Bernard [2] introduces the terms descriptive and instructive figure captions. According to Bernard, descriptive figure captions provide a verbal description of the content of an image. Our analysis of figure captions in anatomy suggests using this definition in a more broad sense. We take descriptive figure captions comprises to describe a view on a model or a section of the real world. This new definition also comprises phrases describing not only what is visible in the figure but also what is hidden, has been removed or what important objects are close to the depicted portion. Consequently, in our terms descriptive figure captions describe an image and its (spatial) context. Descriptive figure captions are employed e.g. to explain the construction of complex objects.
Descriptive figure captions serve different functions: they summarize the content of pictures and ease the orientation by providing the name of the depicted model and the viewing direction. Furthermore, descriptive figure captions often inform about the thematic focus and describe abstraction techniques applied to an image. Since these modifications often remain unnoticed to the untrained eye, their existence needs to be described. This is probably the most interesting aspect of the figure captions functionality, as it reveals what has happened in the abstraction process that transformed the real world data into the image to communicate information to the user. These parts reflect the abstraction process, i.e. the operations that have been performed on the data to obtain the visualization and their impact on the fidelity of the visualization. Thus, descriptive figure captions guide the interpretation of images and help to determine which operations are still performable on the image.
Instructive figure captions
explain how
an action--which is often denoted by
meta-objects in the
accompanying illustration--could be performed. These captions are
often employed to describe several stages within complex actions.
If an illustration depicts certain stages within an action or process, it represents an abstraction over time. The meta-objects guide the mental reconstruction of the process which leads to the depicted stage or which starts from the depicted stage. Instructive figure captions can provide information over and above the depicted content, e.g.\ causal relationship of steps within a complex action, pre- and postconditions of actions, and the instruments needed to perform this action. Furthermore, instructive figure captions can mention that the depicted stage represents an unwanted situation which arises from a typical complication. Instructive figure captions can be found, for example, in technical documentation and text books on surgery. However, we shall not deal with such instructive figure captions in more detail.
This classification covers a large portion of captions in reference materials, text books and repair manuals. Other figure captions, e.g.\ citations of people shown at a photograph in a journal, are beyond the scope of this classification.