"The deep secret of the brain is that not only the spinal cord but the entire central nervous system works this way: internally generated activity is modulated by sensory input. In this view, the difference between being awake and being asleep is merely that the data coming in from the eyes anchors the perception."

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In the traditionally taught view of perception, data from the sensorium pours into the brain, works its way up the sensory hierarchy, and makes itself seen, heard, smelled, tasted, felt - "perceived." But a closer examination of the data suggests this is incorrect. The brain is properly thought of as a mostly closed system that runs on its own internally generated activity.

— David Eagleman