Visual search habits and the spatial structure of scenes

We compared eye movements during search tasks across three spatial configurations. Participants searched for a line segment oriented 45◦
to the right. Variation in the orientation of distractor line segments determines
the extent to which this target would be visible in peripheral vision: a target
among homogeneous distractors is highly visible, while a target among heterogeneous distractors requires central vision. When the search array is split
into homogeneous and heterogeneous left and right halves, a large proportion
of fixations are “wasted” on the homogeneous half, leading to slower search
times. We compared this pattern to two new configurations. In the first, the
array was split into upper and lower halves. During a passive viewing baseline
condition, we observed biases to look both at the top half and also at the hetergeneous region first. Both of these biases were weaker during active search, despite the fact that the heterogeneous bias would have led to improvements in efficiency if it had been retained. In the second experiment, a “jumbled” search configuration was used in which patches of more and less heterogeneous line segments were scattered across the search space. This configuration allows for more natural, spatially distributed scanpaths. Participants were both more efficient overall, and less variable, relative to the left/right configuration. The results are consistent with the idea that visual search is associated with a distributed sequence of fixations, guided only loosely by the potential visibility of the target in different regions of the scene.