Stimulus Center Bias Persists Irrespective of Its Position on the Display

Since the earliest studies on human eye movements, it has been repeatedly demonstrated that observers fixate the center of visual stimuli more than their periphery, regardless of visual content. Subsequent research suggested only little effect of typical biases in experimental setups, such as observer's position relative to the screen or the relative location of the cue marker. While comparative studies of the screen center vs. stimulus center revealed that both conspire in the process, much of the prior art is still confounded by experimental details that leave the origins of the center-bias debatable. We thus propose methodological novelties to rigorously test the effect of the stimulus center, isolated from other factors. In particular, eye movements were tracked in a free-viewing experiment in which stimuli were presented at a wide range of horizontal displacements from a counterbalanced cue marker in a wide visual field. Stimuli spanned diverse natural scene images to allow inherent biases to surface in the pooled data. Various analyses of the first few fixations show a robust bias toward the center of the stimulus, independent of its position on the display, but affected by its distance to the cue marker. Center bias is thus a tangible phenomenon related to the stimulus.