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Binocular vision

Seeing in reverse

Nature volume 389, pages 235–237 (1997)Cite this article

Recently, people have become fascinated by autostereograms, in which a three-dimensional scene emerges when the eyes misconverge on an apparently random array of dots. We now have stereoscopic virtual reality and stereoscopic ‘vision’ in robots. But what are the brain mechanisms that are involved in stereopsis (binocular vision)2? An advance in our understanding of these mechanisms comes from results by Cumming and Parker3 and by Masson et al. 4, on pages 280and , respectively, of this issue.

When we look at a small object, the images of it fall on corresponding points in the two retinas. Neural signals from these corresponding points then converge on the same ‘binocular cell’ in the primary visual cortex (V1) of the brain5. As a result, the images fuse and we see one object. But if the images differ widely in shape, orientation or luminance contrast, they rival one another rather than fuse. When similar images fall on slightly different retinal regions, we see one object standing out in depth (Fig. 1). This is the magic of stereoscopic vision. The simplest idea is that optic-nerve fibres serving slightly different regions (receptive fields) in each of the two retinas converge onto the same binocular cell. Such a cell is a ‘disparity detector’, because it responds best when the images in the two eyes have a disparity that matches the difference in the positions of the two receptive fields6. Different disparity detectors respond to different signs and magnitudes of disparity. Zero-disparity detectors respond to objects in the plane of fixation, crossed-disparity detectors react to nearer objects, and uncrossed-disparity detectors respond to more distant objects. This is a position-disparity mechanism. Disparities may also be detected by differences in the distribution of excitatory and inhibitory regions within two receptive fields that do not differ in position. This is a phase-disparity mechanism.

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Figure 1: One of the original stereograms constructed by Wheatstone in 1838.
Figure 2: Binocular combination of the two upper displays induces convergence because of the crossed disparity of the black disks.
Figure 3

References

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  1. the Centre for Vision Research, York University, North York, M3J 1P3, Ontario, Canada

    Ian P. Howard

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  1. Ian P. Howard
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Howard, I. Seeing in reverse. Nature 389, 235–237 (1997). https://doi.org/10.1038/38400

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