In a teaching system, a television screen displays a question calling for one of four possible answers, together with one or a pair of dots of light at locations illuminated in a binary pattern indicating the correct answer. Optical sensors detachably secured to the television receiver screen at the locations and concealing the same from view feed a pair of binary signals to a logic decoder. The decoder provides an output signal over one of four output lines to a number of student-actuated responder units, each of which includes four switches for selecting an answer and a lamp that is illuminated to indicate the selection of a correct answer. In an alternative embodiment, each responder unit is disabled after the switches are first actuated to select an answer and is re-enabled only after receiving a reset signal from an additional optically sensed screen dot at the beginning of the next response period.
