The new optical versions add the latest technology to the unique design of the original Perfit Mouse. A high precision optical sensor provides excellent cursor control. The Perfit Mouse Optical (PMO) places a scroll wheel above the thumb support, rather than on the top of the mouse to reduce the strain on the index finger, which is already heavily used for the primary mouse click. At the bottom of the integrated Thumb Support is the innovative two-position side rocker switch which gives quick and effortless access to two additional programmable buttons.