Researchers at Caltech have revealed two illusions illustrating how our brain can be tricked into seeing something that isn't really there. The phenomenon is known as postdiction, and highlights how our perception of reality is actually constructed by our brain retroactively.
The primary illusion developed by the research team is dubbed the Illusory Rabbit. Three beeps, 58 milliseconds apart are played but only the first and third beep are accompanied by a flash, on the left and then the right side of the screen. The study found that most people viewing the illusion perceived an extra central flash alongside the second beep. When the illusion is repeated, without the three beeps, participants tended to only see the two left and right flashes delivering strong evidence that the brain uses postdictive processing to generate its overall perception of reality.
"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," explains Noelle Stiles, first author on the study. "This already implies a postdictive mechanism at work. 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."
The other illusion developed, called the Invisible Rabbit, is an interesting riff on the first. This time three flashes are presented to the participant but only the first on the left, and the third on the right are accompanied by beeping sounds. The study found most people do not see the second central flash, again affirming that the brain retroactively ignores the middle flash when not accompanied by a sound.
A major revelation in the study is that perception seems to be a result of postdictive processing across multiple senses. This is the first time researchers have shown contradictory stimuli from different senses interacting in this way. In addition, this is one of the few times research has demonstrated sound altering a person's sense of visual perception.
"These illusions are among the very rare cases where sound affects vision, not vice versa, indicating dynamic aspects of neural processing that occur across space and time," explains Shinsuke Shimojo, another author on the new research.
The research acts as a compelling reminder that our perception of reality is in fact a delayed construct, created by our brain to make us feel like we are consciously experiencing the present in real-time. Exactly how far behind real-time our conscious awareness actually is though, is still subject to some debate.
Some research suggests the delay in processing visual information is around 80 milliseconds, which is inline with the timing of the Rabbit Illusion video above. On the other end of the scale, pioneering work from scientist Benjamin Libet revealed sensory information can take up to 500 milliseconds to be processed by the brain. Libet's work in particular led to a still controversial theory, referred to as the half-second delay, suggesting our brains fool our conscious minds into thinking we are experiencing sensations instantly when in fact our subjective immediate experience is an artificial construct.
What this science means in relation to human concepts surrounding free will is something that will surely be debated for years to come, but some philosophers suggest this means our conscious awareness of a decision may come some time after our brains have already initiated an action. Libet himself claimed our free will appears in the small time we have to veto a decision already actioned by the brain. This small window of 100 to 150 millisecond is hypothesized as all we have, and some neuroscientists call this not free will, but rather free won't.
Philosophy aside, the new Caltech study certainly highlights how our sensory perceptions can be strangely retroactively modified without our conscious minds aware of the adjustment. When aural and visual stimuli present the mind with contradictory information our brains intriguingly can construct a false perception of events.
The new research was published in the journal PLOS One.
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