The aim of this article is to give a brief introduction to the human visual system, and to briefly explain why we move our eyes and its relevance to eye tracking.
The human visual field spans about 135 x 220 degrees and is, as previously mentioned, divided into three main regions: the foveal, parafoveal, and peripheral regions. We primarily register visual data through the foveal region which constitutes less than 1% of the visual field. Even though this represents only a small part of our field of vision, the information registered through the foveal region constitutes about 10% of what is sent to the brain through our optic nerve (Essen & Andersson 1995). Our peripheral vision has very poor acuity, and is only good for picking up movements and contrasts. Thus, when we move our eyes to focus on a specific region of an image or object, we are essentially placing the foveal region of the eye on top of the area that is currently within the main focus of the lens in our eye. This means that we are maximizing our visual processing resources on the particular area of the visual field that also has the best image due to the optic characteristics of the eye. By letting the foveal region register the image, the brain gets the highest possible image resolution of the interesting area to process as well as the most amounts of data registered by the eye about that area.
Eye movements have 3 main functions which are considered important when we process visual information:
Van Essen, D.C. & Anderson, C.H. 1995. Information Processing Strategies and Pathways in the Primate Visual System. In: An Introduction to Neural and Electronic Networks, 2nd ed., 1995, Academic Press, Zornetzer et al., eds., pp. 45-76.