In his 2010 book, Oliver Sacks writes:
The only way to actually perceive depth -- to see it rather than judge it -- is with binocular stereoscopy (p. 114).Thus, he reports on a woman ("Stereo Sue") who gains stereo vision and with it, for the first time in her life, real experience of three-dimensional visual depth. And of his own loss of vision in one eye he writes:
Stereo vision, however, now that I am mostly monocular, is quite compromised -- completely missing in the upper half or two-thirds of my visual field, though partly intact at the bottom, where I retain some peripheral vision. So I see the lower halves of people in stereoscopic depth, while their upper halves are completely flat and two-dimensional (p. 173).Although Sacks acknowledges the possibility of some individual variation, he also objects to contemporary accounts of depth perception that treat binocularity as only one among several depth cues:
With one eye occluded, I have no sensation of distance or depth whatever (p. 157).
Such [multifactorial] views [of depth perception], while wholly consistent with a behavioral or empirical theory of vision, give no weight to the qualitative and subjective aspects of stereoscopy. Here one needs inside narratives, personal accounts of what it is like to suddenly gain stereo vision after a lifetime of stereo blindness... or to suddenly lose it after a lifetime of seeing in stereo [as in Sacks's own case] (p. 140-1, fn 14).I wonder, though: Why is permanent loss or gain of stereoscopy necessary to the subjective evaluation? Can't the binocular among us at least temporarily mimic monocular experience simply by closing one eye? Does doing so make the world go from three-dimensional to flat?
It doesn't seem that way to me. Nor to most other people I've interviewed. Maybe depth is a little richer and more striking binocularly than monocularly -- but the difference is nothing so radical as the difference between 2-D and 3-D. Some of my interviewees, though, do characterize the monocular-binocular difference as 2-D vs. 3-D; and so do, for example, Ernst Mach (1886/1959) and Brian O'Shaughnessy (2003). Call those who report radically different monocular vs. binocular experience, like Sacks and O'Shaughnessy, the PuffOuters. Call those who report pretty similar monocular vs. binocular experience, like me (see also Chapter 2 of my recent book), the StaySamers.
Question: Is the difference between PuffOuters and StaySamers a real difference in experience? That is, does the world actually seem to puff out and go flat for PuffOuters, depending on whether it is seen monocularly or binocularly, while it varies little for the StaySamers? Or is the group difference mainly a difference in report only, with everyone having pretty much the same monocular vs. binocular experience -- and some people mistaken about or misdescribing their experience (in line with some of my conjectures in Chapter 2)?
We might start to address this question empirically as follows.
Sacks mentions that people differ substantially in how they experience three-dimensionality when presented with stereoscopic pictures -- that is, when presented with slightly offset pictures, one to each eye, producing a three-dimensional effect (like in contemporary 3D movies). For example, when presented with a stereoscopic picture of an impossible M.C. Escher tuning fork, Sacks reports one person saying it looked like the top prong rose about 3-4 centimeters from the plane of the page, while two others reported seeing the prong as 12 cm above the plane and Sacks himself saw it as 5 cm higher still (p. 134-5).
Conjecture: If the PuffOuter-StaySamer difference is a real difference in experience, the PuffOuters should, as a group, report much larger effects of that sort than the StaySamers. That could be tested empirically. Presumably PuffOuters should also better detect subtle stereoscopic effects -- which is even more appealing experimentally because it would permit catch trials to corroborate any differences in subjective report.
Yes, this is totally do-able.