optical illusion

When I worked in the Australian Outback in 1970, I used aerial photographs to assist my production of geological maps and the staking out of mineral claims on behalf of my company. When aerial photos are used for this purpose, it is customary for there to be an 80 percent overlap between adjacent shots. There are two reasons for this practice: to avoid the distortion that occurs close to the edge of such a photo; and to allow the viewer to look at the same point on the ground from two different angles.

The advantage of this last point may not be obvious, but consider that popular Victorian parlour toy the stereoscope. By looking at two slightly different images through a special viewer, the user could see a composite, three-dimensional picture. No one is going to lug a clunky wooden contraption around the bush, but small folding stereoscopes that will fit into a breast pocket have been around a long time.

And the resulting 3D pictures are remarkable. Trees stand out above their shadows, and small natural features are so easy to spot on the photos that it becomes straightforward to find wherever you are on them. However, there is an interesting phenomenon that happens occasionally, usually without warning. Trees and buildings become holes in the ground, and small hills become round hollows. This so-called ‘pseudoscopic’ image can be difficult to shake off when once seen, even though you know what the image should look like.

Which brings me to the point of this post. Last night, I was browsing through my folder of abstract photos when I experienced a similar phenomenon. I have the advantage of knowing the subject of the photo below, but for the purposes of this exercise let me just say that it is either a picture of pebbles on a beach or a kind of grating with irregularly distributed round holes in it. I could see only the incorrect interpretation, and I found it extremely difficult to force the optical processing system in my brain to switch images. However, the switch eventually took place automatically, and unexpectedly.

This is where you, the reader, come in. I hypothesize that each viewer will always see the same image first, that they will have difficulty switching to the alternative, and that this reflects how their brain is wired. Your assistance in either validating or refuting this hypothesis will be greatly appreciated. Please study the picture below before answering the following questions.

Which of the two images did you see first? Please record your answer in the poll on the right. Could you see both images? Did you have difficulty switching to the other image? Although this isn’t directly relevant to my inquiry, do you know what the real subject of this photograph is? I mention this because the origin of the round shapes is a mystery to me, and I have already disproved a couple of initially plausible hypotheses.