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.
Highlights
Neural correlates underlying the size constancy effect have been investigated in both real[5] and virtual 3D world[6]
In the present fMRI study, by constructing a virtual 3D world in the MR scanner via 3D goggles, we aimed to investigate whether and how the functional connectivity between the occipital cortex and the higher order brain regions change as a function of the constant perceived size in virtual 3D space vs. varying perceived size in 2D
In the 2D control condition, bilateral inferior occipital gyrus (IOG) was significantly activated in the “Small” condition as compared to the “Large” condition (Fig. 1b upper panel and Table 1c)
Summary
Neural correlates underlying the size constancy effect have been investigated in both real[5] and virtual 3D world[6]. In the constant perceived size (variable retinal size) condition, where the classical size constancy effect occurs, no significant adaptation effect was reported It remains unclear how the lower level visual cortex interacts with higher-level cortical regions to explain the size-distance invariant object representations. In the present virtual 3D condition, the same 3D object was positioned either close to or far away from the observers so that the further object was perceived as the same object as the closer one (of the same physical size) despite their differences in retinal size, i.e. size constancy. With regard to the neural correlates underlying size constancy in the virtual 3D condition and the perception of differential object sizes in the 2D control condition, we expected to observe differential patterns of functional connectivity between the commonly activated occipital visual areas and higher-order cortical areas
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