In Parapsychology
Within the field of parapsychology, telepathy is considered to be a form of extra-sensory perception (ESP) or anomalous cognition in which information is transferred through Psi. It is often categorized similarly to precognition and clairvoyance. Various experiments have been used to test for telepathic abilities. Among the most well known are the use of Zener cards and the Ganzfeld experiment.
Zener cards are cards marked with five distinctive symbols. When using them, one individual is designated the "sender" and another the "receiver". The sender must select a random card and visualize the symbol on it, while the receiver must attempt to determine that symbol using Psi. Statistically, the receiver has a 20% chance of randomly guessing the correct symbol, so in order to demonstrate telepathy, they must repeatedly score a success rate that is significantly higher than 20%. If not conducted properly, this method can be vulnerable to sensory leakage and card counting.
When using the Ganzfeld experiment to test for telepathy, one individual is designated the receiver and is placed inside a controlled environment where they are deprived of sensory input, and another is designated the sender and is placed in a separate location. The receiver is then required to receive information from the sender. The exact nature of the information may vary between experiments.
Some parapsychologists still proposed that telepathy may have a physical explanation. The Italian neurologist Ferdinando Cazzamali in the 1920s had claimed that telepathic communication occurred due to a type of electromagnetic radiation. However the neurophysiologist William Grey Walter in his book The Living Brain (1953) wrote that electrical 'brain- waves' are too weak to explain telepathy. Hans Berger also held this view but extended the theory by proposing that telepathy occurs when "electrical energy in the agent's brain is transformed into 'psychic energy' which can be diffused to any distance, passing through obstacles without attenuation".
In 1974 Michael Persinger proposed that extremely low-frequency (ELF) electromagnetic waves may be able to carry telepathic and clairvoyant information. Gerald Feinberg also suggested that telepathy may exist due to as of yet undiscovered elementary particles which he called 'psychons' or 'mindons'.
In recent years the parapsychologist Charles Tart has accepted the existence of telepathy but claims that it is nonphysical in nature and can not be fitted into any physical theory.
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