Physiological Psychology - Lateralisation & Language

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PSY202: Physiological Psychology - Week 6

Part A: Lateralisation & Language

References: Text Chapter 13

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Topics Covered in Week 6

1. Lateralisation
  • Left & Right Hemispheres
  • Lateralisation of Hemispheres
  • Lateralisation & Emotion
  • Connecting the Hemispheres - The corpus callosum
  • Split Brain
2. Language
  • How did humans evolve language?
  • Localising language in the brain
  • Language Disorders

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Lateralisation: The Left & Right Hemispheres

  • The human brain is divided into two nearly symmetrical hemispheres, i.e., Left and Right.
  • Each hemisphere performs distinct functions:
    • Left Hemisphere: Controls right side of the body
    • Right Hemisphere: Controls left side of the body
    • Exception: Both hemispheres control the trunk and facial muscles

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Functionality between Hemispheres

  • Information is exchanged via:
    • Corpus Callosum
    • Anterior Commissure
    • Hippocampal Commissure

Note: The corpus callosum allows each hemisphere access to information from both sides.

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Sensory Processing by Hemispheres

  • Each hemisphere receives input from the opposite visual field:
    • Vision: Left retina to Left Hemisphere sees Right visual field
    • Auditory: Both ears send information to both hemispheres, but the contralateral ear is prioritized
    • Taste: Both hemispheres receive taste input from both sides of the tongue
    • Smell: Each hemisphere processes information from the same side nostril

Visual and Auditory Connections to Hemispheres
![Image credit: Dr. Dana Copeland/Wake Pathology]

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Structural Differences between Hemispheres

  • The L and R hemispheres are not mirror images.
  • A specific area in the temporal lobe called the planum temporale is larger in the left hemisphere (65% of individuals).
  • Critical for speech comprehension.
  • Differences in hemispheric structure are evident in pre-term babies.

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Hemispheric Specialisations

Left Hemisphere
  • Specialisation: Speech production (95% of right-handers, 80% of left-handers)
  • Focus: Details
Right Hemisphere
  • Specialisation: Emotional and spatial processing
  • Focus: Overall patterns

Tasks:

  • Identifying small letters activates the left, while recognizing larger patterns activates the right.

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Lateralisation & Language

  • Division of Labour: Known as lateralisation
    • Left hemisphere dominant for language: 95% right-handers, 80% left-handers
    • Question: Is it advantageous for one hemisphere to control speech?
    • Evidence suggests that bilateral speech control may lead to stuttering as it produces competing messages for muscle control.

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Lateralisation & Emotion

  • Right hemisphere’s role in processing emotional contexts may explain its dominance in emotion.
  • Damage to the right hemisphere can lead to:
    • Poor emotional recognition
    • Poor understanding of humor and sarcasm
    • Monotone speech

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Emotional Processing Study

  • Intact participants scored ~50% on honesty detection in videos.
  • Participants with left hemisphere damage scored ~60%, relying more on right hemisphere interpretation.
  • WADA Procedure: When the right hemisphere was inactivated, individuals remembered facts but no emotions from significant events, indicating its role in emotional recall.

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Corpus Callosum: Information Connection

  • Main Link: Corpus Callosum
  • Smaller Links: Anterior commissure and hippocampal commissure
  • The corpus callosum grows and thickens during childhood and adolescence due to increased myelination.
  • Callosal Agenesis: Condition where individuals are born without a corpus callosum, altering the development of brain areas.

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Corpus Callosum Damage & Surgery

  • Damage disrupts interhemispheric communication.
  • Corpus callosotomy can be used to treat epilepsy, which affects ~1-2% of the population.
  • Surgery reduces the spread of seizures to one side of the body.
  • Split-brain patients: Individuals have undergone a hemispherectomy

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Language Characterization

  • Uniquely human due to its productivity - the ability to create new combinations for new ideas.
  • Primate communication lacks grammatical structure; chimps taught sign language display limited understanding.
    • Terrace et al. (1979) investigated sign language use in chimps and found no evidence of grammatical structure.

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Evolution of Language

  • Language may have evolved as a by-product of larger brain size or as a distinct mechanism.

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Language as a Specialisation

  • Theories by Chomsky and Pinker: Language Acquisition Device - an innate mechanism for language learning.
  • Social elements suggest dependence on language for survival during childhood.

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Case Study: Williams Syndrome

  • Genetic disorder with a prevalence of 1 in 20,000.
  • Despite cognitive impairments (IQ 50-60), individuals often retain fluent speech and social skills.
  • Illustrates the dissociation between intelligence and language ability.

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Language Development Critical Period

  • Sensitive Period: Early exposure to language is crucial for development; lack of exposure limits adult language learning capabilities.
    • Second language acquisition is best before age 12.

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Localising Language in the Brain

  • Language localisation studies rely on:
    1. Lesion studies from organic brain damage
    2. Brain stimulation in conscious patients
    3. Imaging/electrophysiological studies
  • Language impairment is commonly associated with left hemisphere damage (90-95% cases).

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Lesion Studies

  • Paul Broca (1861): Case of “Tan” - post-mortem revealed damage to the left inferior frontal area (Broca’s Area).
  • Carl Wernicke (1870s): Described language comprehension issues related to damage between the auditory cortex and angular gyrus (Wernicke’s Area).

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Broca’s Aphasia (Nonfluent Aphasia)

  • Impairs language production; individuals are slow and struggle with various forms of communication, including sign language.
  • English speakers might omit function words and grammatical endings.

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Wernicke’s Aphasia (Fluent Aphasia)

  • Poor language comprehension despite fluent articulation.
  • Patient struggles to find the correct words (anomia) despite normally recognizing objects.

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Electrical Stimulation Studies

  • Conducted by Penfield during epilepsy surgery.
  • Found areas affecting speech in Broca’s and Wernicke’s areas, and stimulation impacts voluntary control of facial movements.

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Findings of Stimulation Studies

  • Current understanding shows significant interindividual variation in size and function of language regions.
  • Stimulation can disrupt more than just Broca’s and Wernicke’s areas, indicating a broader network is involved in speech production.

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Subcortical Structures in Language

  • Identified involvement of left thalamus nuclei in language function.
  • Damage to these areas can cause dysphasia, altered speech rate, and declined verbal IQ.

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Imaging Studies by Petersen et al.

  • Utilized PET scans while participants performed various language tasks.
  • Findings indicated activation of different regions depending on task type (visual vs auditory vs output tasks).

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Language Activation Areas

  • Activation of posterior areas (Wernicke’s) mainly for pronounceable words.
  • Significant engagement of the left inferior frontal area (Broca's) for language output tasks.

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Language Generation Pathways

  • Identifies two distinct pathways for word generation, suggesting complexity beyond a binary model of just Broca’s and Wernicke’s areas.

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Language Disorders Overview

  • Aphasia: A disorder affecting speech, writing, or reading due to brain damage.
  • Paraphasia: Production of unintended syllables or words during speech.

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Types of Aphasia

  1. Fluent Aphasia: Good speech flow but challenges in comprehension or repetition (e.g., Wernicke’s, conduction).
  2. Non-fluent Aphasia: Difficulty articulating yet relatively good comprehension (e.g., Broca’s, global).
  3. Pure Aphasia: Selective impairments affecting reading or writing (e.g., alexia).

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Wernicke-Geschwind Model of Aphasia

  • Speaking a heard word: Involves auditory analysis (primary auditory cortex -> Wernicke’s area -> Broca’s area -> motor cortex).
  • Speaking a written word: Involves visual processing (visual cortex -> angular gyrus -> Wernicke’s area -> Broca’s area).

Figures & Diagrams

  • Include diagrams showing the flow of information between brain regions in the Wernicke-Geschwind model to further enhance understanding of language processing and disorders.