It was almost like Britney Spears‘ brain was killed by a poisonous mushroom at the “X Factor” auditions in North Carolina … ’cause she went CRAZY for a Vanilla Ice wannabe who busted out “Ice Ice Baby” … and it was all caught on tape.

It all went down BEFORE that crazy lightning storm in Greensboro … Britney was so moved by some guy’s rendition of the ’90s rap … she actually got out of her seat and started shakin’ her ass with the music.

Simon, L.A. and Demi weren’t feelin’ it as much. They just don’t understand.

Word to your mother.