In Larry Niven's Known Space stories, a wirehead is someone who has been fitted with an electronic brain implant (called a "droud" in the stories) to stimulate the pleasure centres of their brain. In the Known Space universe, wireheading is the most addictive habit known (Louis Wu is the only given example of a recovered addict), and wireheads usually die from neglecting themselves in favour of the ceaseless pleasure. Wireheading is so powerful and easy that it becomes an evolutionary pressure, selecting against that portion of Known Space humanity without self-control. Wireheading need not use an actual brain implant; the pleasure centre can be remotely activated by a small device called a "tasp" (important in the Ringworld novels).

http://en.wikipedia.org/wiki/Wirehead_(science_fiction)