ACT-RS: A Neuropsychologically Inspired Module for Spatial Reasoning

Christian Schunn (schunn@gmu.edu) 
Department of Psychology, 
George Mason University
MSN 3F5, Fairfax, VA 22030-4444
Anthony Harrison (aharris8@gmu.edu)
Department of Psychology, 
George Mason University
MSN 3F5, Fairfax, VA 22030-4444

Abstract
We present an extension to ACT-R, called ACT-RS which adds a neurologically-inspired module for representing and interacting with space. The module includes four functionally different representations of external space, that vary along dimensions of input modalities, tasks supported, format, resolution, extent, and features encoded.
Introduction
The current version of ACT-R (and indeed all current problem-solving architectures) encompasses a rather simple mechanism of vision and visual representation. An add-on to the modeling environment, ACT-R/PM, incorporates a more advanced theoretically-driven account of vision. However, it is still concerned entirely with representing what is viewed within a two dimensional space. The visual world of the model is bounded by the edges of the screen on which it is displayed. It is our aim to extend this theory to allow for complex reasoning with and about spatial objects.
Model Components
As a starting point for this model, we adopted Previc's neuropsychological theory of 3-D space (199x), which is based on large amount of data from behavioral experiments, brain damage performance dissociations, and animal work. Previc's model is the most recent extension of such neuropsychological accounts (e.g. Rizzolatti & Camarda, 1987; Rizzolatti et al., 1985; Grusser, 1983; Cutting & Vishton, 1995). His model has four distinct regions of spatial processing, each with a separate form and function. The first region is called the perispersonal space. It extends about two meters beyond the individual through the central 60 degrees of the person's body and is used for reaching and grasping. The second region, extrapersonal focal, is retinotopic and is tied exclusively to foveation. The third region, extrapersonal action, spans a full 360 degrees around the individual, extending to the spatial bounds of the region that the person is occupying. Finally, there is the ambient extrapersonal space. This region only spans the front most 180 degrees of the spatial region and uses an exocentric representation.
Characteristics of Each Spatial Region
Each of the four spatial regions has distinctive functions and representations. In the design process, we considered the rational affordances of each of these regions while keeping their neurological distinctions in mind. This is informing a model that will permit computational efficiency in the face of rational necessity. The following sections deal more closely with these issues. The regions are present in order of increasing computational intensity.
Ambient (Extrapersonal) Space
Ambient space processing appears to emphasize the visual periphery extending to the most perceptually distant portion of the world, be it a windowed room or a row of bleachers in a football field (Dichgan & Brandt 1978; Leibowitz & Post, 1982; Previc & Neel, 1995). This information is used to ensure proper postural control and a locational reference within a given region. This system allows us to spatially orient our bodies in an earth-fixed manner without having to consciously attend to our position or motion (Leibowitz & Post, 1982). Surprisingly, the preconscious representation is often more veridical than the consciously registered representation (Loomis, et al., 1992; Proffit, Bhalla, Gossweiler, & Midget, 1995).
The core ambient space representation in our model is simply a geometric region that is determined by the bounds of the most perceptually distant borders. The agent's position and vertical orientation within this geometric region are automatically updated based on peripheral visual feedback as well as self-controlled locomotion. This representation of absolute position is then utilized by the other systems to interpret as well as to extrapolate the representations of other objects in the world.
Action (Extrapersonal) Space
The functional role of action space is to support navigation and target orientation with respect to a gaze-centered view. While the lateral limit of action space spans a full 360 degrees, there is a large compression effect outside the visible field of view (Beer, 1993). Additionally, while action space extends from around two meters almost to the limit of visual space, important distance cues begin to degrade after a certain radius limiting the actual functional range of action space to around 15 meters (Grusser, 1983; Fukusima,  Loomis, & Da Silva, 1997).
In our model, action space representations contain just enough information to inform effective navigation and object avoidance. This representation includes three angle-range pairs. These angle-range pairs are all relative to the eye point of the agent and represent the relative position of the leftmost, rightmost and topmost (or bottommost for suspended objects) edges. The representations of objects that fall within the biased field of view (200 degrees, 2-15m, upper-field bias) are automatically updated with respect to the agent's head and body positions. Those that fall outside of this region are not updated and if queried, must be consciously re-encoded. This might produce many of the observed biases and distortions in dead reckoning and object localization tasks (e.g. Fukusima,  Loomis, & Da Silva, 1997). 
Focal (Extrapersonal) Space
Focal space is the region typically included in architectures of cognition (including ACT-RP/M). It is concerned with object recognition and attention to finer detail. This system is most closely regulated by conscious control and is influenced by top-down processing.
Object representations in the other spaces have no object recognition mechanism (i.e., objects are represented as blob objects rather than instances of particular categories). It is entirely up to the focal region to tag representations with their actual identity. To support this recognition process, we assume a recognition-by-components account in which object parts are represented by geons (Biederman ref). To facilitate object recognition with size and rotation invariance, we assume that the geons contain no metric information, and no real 3-dimensional extent.
Peripersonal Space
Finally there is the most three dimensional and metric of all the spaces, peripersonal space. This region is concerned with object manipulation. Once again, we assume a geon-based representation of object components, but this time the geons have detailed 3-dimensional metric information including surface texture information.
Computational Limitations
The previous descriptions paint a picture of four parallel systems that are almost boundless in their ability to automatically represent their particular piece of the picture. This is not accurate. Each of the regions has a series of limitations. The first of these is their uptake span. New objects are automatically encoded and updated within this angle span. Next, there is the wider maintenance span within which previously encoded representations are maintained and updated in memory. Anything outside of these regions must be consciously interpreted through indirect methods. Each of the regions also has biases towards specific parts of the visual field, representing a prioritization mechanism within the uptake and maintenance bounds. The finer details of this mechanism still have yet to be resolved.
Application and Testing
We are currently implementing all the components of ACT‑RS  and expect to test the system in a 3-dimensional navigation tasks first. We then hope to apply the model to complex spatial problem-solving like the submarine sonar domain.
Acknowledgments
This work was funded by grant #XYZ to the first author from the Office of Naval Research through Susan Chipman.
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Beiderman…
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