Category Archives: Visual perception

Jello in a spirit level

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I decided that rather than try to describe what my vision has been and currently is, I would post some images. You may find this hard to beliee, but these images are not from a biology textbook; I drew them myself.

First, this is what normal vision (sort of) looks like:

Each of your eyes see its own image of this odd-looking dog with the odd-looking gait. They are slightly different, but your brain seamlessly merges them into one coherent image. You are normally not aware that there are two separate images in the overlapping area, but the differences in the images are what allows you to perceive the different distances to various objects that you see.

After my surgery, the vitreous in my right eye was replaced with sulfur hexafluoride (SF6). The optical properties of SF6 are different from that of the vitreous, and the eye can’t focus an image through it. That means that my right eye’s image was blurry, and objects in my field of view seemed to be in a different place from what my left eye was reporting. This yielded an image kind of like this:

Imagine that I was looking at a table. My left eye saw it clearly, while my right eye saw a blurry, displaced images. My brain couldn’t figure out how to merge these images, so it basically didn’t. It just gave me two different images at the same time. This made it essentially impossible to read or to drive. Since I spent Thursday afternoon after the surgery through Sunday with my head down, reading was about all I could do. I solved this problem by closing my right eye.

As the body absorbed the SF6, the bubble shrank. For several days, this is what I saw with my right eye:

The very top of my visual field was clear, while all of the rest was blurry. There was not enough clear vision to allow my eye to focus at normal distances, so I still had to close my right eye to read, and I couldn’t drive.

Eventually, the bubble shrank to the point that it was beneath almost everything I needed to see, leaving something like this:

This image is from what I imagine driving though the foothills of the Sierras would look like.

And this is what my vision is like as I write this on Saturday night. I can look up where I’m writing and can see the mess of objects on our dining table, and, most importantly, i can tell where they are. I can reach out and touch the salt shaker without making several tries. Unfortunately, if I look down, I am looking through the bubble and I have no depth perception. A Walmart employee had to help me put my credit card in the reader tonight because I kept missing the slot

If I look straight down, I can see the entire bubble as a circle filling maybe 80 percent of the entire field of view. The bubble jiggles when I shake my head, like a bowl of Jello. When I look up, the bubble reminds me of the bubble in a spirit level, finding its own level as I move my head around.

But, you are wondering, if this is a gas bubble within an eyeball otherwise filled with fluid, why is it floating at the bottom of my eye rather than the top? That’s an astute question, and I’m glad you asked. I wondered about that myself for a while, and then I remembered that the image the eye’s lens projects on the back of the eye is inverted; it’s upside down. The brain perceives it as right-side-up. Since the bubble is at the top of an upside down image, the brain thinks the bubble is actually at the bottom of my eye, thus fooling both itself and me.

Since I am still recuperating, I continue to spend a lot of time reading on my phone. Looking down still moves my bubble up (actually, down) into my field of view, so it’s still a real distraction. My solution now is to put blue masking tape on the right lens of a cheap pair of reading glasses.

I can still see the bubble in my right eye; in fact, in a lighted room, I can see the bubble with both eyes closed. But I can do a pretty good job of ignoring it. Right now there is a ghost image of the bubble covering about the bottom third of my laptop’s screen. It’s somewhat distracting, but much less than it would be without masking tape on my glasses.

There is one discouraging development. At my post-op appointment, while waiting for the doctor to come into the examination room, I discovered that I could focus on an object very close to my eye. I put a medical alert bracelet I was wearing up close and could read the fine print. It seemed to me that the blind spot and distortion I had been experiencing were gone. That made me very happy. Unfortunately, I have since determined that I still have a blind spot and distortion at my fovea. If I look at a straight line, I see something like this:

The images is blurred and distorted, and whatever is at the very center of the fovea is not visible. If I look directly at a small object, it disappears and is replaced by whatever surrounds it in my visual field. A small stone on a concrete surface disappears, and where it should be looks like more concrete. The effect might not be quite as bad as before my surgery, but it’s still there.

I have read that one’s vision after a vitrectomy can continue to improve for up to six months, so maybe it will get better. If it doesn’t, I suppose, and hope, that my brain will accomodate that deficiency and begin to ignore the blurring and distortion, replacing it with the better image from my left eye.

What color is this

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I have been making leashes for the dogs out of a long piece of rope I got at one of our local building supply companies. The dogs started out with store-bought leashes, but Zoe bites at hers, and she chewed it so much that I finally threw it away. I can make a lot of leashes from a fairly cheap length of rope instead of having to buy a new leash every month or so.

I made a short leash for Sam and about a five-foot leash for Zoe. Then I made a longer addition for Zoe’s so I could let her run in the front yard with Sam. The total length for Zoe’s is about 12 to 14 feet, which is long enough that she can get up to a good speed by the time she stretches the leash to its full length. And she did that Monday afternoon. She was going so fast I had to release the leash or have my arm dislocated. She kept going full speed into the woods and disappeared with Sam following.

This was the fifth time they have run away. The first time they did it they came home after dark and sat around in the garage until I happened to look. The next time Sam was gone a full day and Zoe was gone four days. The third time a neighbor caught Zoe and called us. The fourth time, which was only about a week ago, a couple living at the bottom of the mountain caught them and called. This was the fifth time. This time I posted on Facebook. They disappeared at about 2:15 or so. Eventually they were spotted a couple of miles away, and then more than four miles away, but by the time I got to the places where they were sighted, they were gone.

They finally came home at about 10:30. Sam had rolled in something nasty and smelly, so I had to bathe him before I could let him in the house. Zoe was wet and a little smelly, but I didn’t want to undertake as big a job as washing her would be that late in the evening. She got her bath today, Tuesday.

The thing I found most interesting (not the thing I found most infuriating) in this case was the eyewitness accounts of the people who responded to my post on Facebook. One man who saw them in his yard said the big one was dragging an orange leash. A woman who saw them later in the day said the big one was trailing a bright red line. This is the rope I use for their leashes.

And this is the stub of Zoe’s leash, the only part she brought back home. It looks kind of like a fishing lure.

What color would you call this? I call it blue because to my eye it’s mostly blue. The red/orange is eye-catching, maybe even more so in bright sunlight, but I was surprised when they talked about a red or orange leash.

The top photo shows the knot I use to make the hand loop. It’s called a bowline loop. Boy Scouts and sailors probably know this one, but it was new to me. I found it when searching for knot-tying instructions. Since this is a nautical knot, you might expect “bow” to be pronounced like the bow of a ship, but in the case of this knot, it’s actually pronounced like bow in “bow and arrow.” It has the advantage of not slipping, so the loop doesn’t close up on your hand when the dog pulls on it. The knot I used to attach the clip is called a clip knot, which seems appropriate.

I could probably tie another bowline loop without looking at the instructions, which means it’s a pretty simple knot. I would have to look up the clip knot, although it’s also a simple knot.

Crepuscular rays from behind

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Leah called me out onto the front porch Tuesday evening right at sunset to see the sky. This is what we saw.

The Sun was setting directly behind us, near the horizon but high enough to illuminate the thunderstorm in the distance. Apparently there were some clouds between us and the Sun, and the shadows of those clouds spread across the entire sky.

Here’s a closer shot at the moon, the clouds and the sky.

The normal view of crepuscular rays is of bright, fan-shaped rays shining out from the Sun through gaps in the clouds. What these images show are the dark areas between the rays. “Normal” crepuscular rays are, as I said, fan shaped, with the narrow part near the Sun, getting wider as you look further from the Sun. These dark “rays” appear wider closer to us, the viewers, and the Sun behind our backs, and they get narrower as they go away from the position of the Sun. How can that be?

Well, my explanation is that the fan-shaped appearance of “normal” crepuscular rays is an optical illusion. The rays are nearly parallel, widening only a small amount. However, they look wider as they get closer to us, just like a road looks wider where your car is and narrower further away. The rays look like they are fan shaped because we tend to see the phenomenon as two-dimensional, as if the rays were pasted on the sky in the far distance. Instead, the rays are actually shining towards us.

This is what we see in these pictures; the rays have shot out from the Sun around the clouds and passed over our heads, disappearing into the distance in the east. Since the rays (or the darker areas between the rays) are very close to parallel, we see them as wider directly overhead and appearing to get narrower as they disappear into the distance. I am not sure I remember seeing this phenomenon before.

If we could have seen the entire dome of the sky, I think we would have seen crepuscular rays appearing narrow at the Sun, wider as they approach and pass over our heads, and then getting narrower again as they shine off into the distance. That would have been a sight.

These rays stayed visible for about a half an hour. I watched as the shadow of the Earth rose up on the bright cloud in the distance until the cloud was a barely-discernible gray mass on the horizon. I thought about the Sun moving towards the west and the shadow climbing up the cloud, and it occurred to me that the Sun was not, of course, moving; it was us. The surface of the Earth was flying at around 860 miles per hour around its axis, at the latitude where we live, carrying us and dragging the entire atmosphere along with it. So it was not the Sun that was moving, gradually hiding the clouds, but the clouds themselves that were retreating from the Sun.

Later Tuesday night that cloud (or one very like it) gave us a show as lightning flashed inside the cloud.

Projectile points

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Finding an Indian arrowhead in this part of Georgia is exciting but not particularly unusual. According to the archeologists, Indians have lived in this part of Georgia for more than 10,000 years and they were making stone implements for essentially all that time. For most of those thousands of years, the stone implements were spear points, axes, scrapers, knives and maybe other things, but not arrowheads. Arrowheads appeared only around 2,000 years ago.

Here is an arrowhead fragment I found around here a couple of years ago along with a complete arrowhead that Leah found when her parents were building their house in the early ‘60’s. I outlined the complete point and laid the fragment in it.

two points compared

It seems clear to me that the fragment is part of a full point that would be very similar to the complete point. I’m going to call the point I found an “arrowhead” because it seems to be the right size and shape, but it might, in fact, be something else. I suspect that an expert would be able to identify the marks that the maker left when the point was chipped from the original source stone. But to me the shape in general is enough to convince me that it really is part of an arrowhead.

Here the fragment is in my hand to give an idea of the size.

point in hand

The fragment appears to be made from chert. One Web site says that brown chert is common in southern Georgia, while gray or black chert is common in northwestern Georgia. The same site says that brown chert turns reddish when heated. The fact that Leah’s arrowhead is black is consistent with where she found it, but what about the reddish color of the fragment I found? Was it obtained in trade with Indians living further south?

I don’t know how many Indians lived around the Rome area through those thousands of years, but if you assume that as few as a couple of people who made or used projectile points lost or broke a couple each year, that would mean there were tens of thousands of projectile points scattered in the area. Maybe not as many arrowheads, but I’m going to make an uneducated guess that Indians might have lost or broken arrowheads at a higher rate than other stone implements. My guess is based on the relatively small size and the fact that arrowheads are shot from bows in ways that might make them hard to find if they missed their target. Even if that assumption is not true, if you assume a couple of lost or broken arrowheads every year from a given small population, there should still be at least a couple of thousand points in the area where that small population lived.

They would have to have been lost mainly in areas where they were made or used, so they are probably concentrated in some areas and scarce in others. The Rome area should be such a point. We know for sure that there was a reasonably large population not far from Rome, up the Etowah River near Cartersville, because they built large earth mounds there somewhere around 1,500 years ago. We also know that Indians caught fish in the rivers around here, because of the fishing weirs that still exist in the Oostanaula and Etowah rivers. It seems reasonable that the two rivers that meet in Rome would have provided a constant, reliable source of water for human and animal needs. Today there is a lot of game around our mountain, deer, squirrel, rabbit and turkey at least. I assume, maybe wrongly, that the same types of animals would have been here for thousands of years. There seems to be good reason for Indians to have uses their stone implements in this area, so, in my view, it’s not surprising that I found a projectile point. And the simple fact is that I did.

That leaves only the question of how I found it.

I was looking down as I walked. The arrowhead was lying on the ground surrounded by pieces of rock about the same size and color. If you had been trying to find a good place to hide an arrowhead in plain sight, it would have been a good choice. But my eye was drawn to it, and when I saw it I knew immediately what it was, despite the fact that it was not even a complete arrowhead.

I know in general how I did it. Humans have a remarkable ability to recognize patterns. The human visual system – the eye and brain – do this job so well that we seem to be forced to find patterns even when there are none. For example, constellations and images of Jesus on a piece of toast. But still, finding it was an amazing feat, even if I do say so myself.

I didn’t have a camera with me when I found the fragment, but I went back some time later and put it down in an area similar to where I found it. After putting it down I’m not sure that I could have found the fragment if I had walked away and come back the next day, even though I knew where I put it.

hidden point Here it is with a cheater arrow.

hidden point pointed out

Here it is with a few rocks chosen from the area in the photograph.

point with rocks

It’s still kind of hard to see it, but you do the same thing pretty much every day. If you have ever become interested in something, say nice, round rocks, or box turtles, you are probably familiar with the way it seems like you start seeing them everywhere. They were always there, but your pattern recognition system has been trained to see them. Unconsciously you have identified some features of the thing you’re interested in, and your visual system automatically, with no conscious effort on your part, uses those features to discriminate between your object of interest, and everything else in the world.

I was not looking for anything in particular, much less arrowheads. And besides, it wasn’t even a whole arrowhead. Leah remains unsure that it is an arrowhead fragment. I understand pattern recognition, but I still don’t know how I recognized it so quickly and easily.

I do know that if you could turn what I did into a computer program, you could probably get a job in the field I used to work in (missile defense).

When is a dog not a dog?

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Human visual perception is a funny thing. You can be absolutely certain that you see a particular thing, and unless  you look a little further you may never know how wrong you were.

On a recent walk with Zeke, I saw this.

Look into the woods

Look into the woods

Does that dark object slightly above mid-picture look like anything to you? For a few seconds I was convinced it was a dog sitting on its haunches. I couldn’t tell what kind it was, but I thought either a German shepherd or a doberman. Once my mind had made that identification, that’s what it looked like. I began to fill in details and the longer I looked, the more it looked like a dog.

Then I asked myself why a dog would be sitting out in the woods like that, calmly watching us approach, without a motion. So I went closer.

A log is not a dog

A log is not a dog

Up close it doesn’t look much like a dog.

Why would I have identified this as a dog? From a distance this object had the rough outline of a sitting dog. Even the coloring suggested a dog, although part of what I saw as coloring was actually the leaves behind it, seen through the gap between its “legs” and its “body.” It was considerably larger than a dog, but at that distance, the scale was not immediately obvious. Also, abandoned dogs are far from rare on our rural mountaintop. And, probably most important, my wife and I have been discussing the possibility of getting another doberman. So, wishful thinking?

Would you have seen a dog, or something else?