The Human Brain Can Get Confused About What's Real and Imagined, Says New Study

A new study into a phenomenon known as the ventriloquist illusion has shown that simply imagining an object while you hear a sound can change how you later perceive that sound.

The brain must constantly process the sensory information it is bombarded with in order to present us with a picture of reality. But this doesn’t always quite go to plan.

Instead, the brain sometimes creates something that psychologists call the ventriloquist illusion. As the name suggests, this is the effect harnessed by a ventriloquist and their dummy. It occurs when we see an object and hear a sound from a different source but the brain wrongly understands it as coming from the visual stimulus.

And if we experience this pairing repeatedly the illusion will persist as an aftereffect, even when the visual stimulus is gone and we only hear the sound.

"The sensory information we imagine is often treated by the brain in the same way as information streaming into us from the outside world," explained researcher Christopher C. Berger of the California Institute of Technology in a statement.

"Our work shows that what we imagine in our 'mind's eye' can lead to changes in perception across our sensory systems, changing how we perceive real information from the world around us in the future."

Researchers at the Karolinska Institutet in Sweden and California Institute of Technology used brain imaging to investigate the ventriloquist illusion and whether it would work with imagined visual stimuli. The team published their findings in Psychological Science, a journal of the Association for Psychological Science.

First, study participants were played white noise from one of three locations behind a screen, and were instructed to imagine a circle on a specific location on the screen. They were then played bursts of white noise and asked to determine whether it was coming from the right- or left-hand side of the screen.

The scientists found that the participants based their answer on what they had experienced during the first part of the test. This proved their hypothesis that our imagination is all that is needed to create a ventriloquist illusion after-effect. 

But the research also showed that the effect only worked when the sound was the same. It didn't work when the white noise was replaced with a tone, for instance. 

"We were surprised to find that the effects on participants' perception of acoustic space were almost as strong for imagined stimuli as they were for real visual stimuli," said Berger. "That is, what we imagine seeing can affect our future perception of sound as much as what we actually see."

The study authors believe their results have a wide range of uses, from helping rehabilitate stroke patients to creating neural prostheses.