Abstract: Apparatus for testing the response of a viewer to questions or problems displayed on a video screen comprises apparatus to modulate the video signal to display coded patches on the screen. The patches are coded by alternately raising and lowering the luminance signal of the video without destroying spacial variations of the signal in the patch. The coding is detected by a probe having a light guide pointer conducting light from the video screen to a photo-detector in the casing of the probe. The coded luminance variations are read by the probe and produce response indications such as sound or light signals which qualify the response. The apparatus is useful as a teaching aid.