I first encountered the McGurk effect in a neuroscience class. My professor played a video of a person moving their lips as if saying “ga,” while the audio track played “ba.” Most students heard neither. They heard “da or ka” — a third sound created by the brain.

My experience was different. When I focused on the sound, I heard “ba.” When I watched the lips, I perceived “ga.” My perception followed where my attention was, rather than blending the two into something else. At the time, I didn’t think much of it. But, it was a useful clue.

As I learned more, I realized that sensory illusions aren’t tricks at all. They’re shortcuts the brain sometimes uses. Most of the time, perception works because the senses cooperate. Sight, sound, and touch usually point to the same event, arriving close together in time. When that happens, the brain doesn’t need to think very hard about what belongs together. The world feels stable. Neuroscientists call this multisensory integration. In everyday terms, it’s the brain weaving separate signals into a single story. Most of the time, we never notice the weaving. Illusions are what happen when the stitching shows.

Take another familiar illusion, the stream-bounce illusion. You watch two dots move toward each other on a screen. When they meet, some people see them stream through one another. Others see them bounce apart. Add a brief clicking sound at the moment they meet, and something interesting happens. Suddenly, a bounce feels obvious. Nothing about the visual scene changed. The sound didn’t add detail—it added certainty. The brain had an ambiguous situation and resolved it using whatever information was available.........

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