Cerebral Cortex & Hemispheric Specialization Lecture

Cerebral Cortex – General Overview

• Outer‐most “gray matter” of the forebrain; seat of higher-order cognition that is uniquely human (planning, language, symbolic thought, self-awareness, etc.)
• Localization of function: different cognitive/behavioral abilities map onto different cortical areas, yet work in concert through dense interconnections.
• Cortex is divided into two hemispheres, each containing four lobes.

The Four Lobes – Location & Core Functions

1. Frontal Lobes (anterior / “front”)

• Executive functions: planning, decision making, impulse control, judgment, emotional regulation, "executive function" umbrella.
• Motor Cortex: orchestrates all voluntary muscle movements (including the fine motor choreography of speech).
• Broca’s Area (left frontal): language production; damage → Broca’s aphasia (intact comprehension, laborious speech).
• Damage Profile (e.g., Phineas Gage, sports concussions, TBI): personality change, risky decisions, emotional disinhibition, loss of social “filter.”

2. Parietal Lobes (posterior to frontal / “top-middle”)

• Somatosensory Cortex: integrates touch, temperature, pain, proprioception (body position), and helps build a coherent map of limbs-in-space.
• Supports movement guidance & multisensory perception (e.g., “alien-hand” disorders when callosal/parietal circuitry is disrupted).
• Damage → spatial neglect, disordered body awareness, impaired sensory discrimination.

3. Occipital Lobes (posterior / “back”)

• Primary & secondary visual processing centers.
• Impact trauma to back of head → transient “seeing stars” or blackout due to occipital disruption.

4. Temporal Lobes (inferior-lateral / “sides”)

• Auditory cortex: hearing & complex sound analysis.
• Wernicke’s Area (left temporal): language comprehension; damage → fluent but nonsensical speech (Wernicke’s aphasia).
• Memory systems: Hippocampus (located medially in temporal lobe) – consolidation of episodic & spatial memories.
• Elements of sexuality, affective bonding (lust vs. love networks; covered further in Unit 4).

Signature Case Study – Phineas Gage

• Railroad foreman (1848) survived tamping-iron blast that destroyed portions of ventromedial frontal cortex.
• Pre-accident: responsible, affable. Post-accident: irritable, profane, impulsive → seminal evidence for frontal-lobe role in personality & executive control.
• Methodological importance: lesion studies remain a core tool for inferring brain–behavior relations.

Traumatic Brain Injury & Modern Parallels

• Contact sports (e.g., professional football) often yield repeated frontal lobe impacts → chronic deficits in impulse control, mood regulation, and decision making.
• Highlights ethical debates on sports safety, long-term healthcare, and informed consent for athletes.

Neurotransmitter Interlude – Dopamine, Parkinson’s & Schizophrenia

• Substantia Nigra (midbrain, not cortical) supplies dopaminergic inhibition that keeps muscles from constant contraction.
• Parkinson’s disease: degeneration of dopaminergic neurons → resting tremor, rigidity, progressive motor decline.
  • Treatment: L\text{-dopa} (dopamine precursor) crosses blood–brain barrier.
  • Overmedication → excess dopamine → positive-symptom schizophrenia-like hallucinations; clinical proof of dopamine hypothesis of psychosis.
• Demonstrates system-wide drug effects: boosting one neurotransmitter affects all neural circuits that use it.

Language Production vs. Comprehension – Why Two Sites?

• Speech = highly complex motor sequence (lips, tongue, larynx). Hence production center (Broca) is adjacent to motor cortex in frontal lobe.
• Comprehension involves auditory decoding & semantic integration, located near primary auditory areas (Wernicke) in temporal lobe.
• Both centers are usually left-hemispheric ("LL" = Language → Left).
• Handedness: even most left-handed individuals keep language functions on the left; only a small minority have right-hemisphere language, and they are disproportionately left-handed.

Motor, Sensory & Visual Pathways – Laterality Rules

• Contralateral (opposite-side) organization for motor output & somatosensory/visual field input.
  • Right body/visual field → left hemisphere; left body/visual field → right hemisphere.
• Ipsilateral (same-side) exceptions: olfaction (smell) projects to same-side hemisphere initially.
• Bilateral input: Each eye sends data to both hemispheres via the optic chiasm below cortical level, preserving vision even without a corpus callosum.

Hemispheric Specialization – "Two Minds in One Skull"

Left Hemisphere (Analytic/Logical)

• Language (speech, writing, grammar).
• Sequential, stepwise processing (mathematics, logical analysis).
• Controls motor functions of right side; receives right-side sensory input.

Right Hemisphere (Holistic/Creative)

• Spatial reasoning, mental rotation, and complex scene analysis.
• "Analysis by touch" – identifying objects via haptics with eyes closed.
• Artistic, musical, metaphorical thinking; emotional prosody of speech.
• Controls motor functions of left side; receives left-side sensory input.

Corpus Callosum – The Bridge

• \approx 2\times10^{5} to 5\times10^{5} myelinated axons ("millions of fibers") connecting hemispheres.
• Allows rapid sharing of perceptual, cognitive, and emotional data → unified sense of self.
• Split-brain patients (surgical or congenital agenesis): hemispheres act semi-independently, revealing distinct "wants" or knowledge sets (e.g., left hand draws object the verbal right hand cannot name).

Key Lateralization Vocabulary

• Contralateral: opposite side processing (e.g., visual fields, motor control).
• Ipsilateral: same side processing (e.g., olfaction).
• Bilateral: information sent to both sides (e.g., retinal projections).

Preview & Connections

• Jill Bolte Taylor’s stroke (TED Talk): first-person account of losing one hemisphere’s function; illustrates subjective split-brain phenomenology and right/left specializations.
• Upcoming Unit 4: neural circuitry of lust vs. love – integrates temporal lobe (amygdala, hippocampus) with frontal regulatory loops.
• Ethical/clinical implications: balancing neuropharmacology (SSRIs, L-dopa) for targeted benefit vs. system-wide side effects; sports policy on head trauma.

Mini-Summary Cheat Sheet

• "LL" = Language Left; frontal = Plan & Act; parietal = Feel & Locate; occipital = See; temporal = Hear & Remember.
• Dopamine low → Parkinson’s; dopamine high → psychosis-like symptoms.
• Corpus callosum = cognitive "zipper"; without it, hemispheres reveal separate competencies & preferences.