Electroencephalography

Electroencephalography, or EEG, was an Earth 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 made on the Third Doctor, showing no activity. (TV: Spearhead from Space)

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