Localization of shapes: eye movements and perception compared

Abstract

The localization of spatially extended objects is thought to be based on the computation of a default reference position, such as the center of gravity. This position can serve as the goal point for a saccade, a locus for fixation, or the reference for perceptual localization. We compared perceptual and saccadic localization for non-convex shapes where the center of gravity (COG) was located outside the boundary of the shape and did not coincide with any prominent perceptual features. The landing positions of single saccades made to the shape, as well as the preferred loci for fixation, were near the center of gravity, although local features such as part boundaries were influential. Perceptual alignment positions were also close to the center of gravity, but showed configural effects that did not influence either saccades or fixation. Saccades made in a more naturalistic sequential scanning task landed near the center of gravity with a considerably higher degree of accuracy (mean error <4% of saccade size) and showed no effects of local features, constituent parts, or stimulus configuration. We conclude that perceptual and oculomotor localization is based on the computation of a precise central reference position, which coincides with the center of gravity in sequential scanning. The saliency of the center of gravity, relative to other prominent visual features, can depend on the specific localization task or the relative configuration of elements. Sequential scanning, the more natural of the saccadic tasks, may provide a better way to evaluate the "default" reference position for localization. The fact that the reference position used in both oculomotor and perceptual tasks fell outside the boundary of the shapes supports the importance of spatial pooling, in contrast to local features, in object localization.

The distribution of attention within objects in multiple-object scenes: Prioritization by spatial probabilities and a center bias

“…A final hypothesis is that attention to the centers of objects is unrelated to motion, and that attention is concentrated at the centers of objects even if they are not moving. This would be consistent with the findings, discussed above, that a bias toward centers is prevalent in many perceptual and attention tasks (e.g., Vishwanath & Kowler, 2003Zhou et al, 2006).…”

“…This result is more consistent with the idea that the center bias is a pervasive phenomenon in visual perception. This attentional bias toward the centers of moving and stationary objects could be related to the finding that saccades to stationary objects or arrays land near the centers of those objects or arrays (see, e.g., Vishwanath & Kowler, 2003. It has been suggested that saccade landing locations are computed by averaging across all stimulated locations within the target object or array (e.g., Vishwanath & Kowler, 2003.…”

“…A final hypothesis is that attention to the centers of objects is unrelated to motion, so that attention is concentrated at the centers of objects even if they are not moving. Studies on saccadic eye movements (e.g., Vishwanath & Kowler, 2003 have found that saccades to stationary objects or arrays land near the centers of the objects or arrays, and Zhou et al (2006) found an attentional bias toward the center of a scene. Together, this research suggests that a center bias may be a pervasive phenomenon in visual perception.…”

“…This suggests that when target location is uncertain, the entire relevant object is given enhanced attentional priority, but when target location is certain, other locations on the object are not given enhanced attentional priority. However, it should be noted that Chen and Cave (2006) suggested that, in Shomstein and Yantis's (2002) study, the dic eye movement is directed to an extended object, the line of sight lands, on average, near the object's center of gravity (see, e.g., Vishwanath & Kowler, 2003. Similarly, when two or more targets are presented in close proximity, the initial saccade is deviated toward the center of gravity of the configuration (e.g., Coëffé & O'Regan, 1987;Findlay, 1982).…”

The Role of Part Structure in the Perceptual Localization of a Shape

“…A reference point for assigning a location to an object is often the centroid of the shape (also known as center of gravity, COG) Kowler 2003, 2004;Watt andMorgan 1983a, 1983b;Westheimer and McKee 1977;Whitaker et al 1996Whitaker et al , 2004. The visual system can estimate the COG by averaging the luminance distribu tion of a target shape, or by averaging its local contrast energy (Guez et al 1994;McGowan et al 1998;Melcher and Kowler 1999;Vishwanath and Kowler 2003).…”