Split-Brain & Hemispheric Specialization

Hemispheric Specialization

• Two cerebral hemispheres perform partially distinct functions
  • Left: explicit, analytic, language-based, conscious “verbal self”
  • Right: implicit, holistic, non-verbal, non-conscious influences
• Communication bridge = corpus callosum (≈ hundreds of millions of axons)
  • Coordinates wants, drives, personalities of both sides so they normally “get along”

Split-Brain Operation

• Surgical name: corpus callosotomy (aka “split-brain surgery”)
• Instrument: heat/cutting probe analogous to a soldering iron
• What is cut: ONLY the corpus callosum (cortex & subcortical pathways remain intact)
• Indication: intractable epilepsy where seizures spread bilaterally
  • Epileptic seizure = uncontrolled, runaway electro-chemical activity
  • Cutting callosum stops spread → reduces frequency/intensity of generalized seizures
• Frequency: extremely rare but clinically effective

Visual System Rules (CRUCIAL FOR TESTING)

• Visual fields (not eyes) are contralateral
  • Left visual field (LVF) → right hemisphere (RH)
  • Right visual field (RVF) → left hemisphere (LH)
• Eyes are bilateral
  • Each eye sends information to both hemispheres via the optic chiasm
• Optic chiasm sits below corpus callosum, so crossing STILL occurs after callosotomy
• Consequence: to send info to one hemisphere in a split-brain patient, present it briefly (< 200 ms) only in one visual field; simply closing an eye will not work

Classic Split-Brain Testing Paradigm

1. Patient fixates a central dot/cross.
2. Two images/words flashed simultaneously:
   • One in LVF (→ RH)
   • One in RVF (→ LH)
3. Verbal report → samples LH knowledge (language lives in LH for ≈ 95 % of right-handers)
4. Non-verbal action with LEFT hand → samples RH knowledge
   • Left hand motor control is contralateral (RH → left limbs)

Example: Joe (callosotomized)

• “CAR” flashed in RVF → LH receives → Joe says “car.”
• “KEY” flashed in LVF → RH receives → Joe cannot say what he saw but can
  • Draw a key with his left hand
  • Point to a key among objects using left hand
• Demonstrates double-consciousness:
  • LH conscious verbal self unaware of RH perception
  • RH acts non-verbally, influencing behaviour outside awareness

Everyday Manifestations

• Closet anecdote (patient in older video)
  • LH (verbal self) plans to wear blue shirt; controls right hand.
  • RH prefers green shirt; controls left hand → left hand pulls green shirt; LH rejects it angrily.
  • Illustrates independent motives & “two minds” inside one skull.
• Alien Hand Syndrome
  • Typically affects left hand (RH) in individuals with disrupted inter-hemispheric communication (e.g., partial callosal damage)
  • Hand feels “not mine,” may perform complex, sometimes aggressive acts
  • Highlights unconscious RH motor programmes outside LH awareness

Implicit vs Explicit Processes

• Explicit/Conscious ≈ largely LH, cortex-based (oversimplification but pedagogically useful)
• Implicit/Unconscious ≈ RH + subcortical circuits (basal ganglia, cerebellum, etc.)
  • Include habits, conditioning, priming — shared by many non-human species
• Important caveat: not ALL unconscious = RH; many implicit processes are sub-cortical

Motor & Sensory Contralaterality Beyond Hands

• All voluntary limbs contralateral
  • Right leg movement & sensation ↔ LH motor/somatosensory cortices
  • Left leg ↔ RH
• Same rule applies to face, trunk, etc.

Neuroplasticity & Extreme Surgery

• Hemispherectomy: complete removal of one cerebral hemisphere (usually to treat catastrophic epilepsy in children)
  • Patients can survive and often recover most functions if done early → showcases massive brain plasticity

Language Disorders (Aphasias)

Broca’s Aphasia (Non-fluent)

• Lesion: Broca’s area, left frontal lobe (near motor cortex)
• Deficit: language production (slow, effortful speech)
• Comprehension: largely intact → patient can follow commands
• Writing possible (especially with left hand if right side paresis from stroke)

Wernicke’s Aphasia (Fluent)

• Lesion: Wernicke’s area, left temporo-parietal junction (often labelled temporal)
• Deficit: language comprehension → produced speech is grammatically smooth but semantically meaningless (“word salad”)
• Patient unaware of deficit
• Stroke prevalence: higher in LH because heart is slightly left-of-midline → emboli/clots more likely to lodge in left MCA territory

Recurring Exam Logic (Instructor’s Emphasis)

• Always start with requested BEHAVIOUR (say, draw, point with which hand?).
• Determine hemisphere responsible for that behaviour (motor control or language).
• Trace which visual field delivered the information to that hemisphere. Example walkthrough for “Right hand draws what it saw”:
  1. Right hand → LH motor cortex.
  2. LH only “knows” what appeared in RVF.
  3. Correct answer = stimulus in RVF.
• Practice repeatedly; questions will appear on exam.

Key Take-Home Points

• Corpus callosum integrates two semi-autonomous minds.
• Split-brain studies prove lateralization yet demonstrate covert cooperation.
• Visual field vs eye distinction is essential for interpreting experiments.
• Behavioural dissociations (speech vs drawing) map directly onto hemispheric specializations.
• Brain plasticity can compensate for massive cortical loss, including entire hemisphere removal.