The strategy of focusing on higher-level processes might seem eminently reasonable for a theory of higher-level cognition. However, the strategy creates two stresses for the plausibility of the resulting models. One stress is that by assuming a processed representation of the input the theorists are granting themselves unanalyzed degrees of freedom in terms of choice of representation. It is not always clear whether the success of the model depends on the theory of the higher-level processes or the choice of the processed representation. The other stress is that the theorist may be ignoring significant problems in access to that information which may be contributing to dependent variables such as accuracy and latency. For instance, the visual input may contain more information than can be held in a single attentional fixation, and shifts of attention (with or without accompanying eye movements) may become a significant but ignored part of the processing. For these reasons we were encouraged to join the growing number of efforts (e.g., Kieras & Meyer, 1994; Wiesmeyer, 1992) to embed a theory of visual processing within a higher-level theory of cognition.
The choice to focus on vision was largely strategic - reflecting the fact that most of the tasks that ACT-R has modeled involved input from the visual modality. To be more exact, most tasks have involved processing input from a computer screen and so we have developed a theory of the processing of a computer screen. However, while we started with a theory of the visual interface, we have more recently become concerned with a more general theory of the coordination of perception, action, and cognition. This more general system will be described in the next chapter on ACT-R/PM. Here we will describe ACT-R's visual interface and the evidence for its connection to the cognitive system. The visual interface is the most developed part of ACT-R/PM and greatly influenced its overall design.