Question
A 68-year-old man is brought to the emergency department 30 minutes after collapsing on the street. On arrival, he is obtunded. His pulse is 110/min and blood pressure is 250/120 mm Hg. A CT scan of the head shows an intracerebral hemorrhage involving bilateral thalamic nuclei and the third ventricle. Cortical detection of which of the following types of stimuli is most likely to remain unaffected in this patient?
| A. |
Gustatory
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| B. |
Olfactory
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| C. |
Visual
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| D. |
Auditory
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Show Answer
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Correct Answer � B
Explanation
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Answer B) Olfactory
With the exception of one pathway, all sensory information pathways are relayed by thalamic neurons before reaching functionally distinct areas of the cortex. The pathway in question projects to the cortical area that is often the first to be affected in patients with Alzheimer disease.
Olfactory
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The olfactory bulb is the first central relay station for the olfactory pathway, which processes odor information.
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It receives afferent input from the olfactory nerve and projects to the primary olfactory cortex.
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Although the olfactory sensory neurons do not project directly to the thalamus, the mediodorsal thalamic nucleus receives and sends information to the olfactory cortex.
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Damage to the mediodorsal thalamic nucleus does not impair cortical olfactory detection (which would result in anosmia), but it does impair other olfactory functions, such as olfactory identification or discrimination (which may result in dysosmia).
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Olfactory dysfunction is a common finding on neurologic examination in patients with Alzheimer disease.
Gustatory
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Gustatory information is conveyed via the ventral posteromedial nucleus, the thalamic relay center for taste.
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The ventral posterior medial nucleus receives afferent input from the trigeminal nucleus and projects to the primary somatosensory cortex.
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Hence, a bilateral thalamic lesion would affect taste.
Visual
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Visual information is conveyed via the lateral geniculate body, the thalamic relay center for the optic pathway.
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The lateral geniculate body receives afferent input from optic nerve via the superior colliculus and projects to the primary visual cortex.
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Therefore, a bilateral thalamic lesion would affect vision.
Auditory
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Auditory information is conveyed via the medial geniculate nucleus, the thalamic relay center for the auditory pathway.
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The medial geniculate nucleus receives afferent input from the inferior colliculus and the superior olivary nucleus and projects to the primary auditory cortex.
Therefore, a bilateral thalamic lesion would affect auditory sensation.
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