What’s happening when you think there’s a bug crawling on your leg, but nothing’s there?
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Your brain is basically a machine that compares two models of external reality- the model created by your senses, and a predictive model that guesses what the sensory model is going to look like. If that sounds weird it’s because it is.
When you feel a phantom bug on your leg, its because your sensory model showed some movement on your leg- wind brushing your hair or something- and the predictive model thought that seemed like the sort of data that would show up when a bug starts to walk on you, so it predicts that the sensory data will continue along those lines. So the predictive model adds a “bug walking on your leg” sensation, and you literally experience that sensation for a brief moment before the sense data shows that there isn’t actually a bug.
I see people in the comments talking about how autism can cause a really irritating form of this phenomenon where the bug crawling sensations keep coming back over and over again- there are some interesting hypotheses about autism and how it may in part be a pathology of the sensory/predictive models where the balance is thrown off in favor of the sensory model and the predictive model can’t smooth over irrelevant stuff or stuff that doesn’t make sense. Hence autistic people continually feeling the lining or tags on socks and clothes- the predictive model is being constantly outweighed by the sensory model and it can’t, as it does in neurotypical people, whiteout the irritating sense data from the tag.
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