Study Notes on Hemispheric Specialization

Chapter 4: Hemispheric Specialization

Exaggerated Asymmetries

  • The left hemisphere is characterized as:

    • Analytical

    • Decision-making

    • Logical reasoning

    • Practical tasks

    • Scientific approaches

    • Local representation

    • Example phrase: "I AM THE LEFT BRAIN"

  • The right hemisphere is characterized as:

    • Creative

    • Artistic endeavors

    • Vivid imagination

    • Passionate expression

    • Global representation

    • Example phrase: "I AM the RIGHT BRAIN"

Hemispheric Functions

Left Hemisphere Functions

  • More language function

  • Controls the right motor functions

  • Processes the right visual field

  • Engaged in inference and logic tasks

  • Local representation of information

Right Hemisphere Functions

  • Specializes in visuospatial processing

  • Primarily responsible for face processing

  • Manages spatial attention

  • Governs the left motor functions

  • Interprets the left visual field

  • Engages in the global representation of information

Clinical Applications of Hemispheric Specialization

  • Amobarbitol: A substance used to induce rapid and brief anesthesia to disrupt function in one hemisphere of the brain.

    • Application: Used to identify sources of epileptic seizures.

    • Insight: Provides valuable information regarding hemispheric specialization by temporarily anesthetizing the left hemisphere, which prevents verbal communication about held objects.

Anatomical Correlates of Hemispheric Specialization

Anatomical Asymmetries

  • Notable differences in the anatomy of the two cerebral hemispheres:

    • Enlargement of anterior portion of the right hemisphere.

    • Enlargement of posterior portion of the left hemisphere.

    • Increased size of the planum temporale in the left hemisphere, which is part of Wernicke’s area responsible for language understanding.

    • Thalamic structures are larger on the left side.

Corpus Callosum Functions

  • The corpus callosum connects both hemispheres and consists of three parts:

    • Genu: Anterior portion

    • Body: Middle portion

    • Splenium: Posterior portion that connects to occipital lobe

  • The corpus callosum contains larger axonal fibers and maintains topographical organization between hemispheres.

The Commissures

  • Commissures: Structures that interconnect the brain's hemispheres.

    • Anterior commissure: Connects parts of the temporal lobe and amygdalae, incorporating fibers from the olfactory tract and the neospinal thalamic tract.

    • Posterior commissure: Contributes to the pupillary light reflex.

Functional Consequences of Split-Brain Procedure

  • Cortical Disconnection: The surgery known as corpus callostomy cuts the corpus callosum to interrupt communication between the two hemispheres, often employed to treat intractable epilepsy.

    • Challenges in verifying the extent of transcortical sectioning in patients with prior neurological damage.

    • Most tests involve briefly flashed visual stimuli, exploiting the strict lateralization of the visual system.

Behavioral Outcomes in Split-Brain Patients

  • After visual stimuli presentation:

    • Reports are usually only from the right visual field (connected to the left hemisphere).

    • Patients cannot recognize objects in the left hand, as the sensory information is disconnected from the speech-dominant hemisphere.

    • Patients display nonverbal responses showing understanding via the right hemisphere in involuntary actions.

  • Example responses:

    • A patient can draw a saddle using the left hand but cannot verbally describe the same object due to lack of right hemisphere communication.

Letter-Priming Effect

  • Experiment assessing how each hemisphere accesses the lexicon:

    • Task: Identify whether the letter presented was an 'H' or 'T', primed with lowercase letters to test reaction time (RT).

    • Results demonstrate faster RT in words presented to the right visual field, indicating a right hemisphere deficit in letter recognition.

Recognition Tasks: Patient J.W.

  • Examined recognition tasks show a discrepancy between recognition of self and familiar others:

    • The left hemisphere shows better recognition of self.

    • The right hemisphere excels in recognizing familiar others.

  • Data represent a performance metric showing a strong preference for the respective recognition capabilities of each hemisphere.

Right and Left Hemisphere Differences

  • Right Hemisphere Functions:

    • Understands language but lacks syntax processing.

    • Limited inference-making capabilities.

    • Superior recognition of unfamiliar faces (FFA larger on the right).

    • Processes emotional content predominantly with right lateralization.

    • Limited ability to interpret emotional prosody.

  • Left Hemisphere Functions:

    • 96% of right-handers are left-dominant in language processing.

    • 65% of brains show larger planum temporale structures in the left hemisphere.

    • Engages in syntax and semantic processing, logical inference making, and controls right-side motor actions and visual fields.

Conclusion on Hemispheric Specialization

  • Hemispheric specialization extends beyond language to visual processing and problem-solving frameworks, revealing a competition for optimal cognitive performance between brain hemispheres.

  • Anatomical correlates of hemispheric specialization include identifiable asymmetries in structure and function across cognitive tasks.