During the 20th century a psychiatrist specializing in medical hypnosis, Milton Erickson, was one of the leading researchers in the area. He was the founder of the American Society for Clinical Hypnosis and he was also a major influence on neuro linguistic programming (NLP) conducting research on the power of the unconscious mind including numerous colour studies.
In 1938 Erickson and Erickson conducted a study involved 4 patients. They all received hypnotic suggestions that a white piece of paper was actually of another colour and then received suggestions to see the complementary colour of the preceding sheet of paper. All 4 participants hallucinated the complementary colour.
A year later another colour study was conducted involving a further set of participants. This study surrounding the subject of colour blindness and involved 6 participants with normal vision. This time the hypnotic suggestions were designed to induce colour blindness. An Ishihara test was conducted to find the extent of colour blindness. One participant associated the number 3 with the colour red and after receiving hypnotic suggestions, was not able to comprehend the number 3. This study led Erickson to determine that colour blindness originates in the cortical area of the brain and hypnosis can influence it.
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