Searchers adjust their eye movement dynamics to the target characteristics in natural scenes

Abstract: When searching a target in a natural scene, both the target's visual properties and similarity to the background influence whether (and how fast) humans are able to find it. However, thus far it has been unclear whether searchers adjust the dynamics of their eye movements (e.g., fixation durations, saccade amplitudes) to the target they search for. In our experiment participants searched natural scenes for six artificial targets with different spatial frequency throughout eight consecutive sessions. High-spatial frequency targets led to smaller saccade amplitudes and shorter fixation durations than low-spatial frequency targets if target identity was known before the trial. If a saccade was programmed in the same direction as the previous saccade (saccadic momentum), fixation durations and successive saccade amplitudes were not influenced by target type. Visual saliency and empirical density at the endpoints of saccadic momentum saccades were comparatively low, indicating that these saccades were less selective. Our results demonstrate that searchers adjust their eye movement dynamics to the search target in a sensible fashion, since low-spatial frequencies are visible farther into the periphery than high-spatial frequencies. Additionally, the saccade direction specificity of our effects suggests a separation of saccades into a default scanning mechanism and a selective, target-dependent mechanism.