Sarina Douglas was a genetically-engineered Human. As a child, she had undergone an illegal procedure called accelerated critical neural pathway formation which dramatically increased her intelligence beyond what Humans would call genius level. Unfortunately, though her cerebral cortex was enhanced to process information faster, it couldn't initially get that information from her visual and auditory systems fast enough, and Sarina was unable to focus on her world. Outwardly, this made her appear mute and oblivious to her surroundings. (DS9: "Statistical Probabilities")