The human brain has a multi-stage filtering system for stimuli. Information coming in from the senses is analyzed through the hypothalamus, which decides how crucial a given stimulus might be.

If the Hypothalamus "decides" that a given stimulus is not important, it sends no signal to the Reticular Activating System, and the stimulus remains outside of conscious perception. The stimulus is below the thalamic gain threshold.

That "decision" might be based on the amplitude, the the rate of amplitude change, or the perceived importance (danger or benifit) of the stimulus.

If the stimulus is above the thalamic gain threshold, the hypothalamus sends a message to the Reticular Activating System, or RAS, which in turn activates the rest of the brain, alerting it to the stimulus.

The thalamic gain level is variable, even in individuals, according to fatigue level and interest. But there is a fundamental set point (similar to the pain threshold) from which circumstantial variations are pegged. This fundamental set point is inborn.

A person with a fundamentally high thalamic gain point can remain interested in relatively low stimulus for long periods of time. A person with a relatively low thalmic gain point requires a stronger stimulus to alert the RAS. This results in some unusual attention phenomena in ADD people.

For example, an ADD person might fail to perceive spoken information at all, if the subject matter is not intensely interesting to him. He will literally hear nothing. He is not ignoring the instruction because (perceptually speaking) there is nothing to ignore. This can be anything from annoying to catastrophic depending on the information that is missed.

On the other hand, an ADD person might notice an extremely small or seemingly insignificant detail if it is something that is interesting. This is because the cerebral cortex takes a role in assigning levels of importance to stimul, which affect the hypothalamus' evaluation of that stimuli.

An example that everyone can relate to is the alarm clock. Many people report being able to sleep through their mate's alarm clocks, while being awakened by their own. Another example is the mother who can ignore a truck going by outside while hearing the smallest sound uttered by her baby.

~Thom Hartman proposes that the brain has a "need" to be awake. The neurological basis for this is found in Robert Orenstein's book, Roots of the Self

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