Cerebral Cortex Functions
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Direct answer
Language lives in the left hemisphere of about 95% of right-handed people, and the cortical map that examiners test is built on Brodmann's numbered areas: area 4 for primary motor, 6 for premotor and supplementary motor, 3, 1 and 2 for primary somatosensory, 17 for visual, 41 and 42 for auditory. Broca's area (44, 45) in the dominant frontal lobe produces fluent speech output, Wernicke's area (22) in the dominant temporal lobe decodes it, and their connecting arcuate fasciculus carries repetition — the three stations that let you classify any aphasia at the bedside in under a minute.
What you must remember
- Primary motor cortex (area 4, precentral gyrus) is somatotopically arranged as the motor homunculus — upside down, with disproportionate hand, tongue and lip areas; stimulation gives discrete movements, not ideas of movement.
- Somatosensory cortex in the postcentral gyrus (areas 3, 1, 2); visual cortex (17) lies along the calcarine sulcus, each hemisphere serving the opposite visual hemifield; auditory cortex (41, 42) in Heschl's gyri receives bilateral input.
- Broca's aphasia: non-fluent, effortful speech with intact comprehension — the patient understands your question and struggles to answer, frustrated.
- Wernicke's aphasia: fluent, grammatical but empty jargon with impaired comprehension — the patient answers fluently and is unaware of errors.
- Conduction aphasia: arcuate fasciculus lesion — comprehension and spontaneous speech preserved, repetition selectively abolished.
- Angular gyrus (39, dominant) lesions produce Gerstmann syndrome: agraphia, acalculia, right-left disorientation and finger agnosia.
- The non-dominant (usually right) parietal lobe owns spatial attention — its lesion causes contralateral hemineglect, constructional and dressing apraxia.
- Prefrontal cortex governs working memory, judgement and social restraint — Phineas Gage's iron rod injury is the classic lesion story; Sperry's split-brain studies showed the left hemisphere names, the right perceives space.
Localising an aphasia at the bedside
Ask three questions in order. Is speech fluent? Effortful, telegraphic output with the patient visibly frustrated points to Broca's area in the left inferior frontal region. Next, does the patient understand? Hand him a simple command — "pick up the paper, fold it, place it on the chair" — preserved in Broca's, lost in Wernicke's, where fluent jargon pours out with no awareness of error. Third, can he repeat a phrase such as "no ifs, ands or buts"? If repetition fails while everything else works, the arcuate fasciculus is disconnected — conduction aphasia, the pattern that proves fluency and comprehension are not enough for repetition.
Now let anatomy finish the case. A patient with non-fluent speech, right arm weakness and right lower facial weakness has a left middle cerebral artery territory infarct involving the frontal operculum; add a right homonymous hemianopia and the whole cortical MCA territory is implicated. Add one more layer of sophistication with transcortical aphasias: repetition is preserved when the perisylvian core is intact but isolated from surrounding cortex — transcortical motor aphasia mimics Broca's except that the patient echoes your sentences easily. Alexia without agraphia completes the disconnection set: a left posterior cerebral artery infarct of the splenium and left visual cortex leaves a patient who can write but cannot read what he has just written.
How the exam frames cortex questions
One-liners dominate: area for primary motor (4), for vision (17), for audition (41), Broca (44, 45), Wernicke (22) — the numbers are free marks and should never be guessed. Conceptual traps follow: hemineglect follows a right (non-dominant) parietal lesion even though the left field is affected, because the right hemisphere attends to both sides and the left to the right only. Handedness data are worth quoting: about 95% of right-handers and roughly 70% of left-handers are left-hemisphere dominant for language. Finally, homunculus questions reward the clinical link: why does a cortical hand-area lesion cause monoplegia when subcortical lesions cause hemiplegia — because the cortical map is discrete and the descending fibres have converged below.
Frequently asked questions
Which Brodmann areas constitute Broca's and Wernicke's regions?
Broca's expressive area occupies areas 44 and 45 of the dominant inferior frontal gyrus; Wernicke's receptive area is area 22 of the dominant superior temporal gyrus.
How does conduction aphasia differ from Broca's aphasia?
Both affect expression, but conduction aphasia has fluent spontaneous speech, good comprehension and selectively abolished repetition from arcuate fasciculus disconnection.
What functions localize to the non-dominant parietal lobe?
Spatial attention and body image, so lesions produce contralateral neglect, constructional and dressing apraxia, and impaired prosody.
What is Gerstmann syndrome?
Dominant angular gyrus lesion causing finger agnosia, right-left disorientation, acalculia and agraphia, with or without alexia.
Why can a split-brain patient name an object felt in the right hand but not the left?
Somatosensory input crosses to the opposite hemisphere; only the language-dominant left hemisphere can name, and the corpus callosum that would transfer the right hemisphere's information has been sectioned.