Object category understanding via eye fixations on freehand sketches

HEN shown photographic images under a free-viewing (i.e task-free) paradigm, human eyes preferentially fixate on image locations which are visually salient. Multiple studies [1]-[5] have demonstrated that this fixation mechanism is bottom-up, predominantly driven by image content and richness of detail (color, texture etc.). This explanation, while satisfactory for photographic images, seems inadequate for certain categories of images such as line drawings. In particular, one class of line drawings - hand-drawn sketches - are sparse and largely devoid of detailed content. In addition, they are typically binary images containing virtually no color-based information (see Figure 1). Even so, multiple studies have demonstrated a "fixations-intonothing" phenomenon [6]-[9], wherein the eye fixations on the same stimulus by multiple subjects fall on empty regions, yet exhibit enough regularity to make gaze-based inferences. One possible explanation is that the first eye fixation conveys all there is to know ('Gestalt') about the underlying scene semantics [10] and the regularity in rest of the fixations is a statistical anomaly. However, a more intriguing explanation is that these empty region fixations aim to implicitly verify the overall consistency of the scene content depicted in the sketch [11], [12]. Which of these explanations is correct?