Neural Correlates underlying Size Constancy in Virtual Three-Dimensional Space

Abstract

The perceived size of an object remains relatively constant although its retinal size keeps decreasing as the object moves away along the depth dimension of the 3D space, i.e. size constancy. Neural mechanisms generating size constancy in virtual 3D space, however, remain poorly understood. By constructing a virtual 3D world in the MR scanner, we positioned the same 3D objects either near or far from the observers so that the near and far objects were perceived as having the same physical size despite their differences in retinal size. To control for the effect of differential retinal size, an additional 2D condition was introduced: a large and a small object, with matched retinal images as the near and far objects in the 3D condition, respectively, were presented on a 2D screen. Differences in retinal size activated overlapped areas in bilateral inferior occipital gyrus (IOG) in both experiments. The overlapped areas in IOG, however, showed different patterns of functional connectivity with different neural networks, depending on the perceived size of objects. In particular, IOG showed enhanced connectivity with bilateral superior parietal cortex in the 2D condition, but with inferior temporal and prefrontal cortex in the virtual 3D condition, i.e., size constancy.

Figure 1

The main effect and conjunction result in the virtual 3D and 2D experiment. (a) Main effect of the depth of objects in the virtual 3D experiment. Red: “Far > Near”; Blue: “Near > Far”. (b) Main effect of the physical size of stimuli in the 2D experiment. Dark red: “Small > Large”; Dark blue: “Large > Small”. (c) The conjunction analysis between (a) and (b).

Figure 2

PPI analysis based on neural activity in bilateral IOG (dark red) with the “Small > Large” contrast as the psychological factor in the 2D experiment. The right upper panel: the left IOG showed higher neural coupling with the right SPC in the “Small” condition than in the “Large” condition. PPI analysis based on neural activity in the left middle occipital gyrus of an individual subject (Subject 2D_S24) is shown. Mean corrected neural activity in the right SPC was plotted as a function of the mean corrected neural activity in the left IOG in the “Small” and “Large” conditions, respectively. The right IOG showed higher neural coupling with bilateral SPC in the “Small” condition than in the “Large” condition. PPI analysis based on neural activity in the right IOG (Subject 2D_S24) was shown. Mean corrected neural activity in bilateral SPC was plotted as a function of the mean corrected neural activity in the left IOG in the “Small” and “Large” conditions, respectively.

Figure 3

PPI analysis based on neural activity in bilateral IOG (red) with the “Far > Near” contrast as the psychological factor in the virtual 3D experiment. The right upper panel: the left IOG showed higher neural coupling with the right IFG, the right SFG and the right ITG in the “Far” condition than in the “Near” condition. PPI analysis based on neural activity in the left IOG (Subject 3D_S13) was shown. Mean corrected neural activity in the right IFG and the right ITG was plotted as a function of the mean corrected activity in the left IOG in the “Far” and “Near” conditions, respectively. The right lower panel: the right IOG showed higher neural coupling with the right IFG, the right SFG and the right ITG in the “Far” condition than in the “Near” condition. PPI analysis based on neural activity in the right IOG (Subject 3D_S13) was shown. Mean corrected neural activity in the right IFG and the right ITG was plotted as a function of the mean corrected activity in the right IOG in the “Far” and “Near” conditions, respectively.

Figure 4

The accuracy of three types of task in 2D and 3D experiment. (a) The accuracy of three types of task in 2D experiment. (b) The accuracy of three types of task in 3D experiment.

Figure 5

Experimental settings in the present study. (a) Top view of the 3D objects in the virtual 3D experiment. (b) Visual stimuli in the 2D experiment were exactly the same as the frontal view of the 3D stimuli in the virtual 3D experiment. (c) Two major differences (hightlighted) between the 3D and the 2D experiment.