Superolateral Surface of the Brain: Functional Areas - Essay

Introduction

• Neuroanatomy through Clinical Cases, 3rd Ed.
• Localization in Clinical Neurology, 8th Ed.

Gross Anatomy of the Superolateral Surface

Major Fissures and Sulci

1. Central sulcus (Fissure of Rolando) - runs downward and forward from the superomedial border, separating the frontal lobe (anteriorly) from the parietal lobe (posteriorly).
2. Lateral sulcus (Sylvian fissure) - runs horizontally, separating the frontal and parietal lobes above from the temporal lobe below. The insula lies buried in its depths.

Lobes Visible on the Superolateral Surface

1. Frontal Lobe

• Precentral gyrus (between the central sulcus and precentral sulcus)
• Superior frontal gyrus
• Middle frontal gyrus
• Inferior frontal gyrus (divided into triangular, opercular, and orbital parts)

2. Parietal Lobe

• Postcentral gyrus (immediately behind the central sulcus)
• Superior parietal lobule (above the intraparietal sulcus)
• Inferior parietal lobule (below the intraparietal sulcus), which is subdivided into:
  • Supramarginal gyrus - curves around the posterior end of the Sylvian sulcus
  • Angular gyrus - curves around the posterior end of the superior temporal sulcus

3. Temporal Lobe

• Superior temporal gyrus
• Middle temporal gyrus
• Inferior temporal gyrus

4. Occipital Lobe

Functional Areas (Brodmann's Areas)

A. PRIMARY MOTOR CORTEX (Area 4)

• Location: Precentral gyrus, frontal lobe, anterior to the central sulcus
• Function: Controls voluntary movement of the contralateral side of the body
• Somatotopic organization: Represented as the motor homunculus - the face and hand areas are on the lateral convexity; the leg and foot areas are in the interhemispheric fissure (paracentral lobule on the medial surface)
• Efferent connections: Projects via the corticospinal (pyramidal) and corticobulbar tracts to lower motor neurons in the spinal cord and brainstem
• Lesion: Contralateral upper motor neuron (UMN) paralysis/paresis

B. PREMOTOR CORTEX (Area 6 - anterior part)

• Location: Anterior to the precentral gyrus (lateral surface)
• Function: Planning and programming of complex voluntary movements; encodes the "strategy" of movement before execution by area 4
• Connections: Projects to primary motor cortex and directly to corticospinal neurons
• Lesion: Contralateral limb apraxia, difficulty with complex sequential movements

C. SUPPLEMENTARY MOTOR AREA (Area 6 - medial part)

• Location: Mesial frontal lobe (mostly on medial surface, but its anterior part is visible on the superolateral surface)
• Function: Bilateral planning of voluntary movements, especially complex bimanual tasks; involved in motor sequence learning

D. FRONTAL EYE FIELD (Area 8)

• Location: Posterior part of the middle frontal gyrus, immediately anterior to the premotor cortex
• Function: Controls voluntary conjugate eye movements (saccades) to the contralateral side
• Lesion: Eyes deviate ipsilaterally (toward the side of the lesion); this resolves over days

E. PRIMARY SOMATOSENSORY CORTEX (Areas 3, 1, 2)

• Location: Postcentral gyrus, parietal lobe, posterior to the central sulcus
• Function: Processes conscious touch, proprioception, pain, and temperature from the contralateral side of the body
• Somatotopic organization: Represented as the sensory homunculus - the face and hand areas are on the lateral convexity; the leg areas are in the interhemispheric fissure
• Key point: Area 3a (deep in the fundus of the central sulcus) processes proprioception; area 3b processes texture and size; areas 1 and 2 process more complex tactile discrimination
• Lesion: Contralateral loss of discriminative touch, two-point discrimination, proprioception, and graphesthesia

F. SOMATOSENSORY ASSOCIATION CORTEX (Areas 5 & 7)

• Location: Superior parietal lobule, posterior to the postcentral gyrus
• Function: Integrates somatosensory information for higher-order tactile discrimination; spatial awareness; recognizing objects by touch (stereognosis); integrating sensory input with motor planning
• Lesion: Tactile agnosia, astereognosis, sensory neglect

G. BROCA'S AREA (Areas 44 & 45)

• Location: Inferior frontal gyrus (opercular and triangular parts), left (dominant) hemisphere, just anterior to the face representation of the primary motor cortex
• Function: Motor planning and production of speech (articulation program); also involved in syntax and grammatical processing
• Lesion (dominant hemisphere): Broca's (expressive/non-fluent) aphasia - halting, effortful speech with intact comprehension but impaired repetition and naming

H. WERNICKE'S AREA (Area 22 - posterior part)

• Location: Posterior superior temporal gyrus, left (dominant) hemisphere, adjacent to the primary auditory cortex
• Function: Language comprehension - decoding of heard speech; critical for associating sounds with meaning
• Lesion (dominant hemisphere): Wernicke's (receptive/fluent) aphasia - fluent but meaningless speech (paraphasias, neologisms), severely impaired comprehension and repetition

I. PRIMARY AUDITORY CORTEX (Areas 41 & 42 - Heschl's gyri)

• Location: Transverse gyri of Heschl, on the superior surface of the superior temporal gyrus (within the Sylvian fissure)
• Function: Initial processing of auditory information; tonotopically organized (different frequencies mapped to different regions); left hemisphere has bias for temporal processing (speech), right for spectral processing (music)
• Lesion: Bilateral lesions required for cortical deafness; unilateral lesion causes slight decrease in hearing on the contralateral side

J. AUDITORY ASSOCIATION CORTEX (Area 22 - Wernicke's area; planum temporale)

• Location: Superior temporal gyrus (posterior to Heschl's gyri), including the planum temporale
• Function: Higher-order processing of auditory information; in the dominant hemisphere, specializes in language; in the non-dominant hemisphere, processes musical and prosodic (emotional tone) aspects of speech
• Anatomical asymmetry: The planum temporale is approximately one-third larger in the left hemisphere in right-handed individuals

K. INFERIOR PARIETAL LOBULE (Areas 39 & 40)

• Location: Curves around the posterior end of the Sylvian fissure
• Function: Integration of somatosensory and auditory inputs; important for phonological processing; in the dominant hemisphere, contributes to reading and writing

• Location: Curves around the posterior end of the superior temporal sulcus
• Function: Integration of visual, auditory, and somatosensory information; reading (linking written symbols to spoken words); calculation; body schema; in dominant hemisphere, critical for reading and writing
• Lesion (dominant hemisphere): Gerstmann's syndrome (agraphia, acalculia, finger agnosia, right-left disorientation)

L. PARIETAL ASSOCIATION CORTEX (Right hemisphere)

• Location: Particularly the right inferior parietal lobule (non-dominant)
• Function: Spatial attention to the contralateral body and extrapersonal space; visuospatial processing
• Lesion (non-dominant / right hemisphere): Contralateral hemispatial neglect (ignoring the left side of space); anosognosia (unawareness of one's own deficits); dressing apraxia; constructional apraxia

M. PREFRONTAL CORTEX (Areas 9, 10, 11, 46)

• Location: Anterior to the premotor cortex; includes the superior, middle, and parts of the inferior frontal gyri
• Function: Executive functions - decision-making, working memory, planning, impulse control, personality, abstract reasoning, and social behavior
• Lesion: Frontal lobe syndrome - personality changes, disinhibition, poor judgment, perseveration, executive dysfunction

Important Gyri and Their Functional Summaries (Table Format)

Clinical Significance and Lesion Correlations

• Contralateral face and arm weakness/sensory loss (sparing the leg, which is supplied by ACA)
• Contralateral homonymous hemianopia (due to damage to optic radiations in the parietal/temporal white matter)
• Aphasia (if dominant hemisphere is affected) - either Broca's (frontal branch) or Wernicke's (temporal branch)
• Hemispatial neglect (if non-dominant parietal lobe is affected)

Hemispheric Dominance (Lateralization)

• Left hemisphere (dominant in ~95% of right-handers and ~70% of left-handers): Language (Broca's, Wernicke's), calculation, reading, writing, analytical reasoning
• Right hemisphere (non-dominant): Visuospatial processing, prosody (emotional tone of speech), face recognition, musical processing, holistic pattern recognition, spatial attention

Summary

1. Primary motor cortex (precentral gyrus) - voluntary movement
2. Premotor and supplementary motor cortex - motor planning
3. Frontal eye field - voluntary gaze
4. Broca's area - speech production
5. Prefrontal cortex - executive functions
6. Primary somatosensory cortex (postcentral gyrus) - conscious sensation
7. Somatosensory association cortex - higher tactile processing
8. Inferior parietal lobule (supramarginal + angular gyri) - multimodal integration, language, spatial processing
9. Primary auditory cortex (Heschl's gyri) - auditory processing
10. Wernicke's area - language comprehension
11. Auditory/temporal association cortex - higher auditory processing
12. Visual association cortex (lateral occipital) - object and motion recognition

• Neuroanatomy through Clinical Cases, 3rd Edition (Blumenfeld) - Ch. 2 & 10
• Localization in Clinical Neurology, 8th Edition - Ch. 20 (Anatomy of the Cerebral Cortex)