An optical mouse includes an image sensor for providing image data via an analog-to-digital converter to a correlation circuit and a motion estimation circuit to provide output signals representative of motion of the mouse. The output signals may be disabled when the mouse is lifted away from the working surface. This may be achieved by high-pass filtering the signals, summing each frame in a summer to provide a single value, and comparing this to a threshold. If the filtered and summed value exceeds the threshold, this may indicate that the image contains in-focus objects, and that the mouse is on the working surface.