Quantifying center bias of observers in free viewing of dynamic natural scenes

Human eye-tracking studies have shown that gaze fixations are biased toward the center of natural scene stimuli (center bias). This bias contaminates the evaluation of computational models of attention and oculomotor behavior. Here we recorded eye movements from 17 participants watching 40 MTV-style video clips (with abrupt scene changes every 2-4 s), to quantify the relative contributions of five causes of center bias: photographer bias, motor bias, viewing strategy, orbital reserve, and screen center. Photographer bias was evaluated by five naive human raters and correlated with eye movements. The frequently changing scenes in MTV-style videos allowed us to assess how motor bias and viewing strategy affected center bias across time. In an additional experiment with 5 participants, videos were displayed at different locations within a large screen to investigate the influences of orbital reserve and screen center. Our results demonstrate quantitatively for the first time that center bias is correlated strongly with photographer bias and is influenced by viewing strategy at scene onset, while orbital reserve, screen center, and motor bias contribute minimally. We discuss methods to account for these influences to better assess computational models of visual attention and gaze using natural scene stimuli.