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A psychic link was a link between two individuals.
The Doctor had a psychic link with his other incarnations whenever they were in the same part of space and time. This allowed them to share thoughts and information amongst themselves, or even combine their mental efforts against an opponent. (TV: The Three Doctors, The Five Doctors, The Two Doctors)
Prisoner Zero used psychic links with the minds of several people around Leadworth. They were put into comas as a result and taken to Royal Leadworth Hospital. This allowed him to take on the form each individual person was dreaming about. Rory Williams noticed the patients around the village and took photos of Prisoner Zero on his mobile phone. The Eleventh Doctor used these pictures to help the Atraxi locate Prisoner Zero. Prisoner Zero told the Doctor he would take on another form, but the Doctor pointed out that it took months to form such a link. Prisoner Zero activated a psychic link with Amy Pond, as he had spent twelve years in a room in Amy's house. The Doctor used the link to force Prisoner Zero to take on its own form, alerting the Atraxi to its location. (TV: The Eleventh Hour)