Quantifying depth bias in free viewing of still stereoscopic synthetic stimuli

In studies of 2D visual attention, eye-tracking data show a so-called “center-bias”, which means that fixations are biased towards the center of 2D still images. However, in stereoscopic visual attention, depth is another feature having great influence on guiding eye movements. Relatively little is known about the impact of depth. Several studies mentioned that people tend to look at the objects at certain depth planes. Therefore, it is reasonable to suppose the existence of a “depth-bias”. But studies proving or quantifying this depth-bias are still limited. We conducted a binocular eye-tracking experiment by showing synthetic stimuli on a stereoscopic display. Observers were required to do a free-viewing task through active shutter glasses. Gaze positions of both eyes were recorded for obtaining the depth of fixation. Stimuli were well designed in order to let the center-bias and depth-bias affect eye movements individually. Results showed that the number of fixations varied as a function of depth planes. There was also a relationship between the duration of fixation and the depth plane where the objects were located