However, it is important to remember that while a medium can be defined and distinguished by a characteristic cluster, or profile, of symbol systems and processing capabilities, some of these capabilities may not be used in a particular learning episode (Salomon and Clark, 1977). For example, a particular video presentation may use few or no representational symbols (e.g., a "talking head" presentation). Or, a viewer may allow a video disc presentation to play straight through and not use the available search capabilities. In these cases a "virtual medium" is created that consists of the profile of symbol systems and processing capabilities that were actually used during the session: a television becomes, in effect, a radio; a video disc player becomes broadcast television. It is only the capabilities of the virtual medium that can be expected to have an effect on learning processes and outcomes.