The question is, how much moral value you want to grant to a life of a monkey (not ape in this case). Most people wouldn't give it anything close to human-life level, at which point we're potentially open for experiments like these.

Assuming they're not doing it just for the kicks, but actually trying to make helpful discoveries about brains, the moral calculus will favour this experiment.

And if you do think a monkey's life is as important as human life, then well... you'll find many other more important moral violations to solve that have nothing to do with scientific experiments.