Hemispheric Specialization, Split-Brain Research, and Memory Systems

Hemispheric Specialization & Redundancy

• Classic claim: “Language = left hemisphere.”
  • Reality is fuzzier; right hemisphere (RH) can perform limited language tasks (e.g., spelling simple words, naming familiar objects).
  • Functional redundancy across cortex → if one module is damaged another, partly-overlapping module can assume some functions (neuroplasticity).
• Jill Bolte Taylor example: after LH stroke her RH still handled rudimentary language, illustrating overlap.
• Key caution: do not over-simplify hemispheric labels ("LH = language, RH = creativity").

Split-Brain Experiments (Corpus Callosotomy)

• Surgery severs corpus callosum; hemispheres can no longer share cortical information.
• Goal in lab: deliver stimulus to only one hemisphere (possible only for vision & somatosensation—ears and eyes themselves are bilateral).
• Method to target RH:
  • Keep gaze on central fixation.
  • Flash image in left visual field (LVF).
  • NOT by covering an eye; each eye projects bilaterally.
• Clicker question recap:
  • A) Left visual field (correct)
  • B) Left eye (wrong)
  • C) Either/both (wrong)
• Mnemonic: “Visual field = contralateral; Eyes & Ears = bilateral.”
  \text{LVF} \rightarrow \text{RH},\; \text{RVF} \rightarrow \text{LH}
• Optic chiasm (sub-cortical) transmits each eye’s info to both hemispheres; corpus callosum lies in cortex, so closing one eye cannot force lateralisation.

Joe Demonstration (Video Recap)

• Stimuli: HAMMER (RVF → LH) | SAW (LVF → RH).
• When asked verbally → “hammer” (LH has language output).
• When asked to draw with left hand (LH controls right hand; RH controls left hand) → draws a saw.
• Asked why? “I don’t know.” – RH knowledge inaccessible to LH language.
• RH could also spell “S-A-W” with left hand → limited language capacity.

Unconscious vs Conscious Self

• LH = narrative, verbal, “conscious” interpreter.
• RH = non-verbal, “nonconscious” intentions (e.g., patient Vicky’s LH said “I don’t want that hat,” but LH hand still grabbed it).

Neuroplasticity & Functional Compensation

• Damaged regions can be taken over by adjacent/contralateral regions—depends heavily on age.
• Younger brains (≈ < 20 y) exhibit stronger plasticity → better recovery from lesions/TBI.

Memory Systems Overview

• Explicit / Declarative
  • Episodic (events) & Semantic (facts).
  • Hippocampally mediated.
• Implicit / Non-Declarative
  • Procedural skills, priming, classical conditioning, non-associative learning.
  • Not hippocampally mediated; distributed across basal ganglia, cerebellum, neocortex, amygdala, etc.

Wilder Penfield & The Homunculi

• Neurosurgeon (1891–1976); pioneered awake cortical stimulation.
• Discovered motor & somatosensory homunculus (topographic map):
  • Large cortical area for hands & lips (dense receptor/motor units), small for elbows.
• During epilepsy/tumor operations, gently stimulated cortex with electrodes:
  • Mapped motor twitches or somatic sensations.
  • Unexpectedly evoked vivid, multimodal memory flashes (sounds, voices, music) in a minority of patients.
  • Patient perceived music as if externally present; full sensory richness.
  • Demonstrated that triggering a node in the network can "enter the circuit" of a memory (cf. Max Cynader’s TED Talk).

Implications of Penfield

• Memories are distributed circuits, not single "files" stored in one spot.
• Stimulation ignites a pattern spanning auditory, visual, emotional cortices → reconstructs experience.

Engram & Circuitry – Lashley’s Legacy

• Karl Lashley searched for locale of a memory engram via rat maze lesions.
  • Found performance deficits correlated with size, not site of the lesion.
• Supports distributed‐circuit view above; Penfield provides human evidence.

Ribot’s Law (1881)

• In amnesia, earlier (remote) memories survive better than recent ones.
• Example: H.M. (Henry Molaison) & Clive Wearing retained childhood facts/events yet could not form new episodic memories (anterograde amnesia).

Nomadic Memory Hypothesis

• Describes time-dependent migration of memory traces.
  1. Initial Encoding
  • Requires hippocampus + neocortex; fragile.
  1. Systems Consolidation (days → years)
  • Repeated reactivation (esp. during sleep) strengthens cortico-cortical connections.
  1. Remote Memory
  • Retrieval relies primarily on distributed neocortical network; hippocampus becomes optional.
• Thus patients with hippocampal destruction lose capacity for new explicit memories yet preserve many pre-lesion memories.
• Consolidation is gradual—no strict 24 h boundary ("50 First Dates" amnesia is fictional).

Clarifying Movie Myth (“50 First Dates”)

• Condition portrayed (full-day memory that resets nightly) does not exist; only short-term (≈ 30 s) or long-term (> 30 s) systems.

Practical & Ethical Takeaways

• Awake brain surgery necessitates mapping individual functional topography to avoid critical eloquent cortex.
• Electrical stimulation can intrude on subjective experience—raises questions about autonomy, privacy of thoughts.
• Neuroplasticity research guides rehabilitation protocols post-stroke/TBI.

Numerical / Technical Nuggets

• Optic routing: \text{LVF}\to\text{RH},\;\text{RVF}\to\text{LH} (contralateral) vs. each eye \to both hemispheres (bilateral).
• Possible neuronal connection permutations > number of atoms in the observable universe (demonstrates circuit complexity).

Upcoming Lecture Road-Map (pre-Unit 3 exam)

1. Mechanisms of memory storage & consolidation in the brain.
2. Brain injury, surgical lesions, and recovery (neuroplasticity).
3. Cellular/biochemical basis of neural change (synaptic plasticity, LTP, pruning, etc.).

Mantra for visual lateralisation:
\text{Visual Field} = \text{Contralateral}, \; \text{Eyes/Ears} = \text{Bilateral}.