“However, the nature of this association varied widely,” noted Sakari Kallio from the University of Turku. “Two participants reported that they visually experienced the symbols as having the suggested color: in one case with full self-awareness of doing so and in another case not."
The third person wasn’t aware of the suggestions and didn’t experience any color change, but still had difficulties naming the real colors of the symbols. Meanwhile, the controls failed to reproduce the synesthetic experience.
Interesting as these results are, the team highlights that further studies are needed to determine whether synaesthesia really can be brought on by hypnosis. After all, a sample size of four is far too tiny to go on. It’s also difficult to link the experiences from hypnotic suggestion to synaesthesia itself.
Nevertheless, the findings in combination with other studies may help in terms of finding out more about perception and sensation. "Perhaps most importantly, the results showed both definite similarities and clear differences to naturally occurring synaesthesia,” said Kallio.