The ganzfeld is a sensory deprivation technique developed by psychologist Wolgang Metzger in the late 1920s. The term “ganzfeld” comes from the German word for “total field,” and it works by presenting the subject with completely uniform sensory stimuli and then asking them what they see. This is most commonly accomplished by placing half-ping-pong balls over the subject’s eyes and headphones playing white noise (static) over the ears.

Parapsychologists used this technique in the mid-1970s to see if it was conducive to ESP. Charles Honorton, of the Maimonides Laboratory, had recently announced promising results from his research into ESP and dreams, but he was frustrated by the length of time it took to complete one trial. The ganzfeld was chosen because it was faster and was thought to induce a hypnogogic-like state in the user, allowing ESP to function. As a result, while the subject (or “receiver”) was in the ganzfeld state, another person was looking at a target (usually a photograph or videoclip). After a half-hour, the subject would be given the target as well as a number of decoys and asked to select the one that looked the most like the images they saw. Typically, three decoys are used in addition to the target, yielding a score of 25% by chance.

The ganzfeld protocol quickly became a popular choice for testing for psi, and it has been in continuous use since the first experiment was published in 1974. The evidence for psi in the ganzfeld is typically divided into three sections: 1974-1982 (Honorton’s 1985 meta-analysis); 1982-1989 (Honorton’s Psychophysical Research Laboratory results); and 1991-1999 (Bem, Broughton, and Palmer’s meta-analysis).