Every member of Tigan society was implanted with an interface, just behind the ear. Via the interface information from the Tigan central computer was downloaded straight into the cerebral cortex using minute tachyon bursts. Entertainment, communications, financial transactions were all delivered straight into the minds of the Tigans.
However, as Data pointed out in 2364, the system was not a closed one, and as such was vulnerable to corruption. Indeed the system had been corrupted for centuries prior to Data's comments; when testing Tigan's planetary power production facility, the gravimatrix, a graviton pulse was emitted, it wiped the Tigan central computer and the information had to be re-input by hand. Politicians at the time took the opportunity to make a few alterations to Tigan’s recorded history, which via the interface were recorded into the Tigans minds, them none the wiser. Over centuries the process was repeated, more and more frequently, history being rewritten at the whim of those in power. The practice was finally brought to an end in 2364 when Edic, a man who's family had long been involved in the conspiracy, saw the wrongs that had been done and rebelled against his fellow conspirators. (TNG comic: "History Lesson")