Tag: eyes

Spider Eyes are Nature’s Marvels

Now I do not exactly remember where and how I started my journey down this rabbit hole. But the deeper I went the more interesting it became. It was a great learning experience. I’m clearly not an expert. Here I share the understanding I developed of the spider eye over the few hours of exploration. For this I referred to various sources all of which are mentioned in the links. And if you know more or would like to add something interesting to the article please let me know in the comments below.

The  first thing about spider eyes is that 99% of spiders have 8 eyes. A little less than 1% of them have 6 eyes. In some fringe species there are 4, 2 or no eyes at all. Apparently, based on the pattern these eyes are arranged in, on their cephalothorax (let us mortals call it the ‘head’ to make things simple), the family to which the spider belongs can be determined. Some blessed human, made the following schematic to help us do exactly that. In case you ever feel the need to do so, here it is:

And in much greater detail, right here.

For their small size and the limited number of photocells, spider eyes, especially the jumping spider’s (Salticids) eyes perform surprisingly well. Their resolution is better compared with larger mammals than with insects. In the human world a camera of such standards this would simply be an engineering miracle. You will understand why I say that soon…

In the image above if you locate the family Salticidae, you will see those two large eye in the front which are particularly very interesting. These are called the principal eyes (or anterior median eyes) and are the ones that allow high resolution vision. So much that the spider would be able to resolve two spots on a screen 20 cm away from the spider, sitting just 0.12 mm apart from each other. An acuity of about ten times that of a dragonfly – 0.04°.

The brain of this spider, show in blue in the image below is pretty big for its size. The proportion of the volume of brain to body is more or less similar to that of human beings. The brain of Salticids also have a rather large region dedicated for visual processing.

The principal eyes we are talking about are in the shape of elongated tubes as seen below, in the front of which is a hard lens and at the other end is a layer of photocells. Inside the tube, near the retina is another little lens which moves back and forth along the tube like a telephoto lens system. These elongated tubes are like the tubes of a binocular which allow for a higher resolution using a small package.

However the downside of such a tube like architecture is that it limits the field of vision. Here’s how that problem is dealt with.

The front part, with the big corneal lens is fixed. It has a long fixed focal length. The farther end where the retina is located, is connected to these muscles shown in red. These muscles allow for the tube’s farther end to move around in several degrees of freedom to make quick movements and scan a larger image in its head, one small field of view at a time.

In the video below you can see the retinal end of the black tubes moving around inside the translucent exoskeleton of the spider as the spider forms a high resolution complete image of its surroundings, one small field of view at a time.

If you peer deep into their eyes you will see a dark (black) when you are looking into the small retina. However when the farther end of the tube moves, you see a honey brown color with spots. This is the inner wall of the tube that you are seeing in the following video.

Then the retina itself is another biological marvel. Unlike our single layered retina, the Salticid’s retina is made up of four layers. The four layers are arranged one behind the other. This lets the nature pack more photocells in a smaller area and also helps the spider see in color as different colors (different wavelengths) with different refractive indices are focused in different planes.

Counting from the rear end, the spider uses different layers of retina to obtain different colors of the image. The retina’s layer 1 and 2 to get the green color (~580 nm – 520 nm wavelengths), blue color using the layer 3 (~480 – 500 nm wavelengths) and layer 4 for ultraviolet (~360 nm).

An important detail in the above image reveals how spiders manage to keep focus on different objects at different depths, in focus. The layer one has photocells arranged in a step fashion, with varying distance from the lens which makes sure that all objects are focused on at least one part of the layer 1.

The other problem of distance estimation which matters a lot for jumping spiders is again solved rather elegantly by the same apparatus. Humans use their stereo vision – two eyes which are far apart to estimate distance. Other animals move heads to do the same but I’m not getting into that.

Jumping spiders employ a completely different algorithm, utilizing degree of blur cues. For which the second layer plays a crucial role. The second layer would have received a sharp blue image, but they are not sensitive to blue light like I mentioned above. The green they detect is rather blurred at that plane. It turns out that the amount of blur depends on the distance of the object and helps the spider determine the depth by processing the amount of blur in the image. Hence allowing it to jump and hunt accurately.

If you are a university student with free access to journals, I think a quick look at the paper titled: “‘Eight-legged cats’ and how they see – a review of recent research on jumping spiders,” will help you delve into greater detail.

Psst: Someone has it uploaded on research gate for free access for I don’t know how long: here.

Please leave a comment below to let me know your thoughts on this, or if you have any ideas for future posts. I plan to reward the top commentators every month so do not forget to say something.

Stopped Clock Illusion

By Anupum Pant

When you quickly move your eyes to focus on the seconds hand of an analogue clock, have you ever noticed that the first second you see actually seems to linger for a slightly longer time? Yes, it does. And there’s a reason why it happens.

When you rapidly move eyeballs to focus from one point to the another, it’s called a Saccade. If you ever try doing this rapid movement with a camera, a motion blur occurs in between the first point focus and the last point focus.

Unlike cameras our eyes (work closely with the brain) has a built-in mechanism to erase this motion blur. The brain erases all the motion blur during those few milliseconds and replaces the motion blur frames with the final image in the end.

This is why you see the longer first second when you quickly focus your eyes on the seconds hand – the stopped clock illusion or chronostasis. This also explains why you can never see your eyeballs moving when you try to spot their movement while staring at your own eyes on a mirror.

Michael Stevens from Vsauce explains…

Seeing Your Own Eye Blood Vessels

By Anupum Pant

Blind spots are fine and I’ve known for years how to spot your own blind spot. You can make 2 spots on a paper separated by 4-5 inches, close your right eye and look at the right side spot with your left eye. If you do that and move forward or backward ( and rest at about 15 inches from the surface you drew on), you’d find a point where your left eye’s peripheral vision would not render the left side spot. You’d have found your blind spot.

But there is something more interesting, I never knew. You can actually see the blood vessels of your eye, with your own eye. Here’s how…

Take a sheet of paper (or card), and poke a pin hole in it. Then close one eye and holding paper close to your eye, jerk around the paper in little circles. At the same time, make sure you are looking at a bright white area through that hole. You could open up MS paint, make the whole canvas white and stare at it through the hole. Try to focus on the white screen and not the paper (or card)…

The video probably explains it better.

The Best Illusion of the Year 2014 Award

By Anupum Pant

You probably know the static Ebbinghaus illusion – where a circle appears bigger around smaller circles even when it is of the same size. It’s static because it works without moving. Well, if you don’t know, you should because it helps you lose weight in a very subtle manner.

A slight variation involving movement of the Ebbinghaus illusion won the best illusion award for the year 2014. Yes, there are annual awards for the best illusions (I never knew that!). This one which won the award was submitted by researchers from the University of Nevada Reno.

The new variation is called the Dynamic Ebbinghaus effect. This is what happens in it…

best illusion animation

There’s an arrangement of circles, exactly like the Ebbinghaus illusion, but there’s just one of the sets from the static illusion discussed above. While this arrangement of circles move, the central circle remains of the same size and the surrounding circles change in size.

Now, if you look into the central circle, you’ll see that it changes size too. In reality, it doesn’t. This effect is weaker when you look directly into the central circle. To make it more pronounced, you can shift your focus to the side and look at it through your peripheral vision. It’s totally mesmerizing. No wonder it won.

It works even when you  know about it.

Eyes of the Mantis Shrimp – Colours and Hexnocular Vision

By Anupum Pant

Of course there’s a lot of other things to talk about the Mantis Shrimp. But today, I’m going to only talk about its eyes.

Colours

The eyes of a Mantis Shrimp are one of the most advanced eyes on the planet. To realize how extraordinary their colour vision is, you need to have some perspective on what we are talking about.

Colour is just a trick of our mind. What we see is really out there, there’s no way to know for sure if it is the reality. Or, there’s no way for us to explain what we really see.

For instance, imagine how we see the world, say particularly, the colour red and all its derived colours. Now, what you see is very different from how a colour blind person or a dog sees it. Dogs and about 10% of men who are colour blind can’t see colours like we do. That is because, instead of 3 cones (red, blue and green sensitive ones), they just have two. If you and a dog would point their eyes towards the same rainbow, both of you would see a very different image (if you are not colour blind).

A dog probably would see a rainbow which would start with a blue colour and then there’d be green in it for a dog. Nothing else. That is because it has no red sensitive cones. A single difference in the number of types cones can make such a huge difference in the colour vision. Addition of the single red sensitive cone enables us to see a whole set of new colours.

Some women (estimated to be about 2-3%of the world’s population!) have a super-human ability that makes them able to see a whole set of new colours. Like we see a million different colours, these women can probably see 100 Million different colours. It’s hard to imagine what they really see. Probably that is why they say men are so bad at colours.

Similarly, consider a butterfly. They have 5-6 different kinds of cone receptors. So, when they look at a rainbow, they probably see a range of colours between the blues and the greens and the greens and the yellows. Of course, it can also see an ultraviolet beyond the violet. Incredible enough.

mantis colour range

The Mantis Shrimp, an animal of the size of your finger, has one of the most amazing colour visions. It has 16 different types of cones. You can’t even start to imagine how the world looks to them. And suppose they try and see a rainbow, they’d see a really rich set of colours. No other animals we know have even a visual system that is half as advanced.  There’s no reason they must have this ability.16 is just too many cones!

Needless to say, these technological marvels can see ultra-violet light, infra-red light, and some can even see polarised light.

Hexnocular vision

tumblr_ljijxhMzSC1qfcmjd

Now, we see with our two eyes and call it a binocular vision. We have 2 eyes and 1 focal point each. So, to see in 3-D, we need both out eyes.

Mantis Shrimp, however, has 2 eyes with 3 focal points each. Each of its eye is divided into 3 sections and can see 3 different images, using the 3 different sections. It doesn’t need 2 eyes to see in 3-D. One is enough. Besides that, it is able to judge depth much better than we are able to do it. Think of an image stitched out of 6 different eyes.

Seeing With Your Tongue and Listening to Colour

By Anupum Pant

I’m always fascinated when I see one sense organ do the work of some other sense organ. Like breathing from your eyes ( not really) or seeing with your ears (really) listening to colours etc…

not available in your country

Solving The Technical Problem (Not available in your country): Today, I stumbled upon a video whose title was “Blind Learn To See With Tongue“. It was uploaded on YouTube by CBS – an American TV network. The sad part is that they had tweaked the videos settings which did not allow me to watch it. It wasn’t available in India.

Whenever someone says I can’t do something, I’m almost always prepared for it. This time, I had this extension installed on Chrome called ZenMate. It’s perfectly legal (available on chrome store) and works very smoothly. It allows you to surf the internet with total control. With it installed, you can totally forget about your physical location and fool the websites which place a location restriction for access, like Spotify and Youtube’s “not available in your location” videos. I haven’t tried other things but it should allow Indians to access stuff from websites like Hulu, Pandora and Netflix. (Even if it may seem out of place, I wasn’t paid by ZenMate to write this. I really recommend it.)

The Customary David Eagleman talk

Now, whenever I come across something that has to do with seeing things with an organ that is not really meant for seeing, I remember this very-old TED talk by David Eagleman. And I like to attach it to my blog because I can’t really explain this amazing ability of the Human brain as well as he does. He basically segues his talk to discuss how brain can learn to interpret various kinds of signals to produce an image. So, here it goes. Watch it and read on…

Since it is clear that seeing is the ability of the brain, not eyes, we can comfortably move on to see how you could even see with your tongue – Tasting the light.

Seeing with the Tongue

A device called BrainPort can help you do that. The contraption consists of a camera that sits on your forehead and sends information to a small computer. The information is processed, converted into electrical pulses, and then sent to an array of electrodes touching your tongue. The brain processes these signals and converts them into an image.

At first, of course the brain doesn’t know the trick to process visual signal from the tongue, but it learns. Gradually the device becomes a part of your body and you start seeing with your tongue! Just like Neil Harbisson can listen to colour. In fact, he can see more colours than our eyes can see because the technology allows him. He can see infrared and ultraviolet too!

Listening to Colour – TED talk


Hit like if you learnt something from this article.

Can Your Eyes Breathe?

By Anupum Pant

Wait! Who says eyes breathe

The transparent front part of the eye that covers the iris and pupil is called Cornea. Cornea contributes a lot to the focusing power of the eye. That means, light has to pass through it without obstruction. To do that it has to remain completely clear. Consequently, to remain transparent, it can’t have any impurities nor can it have any blood vessels – that would have made it less transparent.

Every organ needs oxygen to run the cell processes with the energy that comes by oxidizing nutrients contained in the cells. To receive oxygen, they need to have access to blood. Since Cornea does not have any blood vessels, it cannot receive oxygen from blood. So what does it do to stay alive?

It absorbs oxygen directly from the air through diffusion. Oxygen gets dissolved in the tears and then diffuses across the cornea. However, the amount of diffused oxygen is so less that it is just enough for only the cornea cells. This can’t be supplied to other parts of the body. And this is exactly the reason you would get yourself killed if you plug your nose and your mouth, expecting your eye would keep you alive by breathing in oxygen.

In a sense, you could call it breathing. But it isn’t exactly ‘breathing’. Breathing, according to the medical definition means:

The process of respiration, during which air is inhaled into the lungs through the mouth or nose due to muscle contraction and then exhaled due to muscle relaxation.

Clearly, as the oxygen received by cornea doesn’t come from the mouth or nose and never goes through the lungs, it cannot be called breathing.

Other ways?

Let us consider the second possibility. At the inner corner of the eye there is a very thin tube that connects to the nose. Through this tube (A.K.A the Punctum), your eye is able to drain off excess tears. Punctum is the reason, you have to blow your nose when you cry. Punctum is also the tube that enables this guy to squirt milk out of his eyes (By sucking it through his nose first) [Video – Pretty disgusting to watch]

Also, you can blow an extremely tiny volume air out of these corner eye holes. But, you can’t breathe in through them.

Conclusion

Although the cornea of your eye has the ability to absorb oxygen directly from the air, you cannot technically call it breathing.

Related articles

 

Enhanced by Zemanta

A Bat’s Inverted Sleep Position

by Anupum Pant

I have written about sloths in the past. In that post, we appreciated the way their bodies are engineered to stay inverted for most of their lives. It turns out, a bat’s body is designed (rather evolved) in a similar way, which enables them to relax and sleep upside down. In this post, I would like to discuss – why did they evolve this way and how do they do it?

If you are interested to know more about bats, you will definitely like this post from the archives. [Bats can See]

How can bats manage to sleep like this?

Humans sleep in a horizontal position, cows sleep with their eyes open, horses sleep in a standing position, and of course, bats sleep in an inverted position. What makes an animal sleep in the position they do, is basically their anatomy – the way their bodies are designed. While sleeping upside down might seem as an anomalous behavior to us, it is a normal position for the bat’s body. Like we don’t exert energy when we are lying down, bats’ bodies don’t consume extra energy for hanging down like that.

Firstly, a bat’s claw is like a hook. A better way to understand why this helps is, to look at a converse behavior – the way a human hand works. We use up energy to contract tens of muscles and make up a hook with our fingers; this is not a normal state of our hand. Also, our relaxed hands are open where we don’t exert any energy and we sleep with our hands in that position. A bat’s claws are designed in a completely inverse fashion; they are hooked in the normal position. They don’t take up energy to make them into hooks, they are like that. And they sleep like that – which enables them to hang without using energy.
So, unlike our hands, a bats’ closed fist is their relaxed position. They have to contract tendons and use energy to open them up. This anomalous talon design allows them to hang in a relaxed position.

Bat's Talons - Normal position

Secondly, unlike every other bird, a bat can’t take off from an upright position, or from the ground. They have to be inverted to start flying. This is because they have relatively weaker wings which can’t make them fly from a stationary position. Think of an X-51A Waverider, which has to be carried on a B-52 plane and dropped down to start a flight. They drop down for a very small amount of time and beat their wings vigorously to start a flight. Since, they have to wake up inverted to go flying and catch a meal, they go to sleep like that.

Why did they evolve this way?

They’ve evolved this way to simply stay away from the predators:

  1. By hiding up in a place where not many predators would look – under a bridge, roof of the cave and dark tree canopies. Also, at places like these, they don’t have to compete with other birds for a place.
  2. And by escaping quickly in case of an attack by attaining instant flight [see above].