Perceiving the center of irregular contour quadrangles

Perceived object stability depends on multisensory estimates of gravity

Background

How does the brain estimate object stability? Objects fall over when the gravity-projected centre-of-mass lies outside the point or area of support. To estimate an object's stability visually, the brain must integrate information across the shape and compare its orientation to gravity. When observers lie on their sides, gravity is perceived as tilted toward body orientation, consistent with a representation of gravity derived from multisensory information. We exploited this to test whether vestibular and kinesthetic information affect this visual task or whether the brain estimates object stability solely from visual information.

Methodology/Principal Findings

In three body orientations, participants viewed images of objects close to a table edge. We measured the critical angle at which each object appeared equally likely to fall over or right itself. Perceived gravity was measured using the subjective visual vertical. The results show that the perceived critical angle was significantly biased in the same direction as the subjective visual vertical (i.e., towards the multisensory estimate of gravity).

Conclusions/Significance

Our results rule out a general explanation that the brain depends solely on visual heuristics and assumptions about object stability. Instead, they suggest that multisensory estimates of gravity govern the perceived stability of objects, resulting in objects appearing more stable than they are when the head is tilted in the same direction in which they fall.

Visual Estimation of Three- and Four-Body Center of Mass

Center-of-mass perception in dot patterns arranged into triangular and quadrilateral configurations was investigated. In Exp. 1, the length, orientation, and direction of dots forming right triangles were varied. Errors in center-of-mass perception by 19 participants increased with the length of the triangles. In Exp. 2,27 participants estimated the centers of squares and rectangles made up of four dots. Accuracy was best for squares that contained greater reflective and rotational symmetry. The orientation of responses was generally downward in both experiments, indicating a gravitational influence. The results are consistent with a process model by which observers use internal pattern structure like medians and axes when estimating the center of an object.

Perception of two-body center of mass

Participants estimated the perceptual center of mass between two horizontally oriented black dots varying in size and distance. Experiment 1 showed that estimates, measured as distance from the larger dot's center, decreased with an increase in size ratio between the dots and a decrease in the distance between them, as predicted by the physical center-of-mass equation. The results were replicated and extended in further experiments with different ratios and distances. In all experiments, the true center was consistently overestimated, either because the stimuli were perceived in a low dimensionality or because of a hesitancy to place estimates near the larger dot's edge. In Experiment 4, the center was deliberately placed near this edge for one- and two-dimensional solutions. Participants still overestimated, indicating an "edge effect" as responsible.

Vernier judgments in the absence of regular shape information

Vernier acuity is a form of hyperacuity in which the threshold offset between a test object and a reference object is smaller than the size of a foveal cone. Because the test and the reference objects usually have regular shapes (e.g. rectangular, triangular or circular), relatively few studies have addressed the role of shape information in determining hyperacuity thresholds. In this study, we investigated the effect of shape information on hyperacuity performance using targets of irregular shape with different skew and symmetry properties. Vernier thresholds smaller than 10 arc-sec were obtained for closely spaced asymmetric irregular-shape targets. Thresholds for dots and asymmetric irregular shapes increased with increase in center-to-center gap between the targets. Unlike dots, the thresholds for asymmetric irregular shapes also increased with target area. Although the thresholds for asymmetric irregular shapes were higher than those for dots, thresholds for symmetric irregular shapes were similar. Target skew below a certain level had a negligible effect on Vernier thresholds for asymmetric shapes. Our results suggest the existence of feature-independent neural circuitry that can support hyperacuity thresholds and are consistent with the use of the centroid as a primitive for relative localization.

Different mechanisms may underlie the performance in relative localization tasks

Two different groups of subjects had to adjust two-dimensional stimuli, differing in size, shape and type (dot patterns or irregular contour figures), within a reference circle. The two groups performed under two different instructions. The first instruction stressed matching the centres of the stimulus and the circle, while the second required simply positioning the test stimulus in the middle of the reference circle. In two control experiments the subjects had to determine the position of the centres of each stimulus and of the reference circle. Under the first instruction the accuracy of performance, estimated by the variance of the responses, depended on the stimulus size, shape and type in agreement with previous results and models of relative localization. Under the second instruction, however, accuracy remained invariant. Possible mechanisms of relative localization that might differ at their first stages of localization of the separate stimuli are considered.

A model for identifying the perceptual centre of polygons

An attempt is made to develop a general model of perceptual-centre determination. The model describes the possible 'perceptual operations' involved in determining the perceptual centre of flat figures. The specific steps in the model are: (i) determining the orientation and size of the figure; (ii) determining the 'base' of the figure and its 'top-bottom axis'. It is suggested that anisotropy along the main axes of the figures determines a displacement of the perceptual centre toward the phenomenal top of the shape. A quantitative measure for the displacement of the perceptual centre from the area barycentre of the figure is introduced.

Perceived location of two-dimensional patterns

The study attempted to test the possibility that the center of gravity of two-dimensional patterns is the cue used by a human observer for their localization. Four experiments were carried out. The first, using a matching procedure, required the localization of the center of irregular dot patterns, contour and filled polygons which varied in size and orientation. In the other three experiments the subjects had to point to briefly exposed dot patterns in which overall shape (convex and concave in Expts 2 and 3) and dot density (Expt 4) were manipulated. The performance of these direct localization tasks was found to be as accurate as the performance in previous studies of indirect localization or regular patterns. The results consistently supported the claim that information about position of the center of gravity is used for the localization of visual objects.