An apparatus includes a video display to display video images to a user; a gaze detector configured to detect a gaze direction for one or both eyes of the user while the user views the display; a head tracker configured to detect a head orientation of the user; an image processor configured to generate the video images for display by the video display; the image processor being responsive to one or more control functions dependent upon the gaze direction detected by the gaze detector; and a controller configured to detect a predetermined condition and, in response to detection of the predetermined condition, to control the image processor to be responsive to one or more control functions dependent upon the head orientation detected by the head tracker in place of the one or more control functions dependent upon the gaze direction detected by the gaze detector.