Medicine – The Cerebral Cortex

The cerebral cortex is the outer layer of the cerebral hemispheres and is responsible for many higher neurological functions, including voluntary movement, sensation, vision, hearing, language, memory, cognition, and behaviour.

Different cortical regions have specialised functions, so focal lesions can often be localised from the pattern of neurological deficits.

1. Primary Motor Cortex

The primary motor cortex is located in the:

Precentral gyrus of the frontal lobe.

It is responsible for initiating voluntary movements of the opposite side of the body.

2. Motor Homunculus

The body is represented somatotopically within the motor cortex.

Approximately:

Leg → medial surface.

Arm → superior-lateral region.

Face → lateral/inferior region.

Therefore, a focal cortical lesion can produce weakness affecting a specific body region.

3. Effect of a Motor Cortex Lesion

A lesion of the primary motor cortex produces:

Contralateral upper motor neuron weakness.

Possible signs include:

Weakness.

Increased tone.

Brisk reflexes.

Extensor plantar response.

The exact distribution depends on the affected cortical area.

4. Primary Somatosensory Cortex

The primary somatosensory cortex is located in the:

Postcentral gyrus of the parietal lobe.

It receives sensory information from the opposite side of the body.

5. Sensory Functions

The somatosensory cortex processes information including:

Touch.

Pressure.

Pain-related cortical perception.

Temperature-related cortical perception.

Vibration.

Proprioception.

It also contributes to higher-order cortical sensory interpretation.

6. Sensory Homunculus

Like the motor cortex, the somatosensory cortex is organised somatotopically.

Approximately:

Leg → medial.

Arm → superior-lateral.

Face → lateral/inferior.

A focal lesion may therefore cause contralateral sensory impairment in a particular body region.

7. Higher Cortical Sensory Functions

The parietal association cortex is involved in interpreting sensory information.

Lesions may cause:

Astereognosis.

Agraphesthesia.

Impaired two-point discrimination.

These findings require relatively preserved primary sensation.

8. Primary Auditory Cortex

The primary auditory cortex lies in the superior temporal region, particularly the:

Transverse temporal gyri of Heschl.

This is more precise than simply saying “superotemporal lobe.”

9. Auditory Function

The auditory cortex receives and processes information relating to:

Sound frequency.

Intensity.

Timing.

Speech-related auditory input.

Because central auditory pathways project bilaterally, a unilateral lesion rarely causes complete deafness.

10. Cortical Deafness

True:

Cortical deafness

usually requires:

Bilateral auditory cortical lesions.

The peripheral hearing apparatus may remain intact.

11. Primary Visual Cortex

The primary visual cortex is located in the:

Occipital lobe.

It lies mainly along the:

Calcarine sulcus.

This region receives visual information from the opposite visual field.

12. Visual Field Representation

The left occipital cortex processes the:

Right visual field.

The right occipital cortex processes the:

Left visual field.

Therefore:

Unilateral occipital lesion → contralateral homonymous visual field defect.

13. Effect of Bilateral Occipital Damage

Severe bilateral occipital cortex damage can cause:

Cortical blindness.

Pupillary responses may remain normal because the pupillary light reflex pathway does not require the visual cortex.

14. Olfactory Cortex

The original note states:

Olfactory cortex → frontal lobe.

This is incomplete.

The primary olfactory cortex is mainly located in the:

Medial temporal region

and nearby basal forebrain structures.

Important areas include the:

Piriform cortex.

Uncus.

Amygdala-related regions.

15. Orbitofrontal Olfactory Processing

The frontal lobe is still important in smell, especially the:

Orbitofrontal cortex.

This region contributes to:

Conscious identification of odours.

Discrimination of smells.

Integration of smell with taste and reward.

So it is better to distinguish:

Primary olfactory cortex → mainly medial temporal/basal regions.

Higher olfactory processing → orbitofrontal cortex.

16. Broca Area

Broca area is located in the:

Dominant inferior frontal gyrus.

In most people, the dominant hemisphere is:

Left.

Broca area is involved in:

Speech production.

Language output.

Motor organisation of speech.

17. Broca Aphasia

Damage to Broca area produces:

Non-fluent aphasia.

Speech becomes:

Slow.

Effortful.

Agrammatic.

Reduced in output.

Comprehension is relatively preserved, especially for simple language.

18. Wernicke Area

Wernicke area is classically located in the:

Dominant posterior superior temporal region.

It is strongly involved in:

Language comprehension.

Understanding spoken words.

Semantic processing.

19. Wernicke Aphasia

Damage to this region produces:

Fluent aphasia.

Speech may remain fluent but become:

Meaningless.

Paraphasic.

Filled with incorrect words.

Comprehension is impaired.

20. Broca versus Wernicke

Broca area:

Dominant frontal lobe.

Speech output.

Non-fluent aphasia if damaged.

Comprehension relatively preserved.

Wernicke area:

Dominant posterior temporal region.

Language comprehension.

Fluent but poorly comprehended and poorly meaningful speech if damaged.

21. Language Network

Modern understanding recognises that language is not confined to only Broca and Wernicke areas.

Language depends on a distributed network involving:

Frontal cortex.

Temporal cortex.

Parietal cortex.

White-matter connections between them.

The classic Broca-Wernicke model remains useful clinically but is simplified.

22. Arcuate Fasciculus

One important connection between posterior and anterior language regions is the:

Arcuate fasciculus.

Damage can produce:

Conduction aphasia.

Typical features include:

Fluent speech.

Relatively good comprehension.

Markedly impaired repetition.

23. Frontal Association Cortex

The frontal association cortex contributes to:

Planning.

Judgment.

Executive function.

Motivation.

Personality.

Behavioural inhibition.

Lesions may cause apathy, disinhibition, poor planning, and perseveration.

24. Parietal Association Cortex

The parietal association cortex contributes to:

Spatial awareness.

Praxis.

Calculation.

Reading and writing networks.

Body awareness.

Lesions can produce neglect, apraxia, astereognosis, and Gerstmann-type features depending on the hemisphere.

25. Temporal Association Cortex

The temporal association cortex contributes to:

Memory.

Language comprehension.

Auditory recognition.

Emotional processing.

Object and face recognition networks.

Lesions may therefore cause memory disturbance, aphasia, auditory agnosia, or behavioural changes.

26. Occipital Association Cortex

The visual association cortex surrounds the primary visual cortex.

It helps interpret:

Objects.

Faces.

Colour.

Movement.

Damage may cause:

Visual agnosia.

Prosopagnosia.

Colour-processing abnormalities.

27. Cerebral Cortex – Note Form

Primary motor cortex:

Precentral gyrus.

Frontal lobe.

Controls voluntary movement of the contralateral body.

Primary somatosensory cortex:

Postcentral gyrus.

Parietal lobe.

Receives contralateral body sensation.

Primary auditory cortex:

Superior temporal lobe.

Heschl gyri.

Processes sound.

Primary visual cortex:

Occipital lobe.

Around calcarine sulcus.

Processes contralateral visual field.

Primary olfactory cortex:

Mainly medial temporal/basal forebrain regions.

Piriform cortex and uncus important.

Higher olfactory interpretation:

Orbitofrontal cortex.

Broca area:

Dominant inferior frontal gyrus.

Speech production.

Wernicke area:

Dominant posterior superior temporal region.

Language comprehension.

28. Important Corrections to the Original Notes

The auditory cortex is more precisely located in the:

Heschl transverse temporal gyri of the superior temporal lobe.

The primary visual cortex is specifically located around the:

Calcarine sulcus of the occipital lobe.

The statement:

“Olfactory cortex = frontal lobe”

is too simple.

A better description is:

Primary olfactory cortex → mainly medial temporal and basal regions.

Orbitofrontal cortex → higher conscious olfactory interpretation.

Broca area is better described as responsible for:

Speech production and language output

rather than speech alone.

Wernicke area is involved in:

Language comprehension

rather than merely “word comprehension.”

Key Clinical Pattern

Remember:

PREcentral = MOTOR.

POSTcentral = SENSORY.

TEMPORAL = HEARING.

OCCIPITAL = VISION.

BROCA = SPEECH OUTPUT.

WERNICKE = LANGUAGE COMPREHENSION.

And one useful correction:

PRIMARY SMELL = MEDIAL TEMPORAL/BASAL REGION, while ORBITOFRONTAL CORTEX helps identify and interpret odours.