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In a message dated 2/16/99 7:09:56 PM Eastern Standard Time, [log in to unmask] writes: << IT is an interesting idea that pallidotomy is the cutting of a > overactive movement feedback loop . It might explain PD freezing as > an overactive feedback . > However feedback loops are designed to increase control. So the > destruction of a feed back control would naturally result in less > control . >> This is assuming the most common use of feedback loops: NEGATIVE FEEDBACK. (f the output tries to go higher (or lower), the feedback system senses this and sends back a signal reducing (or raising) the gain and the outputs returns to the desired value. A highly fedback system can give great stability -- maintaining the status quo. There is another type of feedback loop: POSITIVE FEEDBACK. This functions just the opposite way; when the output goes higher for any reason, a signal is sent back INCREASING the gain -- giving and even larger output, which is fedback, giving an even larger output, and so on, and so on.....Usually the overall system reaches its maximum limit (or breaks down somehow). The example of this that most people are familiar with is when someone is setting up a PA or Sound system in a hall often times a sudden screech occurs. This is usually when the microphone "sees" the output of the amplifier, and amplifies it over and over. The system reaches its maximum volume (and for other reasons the sound is badly distorted -- SATURATION). If you interrupt the "feedback loop" -- make it so the output of the amplifier no longer goes directly into the mike, it quiets down. I can see why this second type, positive feedback, could be used in modeling tremors. Electronically, this is Oscillator Theory and the next step is to look at the "gain" of the system -- the state of the response to your meds (amount of dopamine present?) NOW as a physicist/engineer I am over my head in the biology/physiology/pharmacology of the brain and will stop here. You have just received the intro to a part of my Electronics course on Control and Oscillators -- doesn't that make you want to rush back to college and study physics and engineering -- or some of the other fun things that go on there. Phil C., CG for wife, Carol 62,15