's) Aphasia: Nonfluent speech, good comprehension, uncommon paraphasia, poor repetition and naming

  • Fluent (Wernicke's) Aphasia: Fluent speech, poor comprehension, common paraphasia, poor repetition and naming

  • Global Aphasia: nonfluent speech, poor comprehension, variable paraphasia, poor repetition and naming

  • Conduction Aphasia: Fluent speech, good comprehension, common paraphasia, poor repetition and naming

Caveats!!

• No aphasic patients have damage restricted to Broca’s or Wernicke’s areas

• Aphasics almost always have damage to subcortical white matter

• Large anterior lesions most likely to produce expressive symptoms

• Large posterior lesions most likely to produce receptive symptoms

• Global aphasia is usually related to massive lesions of several regions

• Aphasics sometimes have damage that does not encroach on Wernicke-Geschwind areas

Damasio’s PET Study of Naming

• Damasio and colleagues (1996): PET study of naming

  • Images of famous faces, animals, and tools

  • Activity while judging image orientation subtracted from activity while naming

• Left temporal lobe areas activated by naming varied with category

• Activity seen well beyond Wernicke’s area

Cortical Stimulation Studies

• Electrical stimulation studies show disruptions in speech production and comprehension based on stimulation sites, giving insight into language processing regions.

Language Acquisition Debate

• Nature vs. Nurture: Language appears to be both innate and learned through experience.

Heredity of Language

• FOXP2 Gene: Related to language development; variations in this gene affect communicative abilities in animals and humans

• Language links:

  • Neanderthals shared our version of FOXP2; chimpanzees do not

  • Mice with FOXP2 gene mutations do not evoke communicative ultrasonic vocalizations

  • When FOXP2 expression is blocked, young male birds fail to learn and recite bird song

Critical vs. Sensitive Periods

• Critical Period: a period in development when exposure or practice must occur for skills to develop

  • e.g. visual processing - the plasticity of the brain will reconnect a baby’s neurons from a blind eye; thus the eye must be corrected quickly, or normal sight will never be regained

• Sensitive Period: a period in development when exposure or practive will most facilitate the development of skills

  • e.g. social behaviour - an individual who experiences extreme stress as a child may show social deficits/abnormalities; but these abnormalities can be overcome (sort of; sometimes)

Sensitive Period for Language

• Language skills are most easily developed during early childhood to late adolescence; challenges in language acquisition exist for older learners.

Cerebral Laterilization of Function

• Each hemisphere specializes in different functions; for example:

  • Left Hemisphere: controlling ipsilateral movement, language capabilities (e.g., Broca's area).

  • Right Hemisphere: Spatial, emotional, and musical abilities, some memory tasks

Split-Brain Research

• Corpus callosum — largest cerebral commisure

  • Transfers learned information from one hemisphere to another

• Cutting the corpus callosum can allow study of the independent functioning of each hemisphere

  • Findings:

  • Each hemisphere can learn and exhibit functions independently.

  • Patients can respond differently based on which hemisphere has perceived stimuli (e.g., right visual field = can verbalize, left = can't)

Commissurotomy in Human Epileptics

• Many never have another convulsion

Cross-Cuing and Dual Attention

• The phenomenon where one hemisphere can inform the other through observable facial feedback (cross-cuing) is crucial for understanding lateralization

Doing Two Things at Once

• Each hemisphere of a split-brain patient can learn independently and simultaneously

  • Helping-hand phenomenon: presented with two different visual stimuli, the hand that “knows” may correct the other

  • Dual foci of attention: split-brain hemispheres can search for target item in array faster than intact controls

  • Chimeric figures task: only symmetrical version of right half of faces recognized (indicated competition between hemispheres

Chimeric Figures Test

• The left hemisphere of a split-brain patient sees a single normal face that is a completed version of the half face on the right. At the same time, the right hemisphere sees a single normal face that is a completed version of the half face on the left

The Z Lens and Chimeric Figures

• Tools to explore lateralized processing of visual stimuli effectively by isolating input to one hemisphere at a time.

Summary of Key Concepts

• Neural circuits involved in language processing reveal parallels between human language capabilities and other species.

• Both biological and experiential factors contribute to language development, with certain genes and critical periods influencing abilities.