Controlling a head-mounted display includes providing a head-mounted display, the head-mounted display includes a switchable viewing area that is switched between a transparent viewing state and an information viewing state. The transparent viewing state is transparent with respect to the viewing area and enables a user of the head-mounted display to view the scene outside the head-mounted display in the user's line of sight. The information viewing state is opaque with respect to the viewing area and displays information in the switchable viewing area visible to a user of the head-mounted display. An external environmental state detector provides an external stimulus notification in response to a detected change in the external environment and causes the viewing state to automatically switch in response to the external stimulus notification.