So, good evening everyone and thank you for joining me in this webinar. I think I've been with Alphabet for about 4 years now, but this is my first pre-recorded webinar. Another indication of the changes that the corona pandemic brought to our lives and I take the opportunity to wish you and yours best of health.

Tonight we'll be talking about vision in animals. I'm not going to turn you into vision scientists, but I'm sure that as veterinarians, you get asked often either by your clients or by friends, about vision in animals. You get asked questions like, who has sharper vision, my dog or my cat?

The answer to this 1 may surprise you. Do dogs really see black and white as you can see up here on the screen, and all sorts of fun facts about animal vision, that you can use to answer these questions, or if you ever find yourself in a very boring dinner or a blind date, I shouldn't say blind date in thalmology lecture actually. So if you ever find yourself in a very boring dinner or an exciting date and you I want to impress people around you with all sorts of fun facts about A vision.

This talk is for you. As always, I have no financial relationship to disclose, but as always, I do use the opportunity as, to disclose as shameless advertising that many of the pictures I'm showing are taken from a textbook I've co-authored together with Paul Miller from Wisconsin and David Max from UC Davis. Right, so we'll cover a variety of subjects and the first one is colour vision, going back to the type of lecture to dogs see in black and white, and I think it's a common misconception of the public, that Dogs and cats see in black and white.

Actually, they are wrong. Dogs, cats, and other species do have colour vision. It is more limited than our colour vision, but definitely they do not see in black and white as you will see in a few minutes.

But before we talk about colour vision in our patients, we have to remember and understand what is the physiological basis for colour vision. So I'm sure all of you remember from your school days that vision is facilitated in the retina due to two types of photoreceptors, the rods and the cones will Mention these two types of photoreceptors quite often in today's talk, and the outermost segment of these rods and cones contain a special molecule, which you can see here, called a photopigment. And this photopigment actually has two components to it, an obscene and a retinal.

The obscene is the part of the molecule that actually absorbs the incoming light particles. It absorbs the photons and then the retinal takes the energy of these light particles and transduces it into A neurological signal which is then processed in the retina and eventually transmitted to the brain. But vision really starts with the absorption of photons by the obscene molecule in the outer segments of the rods and the cones.

Now, rods have just one type of obscene, absorbing light at about 500 nanometers, which is the green shade of the spectrum. Rods are pretty well conserved in evolution and this number 500 nanometers is true for virtually any species on Earth. In cones, on the other hand, we do have some evolutionary divergence and we have different classes of cones which are characterised by the type of obscene that they have.

In us humans, we have 3 types of obscenes in our cones. So we really have 3 cone populations, 1 population with an obscene absorbing red light at about 560 nanometers. Another a second.

Absorbing green light at 500 nanometers and the third one absorbing blue light at about 450 nanometers and that's why we always say that red, green, and blue are the primary colours of our vision. However, the numbers that you see here are not all or nothing absorption numbers for our obscenes. Rather, each obscene has a bell-shaped absorbent spectrum that you can see here.

The peak sensitivity is the numbers that you saw in the Previous slide. So for example, as I said, rods have a peak sensitivity at about 500 nanometers, but you can see here that they can also absorb light from about 420 nanometers all the way up to about 575 nanometers at decreasing efficiency. Now, the important concept to understand in the fundamental basis of colour vision is that the richness of our colour vision and the number of shades that we can see is determined by the number of obscenes that we have in our retina, especially in our cones.

And the degree of overlap of their absorption curves. And this is demonstrated here in this next slide where we are still with the three obscene populations of the three cone classes I've taken out the rods to simplify things. So here are the three cone populations.

Now, let's take a situation whereby Red orange light, excuse me. Enters the eye. Orange light with a wavelength of about 575 nanometers stimulates 99% of the red cones.

At the same time, it stimulates about 42% of the green cones. It doesn't stimulate any of the blue cones cause blue light ends here at about 540 nanometers. So the so-called formula to see orange is 99 red, 42 green, and zero blue.

Mix them, mix red green in these proportions and you'll get orange light. Or orange colour. Yellow light?

Well, yellow, as you may remember, back when you were drawing in kindergarten, you mix all sorts of colours. Yellow is really a mixture of red and green, and as you can see here, yellow is really 80. 3% red, 83% green, and again 0% blue.

So 83 red, 83 green, 0 blue gives you yellow and green, why green is not green at all. Green is a mixture of 67% green, but at the same time we also have 36% blue and 31% red, and that would be the formula for seeing green. Now, the amazing thing about our retina is that it can detect changes as small as 1 nanometer in wavelength.

Let's go back to this example of the yellow light, which as I said, is 83% green, 83% red. If this yellow moves just 1 nanometer, to the right, so it'd be here, it would now stimulate 84% red and 82% green, so the formula has changed slightly. As Human observers, we would describe it subjectively as this yellow light is now slightly, slightly more red, OK, and slightly, slightly more red, in fact, it's just 1 nanometer more red because we have changed the proportions of the red and the green cones that are stimulated by this light.

So as humans really have 3 elementary colours and the richness of our colour vision that you are seeing here is enabled by the overlap of the absorption spectrum of these three primary colours, which is why I would say that as humans have one cone population, then we would have monochromatic vision, one coloured vision, and then we would see just shades of that single colour, in this case, shades of green and yellow. If we would have 2 cone populations, why we would have the chromatic vision, 2 cone vision, in this case, as you can see, an eye with a blue cone and a green cone cone population. So this would be the colour spectrum of this the chromatic eye as opposed once again to the 3 chromatic vision that we primates have.

Another important concept in colour vision before we start describing what actually happens in our animals is so-called colour blindness. We often say that the person is colour blind. True colour blindness is actually a very rare condition.

I have here a picture of a sheep undergoing ERG recording cause I actually found a herd of date blind sheep or colour blind sheep, but in humans it's a very rare disease with a prevalence of about 1 to 30,000 people. Usually when we say that the person is colorblind, they are in fact dechromas. They have two cone populations, so they're not truly colorblind, they're just missing one of the three cone populations that a normal person would have.

Usually a colour blind person would miss and would be missing either the green cone population, in which case they'll not be able to see the colours. 74. I hope everyone back home can see the 74 here or they'll be missing the red cone population, in which case they can't see the 42 here.

So colour blindness is not true. Colour blindness, you have limited colour vision, de chromatic colour vision as demonstrated here, and as it says here, it is a sex-linked disease usually affecting males and actually a fair number of males. So now that we understand what colour, how colour vision is enabled, we can talk about what actually happens in the animal kingdom.

So there are in fact some animal species that are truly colorblind that they have no cones. So they see only with rods, and if you have only rods, this is what your world would look like. You have no colour vision, you have very poor visual acuity, but it's really a very rare condition, .

It is to be found mainly in deepwater fish, and that's because these fish live in deep water, very little light penetrates to such depth and therefore they have no need for their cones. They have only rod vision, and it's always in the animal kingdom, there is one exception, so there is one lizard species that has no cones, but true colour blindness is really a very rare condition. Some animals are monochromats.

Again, reminding you, monochromats, they have one cone population. We're talking about some nocturnal species including raccoons and hamsters and interestingly enough, all of the aquatic mammals, be they whales or dolphins or seal. Lions, etc.

Etc. These animals usually have only red cones, and that's because red light penetrates water better than blue light. And at great depths, you can, you have only red light and infrared light, and that's why evolution helps.

These species developed red cones, so they are monochromats only with a red cone population, which means actually, if you take a moment to think about it, that these animals live in a beautiful blue ocean, but they cannot appreciate its beauty because they are unable to see the blue, they only see red. Some species are tree chromatic species, as I said, as primates have tree chromatic vision with blue, green, and red cone populations also to be found in some fish and reptile species. Some species are tetrachromatic.

Why stop at 3 cone populations if you can have 4 cone populations, this is to be found in many fish and avian species and you only have to look at. Richness of colour in these animals, to understand that colour vision plays a very important role both in their feeding behaviour and in their sexual behaviour, so they have very rich colour vision. The extra cone population, the 4th cone population, usually has an absorbent spectrum in the ultraviolet part of the spectrum.

And if we see the richness of the colour vision in the world around us enabled by just 3 cone populations, we can only imagine how rich their colour vision is when they have 4 cone populations. Here are a couple of Pictures that try to demonstrate this, here on the left is what we would see in the night sky. You see a dim outline here, but you don't really see too well.

Light it up with ultraviolet light and this is what birds would see. Why would birds develop ultraviolet, colour vision? Well, here are a couple of pictures that offer a partial explanation, at least for birds of prey.

These are slides given to me by a colleague here in Israel who's studying avian vision. These are pictures of a field taken from a drone, so a few 100 metres in the air. On the right is a normal picture taken with a normal camera lens, and you can see that there is some.

Fuzzy abnormality in the field here. Put an ultraviolet philtre on the camera and this abnormality really lights up compared this area to this area. So this is how we would see this field.

This is how a bird would see this field. Why the difference? Well, it turns out that in rodents, the urine contains an ultraviolet pigment.

So, in fact, this picture here on the left tells you that there is lots of urine here, which urine of rodents. There are lots of rodent, holes here and this is their public toilet, if you will, and if you are a bird of prey like A falcon, for example, and you're flying over the field, all of a sudden you see this, you know, aha, dinner is here, this is the toilet, this is where the rodent lives. If I hang around long enough, this is where I'll catch my lunch or thinner.

So this is why birds of prey would develop ultraviolet vision, an amazing example of how there is nothing left to chance in evolution or in creation depending on your personal belief. But these are some of the more exotic species. If we move on to our pet population and our patient population, most mammals are dechromatic.

Cats are one of the few non-primate mammalian species that actually have 3 cone populations, but No one has been able to demonstrate tree chromatic behaviour in cats. I'd say it's another typical example of cats. You have the capability by why waste the energy on using it if you can sleep all day, so they have it, but they don't really use it.

Most other mammals have the chromatic vision. Dogs lack the green home population, they lack the green obscene. So here are a couple of pictures showing how The dog would see the world here below versus a human.

This is the colour absorption spectrum of both species, and here are two actual pictures. Here is how we would see the world. Here is how a dog would see the world.

So obviously, if you take out the green, you're also affecting the perception of the blue and the pink as you can see here if you compare both pictures, so. Dogs definitely have a more limited colour vision than we have, but it's definitely not black and white as the general public would think. Another couple of pictures showing canine vision at the bottom and human vision at the top.

And again, take out the green and it affects the perception of blue and yellow, so poor colour vision, but definitely not green, not black and white. If dogs have no green photopigment, it brings up an interesting question, how do guide dogs function? After all, they cannot detect the change in traffic light when it turns green, and actually guide dogs are trained to work by auditory and other visual cues.

They do not see the change in traffic light. So that's dogs, horses and cattle are also dechromatic, but unlike dogs which lack the green photopigment, they lack the red photopigment, the red obscene population. So once again, this is how we would see the world.

This is how horses and cattle would see the world. Sometimes it has very little effect on your colour perception if you take out red. In other situations, the effect may be more dramatic.

So here is on the left how we would see the world, on the right, how a horse would see the world, no red perception. As I said, it holds true for horses. It also holds true for cattle and for cows, which means that really this red colour was actually, is actually the worst possible colour that the Spanish bullfighters could have chosen for their drape cause if there is one colour that the bull cannot see, it is red.

I gave this talk once in Spain and the audience looks back at me, they shrug their shoulders and say, tradition. And who am I to argue with Spanish tradition? Right, so moving on from colour vision, I'm sure we know, you know, that many of our patients, especially cats, seem much better in the dark than we do, and the question, how is this facilitated?

And this is facilitated thanks to 3 mechanisms. First is the amount of light entering the eye at nighttime. Second is the presence of the tepitium, and 3rd, retinal anatomy and physiology and we'll review this one by one.

First, as I said, is the amount of light entering the eye. In order to enter the eye, the light must first pass through the cornea because that's the only transparent part of the connective tissue that holds the eye together. Light cannot enter the eye through the sclera.

It must pass through the cornea and you can see that the surface. The area of the cornea in the cat is approximately twice as much as in humans. Next, the light must pass through the pupil and the fully dilated feline pupil is about 2.5 times as large as that of a human.

So a larger cornea, a larger pupil, more light can Enter a feline eye and in fact, it's been calculated that at night when the pupil is fully dilated, 5 and the amount of light reaching the feline retina is 5 times higher than the amount of light reaching our retina. So more light enters the reaches the feline retina at night. Then there is the tittum, that beautiful reflective layer at the back of the canine eye here on the top and the feline eye at the bottom.

This is a picture given to me by a colleague in Brazil, a pond full of cayman alligators, and here is their typical reflection. You can count the number of pinpoints divided by 2 and that's the number of alligators in this pond. So why doesitium, which is located here behind the retina, why does it allow greater sensitivity at night?

This is shown here in this cartoon which shows you three photons of light entering the eye. On the left, you have a non-typetal part of the eye. The first photon got absorbed here by this photoreceptor.

The second photon got absorbed here by this photoreceptor. The 3rd photon passed through the retina and wasn't absorbed. It got all, it went all the way through and it's now scattering in the eye and since it wasn't absorbed, it's Really wasn't seen.

Now, daytime, there are plenty of photons entering the eye. So if this photon is not seen, no big loss. There is plenty of other photons that enable daytime vision.

However, at night, when there are only a few photons entering our eye, this photon, which wasn't absorbed, represents a huge loss, huge waste. So God or evolution again depending on your belief decided to put a epitium behind the retina. And now this 3rd photon which previously was not absorbed, strikes the epitium which really acts like a mirror and it bounces back onto the retina.

And we've just doubled the probability that it will be absorbed because it gets a second passage and indeed now it's absorbed by this photoreceptor here. So really we increase nighttime sensitivity by doubling the probability that every photon entering the eye will actually be absorbed. Note that there is a price for everything, and the tepitium does increase nighttime sensitivity, but the price is reduced visual acuity which we'll be talking about later.

This photon was Initially supposed to be absorbed here, now it's absorbed here, which means that the presence of the mirror actually scattered the light and if the lights scattered, it means that your visual resolution is decreased. So we said that nighttime vision, improved nighttime vision is facilitated by the increased surface area of the cornea and the pupil by the presence of the tepitum and the third mechanism is retum physiology. And when I say retum physiology, I'm really talking about dark adaptation, the increased sensitivity of photo.

Receptors in the dark, something that we all experience when we enter a dark room or when we are taking a walk in nature at nighttime and there is no light around us. As you spend more time in the darkness, in the dark room, you see that your retinal sensitivity increases due to regeneration of. Photo pigment, you, after a few minutes in the dark, you are able to detect more and more objects.

It is also something that can be recorded electrophysiologically with an ERG. Here is a series of recordings we did in a cat. The cat is in the dark and every 4 minutes we are stimulating the eye with a very dim flash of light.

So we are only recording rod responses and here are the responses after 4 minutes, 8, 1216, and 20 minutes, and you can see that as a cat is spending more time in the darkness, its rein sensitivity is increasing just like your retinal sensitivity is increasing as you spend more time in a dark room. So this is really enabled by the rods because these are the photoreceptors that provide us with nighttime vision. And as you can see here, the feline retina has about 3 times more rods than we do, 160,000.

000 per square millimetre versus 460,000 and if they have more rods, why it gives them greater dark adaptation and gives them greater sensitivity to dim light. How much greater? Well, as humans reach fully dark depth after 30 minutes, which means that after you've spent 30 minutes in the dark, you will not See any better at 30 minutes you've reached your maximum sensitivity, at which time you can detect light as dim as 10-6 candela per square metre.

Cats can continue dark adapting for another 30 to 50 minutes, allowing them to see light as dim as 10-7 candela per square metre and that's a difference of 1 log unit at times 10 difference. Add to that the amount of light that reaches the field. Writing up because of the large cornea pupil and the presence of the tepitum and you can see that they can really see much better at night because of these three mechanisms.

Here are a couple of pictures demonstrating how we would see this spider at night and how a cat would see the same spider at nighttime. OK, on to the next question, people frequently ask me why doesn't my dog watch television? And I know that often there is the odd dog that seems to be interested by what's happening on the television screen, but I think we can agree that dogs and cats largely ignore what's happening on the TV screen, and the reason for that is while we are seeing a picture like that, a dog and cat would see a picture that looks like this, a flickering picture versus the steady image that we are seeing.

And the physiological reason for this difference between humans on the top and cats at the bottom is something that's called a flicker fusion frequency, which is the interval needed by a photoreceptor to recover between responses to two flashes of light. Quite a mouthful, so I'll say it again more slowly. You stimulate a photoreceptor with a flash of light, it responds with an electrical signal.

You've seen an example of the electric signal before. . Stimulate it with another flash of light, it will respond with another signal.

However, it needs a minimum amount of time to recover from the first flash before it can respond to the second flash and That's the flicker fusion frequency, the interval, the amount of time it needs to recover, and this is shown here in the series of traces. Here is the response of the retina to a single flash of light and you see a single robust signal. Here is a response of the retina, the same retina to 4 flashes of light, and you see a distinct response to each and every flash of light.

However, you see that. These signals are obviously much lower than this one and that's because the retina did not have time to fully recover from. This flash before this flash was presented and therefore the amplitude is much lower than this one.

And if you increase the frequency of flashes, well, you still get a distinct response to each and every flash of light, but the amplitude is smaller because now the interval between flash. It's shorter. So again, the retina doesn't have enough time to recover from responding to this flash before this flash is being presented.

And the faster the flashes are being presented, the higher the frequency, the less time there is for the red not to recover and less and less time. Until you get to a point where the flashes are being presented so quickly and so rapidly that the retina doesn't have time to recover and it sees continuous light rather than the distinct and individual flashes, and that's what's shown in these two traces. I apologise for their quality, but the vertical bars that you're seeing.

Here are the light flashes. You can see that in this animal on the left, each and every flash generates a signal in response, but the animal on the right is unable to detect these rapid flashes. They are being presented so fast, the retina doesn't have time to recover from this flash before.

Presented by this flash, it's just seeing a continuous slide, which is why you are seeing this one signal here. We say that the responses of this retina on the right have fused. They have fused from these multiple responses into a single response because of the high frequency of these flashes.

So when does this fusion occur? Well, in rods, it occurs at about 10 Hz, and as I said earlier, these rods are pretty well conserved in evolution, so this figure is true universally for most species. What does it mean?

It means that if you're in a dark room and There is a dim flash going on and off, on and off. You can detect individual flashes up to a frequency of 10 Hz. If they are being presented at a higher frequency, your rods will not have enough time to recover and you will see a dim continuous light in the dark rather than the visual flashes.

Cones have some evolutionary divergence. In humans we are able to detect flickers of up to 30 to 45 hertz, which means that anything higher than 45 Hz at daytime will not be able to see the individual flickers, we will see a continuous image. Dogs Have a higher flicker fusion frequency all the way up to 70 or 80 Hz, they can detect the individual flickers.

This means, for example, that dogs can detect flickering fluorescent light. You may recall that fluorescent light is not a continuous light. It's a flickering light, that flickers at 50 or 60 hertz depending on whether you're in North America or in other parts of the world.

But it's really not a continuous slide. We do see it flickering sometimes when we have a faulty bulb, but in other cases we do see it as a continuous slide and that's because the 50 or 60 Hz flickering is higher than our Flickr fusion frequency. However, put a dog into a room that's lit up by fluorescent light, and the dog can detect the individual flickers of the fluorescent light because it can see 70 or 80 Hz higher than the 50 or 60 hertz at which your fluorescent light is flickering, which means actually that if you're designing The light in your exam room, make it an animal friendly environment by doing away with fluorescent light and using either incandescent lamps or today environmentally friendly LED lamps which do not flicker but not the old fluorescent tubes.

So the fluorescent light in the room is flickering and your TV image is also flickering. TV pictures are not continuous. They are flickering on and off again depending on where you are, but usually the frequency of 48 Hz or high.

We see it as a continuous picture. Once again, dogs, cats, we see the flickering picture. Nowadays, of course, we have the high definition televisions sometimes presented in image, with the frequency of 100 Hz or so.

So if you want to watch television in the company of your Dog, you should invest in one of these more modern high frequency televisions and then you and your dog can enjoy TV together. But if you haven't, then your dog will be seeing this flickering image which we are used to seeing from this old time television. Our next topic, we have two topics left, visual acuity, but before that, I want to talk about visual fields.

What determines if you would see this image or just this image? What is the size of your visual field, for those of you? Wondering this is a picture from Salzburg, Austria and the size of the visual field and whether you're seeing this or this is really determined by the anatomy of your skull and where is the orbit of the eye located.

In predators, be it cats on the left or dogs on the right, we have frontal. Eyes. And because we are not we, because the animals have frontal eyes, it means that actually they have a large binocular field.

A field covered by both eyes. They have a small monocular field and they have a large blind area. Looking at the numbers, you can see that this here is the visual field of the right eye of this cat.

This here is a visual field of the left eye of this cat. This is the area of overlap for both eyes. This is the area of binocular vision about 140 degrees in size.

30 degrees of monocular vision on each side and a large blind area in the back and figures more or less the same for the dog, slight differences here. Why is this important? Why do predators have frontal facing eyes?

That's because we need binocular vision for depth vision. Our depth perception is enabled by actually many visual cues, but most importantly by binocular vision. And if you're a predator, depth vision is very important for you because you have to pounce on the prey in order to catch it.

OK, back to this cat. You're chasing a mouse, you're leaping on the mouse, you really get just one chance. You want to make sure you pounce on the mouse, not next to the mouse, or if you're lying, you want to pounce on the zebra and not next to the zebra.

So you need good depth vision, which is why you want by not good or extensive binocular vision. The price to be paid, as I said, there is a price for everything and that's a large blind area, but you don't care about a large blind area because you're a predator, you're at the top of the food chain, you don't care about what's sneaking up behind you. So what do visual fields look like in our patients?

Here is a picture I showed before of when I spoke about canine colour vision, but now let's look at the visual fields of the dog on the left and the human on the right. You can see that the size of the visual. The field that we experience in the dog experiences is virtually the same, but we have a larger binocular visual area as shown up here and down here as opposed to the dog, but overall we are seeing more or less the same.

However, if you are a herbivore, in this case, a Horse on the left, then you develop a different strategy. If you're a horse or a gazelle or some other type of herbivore, you really don't need that much binocular vision to find your food, to eat grass in the meadow, so you need very little binocular vision. But you are a herbivore, you're a prey species.

You don't want anyone sneaking up on you from behind, so you want a small blind area behind you. So that's why herbivores like dogs, cows, gazelles, whatever, have laterally placed eyes. So this is the visual.

Field of the right eye of the horse. This is the visual field of the left eye of the horse. As you can see, very little binocular vision, but really almost 360 degree binocular monocular vision and a very small blind area which explains why a horse can kick with its hind legs, when someone is approaching from.

Behind someone who would not be seen by this cat. Note another implication of these pictures is that if you nucleate a cat, then actually the cat would lose just 30 degrees of its entire visual field. It would still have all of this vision.

If you nucleate a horse, the impact would be much greater. A horse, if you nucleate. One eye, it loses about 145 degrees of visual field, which is why a one-eyed or horse can very easily panic.

So I showed you earlier what the visual field of a human looks like, what does a visual field of a horse looks like? Well, here are two pictures of a horse with a rider. On the left, the photographer is standing behind the horse and the rider, looking at the city Perth, Australia.

This is what the rider would see. This is what you and I would see. Now the photographer runs around and is facing the rider and the horse, and this is what we would see if the rider would see if she turned around.

So here are the stairs and the fence, etc. And here is what the horse would see. Here is the city.

Here is the fence and the stairs that you're seeing here, really 360 degree panoramic vision for the horse. And talking about visual fields, please remember that visual fields are not only horizontal, they are also vertical as shown in these three pictures of a meadow here. So this is what you and I would see walking through this field of grass, a tall dog.

Would see this, the grass would be at the level of its nose and a small dog like a Pekinese or a chihuahua would think that it's walking in the forest because it has a vertical visual field affecting its perception of the world. Right, our final topic that I want to cover in my remaining time is visual acuity, how sharp is our patients, vision. If I was in the room, I'd say in the lecture room, I'd say that I'm going to discuss three species, dogs, cats, and horses and I'd take a vote, with the audience and most people would say that, yeah, cats have the sharpest vision.

And that's another myth that I am about to bust. But before giving you actual numbers, let's talk about how the visual acuity or high visual resolution is enabled. It's Really, as we know, our sharp high acuity, high resolution vision is facilitated by cones.

Rods give us the nighttime vision, cones give us the high resolution daytime vision. Why is this? Well, here on the left you have the visual pathway of rods and you can see that it's what we call a converging pathway.

The input from a large number of rods converges on a smaller number of bipolar cells, which is a second. Converging on a smaller number of amacrine cells eventually converging on a single ganglion cell, the axion of which goes to the optic nerve head and constitutes a fibre of the optic nerve. The implication of what you're seeing here is that when a photon strikes the receptive field of this ganglion cell.

We don't precisely know where it was. The photon may have struck this rod or this rod or this one. It doesn't matter.

The end result would be the same. This ganglion cell will fire. So when this rod associated ganglion cell fires, we really have no idea where the photon was.

It could have been anywhere in this receptive field. Which is why we say that rods have very poor visual acuity, which is why, as you know, you can't read the book in the dark. You don't have high resolution vision, your rods will not be able to detect individual letters in the dark.

This, as opposed to the cone pathway, which is a much simpler 1 to 1 to 1 pathway, and this provides us with very high visual resolution because if this ganglion cell fires, we know that the photon was here. If the photon were here, then this ganglion cell would fire and if the photon was here, this ganglion cell would fire. So that's why we say that cones and their associated ganglion cells give us high resolution vision thanks to this 1 to 1 to 1 pathway.

Now we humans are very fortunate that in our retina we have a region called the fovea which is populated just by cones. This retina area gives us our high resolution vision, so when I want to see something with highest resolution like reading a book or driving, I turn my oba onto this area. Cats and other species don't, non-primate species don't have fovea.

They do have an area where most of the cones are to be found. It's called the area Centralis, but even in this area Centralis, which is the area of highest concentration of cones, you can see that cones are a minority. Most of the cells are.

This was wonderful 20 minutes ago when we were discussing nighttime vision. They had 3 times as many roads as we do, giving them great nighttime sensitivity, but you got to pack those roads somewhere. Here they are in the areas and trolleys.

The result is that they have fewer cones. How much fewer? Well, we have 200,000 cones.

In our fovea and the 1 to 1 ratio that I described earlier, cats, they have 1/8 the cone concentration that we have and even then, the area centralis doesn't have the 1 to 1 ratio, they have some convergence on the ganglion cell. So really, the number of cones and even more than that, the number of ganglion cells is what dictates their, the visual acuity cause the ganglion cells, as I said, send fibres. Axons to the optic nervehead, they relate the signal to the brain, so it's really the way you should think about this cone associated ganglion cells like pixels on your camera, OK.

If you have just one optic nerve fibre, then Your cortex will know that if the light is on or off, no localization whatsoever because you have just one fibre. If you have 4 fibres, then you would get some visual resolution because you'd know if the photon was top left, top right, bottom left, bottom right, you'll get some spatial recognition. 16 pixels, 16 fibres, better localization, 100 fibres, better localization.

The more fibres in your optic nerve, the more ganglion cells that you have, ah, the better image that is formed on your brain, the more pixels that form their image. So looking at the numbers, we have 1.2 milli ganglion cells in our retina.

Horses, about half a million, dogs, 167,000, and cats for all those who think always that cats have the best resolution. Actually, they have the poorest cause they have just 1/10 the number of pixels that we do, less than the dog, less than the horse. This is shown.

Number of ganglion cells in a cat versus the number of ganglion cells in a human. And don't forget that darn to petum. Again, it was so wonderful at daytime, at nighttime when we wanted to increase nighttime sensitivity, but it scatters the light and therefore further reducing The visual acuity.

So what is the visual acuity in our patients? Well, as humans have 66 vision or 20266 if you were in the metric country, 2020 if you use talking feet and inches, I'll explain what 6/6 is in a minute, but it's basically the gold standard. Horses have 6/10.

What does 6/10 mean? Well, 6/10 means that a horse has to be 10, sorry, 6 metres from a visual target or 20 ft from a target. A human can stand 10 metres away or 33 ft away and it would see the visual target with the same resolution, even though it is 40% further away.

Dogs have 6/22 vision. What does that mean? A dog has to be 6 metres from a visual target.

We, as humans can stand 22 metres away, 3.5 times further away, 20 ft versus 75 ft, and we would still see the image with the same resolution even though we are 3.5 times further away.

And once again, for all those who think that cats have the highest resolution vision, 6/45. We can be over 7 times as far away from the target, and we would see it just as well. Looking at the Snellen chart, that you're all familiar with, I actually measure distances, with the ruler on the computer screen.

We can sit here to read the chart, to read the same line. The dog would have to be here, cat has to be here, and horses doing not too bad here. So actually, at 6 metres, we are seeing this line, cats, ouch.

Dogs, horses, not too bad. So the horse has 60% of the visual acuity of humans. It's doing 1.5 times better than dogs, 3 times better than cats.

Again, one picture is worth 1000 words. So this is how we would see this forest in Germany, how a horse would see the same forest from the hilltop, dog, and look at the poor cat. Just so you understand what 6/45 means, the international definition of blindness is 6/60.

Being blind doesn't necessarily mean that, your retina is completely degenerated. There are many definitions for blindness and one of them is poor visual acuity, 6/60. Your cat is 66.

Over 45, your cat is almost legally blind. Your cat is almost eligible for Social Security benefits. Some species are legally blind.

The rabbit is legally blind, cows 6/105, rats, dogs, if it underwent cataract surgery and we didn't implant an artificial lens, and mice while they are truly, truly blind. And that, and this I love the menace response that we always use to evaluate vision in our patients. Actually, you can see that very little vision is required to generate the famous menace response that we are all using.

At the other end of the scale. We have the raptors. In the raptors, you can see that the fraction is now greater than 1.

Previously, we were talking about fractions smaller than 1. Now they are greater than 1. It means that the kestrel or the falcon or the eagle can be 6 metres from the target.

We have to be 4 or 2.5 or 1.3 metres from the target in order to sit as well as the kestrel or the falcon or the.

So again, going back to measuring distances, we are here. Here's a, here is a horse, and here is the kro falcon and eagle with their better vision. Remember, the cat has 645, the eagle has 6/1.3.

The cat visual resolution is just 3% of the eagles. Terrible, terrible vision for cats. So everyone agrees is terrible.

And then the question comes up in the dinner or the exciting date you're having, can we improve this with glasses? And the answer is no, because all glasses do is really just focused light. If you're farsighted, such as here, you have a plus powered lens in order to bring the image onto the retina.

If you're nearsighted or myopic, such as I am, you put a negative powered glass instead of a lens in front. Of the eye in order to have the eye reach amatropia in order for the image to be focused on the retina. The problem is that most of our patients are amatropic and therefore they do not need glasses.

Glasses will not help them because light is focused on the retina. Their problem is in the retina with the few cones that aren't the petum that degrades their visual resolution. So to conclude, please remember that our patients are not colour blind.

Remember that visual fields are determined by the fitting strategy. Remember that visual acuity is better in horses than in dogs than in cats because of the retinal anatomy and the tepitium, and finally remember to please invest in a good TV. I thank you very much for your attention and if you have any questions about any subject I spoke about, please feel free to mail them to the Alpha office who will forward them to me.

I hope to see you in my next webinar, which I believe is in October. Thank you and good night.