As noted, we utilize a computer vision system as well as audio. A small video camera is positioned such that it can detect frontal faces when the users look on the screen. That way, the system can know when its feedback is being elicited. We can thus tell when the system should give feedback. The computer vision system searches for skin-colored blobs and detects frontal faces using eigenspace techniques. It is a variant of the system described in , and generates a 1Hz signal indicating how likely a frontal face is present in the image.