INTRODUCTION: In the analysis of the processes at play during the performance of human-computer interactions, it is common to consider that there are at least three types of human sub-systems involved. Card, Moran and Newell (Card et al. 1983) proposed that successful human-computer interaction would require the involvement of a percep-tual system, receiving sensory messages from the computer, a motor system, controlling the actions that the user performs to provide input to the computer and a cognitive system, connecting the other two systems, by integrating the sensory input received to determine appropriate user actions. Given the pervasiveness of Graphic User Interfaces (GUIs) in most contemporary computing systems, and certainly in the majority of World Wide Web (Web) sites, the demands placed on the visual channel of the user’s perceptual system have been raised beyond the capabilities of a significant portion of potential Web users.
Typically, a computer system utilizes the user’s visual channel by presenting on a surface (computer screen) patterns of light point sources (pixels) emitting light made up of the mixture of three basic colors: red, green and blue. If all three colors are fully present the resulting mixture will be perceived as a white pixel. If none of the colors is present, at all, the pixel will be perceived as “black” or “off”. Partial mix-tures of red, green and blue will yield the perception of a pixel with a specific color (e.g., purple). Effective human-computer interaction requires that the light patterns formed on the computer screen be successfully processed through a two-stage se-quence: the physical reception of the stimulus (e.g., a desktop icon) and then the interpretation of that stimulus by the user (Dix et al. 1998).
Unfortunately, even the reception of the stimulus may be hindered if the visual system of the user is not performing its expected functions at its full capacity. One could consider the process needed to receive visual information from a computer interface as involving two necessary stages. First the refraction system of the eye must create a proper distribution of light on the retina, to represent the graphical element being viewed. In addition to this requirement, the neural function of the retina must be operative to translate the retinal image into a proper set of neural signals that will be carried to the brain, where they will be ultimately interpreted. This is shown as a simplified diagram in Figure 1.