Great series of posts from Len Press about Stereopsis, 3D vision. I was particularly struck by this passage about how stereopsis relates to spatial judgement, ie how you keep yourself oriented in space.
Stereopsis is also a quale that serves as a barometer of binocular
efficiency, and a compass of sorts when it comes to spatial judgment.
In today’s terms we might say that it’s an important feature of the
visual system’s GPS (Global Positioning System).
Here’s one practical example. I was walking down the block this
morning and heard the sound of jackhammering in the distance. My
attention was drawn to a work area in the region of the red awning,
which is the building that I wanted to enter. Is the work being done at
the entrance, so that I would be better served by entering the rear of
If you have excellent stereopsis you can make that judgment from a
considerably greater distance than if your stereoscopic localization is
poor. Coarse/qualitative/magno pathway or second order stereoscopic
judgment is what we’re talking about here. Booklet tests of stereopsis
like RDS figures or Wirt Circles yielding fine/parvo pathway/first order
stereoacuity won’t necessarily be a good index of your judgment here.
As you draw nearer to the building, particularly if your stereopsis
failed you at a distance, the fact that the work area is beyond the red
awning becomes much more obvious. If the work area was indeed within
the red awning, it may have been only a nuisance that you walked an
extra block to find this out, and would have to backtrack and re-route
to the rear entrance of the building. If you’re driving, and you have
to make an analogous judgment, getting it wrong may cause you to stop
short or swerve into the next lane to avoid encroaching on the work
space. Now your compromised stereoscopic perception can be a potential
road hazard to other drivers instead of a mere nuisance.
For The Whole Series on Stereopsis, see the following: