Measurement-related issues in the investigation of active vision
In humans, as in all higher primates, vision is the dominant sensory modality. The essential role of eye movements in visual perception has been well known for a long time and has been repeatedly demonstrated (e.g., Findlay, 1998; Hayhoe & Ballard, 2005). This natural sampling of information from the environment requires that visual perception is investigated within the framework of “active vision” (Findlay, 1998). Due to the uneven distribution of light-sensitive receptors across the retina, the highest visual acuity is limited to the small foveal area (about two degrees of arc or double the thumbnail size of an extended arm). With increasing eccentricity-in parafoveal and peripheral regions-vision becomes blurred and the perception of color is reduced. These constraints make eye movements mandatory for perceiving the environment. Therefore, saccades-fast ballistic movements-are executed to bring the gaze from one point to another. The relatively stable periods in between are called •xations. The intake of visual information occurs within •xations but is largely suppressed during saccades. In many everyday situations, such as reading
a text or inspecting an image, oculomotor activity can be described as interplay between •xations and saccades.