Visual information is initially represented as 2D images on the retina,
but our brains are able to transform this input to perceive our rich 3D
environment. While many studies have explored 2D spatial representations or
depth perception in isolation, it remains unknown if or how these processes
interact in human visual cortex. Here we used functional MRI and multi-voxel
pattern analysis to investigate the relationship between 2D location and
position-in-depth information. We stimulated different 3D locations in a blocked
design: each location was defined by horizontal, vertical, and depth position.
Participants remained fixated at the center of the screen while passively
viewing the peripheral stimuli with red/green anaglyph glasses. Our results
revealed a widespread, systematic transition throughout visual cortex. As
expected, 2D location information (horizontal and vertical) could be strongly
decoded in early visual areas, with reduced decoding higher along the visual
hierarchy, consistent with known changes in receptive field sizes. Critically,
we found that the decoding of position-in-depth information tracked inversely
with the 2D location pattern, with the magnitude of depth decoding gradually
increasing from intermediate to higher visual and category regions.
Representations of 2D location information became increasingly location-tolerant
in later areas, where depth information was also tolerant to changes in 2D
location. We propose that spatial representations gradually transition from
2D-dominant to balanced 3D (2D and depth) along the visual hierarchy.
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