About computer mouse

Many of you may have opened a computer mouse and have seen two slotted wheels which rotates in one or another direction depending on how you move the mouse.. (I am talking about an ordinary rolling ball mouse) I was wondering how the bidirectional motion of these wheels are detected by only one pair of sensors for each wheel.Or are there two sensors embedded in one single module ? so that IR to one of the sensors is cut first depending on which direction the wheel rotates ..