The vast majority of footage of Koko takes place in the last decade or two. One of my main recurring thoughts while watching the remarkable videos was, "This is absolutely amazing! But how did they teach her?" That was when I happened upon a 1978 documentary entitled Koko-- A Talking Gorilla. It's focus was on her younger years, and the research that was conducted on her adolescent mind. The movie also delves into her social relationships, and the rate at which she learned to speak using American Sign Language (ASL).
Koko-- A Talking Gorilla
https://www.youtube.com/watch?v=0NwCrXVUm_Q
Koko-- A Talking Gorilla
https://www.youtube.com/watch?v=0NwCrXVUm_Q