Visual Illusions: The McGurk Effect and How Your Senses Are Lying to You

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the mcgurk effect visual illusion david eagleman

Neuroscientist David Eagleman, author of the book “Incognito.”

Andrew Parker established vision as the driving force for the “explosion” of new life forms during the Cambrian period with his Light Switch Theory, which suggested the evolutionary importance of vision was initially the predator-prey relationship. Lynne Isabell proposed humans see better than any other species because of the evolutionary pressure of snakes. She concluded the complex human vision is closely connected to human’s superior intelligence and enlarged brain. It means that vision has always been central to evolution and natural selection favored highly complex vision because of its survival benefits.

Although Michael Haneke claims music is the closest art to cinema, film is predominantly a visual art. However, as neuroscientist David Eagleman explains in his book “Incognito,” senses are not one-way assembly lines that operate independently of each other and the brain. Rather, perception occurs by constant feedback between sensory organs and the brain. This can often create misinterpretation of the sensory and create illusions. As Eagleman says:

The different senses influence one another, changing the story of what is thought to be out there. What comes in through the eyes is not just business of the visual system—the rest of the brain is invested as well. In the ventriloquist illusion, sound comes from one location (the ventriloquist’s mouth), but your eyes see a moving mouth in a different location (the ventriloquist’s dummy). Your brain concludes that the sound comes directly from the dummy’s mouth. Ventriloquists don’t “throw” their voice. Your brain does all the work for them.

The McGurk Effect

The McGurk effect is one of the famous examples of a sensory illusion. It establishes the overriding supremacy of vision over hearing:

As the voice-over in the video says, the illusion is remarkable because even knowing how it occurs cannot prevent the false perception of the data. Eagleman explains:

Early ideas of brain functions were squarely based on a computer analogy: the brain was an input-output device that moved sensory information through different processing stages until reaching an end point.

But this assembly line model began to draw suspicion when it was discovered that brain wiring does not simply run from A to B to C: there are feedback looks from C to B, C to A, and B to A. Throughout the brain there is an much feedback as feed-forward—a feature of brain wiring that is technically called recurrence and colloquially called loopiness. The whole system looks a lot more like a marketplace than an assembly line. To the careful observer, these features of neurocircuity immediately raise the possibility that visual perception is not a procession of data crunching that begins from the eyes and ends with some mysterious end point back in the brain. […]

[The McGurk effect] results from the dense interconnectivity and loopiness of the brain, which allows voice and lip-movement cues to become combined at an early processing stage.

Vision usually dominates over hearing, but a counter example is the illusory flash effect: when a flashed spot is accompanied two beeps, it appears to flash twice. This is related to another phenomenon called “auditory driving,” in which the apparent rate of a flickering light is driven faster of slower by an accompanying beeping sound presented at a different rate. Simple illusions like these serve as powerful clues into neural circuity, telling us that the visual and auditory systems are densely tied in with each other, trying to relate a unified story of events in the world. The assembly line model of vision in introductory textbooks isn’t just misleading, it’s dead wrong.