<title>Portable eyetracking: a study of natural eye movements</title>

Visual perception, operating below conscious awareness, effortlessly provides the experience of a rich repr e- sentation of the environment, continuous in space and time. Conscious visual perception is made possible by the 'foveal compromise,' the combination of the high-acuity fovea and a sophisticated suite of eye movements. Our illusory visual experience cannot be understood by introspection, but monitoring eye movements lets us probe the processes of visual perception. Four tasks representing a wide range of complexity were used to explore visual perception; image quality judgments, map reading, model building, and hand-washing. Very short fixation durations were observed in all tasks, some as short as 33 msec. While some tasks showed little variation in eye movement metrics, differences in eye movement patterns and high-level strategies were o b- served in the model building and hand-washing tasks. Performance in the hand-washing task revealed a new type of eye movement. 'Planful' eye movements were made to objects well in advance of a subject's intera c- tion with the object. Often occurring in the middle of another task, they provide 'overlapping' temporal info r- mation about the environment providing a mechanism to produce our conscious visual e xperience. 1. BACKGROUND Despite the seeming ease with which we perceive the world around us, visual perception is actually a co m- plex process that occurs at a level below conscious awareness. Our conscious perception of the enviro n- ment is that of a high-resolution, large field-of-view scene, continuous in space and time. However, because available neural resources cannot sustain such a representation, this perception is illusory. Like many 'optical illusions,' this is the result of the brain's attempt to make sense of the world with only partial information available. Because the process of perception occurs below the conscious level, it does not yield to introspective r e- port. However, monitoring observers' eye movements during a task can provide a tool to better understand visual perception. The retina in the human eye exhibits extreme anisotropy in photoreceptor density, with a small central region of high-resoluti on (the fovea) surrounded by a periphery with much lower spatial resolution. This design requires a mechanism for moving the eyes rapidly to direct the line-of-sight toward objects of interest several times per second. Limited spatial resolution in the periphery forces the ocul o- motor system to execute eye movements in tasks that require high spatial resolution. In natural enviro n- ments eye movements are also made toward task-relevant targets even when high spatial resolution is not required. Such 'attentional' eye movements, made without conscious intervention, can reveal attentional mechanisms and provide a window into cognition.