There is a further reason why people interact with
external representations: to prepare themselves to coordinate internal and external states, structures, and processes.
This feature of interaction is fundamental to our understanding of external representations but rarely studied. See
Kirsh (2009a, c). For example, before subjects use a map to
wayfind, they typically orient or ‘register’ the map with
their surroundings; they put it into a usable correspondence
with the world (Koriat and Norman 1984). Many people
also gesture, point, talk aloud, and so on. In principle, none
of these actions are necessary to establish a correspondence
between elements in the map and the things those elements
refer to. Eye movements, mental projection, and other noninteractive techniques may suffice for map-based navigation. But external interactions are commonplace, and a
major aspect of understanding representations.