What Color is the Number 7?

Researchers from Israel, England, and Spain collaborated on a project that demonstrated that people with average brains are capable of having synesthetic experiences, meaning that triggering one of the senses causes the involuntary use of another.

Examples of this phenomenon include when people consistently see a certain numerical digit as a certain color or when hearing a certain sound triggers the experience of a certain taste.

The findings, published in the journal Psychological Science, contradict the prevailing belief that synesthesia results only within people who have extra synaptic connections in their brain.

Using a technique called posthypnotic suggestion, the researchers showed that it is possible to induce people to have synesthetic experiences. One test to confirm that the participants were truly experiencing synesthesia involved asking those who had been hypnotized to see the numeral "7" as red if they could see the number when it was printed in black against a red background. If the participants were unable to see the digit, the researchers concluded that the hypnotically-induced synesthesia was real.

The research shows that "cross-talk" within the brain can be the cause of synesthetic experiences, not extra brain connections. Coauthor Cohen Kadosh said "this takes us one step closer to understanding the causes of synesthesia and abnormal cross-brain interactions."

• Top 10 Unexplained Phenomena
• Rare but Real: People Who Feel, Taste and Hear Color
• New Insight into People Who Taste Words