Looking for a sci fi short story involving vivisection

I’m trying to find a short story involving a scientist who performs vivisection on apes. I believe it was written in first person. The horror aspect starts coming in when the voices of his coworkers and friends start to distort into a bizarre shriek. He discovers that he has been under the control of aliens (or other dimensional beings) under the veil of the world populous,
And having him perform the vivisection on other humans.

I read this book almost twenty years ago in elementary school. It was on the librarians desk in a basket with dozens of other short stories. It blew my mind and nobody else wanted to hear about it. It put this thought in my head of "who around me is who they say they are". It was also one of my earlier thoughts of what is REALLY going on in the world around me. Even then I was amazed I found such a dark book just tucked away in the mix.