You could call it one of the most magnificent conundrums of our existence: consciousness. How can an experience be so routine, so common to all of us—and yet so utterly unfathomable at its deepest levels?

That enigma has long intrigued neuroscientists such as Christof Koch, author of the cover story, “The Movie in Your Head.” Imaging technology reveals what areas in the brain are buzzing with neural activity when a person is tracking a speeding car, looking at a loved one or eating a chocolate bar. But how does such incessant chemical signaling stitch fleeting sensory impressions into an apparently seamless stream of consciousness? Is the “real world” we know merely an illusion created from those fragments? The show starts on page 58.

Clues about the processing of complex sensory inputs also come from brains that are not “normal.” For people with synesthesia, for instance, sight, hearing and touch can blend in extraordinary ways. The sound of each note plunked on a piano might evoke a different color. Printed letters, words, numbers or even days on a calendar may gleam with hues of their own. Flavor can mingle with shapes. The strains of a violin can feel like a caress.

The condition confers a unique gift not only on people who experience its wonders firsthand but also on researchers. In “Hearing Colors, Tasting Shapes,” beginning on page 16, neuroscientists Vilayanur S. Ramachandran and Edward M. Hubbard describe insights they have gleaned from synesthesia's exotic world.

A vivid sensory rush also underlies humanity's shared “Lust for Danger,” as Klaus Manhart explains, starting on page 24. We crave the pleasurable thrill of risk taking—whether that excitement comes from betting it all in a game of Texas Hold ’Em, watching a suspenseful horror movie or parachuting out of an airplane. The success of our forebears, early human adventurers, gave them a survival edge that remains part of our collective mental hard wiring.

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