Department of Computer Science
Texas A&M University
College Station, TX 77843-3112
Catherine C. Marshall
FX Palo Alto Laboratory
3400 Hillview Road, Building 4
Palo Alto, CA 94304
The move from document-centered hypertext systems to map-based hypertext systems had some unforeseen but far-reaching implications: relationships between nodes could be expressed in more than one way. Maps showed interconnectedness explicitly, usually in the form of a directed graph. But also node proximity came into play; relationships among different nodes or documents could be indicated simply on the basis of their relative location. The use of these map-based hypertext systems to author new information spaces uncovered an interesting phenomenon. Users avoided the explicit linking mechanisms in favor of the more implicit expression of relationships through spatial proximity and visual attributes . Further analysis showed that the use of these spatial and visual cues to imply relationships applied not only to map-based hypertext systems, but also to traditional hypertext systems and in the physical arrangement of paper and notecards .
New interface requirements arose from these observations of practice. Specifically, there was a need to support the expression of the implicit and transient relationships that develop between nodes . With that requirement came the concomitant need to support manipulation, the movement of nodes – and structures of nodes – within the information space. Spatial hypertext systems like VIKI  emerged to support this new method of interacting with information. Commonly used visual attributes like color, shape, and border width can be readily changed to enable "information triage", the rapid interpretation and assimilation of new information .
To better support the manipulation of transient perceptual structures, VIKI includes a spatial parser that recognizes patterns such as vertical and horizontal lists, stacks, and composites. Recognized structures provide easy access to the different levels of perceptual structure within a complicated information space and support the transition of these implicit structures into explicit hypertext .
A spatial interface allows users to take advantage of their visual memory and pattern recognition. Remembering where one saw a document in a visual workspace is a process of recognizing the area in which a document was located at a progressively finer-grained level rather than having to remember the navigational path one took to get to the document. Visual recognition also enables the expression of relation variations -- people recognize visual patterns as being of the same or similar type even when they are not identical; this introduces a way to compose and share imperfect composites.
Another benefit of spatial hypertext is its facility for constructive ambiguity. Where a link in a document-centered hypertext either exists or does not in a particular presentation of the material, placement of a node close but not quite with others can imply some indecision or potential for a relation between the nodes. Allowing people to express ambiguity more easily enables them to perform tasks such as analysis or design where interpretations form as they work with the materials.
It is not only the case that the interpretation of individual documents changes over time in a spatial hypertext, but the visual language representing the interpretation changes as well. A study of users performing short-term interpretation tasks showed that visual languages emerge as users' understanding of the task and their method of approaching the task co-evolved . The use of a visual attribute to represent some abstract information about node content changed over time – for example, color might start off indicating the usefulness of a document but later be used to encode domain-specific characteristics of the content. Such changes indicate that users adapt their solution strategies as they gain more insight into their task. Aquanet and other systems with predefined types and relations make the overhead for making such changes in mid-task overwhelming and very unlikely to occur.
Many tasks require information to be shared among a group of collaborators. Hypertext's application to information sharing has been investigated by many systems including the Virtual Notebook System  and Sepia . The initial study of spatial layouts of information implied a correlation between the number of people sharing an information space and the degree of visual structure apparent in the arrangement. When it was necessary for more people to understand the information space, they created a higher degree of perceptual structure and followed it more strictly. As with sharing information in a navigational hypertext or file system, users must agree on a basic framework in order to effectively communicate. In contrast with a navigational hypertext or file system, effective use of ambiguous and implicit relationships means the spatial hypertext does not require on users to agree on particular relationships or agree on their interpretation.
Theoretical discussions of hypertext have long explored the role of the reader. Spatial hypertext becomes intertwined with these discussions when theorists begin to take a closer look at how readers may interact with texts, taking a step beyond the reader-as-writer foundation. As an important example, Rosenberg has introduced the notion of the reader as gatherer . In his vision of interaction, document-centered hypertexts invite "or" style traversal: a reader can choose to go to this document or that; by contrast, spatial hypertexts invite "and" interactions in which the reader can simultaneously apprehend many different nodes . The representational malleability of spatial hypertext is also considered important; early critiques of the rhetoric of hypertext call for a need to reform the established structural rigidity of hypertext and move to a more exploratory one . In fact, one element of Greco's call for a political praxis of hypertext – the use of hypertext to explore interpretive frameworks and theories – reflects the most basic goals of spatial hypertext . Kolb, a philosopher, further discusses the role of flexible representations of interpretation . The properties of ambiguity and emergence are appealing to those who are interested in reframing existing texts.
Visualizations of hypertext networks have evolved since the original hypertext maps that inspired spatial hypertext. There are now visualizations involving algorithms that cluster nodes or infer a hierarchy to generate visual representations that are simpler and easier to use than the map of the entire link network . Additionally, visualizations have explored the use of three-dimensional representations for hypertext -- including the placement of text into virtual renditions of physical worlds or text-based spatial environments . Within VIKI, the constant shortage of screen real estate and the practice of performing multiple tasks simultaneously led to the development of a multi-focus fisheye view .
The goal of open-hypermedia research is to create a protocol that allows links to exist across different hypertext systems and information stored in one system to be displayed and manipulated in another. For systems that are based on a node and link model of hypertext the primary concerns involve versioning, permissions, and composites. Spatial hypertext, because of the lack of links or other explicit relationships between nodes, challenges the open-hypermedia research protocols to consider information structuring at a more abstract level . Relationships implied in a spatial layout may need to be converted to a link in a document- centered hypertext system while links in such a system need to be converted into a generated layout in a spatial system. Additionally, the ambiguity of links and spatial composites in a spatial hypertext are difficult to express in a node-and-link based system.
The growth of the Web has led to a growing number of systems that provide "Web workspaces" in which users can collect, organize, and otherwise work with information. VIKI itself includes a number of interconnections with the Web including the ability to author presentations containing Web-based information . Additionally, systems like Web Squirrel , D-LITE , Web Forager , and Data Mountain  provide workspaces for analysis and interpretation similar to that of VIKI.
 Bush, V. 1945.As We May Think. The Atlantic Monthly 6, 1 (July 1945), 101-108.
 Card, S. K., Robertson, G. G., and York, W. 1996. The WebBook and the Web Forager: an information workspace for the World-Wide Web. In Proceedings of the ACM Conference on Human Factors in Computing Systems (CHI '96). ACM Press, New York, NY, 111-118.
 Conklin. J. 1987. Hypertext: An Introduction and Survey. IEEE Computer 20, 9, 17-41.
 Conklin, J., Begeman, M. 1988. gIBIS: A Hypertext Tool for Exploratory Policy Discussion. In Proceedings of the ACM Conference on Computer Supported Cooperative Work. ACM Press, New York, NY, 140-152.
 Cousins, S. B., Paepcke, A., Winograd, T., Bier, E., and Pier, K. 1997. The digital library integrated task environment (DLITE). In Proceedings of ACM Digital Libraries '97 Conference. ACM Press, New York, NY, 142-151.
 Dieberger, A., and Bolter J.D. 1995. On the Design of Hyper "Spaces." Communications of the ACM 38, 8 (August 1995), 98.
 Durand D., and Kahn, P. 1998. MAPA: a system for inducing and visualizing hierarchy in websites. In Proceedings of ACM Hypertext `98 Conference. ACM Press, New York, NY, 66-76.
 Engelbart, D.C. 1984. Authorship Provisions in Augment. In Proceedings of the 28th IEEE International Conference, (San Francisco, CA, Feb. 27- Mar. 1, 1984). 465-472.
 Greco, D. 1996. Hypertext with Consequences: Recovering a Politics of Hypertext. In Proceedings of ACM Hypertext '96 Conference. ACM Press, New York, NY, 85-92.
 Halasz, F.G., Moran, T.P., and Trigg, R.H. 1987. NoteCards in a Nutshell. CHI+GI'87 Conference Proceedings (Toronto, Canada). ACM Press, New York, NY, 345-365.
 Kolb. D. 1997. Scholarly Hypertext: Self-Represented Complexity. In Proceedings of ACM Hypertext `97 Conference. ACM Press, New York, NY, 29-37.
 Marshall, C.C., Halasz, F., Rogers, R., and Janssen, W. 1991. Aquanet: a hypertext tool to hold your knowledge in place. In Proceedings of ACM Hypertext `91Conference. ACM Press, New York, NY, 261-275.
 Marshall, C.C., and Rogers, R.A. 1992. Two Years before the Mist: Experiences with Aquanet. In Proceedings of the European Conference on Hypertext (ECHT `92). ACM Press, New York, NY, 53-62.
 Marshall, C.C., and Shipman, F.M. 1993. Searching for the Missing Link: Discovering Implicit Structure in Spatial Hypertext. In Proceedings of ACM Hypertext `93 Conference. ACM Press, New York, NY, 217-230.
 Marshall, C.C., Shipman, F., and Coombs, J. 1994. VIKI: Spatial Hypertext Supporting Emergent Structure. In Proceedings of European Conference of Hypermedia Technologies (ECHT `94). ACM Press, New York, NY, 13-23.
 Marshall C.C., and Shipman, F.M. 1995. Spatial Hypertext: Designing for Change. Communications of the ACM, 38, 8 (August 1995), 88-97.
 Marshall C.C., and Shipman, F.M. 1997. Effects of Hypertext Technology on the Practice of Information Triage. In Proceedings of ACM Hypertext `97 Conference, ACM Press, New York, NY, 167-176.
 Moulthrop, S. 1991. Beyond the Electronic Book: A Critique of Hypertext Rhetoric. In Proceedings of ACM Hypertext '91 Conference, ACM Press, New York, NY, 291-298.
 Nelson. T.H. 1984. Literary Machines, (Edition 87.1, also 6th Edition). The Distributers, South Bend, Indiana.
 Nürnberg¸ P.J., and Ashman, H. 1999. What was the question? Reconciling open hypermedia and World Wide Web research. In Proceedings of ACM Hypertext '99 Conference, ACM Press, New York, NY, 83-90.
 Robertson, G., Czerwinski, M., Larson, K., Robbins, D., Thiel, D., and van Dantzich, M. 1998. Data Mountain: Using Spatial Memory for Document Management, In Proceedings of ACM UIST `98 Conference, ACM Press, New York, NY, 153-162.
 Rosenberg, J. 1996. The Structure of Hypertext Activity. In Proceedings of ACM Hypertext '96 Conference, ACM Press, New York, NY, 22-30.
 Rosenberg, J. 1997. The Interactive Diagram Sentence: Hypertext as a Medium of Thought. Visible Language, 30, 2, 103-116.
 Shipman, F.M., Chaney, R.J., and Gorry, G.A. 1989. Distributed Hypertext for Collaborative Research: The Virtual Notebook System. In Proceedings of ACM Hypertext '89 Conference, ACM Press, New York, NY, 129-135.
 Shipman, F.M., Marshall, C.C., and Moran, T.P. 1995. Finding and Using Implicit Structure in Human-Organized Spatial Layouts of Information. In Proceedings of the ACM Conference on Human Factors in Computing Systems (CHI '95), ACM Press, New York, NY, 346-353.
 Shipman, F.M., Furuta, R., and Marshall, C.C., 1997. Generating Web-Based Presentations in Spatial Hypertext, In Proceedings of ACM Intelligent User Interfaces `97 Conference, ACM, New York, NY, 71-78.
 Shipman, F.M., Marshall, C.C.. and LeMere, M. 1999. Beyond Location: Hypertext Workspaces and Non-Linear Views, In Proceedings of ACM Hypertext `99 Conference, ACM Press, New York, NY, 121-130.
 Streitz, N., Haake, J., Hannemann, J., Lemke, A., Schuler, W., Schutt, H., and Thüring, M. 1992. SEPIA: A cooperative hypermedia authoring environment. In Proceedings of ACM ECHT '92 Conference, ACM Press, New York, NY, 11-22.