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The oxymoronic term blindsight may seem bizarre, but it accurately describes these individuals’ Shakespearean condition: to see, but not to see. A lesion in the primary visual cortex should make a person blind, and it does deprive such patients of their conscious vision—they assure you that they cannot see anything in a specific part of the visual field (which corresponds precisely to the destroyed area of cortex), and they behave as if they were blind. Incredibly enough, however, when an experimenter shows them objects or flashes of light, they accurately point to them. 10 In a zombielike manner, they unconsciously guide their hand to locations that they do not see—blindsight indeed. Which intact anatomical pathways support unconscious vision in blindsight patients? Clearly, in these patients, some visual information still makes it through from the retina to the hand, bypassing the lesion that makes them blind. Because the entry point into the patients’ visual cortex had been destroyed, the researchers initially suspected that their unconscious behavior arose entirely from subcortical circuits. A key suspect was the superior colliculus, a nucleus in the midbrain that specializes in the cross-registration of vision, eye movements, and other spatial responses. Indeed, the first functional MRI study of blindsight demonstrated that unseen targets triggered a strong activation in the superior colliculus. 11 But that study also contained evidence that the unseen stimuli evoked activations in the cortex—and sure enough, later research confirmed that invisible stimuli could still activate both the thalamus and higher-level visual areas of the cortex, somehow bypassing the damaged primary visual area. 12 Clearly, the brain circuits that take part in our unconscious inner zombie and that guide our eye and hand movements include much more than just old subcortical routes.
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Stanislas Dehaene (Consciousness and the Brain: Deciphering How the Brain Codes Our Thoughts)