A computer interface, called a Cyberlink, allows users to control a mouse, keyboard, joy-stick or even more complicated applications like browsers and other programs by thinking about what they want to do. A headband with three built-in sensors is the center of the technology, which is purchased primarily by schools training quadriplegics and others who lack the ability to use hand-controlled devices. Curiously, the engineer who designed the system was motivated, not by dreams of helping the handicapped, but by a vision of controlling his sailboat with his mind while relaxing on deck. Though the company’s Web site says the system is easy to use, others say it is easier for some than others. The manufacturer uses video games, such as Mind Tetris and Brain Billiards, to help users adapt to the interface. As prices fall and the technology improves, commercial game-designers will implement mind-controlled interfaces into their systems.