A positronic brain is a fictional technological device, originally conceived by science fiction writer Isaac Asimov. Its role is to serve as a central computer for a robot, and, in some unspecified way, to provide it with a form of consciousness recognisable to humans. When Asimov wrote his first robot stories in 1939/1940, the positron was a newly discovered particle and so the buzz word positronic, coined by analogy with electronic, added a contemporary gloss of popular science to the concept.

Asimov remained vague about the technical details except to assert that the brain's substructure was formed from an alloy of platinum and iridium. Asimov relied on the reader's knowledge of the capacity of positrons and electrons to be formed in pairs and to annihilate each other, in order to convey the impression that such pair creation and destruction could serve as a metaphor for the evanescence of thought. The focus of Asimov's stories was directed more towards the software of robots (such as the Three Laws of Robotics) than the hardware in which it was implemented.

In the robot era's final days, Spacer roboticist Gubber Anshaw invented the gravitonic brain. It offered speed and capacity improvements over traditional positronic designs, but the strong influence of tradition made robotics labs reject Anshaw's work. (It is tempting to speculate that Roger MacBride Allen, in whose Caliban trilogy Anshaw appears, invented the word "gravitonic" the same way Asimov invented the original, perhaps as a deliberate homage.) Only one roboticist, Fredda Leving, chose to adopt gravitonics, because it offered her a blank slate on which she could explore alternatives to the Three Laws. Because they were not dependent upon centuries of earlier research, gravitonic brains could be programmed with the standard Laws, variations of the Laws, or even empty pathways which specify no Laws at all.

Star Trek

The fictional characters Lieutenant Commander Data, his eldest brother B-4, his "mother" Julianna Soong Tainer, and his evil brother Lore from the Star Trek series The Next Generation, were androids equipped with positronic brains. In one episode Data creates an offspring named Lal with a similar but somewhat more sophisticated brain. After a short time she displays promising advances in emotion and other human behaviours that Data has not been able to master. Sadly, she died of a "rapid positronic cascade failure" shortly after she had been told that Starfleet wanted to separate her from Data.

None of these androids were constrained by Asimov's robot laws, although Data's actions were restricted by ethical programming provided by his creator, Dr. Noonien Soong.

It is stated in the episode "Relics" that older computers were "duotronic," presumably being a combination of electronic and positronic. An aged Montgomery Scott expresses surprise that a purely positronic computer is possible.

Perry Rhodan

In the German science fiction series Perry Rhodan, positronic brains (German: Positroniken) are the main computer technology. The most powerful positronic is called Nathan and covers large parts of the earth's moon.