Caledonian:

... We ask Heath if human beings are as compulsive about pleasure as the rats of Old's laboratory that self-stimulated until they passed out. "No," he tells us. "People don't self-stimulate constantly -- as long as they're feeling good. Only when they're depressed does the stimulation trigger a big response. There are so many factors that play into a human being's pleasure response: your experience, your memory system, sensory cues..." he muses.

http://www.paradise-engineering.com/brain/index.htm

In the case of humans, our reward-pathways are (slightly) more anatomically diffuse than the average rodent. At least with present-day electrode-placement techniques, intra-cranial self-stimulation (ICSS) as practised by laboratory humans doesn't lead to uncontrolled hedonistic excess and death. Only depressed or deeply malaise-ridden human subjects compulsively self-stimulate when wired. Ill-defined "ethical constraints", however, are commonly held to forbid the human use of ICSS rather than to mandate its widespread adoption and refinement for "treatment-resistant" depression - even by avowed utilitarians. So instead of using depressed fellow humans, experimenters use rats. Pleasure-crazed rodents have become the symbolic expression of wirehead hedonism - and of all the pitfalls that "unnatural" pleasure entails.

http://wireheading.com/