| Lobe | Boundaries | Primary Functions |
|---|---|---|
| Frontal | Anterior to central sulcus, superior to lateral fissure | Motor function, executive function, personality, speech production |
| Parietal | Between central and parieto-occipital sulcus | Somatosensory processing, spatial awareness, integration |
| Temporal | Inferior to lateral fissure | Auditory processing, memory, language comprehension, emotion |
| Occipital | Posterior to parieto-occipital sulcus | Visual processing |
| Insula | Deep to lateral fissure (hidden) | Interoception, taste, autonomic function, emotion |
| Limbic | Medial surface (cingulate, parahippocampal) | Emotion, memory, motivation |
| Layer | Name | Cell Types & Connections |
|---|---|---|
| I | Molecular layer | Few neurons; mainly dendrites and axons |
| II | External granular | Small pyramidal cells; corticocortical connections |
| III | External pyramidal | Medium pyramidal cells; corticocortical OUTPUT |
| IV | Internal granular | Stellate cells; thalamocortical INPUT (prominent in sensory cortex) |
| V | Internal pyramidal | Large pyramidal cells (Betz cells in M1); subcortical OUTPUT (corticospinal, corticobulbar) |
| VI | Multiform/Fusiform | Mixed cells; corticothalamic OUTPUT |
Layer IV = INPUT (thalamus → cortex), prominent in sensory areas. Layer V = OUTPUT (cortex → subcortical), prominent in motor areas. Motor cortex has thick layer V (Betz cells), thin layer IV. Sensory cortex has thick layer IV, thin layer V.
| Area | Location | Function |
|---|---|---|
| 4 | Precentral gyrus | Primary motor cortex (M1) |
| 6 | Premotor area | Premotor cortex, SMA |
| 3, 1, 2 | Postcentral gyrus | Primary somatosensory cortex (S1) |
| 17 | Calcarine cortex | Primary visual cortex (V1) |
| 41, 42 | Heschl’s gyrus | Primary auditory cortex |
| 44, 45 | Inferior frontal gyrus | Broca’s area (speech production) |
| 22 | Superior temporal gyrus | Wernicke’s area (language comprehension) |
| 39 | Angular gyrus | Reading, calculation, semantic processing |
| 40 | Supramarginal gyrus | Phonological processing, praxis |
Location: Anterior to central sulcus, superior to lateral fissure
Largest lobe (~1/3 of cortical surface)
Location: Precentral gyrus
Function: Voluntary motor control – direct control of contralateral body movements
Motor Homunculus:
Output:
Clinical:
Premotor Cortex (Lateral Area 6):
Supplementary Motor Area (Medial Area 6):
Clinical:
Functions:
Lesion: Executive dysfunction, poor planning, impaired working memory, perseveration
Functions:
Lesion: Disinhibition, impulsivity, inappropriate social behavior, poor judgment (Phineas Gage syndrome)
Functions:
Lesion: Abulia (lack of will/initiative), akinetic mutism, apathy
Location: Inferior frontal gyrus (pars opercularis and triangularis)
Function: Speech production, grammar, motor programming of speech
Lesion: Broca’s aphasia – nonfluent, effortful speech with preserved comprehension
Location: Posterior middle frontal gyrus
Function: Voluntary saccades to contralateral side
Lesion:
| Syndrome | Location | Features |
|---|---|---|
| Executive dysfunction | Dorsolateral PFC | Poor planning, organization, sequencing, perseveration |
| Disinhibition syndrome | Orbitofrontal | Impulsivity, inappropriate behavior, poor social judgment |
| Abulia/Akinetic mutism | Medial frontal/ACC | Lack of motivation, decreased spontaneous behavior/speech |
| Broca’s aphasia | Inferior frontal gyrus | Nonfluent speech, preserved comprehension |
| Alien limb syndrome | SMA/medial frontal | Involuntary purposeful limb movements |
| Grasp reflex | Frontal lobe (primitive reflex release) | Involuntary grasping when palm stimulated |
Frontal lobe release signs: Grasp reflex, snout reflex, palmomental reflex, glabellar reflex (Myerson’s sign). Suggest frontal lobe dysfunction (dementia, bilateral frontal lesions).
Location: Between central sulcus (anterior), parieto-occipital sulcus (posterior), lateral fissure (inferior)
Location: Postcentral gyrus
Function: Processing of contralateral somatosensory information (touch, proprioception, pain, temperature)
Sensory Homunculus:
Organization within S1:
Clinical:
Functions:
Clinical:
Functions:
Lesion: Conduction aphasia, ideomotor apraxia
Functions:
Lesion: Gerstmann syndrome (dominant hemisphere)
| Syndrome | Hemisphere | Features |
|---|---|---|
| Gerstmann Syndrome | Dominant (angular gyrus) |
4 A’s: • Acalculia • Agraphia • Finger agnosia • Left-right disorientation |
| Hemispatial Neglect | Non-dominant (usually right parietal) | Inattention to contralateral (left) space; may deny deficits (anosognosia) |
| Ideomotor Apraxia | Dominant parietal | Cannot perform learned motor acts to command (but can imitate) |
| Tactile Agnosia (Astereognosis) | Either | Cannot identify objects by touch despite intact sensation |
| Cortical Sensory Loss | Either | Impaired stereognosis, graphesthesia, two-point discrimination |
Hemispatial neglect is MORE COMMON and SEVERE with RIGHT parietal lesions (non-dominant). Left hemisphere attends to right space; right hemisphere attends to BOTH sides. So right parietal damage = severe left neglect.
Location: Inferior to lateral fissure, anterior to occipital lobe
Location: Heschl’s gyrus (transverse temporal gyrus) – hidden on superior temporal plane
Function: Processing of auditory information
Tonotopic organization: Different frequencies mapped along gyrus
Bilateral representation: Each ear projects to both hemispheres (unlike vision)
Clinical:
Location: Posterior superior temporal gyrus (dominant hemisphere)
Function: Language comprehension (spoken and written)
Clinical – Wernicke’s Aphasia:
Functions:
Clinical:
Functions:
Clinical:
| Syndrome | Location | Features |
|---|---|---|
| Wernicke’s Aphasia | Posterior STG (dominant) | Fluent speech, poor comprehension, paraphasias |
| Anterograde Amnesia | Bilateral hippocampi | Cannot form new memories (learning impaired) |
| Klüver-Bucy Syndrome | Bilateral amygdala | Hyperorality, hypersexuality, placidity, visual agnosia |
| Auditory Agnosia | Bilateral auditory cortex | Cannot recognize sounds despite intact hearing |
| Temporal Lobe Epilepsy | Mesial temporal (hippocampus, amygdala) | Aura (déjà vu, fear, olfactory), automatisms, impaired awareness |
| Superior Quadrantanopia | Meyer’s loop (temporal) | “Pie in the sky” – contralateral upper visual field loss |
Temporal lobe epilepsy aura: Rising epigastric sensation, fear, déjà vu, olfactory/gustatory hallucinations, autonomic symptoms. Followed by behavioral arrest and automatisms (lip smacking, fumbling).
Location: Posterior to parieto-occipital sulcus
Location: Calcarine cortex (banks of calcarine sulcus)
Function: Initial cortical processing of visual information
Retinotopic organization:
Blood supply:
Clinical:
Location: Surrounding V1, extending into parietal and temporal lobes
| Syndrome | Location | Features |
|---|---|---|
| Cortical Blindness | Bilateral V1 | Complete vision loss with intact pupillary reflex |
| Anton Syndrome | Bilateral V1 | Cortical blindness + denial of blindness (confabulation) |
| Balint Syndrome | Bilateral parieto-occipital |
Triad: • Simultanagnosia (can’t see whole scene) • Optic ataxia (misreaching) • Ocular apraxia (can’t direct gaze) |
| Prosopagnosia | Bilateral fusiform gyrus | Cannot recognize faces (can recognize by voice) |
| Achromatopsia | V4 (bilateral) | Loss of color vision (world appears gray) |
| Visual Agnosia | Ventral stream | Cannot recognize objects by sight (can recognize by touch) |
| Akinetopsia | V5/MT (bilateral) | Cannot perceive motion (sees world as snapshots) |
Anton syndrome = cortical blindness + anosognosia. Patient denies being blind and confabulates. Due to bilateral PCA infarcts. Also: Macular sparing in PCA stroke = dual blood supply from MCA.
| Structure | Location | Function |
|---|---|---|
| Broca’s Area | Inferior frontal gyrus (44, 45) | Speech production, grammar |
| Wernicke’s Area | Posterior STG (22) | Language comprehension |
| Arcuate Fasciculus | White matter tract | Connects Broca’s and Wernicke’s (repetition) |
| Angular Gyrus | Inferior parietal (39) | Reading, writing, semantic processing |
| Supramarginal Gyrus | Inferior parietal (40) | Phonological processing |
| Aphasia Type | Fluency | Comprehension | Repetition | Lesion |
|---|---|---|---|---|
| Broca’s | Non-fluent | Intact | Impaired | Inferior frontal |
| Wernicke’s | Fluent | Impaired | Impaired | Posterior temporal |
| Conduction | Fluent | Intact | Impaired | Arcuate fasciculus |
| Global | Non-fluent | Impaired | Impaired | Large perisylvian |
| Transcortical Motor | Non-fluent | Intact | Intact | Anterior/superior to Broca’s |
| Transcortical Sensory | Fluent | Impaired | Intact | Posterior to Wernicke’s |
| Anomic | Fluent | Intact | Intact | Variable (angular gyrus) |
Transcortical aphasias have INTACT REPETITION (perisylvian language areas spared). Key feature: patient can repeat but has other language deficits. Often watershed infarcts.
| Disorder | Definition | Lesion Location |
|---|---|---|
| Alexia without Agraphia | Cannot read but can write | Left occipital + splenium (disconnects visual input from angular gyrus) |
| Alexia with Agraphia | Cannot read or write | Angular gyrus (dominant) |
| Apraxia of Speech | Motor programming of speech impaired (effortful, groping) | Premotor/insula (dominant) |
| Dysarthria | Motor execution of speech impaired | Motor cortex, brainstem, cerebellum, nerves, muscles |
Definition: Inability to perform learned skilled movements despite intact motor and sensory function
| Type | Features | Lesion |
|---|---|---|
| Ideomotor | Cannot pantomime gestures to command; can imitate; uses actual objects better | Left parietal, premotor, or connecting white matter |
| Ideational | Cannot sequence multi-step tasks (e.g., making tea); even with actual objects | Left parietal; often in dementia |
| Limb-kinetic | Loss of fine motor dexterity in one limb | Contralateral premotor/motor |
| Constructional | Cannot draw or construct; spatial organization impaired | Usually right parietal |
| Dressing | Cannot orient clothes to body | Right parietal |
Definition: Inability to recognize despite intact primary sensory function
| Type | Features | Lesion |
|---|---|---|
| Visual Object Agnosia | Cannot identify objects by sight; can by touch or sound | Bilateral occipitotemporal |
| Prosopagnosia | Cannot recognize faces | Bilateral fusiform face area |
| Tactile Agnosia (Astereognosis) | Cannot identify objects by touch | Contralateral parietal |
| Auditory Agnosia | Cannot recognize sounds | Bilateral temporal |
| Anosognosia | Unawareness of deficit (e.g., hemiplegia) | Right parietal (usually) |
| Autotopagnosia | Cannot localize body parts | Left parietal |
| Left Hemisphere (Dominant) | Right Hemisphere (Non-dominant) |
|---|---|
|
• Language (most people) • Calculation • Praxis (motor programs) • Logical/analytical processing • Sequential processing |
• Visuospatial processing • Attention (both hemispheres) • Prosody (emotional tone of speech) • Face recognition • Holistic/gestalt processing • Music appreciation |
Handedness and Language:
Territory: Medial frontal and parietal lobes
Structures supplied:
ACA Stroke Syndrome:
Territory: Lateral frontal, parietal, temporal lobes (largest territory)
Structures supplied:
MCA Stroke Syndrome:
Lenticulostriate (deep MCA) stroke:
Territory: Occipital lobe, medial temporal lobe, thalamus
Structures supplied:
PCA Stroke Syndrome:
| Feature | ACA | MCA | PCA |
|---|---|---|---|
| Motor | Leg > arm/face | Face/arm > leg | Usually spared |
| Sensory | Leg > arm/face | Face/arm > leg | Thalamic if involved |
| Visual | Spared | Hemianopia (radiations) | Hemianopia (V1) |
| Language (dominant) | Transcortical motor | Broca’s/Wernicke’s/Global | Alexia without agraphia |
| Other | Abulia, alien limb | Neglect (non-dominant) | Memory loss, visual agnosia |
Watershed (border zone) infarcts: Between ACA-MCA (arm weakness, transcortical motor aphasia) or MCA-PCA (visual cortex sparing central, Balint syndrome). Occurs with hypotension/hypoperfusion.
| Clinical Finding | Localization |
|---|---|
| Broca’s aphasia | Inferior frontal gyrus (dominant) |
| Wernicke’s aphasia | Posterior superior temporal (dominant) |
| Hemispatial neglect | Right parietal (usually) |
| Gerstmann syndrome | Dominant angular gyrus |
| Prosopagnosia | Bilateral fusiform gyrus |
| Anton syndrome | Bilateral occipital (V1) |
| Balint syndrome | Bilateral parieto-occipital |
| Klüver-Bucy | Bilateral amygdala/temporal |
| Abulia | Medial frontal/anterior cingulate |
| Disinhibition | Orbitofrontal cortex |
