Saccadic localization in the presence of cues to three-dimensional shape

Abstract

Saccades directed to simple two-dimensional (2D) target shapes under instructions to look at the target as a whole land near the center of gravity (COG) of the shape with a high degree of precision (He & Kowler, 1991; Kowler & Blaser, 1995; McGowan, Kowler, Sharma, & Chubb, 1998; Melcher & Kowler, 1999; Vishwanath, Kowler, & Feldman, 2000). This pattern of performance has been attributed to the averaging of visual signals across the shape. Natural objects, however, are three-dimensional (3D), and the shape of the object can differ dramatically from its 2D retinal projection. This study examined saccadic localization of computer-generated perspective images of 3D shapes. Targets were made to appear either 2D or 3D by manipulating shading, context, and contour cues. Average saccadic landing positions (SD approximately 10% eccentricity) fell at either the 2D or 3D COG, and occasionally in between, depending on the nature of the 3D cues and the subject. The results show that saccades directed to objects are not compelled to land at the 2D COG, but can be sensitive to other visual cues, such as cues to 3D structure. One way to account for these results, without abandoning the averaging mechanism that has accounted well for performance with simple 2D shapes, is for saccadic landing position to be computed based on averaging across a weighted representation of the shape in which portions projected to be located at a greater distance receive more weight.

Depth cue combination in spontaneous eye movements

“…The fact that spontaneous saccade directions are aligned with the surface depth gradient of a purely disparity-defined plane as shown by Wexler and Ouarti (2008) excludes the possibility that the luminance gradient, always correlated with the perspective tilt axis, guided saccade directions in that and the current study. Earlier, Vishwanath and Kowler (2004) reported that saccade landing positions were related to the 3D geometric cues of the saccade targets revealing that depth cues are important in driving saccades, whereas luminance has been shown to have at the most a moderate effect on saccade landing positions (Melcher & Kowler, 1999;Spering, Montagnini, & Gegenfurtner, 2008). Here we have shown that when two depth cues are present, the axis of spontaneous saccades combines the tilts of the two cues in the same way, and with the same weights, as perception does.…”

“…Other links between 3D perception and saccades have also been found: for instance, landing positions of saccades are affected by the 3D geometric cues of the target (Vishwanath & Kowler, 2004). 2 We believe that the difference between inexperienced and experienced observers is due to the latter having learned not only to maintain strict fixation but to perform visual judgment tasks under fixation.…”

“…3 Between-subject variability is rather commonly observed in these types of tasks (Oruç, Maloney, & Landy, 2003;Vishwanath & Kowler, 2004) and even across modalities (Ho, Serwe, Trommershäuser, Maloney, & Landy, 2009). …”

Differences in fixations between grasping and viewing objects

“…Vishwanath and Kowler (2003) showed that this was especially the case for a visual scanning task, in which participants fixated a number of objects sequentially. Cues to the three-dimensional structure of the shape can influence the gaze location by drawing it to the three dimensional COG (Vishwanath & Kowler, 2004). Salient local features, such as the boundary between two bars constituting an L-shape, have also been shown to attract fixations when observers were asked to make a single saccade to the object or when they were given unlimited time to find the fixation location where they felt to be looking at the object as a whole (Vishwanath & Kowler, 2003).…”

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

“…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). It is generally believed that saccade landing positions are computed by averaging across the structure of the target object or the array of target objects (e.g., Vishwanath & Kowler, 2004). The center of gravity of an object is also used in perceptual localization tasks; when observers perceptually align spatially extended targets, the reference point upon which the alignment is based tends to coincide with the centroid of the luminance or contrast distribution of the target (e.g., Morgan, Hole, & Glennerster, 1990;Whitaker, McGraw, Pacey, & Barrett, 1996;Whitaker & Walker, 1988).…”