Why Some People Can 'Hear' Silent Images

Here's a riddle: If a tree falls in a silent GIF, does it make a sound?

The answer, for many of us, is a resounding yes (even if that sound exists only in our own heads). For proof of this, look no further than the"noisy GIF" that went viral on Twitter last December, showing a metal pylon playing jump rope with its buddies. Every time the pylon landed and the image shook, viewers reported hearing an audible thud, even though the GIF was totally silent.

The phenomenon, Live Science reported at the time, was like a mild form of synesthesia — a sort of cross-wiring of the senses that, in extreme cases, allows people to taste sounds or hear colors. But while less than 4 percent of the population is estimated to be affected with synesthesia to this degree, far more of us are susceptible to "hearing" what we see in little, everyday ways.

According to a new paper published today (March 20) in the journal Cortex, 1 in 5 people reported hearing noises that weren't there when looking at visual stimuli such as blinking car-turn signals, flickering neon signs or people walking down the street. In the paper, researchers provide a new name for the pervasive synesthesia-like phenomenon:visually-evoked auditory response — or vEAR.

"We think that these sensations may sometimes reflect leakage of information from visual parts of the brain into areas that are more usually devoted to hearing," Dr. Elliot Freeman, senior study author and a senior lecturer at City, University of London, said in a statement."In extreme forms of this crosstalk, any abstract visual motion or flashing may be sufficient to trigger the sensation of hearing sounds."

In the new study, researchers recruited more than 4,100 volunteers to complete an online survey, which asked them to rate how much "sound" they perceived in a series of 24 silent film clips (you can watch them all here). Each clip was just 5 seconds long, and showed either a real-world event naturally associated with sound (such as police lights blinkering back and forth or a sledgehammer slowly smashing a television set) or a more abstract visualization of motion (such as an array of multicolored balls swirling around a white field).

After watching each clip, participants rated how strongly the silent footage evoked an audible "sound" on a scale of zero to five. The researchers also asked survey-takers to answer some basic background questions, including, "Do you suffer from tinnitus (ringing in the ears)?", "Do you ever hear music in your head?" and "In everyday life are you ever aware of hearing sounds when you see flashing lights or movement?"

To that last question, 21 percent of respondents answered yes — indicating that about 1 in 5 participants had experienced vEAR before, the researchers wrote.

Results from the video-clip portion of the survey bore out the prevalence of vEAR even further. Not surprisingly, videos that showed events that naturally predicted sound (the flashing police lights, for example) got the highest overall scores for sound sensation. However, even the videos showing what the researchers called "meaningless motion," including a spinning disco ball and other swirling colors, still evoked audible sensations for many respondents. Participants who reported previously experiencing tinnitus or hearing music in their heads were also more likely to hear sounds while watching the silent video clips than those who did not report tinnitus or hearing music.

According to the researchers, the prevalence of this synesthesia-like effect might point to a "relatively direct effect of low-level visual stimulation on auditory processing," suggesting that human audio- and visual-processing systems are more intimately linked than previously thought.

"We like to listen to music synchronized with flashing lights or dance… and incidental sound effects accompanying action in movies, such as the comic 'boing' when a cartoon character slips on a banana skin," the researchers wrote. "These stimuli might reinforce each other via such audiovisual connections."

Originally published on Live Science.

Brandon Specktor

Brandon is the space/physics editor at Live Science. His writing has appeared in The Washington Post, Reader's Digest, CBS.com, the Richard Dawkins Foundation website and other outlets. He holds a bachelor's degree in creative writing from the University of Arizona, with minors in journalism and media arts. He enjoys writing most about space, geoscience and the mysteries of the universe.