A pattern analyser was a device used by the Kraals to gain information and map the subjects memory and intelligence onto their androids.
Sarah Jane Smith was subjected to it by Styggron, who produced an android replica of her. He also learnt information regarding the Doctor's previous triumphs when defending the Earth, leading him to attempt to vaporise the Fourth Doctor.
This failed and, instead, the Time Lord was himself subjected to the analyser, after Guy Crayford persuaded Styggron to do so. The information in his brain was recorded in the Kraal data banks. Styggron told the Doctor that, as he was leaving for Earth, he would not be present to turn of the machine so the stimulation would eventually cause his skull to burst. This was prevented when Sarah freed him. (TV: The Android Invasion)