Neuroanatomy — Cortical Lobe Function and Tracts

Cortical Organization and Functional Hierarchies

• The primary motor cortex (M1), located in Brodmann area 4 at the precentral gyrus, contains the motor homunculus with somatotopic organization; neurons here generate descending commands via the corticospinal tract (90% cross at the medullary pyramids forming the decussation)
• Supplementary motor area (SMA), located on the medial surface anterior to M1, coordinates bilateral limb movements and contributes to motor planning; SMA lesions produce alien hand syndrome and loss of complex motor sequences
• Primary sensory cortex (S1), located in Brodmann areas 1, 2, 3 at the postcentral gyrus, receives thalamic input (VPL nucleus for body, VPM for face) and contains an inverted sensory homunculus; damage produces contralateral sensory loss with preserved reflexes and vibratory sense in dorsal column disease

Language Network and Major Tracts

• Broca's area (Brodmann area 44-45, inferior frontal gyrus) is the motor speech center; lesions produce expressive (non-fluent) aphasia characterized by agrammatism, anomia, and effortful speech with preserved comprehension
• Wernicke's area (Brodmann area 22, superior temporal gyrus) is the receptive language center; lesions produce receptive (fluent) aphasia with fluent but meaningless output, impaired comprehension, and preserved repetition in conduction aphasia
• The arcuate fasciculus is a white matter tract connecting Broca's and Wernicke's areas; interruption produces conduction aphasia (fluent output, impaired comprehension, severely impaired repetition despite relatively preserved naming)
• The superior longitudinal fasciculus (SLF) connects anterior (prefrontal) and posterior (temporal-parietal) language regions; disruption contributes to global aphasia patterns

Prefrontal, Parietal, Temporal, and Occipital Functions

• The dorsolateral prefrontal cortex (DLPFC), supplied by the middle cerebral artery (MCA), mediates executive function, working memory, and impulse control; lesions produce perseveration, loss of initiative, and poor planning (frontal lobe syndrome)
• The orbitofrontal cortex, supplied by the anterior cerebral artery (ACA), regulates emotional control and decision-making; focal lesions produce disinhibition and emotional lability
• The posterior parietal cortex (Brodmann area 5-7) integrates sensory-motor information for limb guidance and spatial awareness; superior parietal lesions cause optic ataxia (inability to visually guide limbs) and simultanagnosia (inability to perceive multiple objects in space)
• The posterior inferior parietal cortex (angular gyrus, Brodmann area 39; supramarginal gyrus, Brodmann area 40) is critical for reading (angular), calculation, and language; lesions produce alexia and agraphia
• The superior temporal gyrus processes auditory language; damage contralateral to dominant hemisphere causes word deafness
• The temporal poles and anterior medial temporal lobes (including hippocampus and amygdala) mediate memory consolidation and emotional processing; lesions produce anterograde amnesia and emotional dysregulation
• The primary visual cortex (V1), located at the calcarine fissure in the occipital lobe and supplied by the posterior cerebral artery (PCA), receives input from the lateral geniculate nucleus via the optic radiations; lesions produce contralateral homonymous hemianopsia (same visual field defect in both eyes)
• Superior optic radiations (parietal radiations, Meyer's loop anteriorly) carry information from the inferior visual quadrant; parietal damage causes superior quadrantanopsia, while anterior temporal (Meyer's loop) damage causes superior altitudinal defect or pie-in-the-sky hemianopsia

Major Descending and Ascending Tracts

• The corticospinal tract originates from M1 and supplementary areas, descends through the internal capsule (posterior limb is critical), brainstem, and crosses at the medullary pyramids (~90%); uncrossed fibers form the ventral corticospinal tract (10%); damage produces contralateral upper motor neuron weakness with hyperreflexia and spasticity; internal capsule lesions produce the classic contralateral hemiparesis (arm > leg often in MCA stroke due to somatotopy)
• The thalamocortical radiations carrying somatosensory information ascend via the medial lemniscus and spinothalamic tract; dorsal column-medial lemniscal pathway carries fine touch and proprioception (decussates in medulla); spinothalamic tract carries pain and temperature (decussates in spinal cord at level of entry)
• Association tracts linking cortical areas include the superior longitudinal fasciculus (connecting frontal, parietal, and temporal regions), inferior longitudinal fasciculus (temporal-occipital connections for visual-semantic processing), inferior fronto-occipital fasciculus (orbital prefrontal to occipital), and the uncinate fasciculus (ventral prefrontal to anterior temporal, critical for semantic and emotional processing)
• Commissural tracts, particularly the corpus callosum (rostral to caudal: rostrum, genu, body, splenium), allow interhemispheric communication; lesions produce split-brain syndrome with inability to transfer information between hemispheres

Structural Lesions Affecting Cortical Function and Tracts

• Ischemic stroke (thrombotic or embolic) affecting major cerebral arteries: MCA territory (most common, causing contralateral weakness, sensory loss, and language/visuospatial deficits depending on hemisphere dominance), ACA territory (medial frontal and supplementary motor area), PCA territory (visual field defects and memory disturbance)
• Hemorrhagic stroke (hypertensive, amyloid angiopathy, vascular malformation, anticoagulation-related) producing focal neurological deficits with mass effect
• Brain tumors (primary or metastatic) causing progressive focal deficits; gliomas preferentially affecting white matter tracts
• Traumatic brain injury with cortical contusions (frontal and temporal poles most common) or diffuse axonal injury (DAI) affecting white matter tracts
• Neurodegenerative diseases: primary progressive aphasia (PPA) variants damage dominant temporal-parietal and frontal regions; frontotemporal dementia affects prefrontal and anterior temporal lobes
• Demyelinating disease (multiple sclerosis) attacking white matter tracts; demyelinating plaques in corpus callosum, internal capsule, or optic radiations produce distinctive deficits
• Infections: herpes simplex encephalitis (HSV-1) with predilection for temporal lobes; neurosyphilis causing general paresis of the insane with widespread cortical atrophy
• Traumatic white matter injury and diffuse axonal injury (DAI) from severe head trauma affecting major fiber tracts at gray-white interfaces
• Metabolic encephalopathies, hypoxic-ischemic injury, and toxic exposures (e.g., carbon monoxide, heavy metals) producing diffuse cortical dysfunction

Motor and Sensory Deficits

• Contralateral upper motor neuron weakness: facial droop (sparing lower face if lesion is contralateral to motor cortex), arm > leg depending on somatotopic location; preserved facial reflexes and increased tone/hyperreflexia distinguish UMN from LMN lesions
• Contralateral sensory loss (fine touch, proprioception if medial lemniscus involved; pain/temperature if spinothalamic lesion) without loss of reflexes or proprioception globally
• Apraxia: inability to perform learned motor acts despite intact strength and comprehension; ideomotor apraxia (damage to premotor cortex or SMA) produces difficulty pantomiming actions while object use may be preserved; ideational apraxia (damage to conceptual motor planning) produces inability to sequence complex multi-step actions
• Ataxia from posterior parietal cortex lesions with relatively preserved strength and reflexes (sensory ataxia pattern)

Language Disturbances

• Broca's aphasia (expressive, non-fluent, agrammatic): effortful, slow, agrammatic speech with normal or slightly reduced comprehension; repetition usually preserved; hallmark is agrammatism (loss of function words and grammatical structure); anomia (word-finding difficulty) prominent; writing affected similarly to speech
• Wernicke's aphasia (receptive, fluent, semantic): fluent output that lacks meaning ("word salad"), severely impaired comprehension, relatively preserved repetition early in course, prominent anomia and paraphasias; reading and writing equally impaired
• Conduction aphasia: fluent spontaneous speech, relatively preserved comprehension, but severely impaired repetition (hallmark); deficit in arcuate fasciculus; anomia present
• Global aphasia: severe impairment in all language domains (expression, comprehension, repetition); typically from large MCA infarcts affecting both Broca's and Wernicke's areas
• Anomia: word-finding difficulty; relatively preserved comprehension and syntax; seen across all aphasias but isolated anomic aphasia suggests angular gyrus or anterior temporal pathology
• Alexia (inability to read) with or without agraphia (inability to write): alexia without agraphia suggests posterior cerebral artery stroke with disconnection of visual cortex from language areas; alexia with agraphia suggests angular gyrus lesion

Visuospatial and Perceptual Deficits

• Homonymous hemianopsia: loss of the same visual field in both eyes (e.g., right homonymous hemianopsia = loss of right visual field in both eyes); indicates lesion contralateral to the visual field loss; cannot occur with monocular vision loss (which indicates retinal/optic nerve pathology)
• Superior quadrantanopsia ("pie in the sky"): loss of superior visual quadrants bilaterally, suggesting disruption of inferior optic radiations (Meyer's loop) in anterior temporal lobe; characteristic of temporal lobe anterior extension of stroke
• Inferior quadrantanopsia: loss of inferior visual quadrants, suggesting disruption of superior optic radiations in parietal lobe
• Cortical blindness with denial (Anton syndrome): bilateral occipital lobe damage causing blindness with confabulation and denial of visual loss
• Neglect syndrome: inability to attend to contralateral side of space (usually right posterior parietal damage causes left neglect, more severe than opposite); manifests as failure to eat from left side of plate, inability to read left margin of page, failure to groom left side of body
• Optic ataxia: inability to visually guide limbs despite intact vision; suggests dorsal stream (parietal) dysfunction
• Simultanagnosia: inability to perceive multiple objects in visual space despite intact individual object recognition; suggests bilateral parietal-occipital junction damage (Balint's syndrome when combined with optic ataxia and ocular apraxia)

Cognitive and Behavioral Deficits

• Frontal lobe syndrome: apathy, loss of motivation, reduced speech output (abulia), poor decision-making, perseveration (persisting with same response despite feedback), disinhibition (orbitofrontal damage), poor planning and executive function
• Utilization behavior: involuntary use of objects in the environment; suggests inferior prefrontal damage
• Akinetic mutism: profound loss of spontaneous speech and movement; suggests bilateral medial frontal or mesencephalic damage (extreme form of abulia)
• Alien hand syndrome: one limb (usually left in right hemisphere dominant individuals) performs purposeful-appearing movements contrary to the patient's intent; suggests medial frontal/supplementary motor area or callosal damage
• Temporal lobe epilepsy: seizures with automatisms (lip smacking, hand movements), olfactory or gustatory auras, depersonalization; suggests anterior temporal lobe pathology

Memory Disturbances

• Anterograde amnesia: inability to form new memories; suggests hippocampal or medial temporal lobe damage
• Retrograde amnesia: loss of previously stored memories; suggests medial temporal lobe or anterior temporal damage

Clinical Bedside Assessment

• Detailed neurological examination with specific attention to: (1) motor system for localization of weakness and identification of UMN vs. LMN pattern; (2) sensory examination for level and pattern of sensory loss; (3) cranial nerve examination (facial droop, visual fields by confrontation) for cortical localization; (4) cognitive and language screening with Mini-Cog or Montreal Cognitive Assessment for gross cognitive impairment, Boston Naming Test for anomia, Token Test for comprehension, repetition testing
• Formal language testing by speech-language pathology when aphasia suspected: assess spontaneous speech (fluency, grammar, anomia), comprehension (single commands vs. complex instructions), repetition (critical for distinguishing conduction from other aphasias), reading, writing
• Visual field testing by confrontation comparing quadrants and comparing patient's vision to examiner's; mapping defects (complete vs. incomplete, altitudinal vs. hemianopic pattern) narrows anatomic localization

Neuroimaging

• Diffusion-weighted imaging (DWI) MRI: gold standard for acute ischemic stroke, showing restricted water diffusion within minutes of symptom onset; hyperintense on DWI with corresponding hypointense ADC indicates acute infarction; sensitivity ~90% within first 24 hours
• Perfusion-weighted imaging (PWI): shows areas of hypoperfusion; PWI-DWI mismatch identifies penumbral tissue suitable for thrombolytic or thrombectomy intervention
• Structural MRI (T1, T2, FLAIR): identifies chronic infarcts, hemorrhage (hyperintense on T1, hypointense on T2), mass lesions; FLAIR sensitive for cortical ribboning in encephalitis
• CT head (non-contrast): first-line for acute stroke to exclude hemorrhage; less sensitive than MRI for acute ischemia but sufficient for clinical decision-making
• MR spectroscopy: shows decreased NAA (marker of neuronal loss), increased lactate (anaerobic metabolism), and increased choline (membrane turnover) in areas of infarction or tumor
• Functional MRI (fMRI) and diffusion tensor imaging (DTI): research tools for mapping eloquent cortex and fiber tract integrity; increasingly used for surgical planning in patients with tumors near language areas
• Angiography (MRA, CTA, or digital subtraction): identifies vascular occlusions, stenosis, or malformations; essential in acute stroke evaluation

Electroencephalography (EEG)

• Focal slowing (theta or delta waves) over lesion site; helps lateralize and localize structural lesions
• Seizure activity: repetitive spike-and-wave discharges; status epilepticus requires emergent intervention
• Periodic lateralized epileptiform discharges (PLEDs): focal spikes at 1-2 Hz interval; associated with acute cortical pathology (stroke, HSV encephalitis, tumor)

Cerebrospinal

Immediate stabilization (any acute focal deficit)

• Airway, breathing, circulation, and fingerstick glucose: hypoglycemia is the classic stroke mimic and must be corrected before the deficit is attributed to infarction.
• Emergent non-contrast head CT: the single best next step in suspected acute stroke — it separates ischemic from hemorrhagic disease and gates all downstream therapy (AHA/ASA Guidelines for the Early Management of Acute Ischemic Stroke).

First-line reperfusion (ischemic stroke)

• IV thrombolysis with a fibrinolytic: alteplase 0.9 mg/kg (maximum 90 mg, 10% as bolus) within 4.5 hours of last known well; AHA/ASA also endorses tenecteplase as a reasonable alternative in selected patients. Blood pressure must be brought below 185/110 mmHg before the bolus and kept below 180/105 mmHg for 24 hours afterward.
• Mechanical thrombectomy: for large-vessel occlusion (ICA, M1), within 6 hours, and up to 24 hours when perfusion imaging shows a favorable core–penumbra mismatch (AHA/ASA).

Escalation and secondary prevention

• Antiplatelet therapy: aspirin within 24–48 hours (delayed 24 hours after thrombolysis); short-course dual antiplatelet therapy with aspirin plus clopidogrel for minor stroke or high-risk TIA, then monotherapy (AHA/ASA Secondary Prevention Guideline).
• High-intensity statin, blood pressure control, and oral anticoagulation if atrial fibrillation is the source: a DOAC for nonvalvular AF, but warfarin if a mechanical heart valve or moderate-to-severe mitral stenosis is present.

Cause-specific and definitive therapy

• Antivirals: empiric IV acyclovir 10 mg/kg q8h for suspected HSV encephalitis with temporal-lobe involvement — start before PCR returns (IDSA encephalitis guideline).
• Surgery: decompressive hemicraniectomy for malignant MCA infarction with midline shift; resection/biopsy with adjuvant therapy for tumors (NCCN CNS Cancers); carotid endarterectomy for symptomatic high-grade stenosis.
• Rehabilitation: early, intensive speech-language, physical, and occupational therapy is the mainstay of recovery for aphasia and neglect once the acute lesion is treated (AHA/ASA Stroke Rehabilitation Guideline); timely reperfusion also improves these outcomes by limiting infarct size, and adjuncts such as constraint-induced language therapy and prism adaptation for neglect are used in selected patients.

Contraindicated / avoid (representative AHA/ASA exclusions to thrombolysis, not exhaustive)

• Intracranial hemorrhage on CT, recent intracranial/intraspinal surgery, ischemic stroke or significant head trauma within the prior 3 months, active internal bleeding or known bleeding diathesis, platelets below 100,000/µL, and blood pressure that cannot be lowered below 185/110 mmHg; glucose below 50 mg/dL should be corrected first because hypoglycemia mimics stroke.
• Aggressive BP lowering in non-thrombolysis candidates (permissive hypertension preserves penumbral perfusion).
• Corticosteroids for cytotoxic edema of ischemic stroke — useful for vasogenic tumor edema, harmful/ineffective here.

Emergencies

• Malignant cerebral edema with herniation: cytotoxic swelling of a large MCA infarct peaks at 48–72 hours; signaled by declining level of consciousness, then ipsilateral blown pupil from uncal compression of CN III and contralateral posturing. Requires osmotic therapy and decompressive hemicraniectomy.
• Hemorrhagic transformation: reperfusion into infarcted, blood–brain-barrier-disrupted tissue, most common after thrombolysis or thrombectomy; signaled by abrupt headache, vomiting, neurologic worsening, or a rising blood pressure — stop the lytic and obtain immediate non-contrast CT.
• Orolingual angioedema from alteplase: bradykinin accumulation, markedly more frequent in patients on ACE inhibitors; tongue and lip swelling minutes into the infusion is an airway emergency.
• Status epilepticus: cortical irritation from infarct, hemorrhage, or HSV encephalitis; suspect in any patient who fails to wake after the deficit stabilizes, and obtain EEG.

Subacute complications of the lesion

• Aspiration pneumonia: pharyngeal weakness and impaired cough; strict NPO until a bedside dysphagia screen is passed is a standard AHA/ASA quality measure.
• Post-stroke seizures and epilepsy: gliotic cortical scar becomes an epileptogenic focus, typically months later; treat recurrent seizures, not a single acute symptomatic one.
• Spasticity and contracture: loss of corticospinal inhibition of alpha motor neurons produces clasp-knife hypertonia, hyperreflexia, and extensor plantar response.
• **Central post-stroke pain (Dejerine–Roussy)**: burning contralateral hemibody pain after thalamic/sensory pathway injury; often with allodynia.
• Hemiplegic shoulder subluxation, falls, and pressure injury: neglect and hemianopia compound the motor deficit; patients with anosognosia deny the deficit and fall.
• Post-stroke depression and vascular cognitive impairment: common, under-diagnosed, and independently worsen rehabilitation outcomes.
• DVT and pulmonary embolism: immobility; prophylaxis with intermittent pneumatic compression, adding pharmacologic prophylaxis once hemorrhage is excluded.

Treatment-related

• Thrombectomy: vessel perforation, dissection, or distal embolization to a previously uninvolved territory — new deficit on the table.
• Acyclovir crystal nephropathy: rising creatinine; prevented by generous IV hydration.
• Antiseizure drugs: rash including Stevens–Johnson syndrome (lamotrigine, carbamazepine, phenytoin; carbamazepine risk associated with *HLA-B\*1502*, for which screening is advised in patients of Asian ancestry), plus CYP interactions with enzyme-inducing agents such as phenytoin and carbamazepine.

• Repetition is the discriminator in aphasia: impaired repetition with fluent speech and intact comprehension = conduction aphasia (arcuate fasciculus). Preserved repetition with otherwise Broca- or Wernicke-like output = a transcortical aphasia (watershed infarct sparing the perisylvian core).
• Homonymous hemianopsia is always retrochiasmal and contralateral; monocular vision loss is never cortical — it is retina or optic nerve. This is the most frequently used distractor in visual-field stems.
• **Superior quadrantanopsia = pie in the sky = Meyer's loop, temporal lobe; inferior quadrantanopsia = parietal optic radiations. Occipital infarcts classically show macular sparing** because of collateral MCA supply to the occipital pole.
• Territory maps to homunculus: MCA infarct spares the leg (face/arm > leg); ACA infarct spares the face and arm (leg > arm) and adds abulia and urinary incontinence from medial frontal involvement.
• Upper motor neuron facial weakness spares the forehead because the upper face receives bilateral corticobulbar input — forehead involvement points to a peripheral CN VII (Bell's) lesion, not cortex.
• Alexia without agraphia = left PCA infarct of the occipital lobe plus the splenium of the corpus callosum: the patient can write a sentence but cannot read what was just written. Alexia with agraphia = angular gyrus.
• Fever, confusion, and temporal-lobe hemorrhagic changes on MRI = HSV-1 encephalitis: the single best next step is empiric IV acyclovir, started before CSF PCR results return (IDSA).
• The first step in any acute focal deficit is non-contrast head CT and a glucose check, not MRI — DWI is more sensitive for early ischemia but CT is what excludes hemorrhage and permits thrombolysis (AHA/ASA).
• Common distractor: giving antihypertensives to "protect the brain" in acute ischemic stroke. Outside the thrombolysis window/threshold, permissive hypertension is maintained; likewise, corticosteroids treat vasogenic tumor edema, not ischemic cytotoxic edema.