The cerebellar hemisphere lesion produces ipsilateral ataxia primarily because:

• A Cerebellar output crosses twice: once in the superior cerebellar peduncle and again in the internal capsule, netting an ipsilateral effect ✓
• B The cerebellum projects to the ipsilateral motor cortex via the superior cerebellar peduncle which crosses in the decussation of the superior cerebellar peduncle
• C Cerebellar output crosses in the pontine tegmentum before reaching the ipsilateral thalamus
• D The cerebellum directly innervates ipsilateral spinal motor neurons via the vestibulospinal tract

Explanation

Cerebellar hemisphere output travels via the dentate nucleus through the superior cerebellar peduncle. These fibers decussate in the midbrain tegmentum (decussation of the superior cerebellar peduncle) to reach the contralateral red nucleus and thalamus (VL nucleus). The thalamic projection then goes to the contralateral motor cortex, whose descending corticospinal tract crosses again at the medullary pyramidal decussation. This double crossing returns control to the ipsilateral side. Therefore, a right cerebellar hemisphere lesion produces right-sided ataxia — the 'ipsilateral rule' of cerebellar signs.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.