Abstract
In a random-dot stereogram (RDS), depth percepts of object surfaces are generated using left-eye and right-eye images that comprise interocularly corresponding random black and white dots. The spatial disparities between the corresponding dots determine the surface depths. If the dots are anti-correlated, such that a black dot in one monocular image corresponds to a white dot in the other, disparity tuned neurons in the primary visual cortex (V1) respond as if their preferred disparities become non-preferred and vice versa, reversing the disparity signs reported to higher visual areas. Humans can perceive this illusory reversed depth in peripheral but not central visual field (Zhaoping & Ackerman 2018), confirming a prediction (Zhaoping 2017) that feedback from higher visual areas to V1, for analysis-by-synthesis in recognition to veto the reversed depth signals from V1 for violating internal knowledges about the visual world, is weaker peripherally.
The current study obtained fMRI responses to the RDSs across the visual hierarchy. A linear decoder is trained to recognize the depth order of a disk against background in correlated RDSs using fMRI activity patterns of a brain region in response to such RDSs. If the decoding performance is better than chance after training, we apply the decoder to the fMRI activity patterns in response to the anti-correlated RDSs to see whether it better reports the reversed than non-reversed depth, and if so, then the brain region is said to signal reversed depth. Reversed depth signals were more likely found in higher (e.g., parietal) than lower (e.g., V1, V2) visual areas, more likely for peripherally viewed RDSs, and more likely for observers who perceived reversed depth (peripherally). Some brain areas, e.g., hV4 and LO, contain the reversed depth signals in central view even though observers could not perceive them, particularly among observers who can perceive reversed depth peripherally.