The Shape of Saccade-based Functional Visual Fields (FVFs): A cautionary note

A Functional Visual Field (FVF) describes an area around the current point of fixation within which some task is done. In visual search, three types of FVF are of interest. The Resolution FVF describes the region within which a specific target can be identified. The Target FVF is a map of the starting point of saccades whose endpoint is a fixation on or near the target of the search, suggesting that the target was found from the previous fixation. If the endpoints of all saccades are normalized to land at one location, the Search FVF is the map of the starting points of all the saccades before the saccade to the target. We measured the Target and Search FVFs for observers as they searched for a T among Ls in regions of different shapes (vertical, horizontal, and triangular). The FVFs reflected the shapes of the regions. These FVF shapes arise as a by-product of a random walk in a constrained space. The results suggest a cautious interpretation of the shapes of FVFs in other studies. The FVF sizes and other aspects of the scanpaths probably remain informative, even if the shape might reflect the shape of the stimulus field.