In a patient whose corpus callosum was sectioned for refractory epilepsy, a picture of a key is flashed exclusively to the left visual field. When asked what he saw, the patient says he saw nothing, yet his left hand correctly picks out a key from a group of hidden objects. This dissociation demonstrates:

• A Anterograde amnesia for visual stimuli
• B Failure of primary visual cortex in the right occipital lobe
• C Left hemispheric dominance for language with right hemispheric perceptual processing ✓
• D Hemineglect of the contralateral visual field

Explanation

Information from the left visual field reaches the right occipital cortex. With the callosum divided, this information cannot cross to the language-dominant left hemisphere, so the patient verbally reports seeing nothing. The right hemisphere, however, still perceives the image and can guide the ipsilateral left hand to select the object. Visual cortex function, memory, and attention are intact, which excludes the other options.

Reference: Principles of Neural Science, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.