Visual Search Asymmetries Are Explained by Visual Homogeneity

Visual search is often asymmetric. Search for a Q among Os is often easier than search for O among Qs. But what image property causes asymmetry in visual search? The most popular explanation is that searching for the presence of a feature is usually easier than searching for its absence – but even this does not explain many asymmetries where no obvious feature can be named. Here, we report a novel and empirically measurable image property – visual homogeneity – that explains visual search asymmetries. For each image, its visual homogeneity is the time it takes to confirm the absence of an oddball target in a search array containing this image. We hypothesized that a search target is easy to find when the distractors are visually homogeneous. More generally, the visual homogeneity of the target and distractor would determine whether a given visual search is asymmetric. To investigate this hypothesis, we asked human participants to search for an oddball target that could be present or absent. We selected 40 popular search asymmetries reported in the literature. Search times were highly consistent, as evidenced by a high split-half correlation across participants (r = 0.92, p < 0.00005). Search asymmetries, measured as the difference between the search times of the hard and easy targets, were also likewise consistent across participants (split-half correlation: r = 0.64, p < 0.0005). Importantly, search asymmetry was accurately predicted by a weighted sum of the visual homogeneity of the target and distractor (r = 0.56, p < 0.0005). These predictions were driven primarily by the distractor, suggesting that search is asymmetric when the distractor is visually homogeneous. Taken together, our findings demonstrate for the first time, an empirically measurable image property that can explain visual search asymmetry.