Hemispheric Specialization
Goals
- Explore historical concepts of cerebral dominance and asymmetries in brain anatomy.
- Understand psychological and experimental studies of lateralization, including visual field and dichotic listening tasks.
- Examine motor control differences between hemispheres.
- Investigate unilateral brain damage and split-brain cases.
- Discuss the “Interpreter” concept and its implications for cognition.
Historical Concepts of Cerebral Dominance
- Early Perspectives:
- Ancient Greek physician Diocles of Carystus (4th century BC): Proposed two brains with distinct functions—right for perception and left for understanding.
- Descartes: Believed the hemispheres were duplicates, except for the singular pineal gland, which he considered the seat of the soul.
- 18th-century Simon Du Pui described humans as “Homo Duplex,” emphasizing the duality of the brain and its connection via the corpus callosum.
- 19th Century:
- Paul Broca (1861): Identified the left hemisphere as dominant for language (e.g., Broca’s area).
- John Hughlings Jackson: Observed that aphasics could produce spontaneous speech (e.g., swearing) but struggled with voluntary speech, attributing this to left hemisphere damage.
- Later Views:
- The left hemisphere was labeled as “dominant” for language, while the right was considered “minor” or focused on perceptual skills.
Anatomical Asymmetries
- Planum Temporale:
- Larger in the left hemisphere in 65% of brains (Geschwind and Levitsky, 1968).
- Asymmetry correlates with language specialization.
- Cerebral Petalias:
- The left hemisphere shows posterior enlargement, while the right hemisphere has a forward shift in the frontal region, creating a “torque” effect.
- Found in hominid fossils (e.g., Australopithecus), suggesting early brain reorganization linked to language.
- MRI Studies:
- Modern imaging confirms asymmetries in language-related areas like Broca’s and Wernicke’s regions.
Psychological and Experimental Studies
Visual Field Studies
- Left and right visual fields (LVF and RVF) project to opposite hemispheres:
- RVF-LH Advantage: Words and language processing.
- LVF-RH Advantage: Faces, gaze direction, and global processing.
- Global vs. Local Processing:
- Global (low spatial frequency): Right hemisphere (e.g., identifying an “H” composed of “F”s).
- Local (high spatial frequency): Left hemisphere (e.g., identifying the “F”s in the larger “H”).
Dichotic Listening
- Right ear (left hemisphere): Advantage for words.
- Left ear (right hemisphere): Advantage for melodies and prosody.
Unilateral Brain Damage
- Left Hemisphere Lesions:
- Impair language abilities (aphasia).
- Preserve perceptual and spatial skills.
- Right Hemisphere Lesions:
- Impair spatial perception and global processing.
- Preserve language abilities.
Global vs. Local Drawing Studies
- Patients with:
- Right Hemisphere Lesions: Draw detailed local elements but miss global structure.
- Left Hemisphere Lesions: Capture global structure but miss local details.
Split-Brain and Disconnection Syndrome
Corpus Callosum Sectioning
- Used as a treatment for severe epilepsy to prevent seizures from spreading.
- Effects:
- Disconnects hemispheric communication.
- Causes striking dissociations in behavior and cognition.
Split-Brain Findings (Gazzaniga’s Research)
- When different stimuli are presented to each hemisphere:
- RVF-LH: Patient can name the stimulus.
- LVF-RH: Patient can draw the stimulus but cannot name it.
- Example Tasks:
- A split-brain patient can draw a saddle (LVF-RH) while verbally denying seeing anything.
- The right hemisphere maximizes (chooses the most frequent outcome in probabilistic tasks), while the left hemisphere matches frequencies, often suboptimally.
The Interpreter Concept
- Proposed by Michael Gazzaniga:
- The left hemisphere creates narratives to explain behaviors, even when it lacks access to the actual cause.
- Confabulation in split-brain patients reflects the Interpreter’s attempts to rationalize right hemisphere-driven actions.
- Possible link to anosognosia: Denial of deficits due to the Interpreter’s narrative generation.
Control of Emotional Facial Expression
- Emotional expressions are lateralized:
- Spontaneous Expressions: Bilateral subcortical control.
- Voluntary Expressions: Left hemisphere dominance.
- Studies using chimeric faces (e.g., left-left vs. right-right composites) show the right hemisphere specializes in processing facial emotions.
Applications and Insights
- Sodium Amytal Test (Wada Test):
- Temporarily silences one hemisphere to determine language and memory lateralization.
- Demonstrates the hemispheric division of function, such as language in the left and prosody in the right.
Key Terms
- Planum Temporale: Language-related brain region larger on the left.
- Cerebral Petalias: Asymmetry where the left posterior and right anterior hemispheres protrude.
- Global Processing: Holistic perception, right hemisphere specialization.
- Local Processing: Detail-focused perception, left hemisphere specialization.
- Interpreter: Left hemisphere’s narrative-generating function in split-brain patients.
This lecture provided a historical, anatomical, and experimental foundation for understanding hemispheric specialization, offering insights into lateralized brain function and its implications for behavior, cognition, and neurological disorders.
