This device consisted of an artificial neural network, designed to imitate the humanoid brain. The construction of a positronic brain was extremely complex, and Dr. Soong was the only scientist to have done so successfully – on at least six occasions: two unspecified prototypes, B-4, Lore, Data, and Juliana Soong (although the latter three were the only truly stable units).
Later that year, Soong's creation, Data, attempted to emulate his father's work, and created the android Lal. However, Lal's positronic net suffered a cascade failure, and ceased functioning. (TNG: "The Offspring")
A plasma shock was enough to overload Data's neural net, (TNG: "Power Play") while a Kedion pulse would cause Data's subroutines to reset to normal if they were altered by outside sources for some reason. (TNG: "Descent, Part II")
Around 2370, Data began a discussion with Deanna Troi about the long-term effects of space travel on his positronic net, which Troi was later disinclined to continue. (ENT: "These Are the Voyages...")
Dr. Julian Bashir implanted Bareil Antos with a positronic matrix to keep him alive after part of his brain stopped functioning due to a bad reaction to various medical treatments Bashir had been forced to perform to keep Bareil available to assist in crucial negotiations with the Cardassians. Bareil awoke but said that the sensations he was experiencing did not seem real and seemed like a distant memory. Later, when other parts of Bareil's brain stopped functioning, Bashir chose to let Bareil die rather than replace the rest of his brain with positronic implants, feeling that he wouldn't be himself if any more of his brain was replaced. (DS9: "Life Support")