Not too long ago, we brought you an optical illusion that made an image disappear before your very eyes. So now, in the interest of fairness, we're about to make you see something that doesn't exist at all.
Researchers at Caltech have come up with two new illusions – dubbed, for no doubt adorable reasons, the "Illusory Rabbit" and the "Invisible Rabbit" – that reveal some of the bizarre effects our senses can have on our brain. Their study is published in the journal PLOS ONE.
The video below shows the first of these – the "Illusory Rabbit". Just focus on the cross at the top of the video, and count the number of flashes you see below it.
Now, if you're like most people, you just saw three lights flash across the video, each accompanied by a high-pitched beep. But we know you're not reading this article to see things that are really there – so what did you actually just watch?
In reality, there were only two flashes. First, a light flashed on the left while a beep sounded at almost exactly the same time. Then, a tiny 58 milliseconds later, a beep was played on its own. Finally, another light flashed on the right, accompanied by a beep, sounding again at almost exactly the same time.
"But IFLS!" we hear you cry. "I saw a third light in the middle! What gives?"
Well, don't cry. We have the explanation, and it turns out the phantom flash's location is actually key to understanding it.
"When the final beep-flash pair is later presented, the brain assumes that it must have missed the flash associated with the unpaired beep and quite literally makes up the fact that there must have been a second flash that it missed," explained lead researcher Noelle Stiles in a statement.
"But even more importantly, the only way that you could perceive the shifted illusory flash would be if the information that comes later in time – the final beep-flash combination – is being used to reconstruct the most likely location of the illusory flash as well," she added.
Similar shenanigans lie behind the research team's second illusion, the "Invisible Rabbit". In this clear attempt at gaslighting, a video actually does show three lights flash across the screen – but this time, only two beeps are heard, causing most people who see it to think they only saw two flashes as well.
The trick to both illusions is, quite simply, speed. In less than one-fifth of a second, the brain is bombarded by input from both our eyes and ears. Scrambling to make sense of the information, it does the best it can to work out what it probably experienced – a phenomenon the researchers call "postdiction".
"By investigating illusions, we can study the brain's decision-making process," said Stiles. "For example, how does the brain determine reality with information from multiple senses that is at times noisy and conflicting?
"The brain uses assumptions about the environment to solve this problem. When these assumptions happen to be wrong, illusions can occur as the brain tries to make the best sense of a confusing situation."
"Postdiction may sound mysterious, but it is not," explained researcher Professor Shinsuke Shimojo. "These new illusions will enable researchers to identify optimal parameters for multisensory integration, which is necessary for both the design of ideal sensory aids and optimal training for low-vision individuals."