To make biological survival possible, Mind at Large has to be funnelled through the reducing valve of the brain and nervous system. What comes out at the other end is a measly trickle of the kind of consciousness which will help us to stay alive on the surface of this particular planet. To formulate and express the contents of this reduced awareness, man has invented and endlessly elaborated those symbol-systems and implicit philosophies which we call languages. Every individual is at once the beneficiary and the victim of the linguistic tradition into which he or she has been born — the beneficiary inasmuch as language gives access to he accumulated records of other people’s experience, the victim in so far as it confirms him in the belief that reduced awareness is the only awareness and as it be-devils his sense of reality, so that he is all too apt to take his concepts for data, his words for actual things.

- Aldous Huxley, The Doors of Perception

And here’s a nice elaborated excerpt from an article about the same thing. This one’s the “money shot,” as they say:

Henri Bergson has suggested that one of the main functions of the brain and nervous system is to eliminate activity and awareness, rather than produce it.

In “The Doors of Perception,” Aldous Huxley quotes Prof. C.D. Broad’s comments on this. He says that every person is capable of remembering everything that has ever happened to him. He is able to perceive everything that surrounds him. However, if all this information poured into our minds at once, it would overwhelm us. So the function of the brain and nervous system is to protect us and prevent us from being overwhelmed and confused by the vast amount of information that impinges upon our sense organs. They shut out most of what we perceive and remember. All that would confound us is eliminated and only the small, special selection that is useful is allowed to remain.

Huxley explains that our mind has powers of perception and concentration that we cannot even begin to imagine. But our main business is to survive at all costs. To make survival possible, all of our mind’s capabilities must be funnelled through the reducing valve of the brain.

Some researchers are studying this effect. They believe that this reducing-valve effect may be very similar to the jamming equipment used to block out offensive radio broadcasts. The brain constantly produces a kind of static, cutting down our perception and reducing our mental activity.

This static can actually be seen. When you close your eyes, you see all sorts of random pictures flashing through your mind. It is impossible to concentrate on any one of them for more than an instant, and each image is obscured by a host of others superimposed over it.

This static can even be seen when your eyes are opened. However, one usually ignores these images since they are so faint compared to our visual perception. However, they still reduce one’s perception, both of the world around him and of himself.

Much of what we know about this static is a result of research done with drugs that eliminate it. According to a number of authorities, this is precisely how the psychedelic drugs work.

And another snippet from someplace else…

Aldous Huxley in “The Doors of Perception” talks about a “reducing” valve in the brain that limits the amount of sensory stimulus that brain allows to be processed. This reducing value, in part, may be the thalamus. In schizophrenia, that reducing valve is more open than it should be; thus a person with schizophrenia receives too much stimulus and their brain has a hard time interpreting the inputted sensory stimulus. This may result in the positive symptoms (hallucinations and psychoses) we see in schizophrenia. When sensory overload occurs, some brain functions may shut down resulting in the negative symptoms (poverty of speech, withdrawl).

Plus another one:

Our normal mode of thinking can be described as survival-thinking. We see a traffic signal, and we think “stop” rather than “pretty red light.” Furthermore, since our mind is designed to act upon things, we normally limit our perception to those things we wish to act upon. This is known as attention, a form of consciousness in which awareness is brought to a sharp but limited focus; we see what we have to see, and we see it the way we need to see it. Both abstraction (”stop”) and attention are designed for action, and so we view the world in terms of our action upon it. Along these same lines, Huxley described the brain and nervous system as a “reducing valve” which receives the flood of sensory input and filters out all that which is not necessary for action, and therefore for survival; were it not for this, we could not function in the world as we know it. To function, we must deceive ourselves as to the actual nature of reality—a form of adaptation which LSD researcher Willis W. Harman has termed cultural hypnosis. “We are all hypnotized from infancy,” wrote Harman, who went on to propose that this was just another way in which to describe enculturation. We accept suggestions from the environment—from our parents and society—and these suggestions shape the manner of our perception; finally, we perceive things in a state of hypnosis: not as they are, but as we are told to see them.

More on this later…