Electroencephalography

Electroencephalography, or EEG, was a technology able to register the electrical activity of the brain (TV: Spearhead from Space) and show the brainwaves of an individual. (TV: Planet of the Spiders)

While in a self-induced coma post-regeneration, the test was performed on the Third Doctor, showing no activity. (TV: Spearhead from Space)

The Third Doctor employed such a test on the clairvoyant Herbert Clegg. (TV: Planet of the Spiders)

The technology of the Kaled security scan was able to run an EEG on the instant. It contributed to demonstrate that the Fourth Doctor and Harry Sullivan weren't native of Skaro. (TV: ''Genesis of the Daleks)

In 5000, at the Bi-Al Foundation, Professor Marius and a nurse took an encephalograph out of the Fourth Doctor, affected by the Nucleus. (TV: The Invisible Enemy)

On Thoros Beta, EEG readings were consulted during an operation of brain transplant by Crozier and the Sixth Doctor. (TV: Mindwarp)

Other references
According to the Fourth Doctor, the Vardans were able to read thoughts and "encephalographic patterns". (TV: The Invasion of Time)