A mnemonic memory circuit was a piece of computer technology that was in wide use in the 23rd century. While trapped in 1930s New York City, Spock attempted to contruct a circuit so that he could access his tricorder and view the recordings that were made when he examined the Guardian of Forever, and try and decipher what changes had been made by Dr. Leonard McCoy when he traveled to this period. Because he was stranded on 20th century Earth, he didn't have access to all the equipment present aboard the USS Enterprise, and was forced to use equipment of the day, such as radio tubes. Spock compared these "zinc-plated, vaccuum tube" period materials to primitive stone knives and bearskins.