Spot the ball: The extent of the functional visual field when searching natural scenes

In visual search with eye movements, the size of the functional visual field -- within which a target may be detected -- is smaller than when the eyes are stationary (Motter and Simoni, 2008, Vision Research 48 2382-2393). How, if at all, does the extent of the functional visual field change in search over a variety of natural scenes? To address this question, observers' gaze was monitored with an infrared video eye tracker (250 Hz) while they searched natural images for a randomly located, neutral grey sphere (Munsell N7). Where a target was correctly detected, observers' fixation fell within approximately 38 of the target, when averaged over scenes, giving a lower limit on the extent of the functional visual field. Despite the agreement between observers in this lower limit, there were notable differences between their detection abilities, as measured by d 0
from signal-detection theory. Idiosyncratic fixation patterns across observers suggested that these differences may be due to variation in individual search strategies. For all observers, the size of the functional visual field altered with the scene. This may be a consequence of natural structures in the scenes acting as potential distractors, analogous to crowding effects in target-distractor
search tasks.
[Supported by EPSRC grant no. EP/F023669/1.]