On the visual span during object search in real-world scenes

Abstract

The current study investigated from how large a region around their current point of gaze viewers can take in information when searching for objects in real-world scenes. Visual span size was estimated using the gaze-contingent moving window paradigm. Experiment 1 featured window radii measuring 1, 3, 4, 4.7, 5.4, and 6.1°. Experiment 2 featured six window radii measuring between 5 and 10°. Each scene occupied a 24.8 × 18.6° field of view. Inside the moving window, the scene was presented in high resolution. Outside the window, the scene image was low-pass filtered to impede the parsing of the scene into constituent objects. Visual span was defined as the window size at which object search times became indistinguishable from search times in the no-window control condition; this occurred with windows measuring 8° and larger. Notably, as long as central vision was fully available (window radii ≥ 5°), the distance traversed by the eyes through the scene to the search target was comparable to baseline performance. However, to move their eyes to the target, viewers made shorter saccades, requiring more fixations to cover the same image space, and thus more time. Moreover, a gaze-data based decomposition of search time revealed disruptions in specific subprocesses of search. In addition, nonlinear mixed models analyses demonstrated reliable individual differences in visual span size and parameters of the search time function.

Keywords:

• Visual search
• Eye movements
• Visual span
• Perceptual span
• Moving windows
• Individual differences

I thank George L. Malcolm for making some of his search scenes available and Wolfgang Einhäuser-Treyer for helpful discussions. I also thank Louise Fraser, Allison Cantor, and Thomas Dixon for assistance in data collection.

I thank George L. Malcolm for making some of his search scenes available and Wolfgang Einhäuser-Treyer for helpful discussions. I also thank Louise Fraser, Allison Cantor, and Thomas Dixon for assistance in data collection.

Notes

¹ The largest radius, 4.1°, resulted in a high-resolution window containing roughly 25% of the full screen area. The authors intentionally refrained from using larger windows, as larger window radii would have left little image area outside the high-resolution window to show effects of a filtered periphery on perception and eye movements.

² The author would like to thank Philippe Schyns for making this suggestion.

³ This two-step fixation check procedure was implemented to obtain clean measures of search initiation times.

⁴ The graph of a quadratic function is symmetric with respect to a vertical line containing the vertex. Note that search time is assumed to decrease with increasing window radius. Therefore, if (C, A) is the vertex of the parabola (Eqn. 1), the relation between window size and search time can be modelled with a quadratic polynomial for 0 < x ≤ C.

⁵ Note that TAM relies on knowledge of the target's specific features to guide search. In the present experiments, however, search was guided by a word search-cue rather than a picture search-cue.