I just stumbled across this forum after plugging in "Reading upside down" on Google.
I'm 41 years old, and I have been reading upside down for the past 16 years. As a kid, I struggled to read. I had trouble tracing left to right on a page. I would get headaches and would get very frustrated. I was very good in math and spelling, and I read mostly Cliff Notes to get through High School and undergrad college. Up till I was 25, I could count the amount of books read for pleasure on one hand. I must say my vocabulary was horrific.
The day I picked up a magazine upside down and started reading transformed me. It felt easy and natural. Since then, I've completed my masters in engineering with a 4.0/4.0 and my MBA (magna *** laude). I moved from Boston MA to England to run a business unit for a public company. More importantly, I have discovered the joy of reading that has eluded me for half of my life.
I cannot do math upside down, so I'm curious as to how the brain is wired for this. I've been searching for years to find if there is a specific diagnosis for this.
My message to you folks you have kids who do this is not to discourage them. The only downside I've experienced is weird looks from fellow plane passengers

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Love to hear from more,
Joe