  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.
