One of the more interesting parts of the brain which seems to be present only in mammals and is tied very closely to dreaming. It appears to be the system which causes voluntary movements to be suppressed while a creature is dreaming, and also seems to be part of what switches REM on and off. In addition, seratonin, the mood-regulating neurotransmitter, seems to be what it uses as its own controlling chemical.

As a case study, various cats had their RAS either destroyed or had their seratonin levels altered. For the former, as they went to sleep, entered REM, and began to dream, they did the feline equivalent of sleep-walking; they jumped around, pounced, hissed and snarled at imagined prey and predators. The latter ones simply were paralysed while remaining conscious, though from anecdotal human experience it can be inferred that they were dreaming (or, more appropriately, hallucinating) while conscious.

Certain people, narcoleptics in particular but also others with odd brain chemistry (such as myself) often have hypnogogic hallucinations thanks to a quirk of their RAS being triggered at seemingly-arbitrary moments, though in my own case I have an HH when I purposefully lay in bed in the morning while awake but allow myself to go back to "sleep" - I stay conscious, but my RAS kicks in, and I begin to dream/hallucinate after a brief uncomfortable (but yet pleasant) period of being paralyzed. It probably helps that I have weird seratonin issues.

The RAS is certainly one of the more interesting "design features" of the conscious brain. IMO, a RAS would imply dreaming, which would imply consciousness, which would imply sentience.

Do androids dream of electric sheep?