Abstract
Traditional approaches examining fixations towards natural scenes either focus on their spatial distribution, or aim to explain their exact sequence. Whereas heat-maps potentially conflate fixations serving different perceptual functions, scan-paths may aim for an unnecessary level of detail. Here, we introduce a novel way of analysing fixations. We hypothesize that fixations mainly serve three goals: Detection, Inspection and Re-Inspection of visual objects. Detection (D) for foveal vision is achieved by the first fixation landing on a given object, Inspection (I) by successive scanning immediately after Detection, and Re-Inspection (R) following the intermediate exploration of other objects. N = 101 participants freely viewed 700 images of everyday scenes. To test consistency within and differences between D, I and R fixations, we generated separate fixation maps for each category and image, separately for two subsets of observers. Results indicate large consistencies for D, I and R fixations across observers, with maps sharing 95%, 93% and 88% of their variance on average. At the same time, the shared variance between D, I and R was only 65% on average. To explore the nature of these differences, we computed the proportions of D, I and R fixations landing on different types of objects and found they could vary substantially. For instance, 57% of Inspection, but only 34% of Detection fixations landed on Text. Finally, we probed individual differences in the proportions of dwell time allocated to D, I and R. These individual biases towards D, I or R were both, considerable and highly consistent across images (all r > .96). Taken together, the DIR model can capture distinct aspects of fixation behavior on an observer-general and individual level. We propose it as a useful level of description for analysing and understanding the diverse perceptual functions of gaze behavior.