Neuro 101: Cerebral Hemispheres

Frontal Lobe

Clinicoanatomic correlation

• Motor function
• Controlled by motor cortex
• Corticospinal tract
  • origin of the primary motor tract from the brain to the spinal cord
• Left side of brain controls right side of body and vice versa

Executive Functions

• Controlled by the prefrontal cortex
• Involve:
  • Focus and concentration
  • Regulation of motor and sensory input
  • Planning and initiating actions
  • Maintaining task performance
  • Self-monitoring and correction

Learned Motor Patterns

• Controlled by the premotor cortex
• Examples include habitual or rehearsed actions such as climbing stairs

Volitional Eye Movements

• Controlled by the frontal eye fields
• Key directional behaviours:
  • Activation of the right frontal eye field turns the eyes to the left
  • Stroke: eyes deviate toward the lesion
  • Seizure: eyes deviate away from the lesion

Speech and Language

• Broca’s area (in the frontal lobe) coordinates speech production
• It works in conjunction with Wernicke’s area (in the temporal lobe) for language comprehension

Broca’s Aphasia

• Results from damage to the dominant hemisphere
  • In right-handed individuals and most left-handed individuals, this is the left hemisphere
• Features include:
  • Halting or hesitant speech
  • Word substitutions
  • Omission of small linking words (e.g., articles and prepositions)
  • Impaired repetition of phrases

---

Temporal Lobe

Important regions

Clinicoanatomic Correlation

Hearing

• Primary auditory cortex: responsible for hearing and initial sound processing
• Associative auditory cortex: interprets spoken and written language
• Cortical deafness: requires bilateral temporal lobe lesions (extremely rare)

Language

• Wernicke’s area: essential for language comprehension
  • Wernicke’s aphasia:
    • Occurs with damage to the dominant hemisphere
    • Produces fluent but meaningless speech
    • Marked difficulty understanding spoken language
• Associative aphasia (conduction aphasia):
  • Results from damage to the arcuate fasciculus
  • Characterised by:
    • Inability to repeat spoken language
    • Difficulty writing coherent language

Emotions and Memory

• Governed by the limbic lobe
• Bilateral lesions of the hippocampus and thalamus can result in profound memory loss

Temporal Lobe Seizures

• Often begin in the first or second decade of life
• Common features:
  • Visual or auditory hallucinations
  • Olfactory hallucinations
  • Automatisms (e.g. lip-smacking, repetitive movements)
  • Déjà vu or jamais vu
  • Clouding of consciousness

---

Parietal Lobe

Clinicoanatomic Correlation

Sensory Function

• Mediated by the primary somatosensory cortex and associative cortices (Brodmann areas 5 and 7)
• Cutaneous sensibility:
  • Pinprick
  • Temperature
  • Light touch
  • Vibration
  • Proprioception
• Cortical sensibility:
  • Two-point discrimination
  • Graphesthesia
  • Stereognosis
  • Double simultaneous stimulation
  • Somatognosia

Extinction

• Inability to perceive multiple simultaneous stimuli of the same type (e.g., tactile or visual)
• Typically occurs in the contralesional field

Neglect

• Failure to perceive or attend to stimuli on one side of the body
• Often occurs with non-dominant hemisphere lesions (usually right-sided)

Parietal Stroke Symptoms

General Signs

• Agraphesthesia
• Loss of two-point discrimination
• Astereognosis
• Extinction

Dominant Hemisphere Involvement

• Right-left confusion
• Finger agnosia
• Acalculia
• Agraphia

These features together comprise Gerstmann syndrome

Other Symptoms

• Asomatognosia (unawareness of parts of one’s own body)
• Dressing apraxia
• Constructional apraxia
• Geographic disorientation

---

Occipital Lobe

Clinicoanatomic Correlation

Calcarine Cortex

• Located within the calcarine fissure
• Critical for visual processing

Cortical Blindness

• Requires bilateral occipital lobe lesions
• May result in Anton’s syndrome:
  • Patient is unaware of blindness
  • May confabulate visual experiences

Disconnection Syndrome

• Occurs due to lesion in left (dominant) visual cortex and corpus callosum
• Causes loss of input from the right visual field
• Corpus callosum damage prevents information transfer to left parietal language areas
• Produces alexia without agraphia:
  • Patient cannot read (alexia)
  • But can still write (agraphia not affected)

---

Integrated Concepts

Sensory and Motor Homunculi

• The homunculus is a visual representation mapping body parts to their corresponding motor and sensory cortical areas
• Areas requiring fine control or sensation (e.g. hands, face) occupy larger cortical regions

Visual Field Deficits

• Can occur due to damage anywhere along the visual pathway: retina, optic nerves, chiasm, tracts, radiations, or occipital cortex
• Lesions in the occipital lobe commonly cause homonymous hemianopia

---

Blood Supply of the Brain

Anterior Circulation

Anterior Cerebral Artery (ACA) Stroke

• Contralateral sensory-motor deficits, especially affecting leg and foot
• Eyes and head often deviate towards the lesion
• Rigidity may be present

Middle Cerebral Artery (MCA) Stroke

• Contralateral hemiplegia, more pronounced in face, arm and shoulder
• Hemianaesthesia (sensory loss)
• Homonymous hemianopia
• Eyes deviate towards the lesion
• Dominant hemisphere involvement:
  • Aphasia (Broca’s, Wernicke’s or global)
• Non-dominant hemisphere involvement:
  • Agnosia
  • Neglect

Posterior Cerebral Artery (PCA) Stroke

• Involves occipital lobe, particularly the calcarine cortex and optic radiations
• Causes homonymous hemianopia

---

Posterior Circulation Stroke

• Typically produces bilateral or crossed symptoms
• Clinical features:
  • Headache
  • Nausea and vomiting
  • Vertigo
  • Nystagmus
  • Limb weakness
  • Extraocular movement abnormalities
  • Ataxia

The “D” Symptoms of Posterior Stroke

• Dizziness
• Dysarthria
• Disequilibrium (or dysataxia)
• Deafness
• Diplopia
• Dysphagia
• Dysphonia

---

Clinical Neurology Essentials

• Coni R. Neuro 101: Neurological Examination. LITFL
• Coni R. Neuro 101: Cerebral Hemispheres. LITFL
• Coni R. Neuro 101: Cerebellum and Basal Ganglia. LITFL
• Coni R. Neuro 101: Brainstem. LITFL
• Coni R. Neuro 101: Cranial Nerves. LITFL
• Coni R. Neuro 101: Spinal Cord. LITFL
• Coni R. Neuro 101: Peripheral Nervous System. LITFL

---

References

Further reading

• Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 8e 2021
• Campbell WW. DeJong’s The Neurologic Examination. 8e 2019
• Fuller G. Neurological Examination Made Easy. 6e 2019
• Kiernan J. Barr’s The Human Nervous System: An Anatomical Viewpoint. 10e 2015
• O’Brien M. Aids to the Examination of the Peripheral Nervous System. 6e 2023
• Patten JP. Neurological Differential Diagnosis. 2e 1996
• Waxman SG. Correlative Neuroanatomy. 23e 1996

Publications

• Carrera E, Michel P, Bogousslavsky J. Anteromedian, central, and posterolateral infarcts of the thalamus: three variant types. Stroke. 2004 Dec;35(12):2826-31.
• Datar S, Rabinstein AA. Cerebellar infarction. Neurol Clin. 2014 Nov;32(4):979-91
• Fama ME, Turkeltaub PE. Treatment of poststroke aphasia: current practice and new directions. Semin Neurol. 2014 Nov;34(5):504-13
• Fanciulli A, Wenning GK. Multiple-system atrophy. N Engl J Med. 2015 Jan 15;372(3):249-63. 
• Kelly PJ, Stein J, Shafqat S, Eskey C, Doherty D, Chang Y, Kurina A, Furie KL. Functional recovery after rehabilitation for cerebellar stroke. Stroke. 2001 Feb;32(2):530-4.
• Kwong Yew K, Abdul Halim S, Liza-Sharmini AT, Tharakan J. Recurrent bilateral occipital infarct with cortical blindness and anton syndrome. Case Rep Ophthalmol Med. 2014;2014:795837.
• Li K, Malhotra PA. Spatial neglect. Pract Neurol. 2015 Oct;15(5):333-9. doi: 10.1136/practneurol-2015-001115. Epub 2015