Kant said it first! https://plato.stanford.edu/entries/kant-reason/

A professor once told me that if someone asks when a concept was first discussed in philosophy, it's always best to reply "Isn't it in Plato?"

https://plato.stanford.edu/entries/innateness-history/#PlaAr...

If that's so, then we're back to Nietzsche's Perspectivism. The experiment is limited by what the experiment entails.

Not quite, because the brain is an empirical object itself. Kants pure intuitions and categories are before any possible experience. Kant would say we can’t conclude anything certain from the empirical observation of the brain, only that before any empirical observation we have those a priori intuitions and categories.