Why Some People See Sound

Some people may actually see sounds, say researchers who found this odd ability is possible when the parts of the brain devoted to vision are small.

These findings points to a clever strategy the brain might use when vision is unreliable, investigators added.

Scientists took a closer look at the sound-induced flash illusion. When a single flash is followed by two bleeps, people sometimes also see two illusory consecutive flashes.

Past experiments revealed there are strong differences between individuals when it comes to how prone they are to this illusion. “Some would experience it almost every time a flash was accompanied by two bleeps, others would almost never see the second flash,” said researcher Benjamin de Haas, a neuroscientist at University College London.

These differences suggested to de Haas and his colleagues that maybe variations in brain anatomy were behind who saw the illusion and who did not. To find out, the researchers analyzed the brains of 29 volunteers with magnetic resonance imaging (MRI) and tested them with flashes and bleeps.

On average, the volunteers saw the illusion 62 percent of the time, although some saw it only 2 percent of the time while others saw it 100 percent of the time. They found the smaller a person’s visual cortex was — the part of the brain linked with vision —the more likely he or she experienced the illusion.

"If we both look at the same thing, we would expect our perception to be identical," de Haas told LiveScience. "Our results demonstrate that this not quite true in every situation — sometimes what you perceive depends on your individual brain anatomy."

The researchers suggest this illusion could reveal a way the brain compensates for imperfect visual circuitry.