Are summary statistics enough? Evidence for the importance of shape in guiding visual search

Abstract

Peripheral vision outside the focus of attention may rely on summary statistics. We used a gaze-contingent paradigm to directly test this assumption by asking whether search performance differed between targets and statistically-matched visualizations of the same targets. Four-object search displays included one statistically-matched object that was replaced by an unaltered version of the object during the first eye movement. Targets were designated by previews, which were never altered. Two types of statistically-matched objects were tested: One that maintained global shape and one that did not. Differences in guidance were found between targets and statistically-matched objects when shape was not preserved, suggesting that they were not informationally equivalent. Responses were also slower after target fixation when shape was not preserved, suggesting an extrafoveal processing of the target that again used shape information. We conclude that summary statistics must include some global shape information to approximate the peripheral information used during search.

Keywords: Extrafoveal processing; Eye movements; Gaze contingent; Shape; Summary statistics; Visual search guidance.

Figure 1

Examples of unaltered targets, seeds, and synthetic images generated from each seed. See text for additional details.

Figure 2

Procedure illustrating a trial from the noise-seed lure condition.

Figure 3

Proportion of trials in which the target (dark gray bars), lure (light gray bars), or non-lure distractors (medium gray bars) was the first object fixated in each of the five experimental conditions. The horizontal dashed line indicates the level of preferential fixation predicted by chance, and error bars indicate one standard error of the mean.

Figure 4

Time from search display onset to first fixation on the target for each of the five experimental conditions. Error bars indicate one standard error of the mean.

Figure 5

Time from first fixation on the target until the button press response for each of the five experimental conditions. Noise-seed and shape-preserved lure conditions were included here for consistency with the other figures, although no effect on target verification time was expected in these conditions. Error bars indicate one standard error of the mean.