PARIETAL LOBE - Short Note (20 Marks)

1. INTRODUCTION

2. FUNCTIONAL ANATOMY - PARTS

CENTRAL SULCUS
      |
 [POSTCENTRAL GYRUS] ← Primary somatosensory cortex (S1) - Areas 3,1,2
      |
 [SUPERIOR PARIETAL LOBULE] ← Areas 5,7 - sensory association, visuospatial
      |
 [INFERIOR PARIETAL LOBULE]
    ├── SUPRAMARGINAL GYRUS (Area 40) ← wraps around lateral sulcus
    └── ANGULAR GYRUS (Area 39) ← wraps around superior temporal sulcus
      |
 [PRECUNEUS] ← medial surface, visual imagery, self-processing

3. BLOOD SUPPLY

• Middle cerebral artery (MCA) - supplies most of the lateral parietal lobe
• Anterior cerebral artery (ACA) - supplies medial/superior parietal regions (precuneus)
• Posterior cerebral artery (PCA) - contributes to inferior parietal region

4. RIGHT vs LEFT PARIETAL LOBE FUNCTIONS

5. FUNCTIONS IN DETAIL

A. Primary Somatosensory Function (Postcentral Gyrus)

• Receives somatic sensation from the contralateral body via thalamus (VPL nucleus)
• Body represented as a sensory homunculus (upside down - face at bottom, leg at top medially)
• Processes: touch, pain, temperature, proprioception, vibration

B. Discriminative/Cortical Sensation (Superior + Inferior Parietal)

• Stereognosis - identify objects by touch (shape, size, texture, weight)
• Graphesthesia - identify numbers/letters written on skin
• Two-point discrimination - distinguish two nearby points
• Tactile localization - identify where on the body you were touched
• These require integration beyond basic sensation - called cortical sensory functions

C. Body Schema

• Internal "map" of the body
• Maintained by the right parietal lobe especially
• Damage causes: neglect, anosognosia, hemiasomatognosia

D. Spatial Orientation

• Integration of visual, tactile, kinesthetic inputs
• Right parietal: personal space and extrapersonal space
• Damage: topographagnosia (cannot navigate), constructional apraxia

E. Language Functions (Left Angular/Supramarginal Gyri)

• Angular gyrus: cross-modal association (visual - auditory - language)
• Supramarginal gyrus: phonological processing, conduction aphasia
• Gerstmann syndrome (left angular gyrus): Acalculia + Agraphia + Finger agnosia + Left-right disorientation (AAFL)

F. Praxis

• Learned skilled purposeful movements
• Left parietal dominant for praxis
• Damage: ideomotor apraxia (cannot mime using a hammer on command)

6. LESIONS AND CLINICAL EFFECTS

A. UNILATERAL PARIETAL LESION (Either Side)

1. Cortical sensory syndrome - loss of discriminative sensation (astereognosis, agraphesthesia, impaired 2-point discrimination) contralaterally - primary sensation often intact
2. Sensory extinction - when stimulated bilaterally simultaneously, the contralateral stimulus is ignored
3. Mild hemiparesis - parietal lobe has some motor connections; damage causes mild weakness/poverty of movement
4. Contralateral inferior quadrantanopia - optic radiations pass through parietal lobe (Meyer's/superior loop)
5. Abolition of optokinetic nystagmus - when target moves toward the side of the lesion

B. LEFT (DOMINANT) PARIETAL LESION

1. Ideomotor apraxia - cannot perform skilled movements to command (e.g., "show me how to comb hair") despite intact motor and sensory function
2. Conduction aphasia - speech fluent, comprehension intact, but cannot repeat (arcuate fasciculus connection between Wernicke's and Broca's via supramarginal gyrus)
3. Gerstmann Syndrome (angular gyrus lesion):
   • Acalculia - cannot calculate
   • Agraphia - cannot write (without motor weakness)
   • Finger agnosia - cannot name fingers
   • Left-right disorientation
4. Alexia with agraphia - cannot read or write (angular gyrus)
5. Tactile agnosia (astereognosis) - bilateral; always worse contralaterally

C. RIGHT (NON-DOMINANT) PARIETAL LESION

1. Hemineglect / Unilateral spatial neglect - ignores everything on the left side of space (the most dramatic and disabling)
2. Dressing apraxia - cannot dress properly (especially left side)
3. Constructional apraxia - cannot copy drawings or assemble block designs
4. Anosognosia - unaware of their own deficit (e.g., denies left hemiplegia - Anton-Babinski syndrome)
5. Hemiasomatognosia - denies ownership of the left limb
6. Topographagnosia - cannot navigate familiar environments, cannot draw maps
7. Prosopagnosia - difficulty recognizing faces (right > left)
8. Lid closure/eye opening resistance - in acute right parietal lesions, patient resists eye opening (may appear drowsy but responds quickly to whispered questions)

7. SPECIFIC SYNDROMES - SUMMARY TABLE

8. CLINICAL ASSESSMENT OF PARIETAL LOBE

Sensory Testing (Cortical/Discriminative)

• Stereognosis - place common objects (coin, key, pen) in patient's hand with eyes closed; ask to identify
• Graphesthesia - draw numbers on palm; ask patient to identify
• Two-point discrimination - use calipers; minimal distance to distinguish two points (normally ~2-5mm on fingertips)
• Tactile localization - touch skin; ask patient to point where touched
• Sensory extinction - touch both hands simultaneously; patient with parietal lesion extinguishes contralateral stimulus

Motor/Praxis Testing

• Ideomotor apraxia test - "Show me how you would use a comb/toothbrush"
• Imitation test - patient imitates gestures
• Object use test - give actual object and ask to use it

Spatial/Visuospatial Testing

• Clock drawing test - patient draws clock with numbers and hands at specific time
• Copying figures - copy intersecting pentagons, cube (Rey-Osterrieth complex figure)
• Line bisection test - bisect a line; neglect patients deviate toward the right
• Star cancellation test - cross out all stars on a page; neglect patients miss left side stars
• Benton Figure Copying Test and Wechsler Block Design - for constructional praxis
• Money's Road Map Test - spatial perception and memory

Language/Dominant Hemisphere (Left Parietal)

• Test calculation: serial 7 subtractions, simple arithmetic
• Test writing: write a sentence to dictation
• Test finger naming: "point to your ring finger"
• Test left-right orientation: "show me your right hand", "point to my left ear"

Neglect Testing (Right Parietal)

• Line bisection, cancellation tasks, drawing (patient omits left side)
• Ask patient to describe room - do they ignore left side?
• Bilateral simultaneous stimulation (extinction test)

Other

• Dressing test - observe patient putting on shirt; note whether they struggle with left side
• Topographic orientation - ask to draw floor plan of house or familiar route

9. QUICK MEMORY TRICKS FOR EXAM

10. HIGH-YIELD SUMMARY FOR 20-MARK EXAM