Influence of disparity on fixation and saccades in free viewing of natural scenes.

Humans select relevant locations in a scene by means of stimulus-driven bottom-up and context-dependent top-down mechanisms. These have mainly been investigated by recording eye movements under 2D natural or 3D artificial stimulation conditions. Here we try to close that obvious gap and presented 2D and 3D versions of natural, pink, and white noise images to human subjects. Importantly, ground truth distance was obtained for all image pairs by laser scanning. Recording eye movements, we investigated the influence of disparity information and of higher order scene correlations on basic saccade properties and on saliency of bottom-up information. Our results show that the removal of higher order correlations changed saccadic rate, length and main sequence, and the subjects' explorative behavior. Introduction of disparity information countered these effects and alleviated differences between image categories. Disparity information had no effect on the saliency of monocular image features like luminance and texture contrast; however, without higher order correlations these features were uncorrelated to fixation locations. An analysis of binocular image features revealed that participants fixated closer locations earlier than more distant locations. Importantly, this also held for 2D natural images. Taken together, we conclude that depth information changes basic eye movement properties and provides a salient image feature.