Takahiro Doi, Maki Takano and Ichiro Fujita
Computation Underlying Stereoscopic Depth Perception Depends on the Temporal Frequency of Visual Patterns: An Experiment of Human Depth Discrimination
Stereoscopic depth perception relies on binocular disparity. The primate visual system uses two computations to encode binocular disparity. One resembles a cross-correlation of the left and right retinal images (correlation computation) , which can falsely encode disparity embedded in stereograms lacking binocularly matched features (e.g., contrast-reversed random-dot stereograms). The other computation encodes disparity by detecting binocularly matched features (matching computation) such that disparity signals are removed from contrast-reversed stereograms . However, it remains unclear how combining the two disparity computations contributes to stereoscopic depth perception.
Here, we hypothesized that both computations contribute to stereoscopic depth perception and that their relative contributions depend on the temporal frequency of visual patterns. We manipulated the frequency of dot pattern changes in random-dot stereograms (RDSs) and examined whether the depth discrimination (near versus far) performance of human subjects followed the predictions made by the correlation computation or those by the matching computation. The two predictions were dissociated by reversing the luminance contrast of dots in one eye at varying proportions. For a full contrast reversal, only the correlation computation contributes to depth discrimination, while for a half contrast reversal, only the matching computation contributes.
When temporal frequency was high (42.5 Hz), subjects perceived reversed depth to a binocularly contrast-reversed RDS and no depth to a half-reversed RDS, consistent with the idea that subjects use disparity information encoded via the correlation computation. As the temporal frequency was decreased, discrimination performance shifted toward the prediction of the matching computation. Subjects perceived no depth to a contrast-reversed RDS and correct depth to a half-reversed RDS. These results suggest that the correlation and matching computations contribute differently to stereoscopic depth perception according to the temporal frequency of the visual patterns. An accompanying model study shows that temporal frequency channels explain the mechanisms controlling the relative contributions of the two computations .
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