As anyone who’s ever seen an optical illusion or heard a nonexistent sound knows, our senses play tricks on us all the time. Another example of this is the oral size illusion. Objects, especially holes, can feel much larger or smaller depending on whether we’re feeling them with our tongue or finger. A recent study published in the Journal of Experimental Psychology: Human Perception and Performance laid out the science behind this phenomenon.
As Discover reports, small holes felt with the tongue seem larger than when felt with a finger (if your mind's already in the gutter, please get it out now; it only gets worse from here). Researchers conducted five experiments to test a hole’s perceived size under different circumstances, including when sensed with the tongue, the index finger, and the big toe. They found that the more pliable the appendage is, the bigger the space it’s exploring seems to be. The squishy tongue has an easier time bending around and conforming to different surfaces, painting a more accurate picture of what it’s feeling and enhancing the size in our minds. A finger doesn’t squeeze into places in quite the same way, so the sensory information it conveys is less accurate. A toe also isn’t as pliable as the tongue and leads the person attached to it to believe they're feeling something that’s smaller rather than larger.
The oral size illusion doesn’t apply to all objects. For example, a previous study found that it was much more likely to be triggered by holes than cylindrical pegs. That could be why when our teeth fall out as kids, the holes they leave behind feel cavernous compared to the teeth that used to be there. That’s just another reminder that we shouldn't always blindly trust the signals our body sends us.
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