Vision loss can trigger vivid hallucinations with full awareness they're false

Sacks devotes extended attention to Charles Bonnet syndrome, a condition in which people with significant vision loss, sometimes total blindness, begin seeing detailed, often elaborate hallucinations: rows of strangely dressed figures, intricate patterns, or scenes that can seem more vivid than ordinary sight ever was. Crucially, most people experiencing this retain full insight that what they're seeing isn't real, distinguishing it sharply from psychotic hallucinations tied to a loss of reality-testing. Sacks traces the condition's discovery back to an eighteenth-century Swiss naturalist who documented it first in his grandfather and later experienced it himself as his own eyesight failed. The underlying mechanism, as Sacks explains it, involves the visual cortex generating activity to fill the void left by diminished input from the eyes, essentially producing imagery from its own idle machinery. Takeaway: vivid, sane hallucination in visual loss is a recognized neurological pattern, not a psychiatric symptom.