Abstract: A detector of an electro-optical mouse having two linear arrays of detector cells at right angles to each other. One linear array images a line and a space in the X direction, while another linear array images a line and a space in the Y direction. Changes in contrast level indicate line crossings. Each linear array has four cells, adjacent cells being 90.degree. out-of-phase with each other. By pairing first and third cells and second and fourth cells of each linear array, then subtracting the output of these, a 180.degree. shift between the cells enhances discrimination of lines and spaces. Lines may be of two colors, or of a single color with optically contrasting line intersections, so long as a total of three contrast levels can be observed by the detector. Counting means is provided for counting line crossings so that observed detector motion on the surface can be made to correspond to cursor motion for a video display.