What do you experience when you watch this gif?
According to an informal poll on Twitter, almost 70% of the people who watch the (silent) gif hear “a thudding sound.”
The gif was created by an artist named HappyToast in 2008, who points out that the effect is almost entirely in the shake of the ground. For me, it results in a bit of a tingling vibration in my ears. Others claim that they literally hear a sound.
It’s been compared to The McGurk Effect—another phenomenon in which your eyes trick your ears. The McGurk Effect relies on your expectations about what comes out a person’s mouth when their lips move in certain ways. If you anticipate a “buh” sound because a person’s lips are touching, you will actually hear that sound, even if the person is actually making a “guh” sound.
The Sound of Silence
I suspect something similar is going on here. When you see something large hitting the ground, and the accompanying shake of the camera, you expect to hear a thud. A lifetime of dropping heavy objects prepares you for *thud*, in the same way a lifetime of watching people talk prepares you for “buh.” Perhaps more importantly, you are used to seeing shaking and thuds paired on screen, because of years of watching movies. Maybe Godzilla is to thank for this effect.
When your brain makes this connection, it's generally a good thing—when one event happens, your brain is already prepared for what usually comes next. That’s useful in the case of a giant monster attack. A study on this phenomenon, cleverly titled “a deafening flash!”, found that not only could 22%* of people shown visual flashes hear nonexistent sounds accompanying them, but those people were better at picking out morse code patterns from the flashes. There was a concrete advantage to the illusion.
Our test Double Trouble relies on your brain preparing to see the colour red when you see the word red (and vice versa)—a phenomenon called the Stroop Effect. A healthy brain is able to suppress that instinct when required, but even people who score highly on the test aren’t quite as fast at identifying the colour of a word when it’s inconsistent the written word. You can’t stop your brain from automatically piecing together sights, sounds, and meaning.
* Maybe the lower number here is because the participants weren't primed with the suggestion that the flashes were supposed to make a sound, unlike the tweets that resurrected this gif. Conscious expectations certainly play a role in the effect too.