The average brain weighs 1300-1400 grams. If 0.5% of that is plastic by weight, your brain contains 6.5 - 7 grams of plastic. The average plastic credit card weighs 5 grams.
https://www.theguardian.com/environment/article/2024/aug/21/microplastics-brain-pollution-health
I gave the study a read. It's in preprint, it's yet to be peer reviewed and the method they used is atypical. Yeah I think disbelief is a pretty reasonable response since it's hard to imagine that much plastic wouldn't be making noticeable changes in behavior/physiology. But it's possible and the research seems rigorous