Viki often communicated with gestures, rather than vocal sounds. Viki’s language ability was her weak point, and this presented the most crucial obstacle in the teaching/learning process. She never developed to the point of human children. Even as a baby, when most human children go through a babbling stage, Viki hardly made a sound. However, the experimenters tried to build on what little vocalization she had made and began a vocal training program at 5 months.

The few sounds she would use in play were assigned meanings, and those meanings were then taught to Viki. Their first step was to teach her to vocalize on command for a reward. It took her 5 months to consistently produce even the crudest of sounds. When the Hayes realized that Viki seemed to be having trouble with the actual formation of sounds, they began to help her by manually forming her mouth to make certain sounds.