information can be presented through speech synthesis, via a Braille display, a force feedback device, or a combination of all of the above. In general, this "translation" requires an additional "layer" of interaction comprising alternative, special interaction techniques; as a consequence, it usually results in longer times for the completion of an operation, than are possible with the visual interface. On the other hand, non-visual interaction techniques remain difficult to visualize. The transient nature of most non-visual media - sometimes linking input and output very tightly - prevents from a simple visualization.
Criteria for visualization include the purpose and content of information, as well as synchronization between the two (or more) presentations. The complexity of these design issues applies also to the software engineering of an application. The specification, implementation and testing of dual user interfaces ( Savidis and Stephanidis 1995), or other forms of "universal" user interface ( Stephanidis et al. 1998), are driving forces for considering the visualization of non-visual user interfaces. From the user's perspective, this requires additional attention to be paid to evaluation, documentation and training.
One can view the non-visual presentation of information as belonging to one of the following two categories: (i) a spot is identified in the non-visual medium; or, (ii) the sequence of spots is connected and forms a trace.
A spot in a non-visual medium occupies a specific time interval and signifies some lexical entity. Limitations occur if there are too many spots, in comparison to the information presented visually.
An example for a trace may be the interaction with a force feedback device, which is based on the rendering of a data model. The actual movements can be shown on a display by recording a trace and rendering this trace within the underlying model ( Basdogan et al. 1998). The trace is made coherent with the visual rendering by putting it into the context of the model. In other words, a trace is made up of objects related to the changes in position of the force feedback display's pointing device. A distinguishable visible object is created when the user takes time to feel the force. Changes in surface texture as felt through the device, however, require a more fine-grained visual attribute, such as color.