OVING a mouse with precision is difficult for those whose hands shake because of motor impairment. Now an I.B.M. researcher has invented an inexpensive adapter that minimizes the impact of such tremors.

Tens of millions of people worldwide experience involuntary hand movements because of conditions like Parkinson's disease and essential tremor.

The adapter, which is the size of a hand-held calculator, plugs in between the mouse and the computer. A microprocessor within the device takes the motion data that normally goes to the computer and applies an algorithm that filters out all the high-frequency motion caused by the tremor.

"It leaves the steady part of the motion alone," said James L. Levine, the research staff member at I.B.M. who invented the device. The adapter can be switched off so that others can use the mouse in the standard way.

The mouse filter is being offered on the Web for about $100 by a small British electronics firm, Montrose Secam (montrosesecam.com). "I was trying to find a solution for my own tremor, which I inherited from my father, " said James Cosgrave, a director of the company. "When I switch on this adapter, I can use the mouse as well as the average person."

The mouse adapter has several controls, including one that rejects the extra mouse clicks of twitchy fingers. Another control makes it easier to double-click, a motion that can be difficult for many people with motor-control problems.

Dr. Levine became interested in the problem of using a mouse about three years ago at a workshop on information technology for older people. "I remembered an incident with an uncle of mine who tried to use our computer, and he couldn't do it because he had so much tremor," he said.

Dr. Levine is an experimental physicist with a specialty in instrumentation. "That's exactly what this problem required - measuring something when there is the noise of hand tremor - so it was natural to think of applying a digital filter," he said.

To test the device, Dr. Levine called on Cathy Bodine, who directs a program at the University of Colorado School of Medicine in Denver that works with people with disabilities. Dr. Bodine contacted a local tremor group as well as a national support group to recruit people who could identify crucial properties they wanted in the prototype, and then test these properties.

"The participants had to click on buttons on the screen," she said, opening and closing programs, as well as drawing the letter X with the device, both with the filter on and off. "There was a huge difference between using the filter and not using it."

Meanwhile, in England, Mr. Cosgrave was trying to find a solution to his own difficulties with a computer mouse. Mr. Cosgrave, a retired airline pilot who continues to fly a small airplane, said his tremor had not interfered with his abilities in the cockpit. "But it causes the cursor on my computer to dance around the screen with a mind of its own," he said.

Clicking on small boxes is particularly difficult. "The little points you have to click on, I can't find those," he said, "but when I switch on the adapter, it's a different matter altogether."

It was Catherine Rice's bulletins on the I.B.M. prototype that first caught Mr. Cosgrave's attention. Ms. Rice is executive director of the International Essential Tremor Foundation in Lenexa, Kan. "Jim Levine had contacted us, asking could we help him find someone who'd be willing to manufacture it," she said. Ms. Rice sent letters and put notices in a newsletter. "That took about six months," she said.

Then Mr. Cosgrave saw a reference and followed up. "I called the foundation because I was thinking of myself, and not from the point of view of manufacturing," he said. But because his company was involved in electronics, he decided to pursue producing the device himself.

About 10 million people in the United States have essential tremor, Ms. Rice said. The average age of onset of tremor is 45. "As you age, the tremor gets worse, but we have a large number of children affected, too," she said. Some people with a gene predisposing them to tremor may not develop it until they are in their 70's; others will be affected far earlier. "It's a misconception that it affects only the elderly," she said.

Unlike some conditions in which a tremor may come and go, essential tremor is always present, she said, except during sleep. "It doesn't start until you start to do something," she said, and then is especially prominent when people extend their hands for fine motor movements like using a mouse. "They can't get the cursor to sit still."

Dr. Bodine says people have been calling since the trials, asking when the mouse will be on the market. "The results of our tests showed that it helped people, minimizing the impact of tremor on the use of the computer," she said.

No additional software is required with the device, called an Assistive Mouse Adapter.

Richard F. Doherty, research director of the Envisioneering Group in Seaford, N.Y., said the I.B.M. device was highly promising. "We have ultraprecision mice for video gamers and for graphics designers laying out movie special effects," he said, but only limited choices in adaptations of mouse controls for people with tremors. Some options, like eye tracking, are relatively cumbersome and expensive. "There's nothing as adaptable and robust as this solution," he said. "It's rugged enough to handle a wide degree of motor skills disorders."