In this video I go into more detail on the retina, describing the two main types of photoreceptors (rods and cones) and how they operate in different levels of light. Next I describe the composition of receptive fields and how retinal ganglion cells communicate differing patterns of light on the receptive fields. Finally I explain lateral inhibition and how it relates to the illusion caused by Mach Bands, which demonstrates that we don’t actually see the “real” levels of light in the world but rather an exaggerated version which emphasizes contrast.

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Video Transcript:

Hi, I’m Michael Corayer and this is Psych Exam Review.

In this video I want to go into more detail on the retina. So the retina is the back surface of the eye where vision actually occurs and this is because the retina has photoreceptors. Photoreceptors are cells that respond to light.

This means that the energy of a lightwave is actually able to stimulate this cell and cause it to fire. We have two main types of photoreceptors in our retinas. We have rods and we have cones and these are named just after their shape. Rods are a little bit more rod-like and cones are little bit tapered on one end so they look a little bit more like cones.

Rods are the more common photoreceptor. You have about 120,000,000 rods in each of your eyes. What rods do is they respond to light or dark and that’s it. They can basically only see in black and white. They can respond to different levels of light and that’s all.

Cones, on the other hand, are more specialized and you have about 6,000,000 cones in each of your eyes and what cones allow you to do is see in color. You can remember that cones and color both start with “c” that can help you remember which photoreceptor can see color. Cones allow us to see fine detail. The way that cones are able to see color is that they can respond to different wavelengths of light.

That’s what I’ll talk about in the next video, we’ll look at the different types of cones. But for now you can see this idea that rods respond to light and cones respond to color of the light if you think about low light situations because the rods are actually a little bit more sensitive than the cones. So in very dim light it’s enough to stimulate rods but not enough to stimulate cones.

You’ve experienced this if you ever wake up in the middle of the night to go to the bathroom and you look around without turning the lights on, you notice that you can see but not very clearly and everything looks grey.

This is because the light is dim so it’s triggering the rods to fire, you’re getting activity from the rods, but it’s not bright enough to stimulate the cones. So you don’t get to see the color. So you look at something on the table a bright red apple that would normally look very vivid in daylight conditions, if you look at it in the dark you can still see it, but it looks grey.

This is because you’re essentially seeing only with your rods, you’re not using your cones. This was also pointed out by Benjamin Franklin when he wrote that “in the dark, all cats are grey” although he meant it in a very different context.

Another way that you can see this difference between rods and cones is if we think about the fovea. So the fovea is the area of the retina that I talked about which has the clearest, sharpest vision. So when you focus on an object, like you’re reading or you want to look at something very intensely, you direct it to your fovea.

The reason for this is that the fovea is entirely made of cones. You have cones in the other parts of your retina but in the fovea it’s all cones. This means you can see very clearly, you can see colors more vividly. But an interesting side effect of this is that you can’t see very very dim light with your fovea. You can experience this if you look at a night sky and you try to find a very dim star, a really dim, small spot of light.

What you’ll probably find, maybe you’ve experienced this already, is when you’re looking at that, if you try to look directly at a very dim star, it seems to get even dimmer, it gets harder to see. But if you look next to it, it actually looks a bit brighter. So you can try that out. What’s happening is, when you look directly at it you’re putting it on the fovea, it’s all cones and the very dim light is not bright enough to stimulate those cones so you might not be able to see it.

But when you direct it to a different part of your retina, now it’s hitting rods and it’s able to stimulate them more, so you’re able to see. So this might explain if you’ve ever seen what seems to be a disappearing star. You see it, then you move your eyes to look at it, then it’s gone. This is the reason why that happens. If you want to observe a very dim star, the secret is to look next to it, don’t look directly at it.

OK, so the way that rods and cones are organized in the retina is they’re organized into groups and these are called receptive fields. So we could imagine, here’s a bunch of rods, let’s say, and this is obviously a bit simplified, but let’s say this is our receptive field, this group of rods here. The way that it works is that the rods then stimulate a few bipolar cells so we go from a bunch of rods to a few bipolar cells and then those bipolar cells go down to a single retinal ganglion cell.

You might be wondering why I’ve drawn it sort of backwards this way the reason for that is that the eyes are sort of backwards again. I said they’re backwards in that light enters the front but we don’t see it until it gets to the back of the eye after it has passed through all this other stuff, and actually that happens again on the retina surface, the light actually passes through the retinal ganglion cells and passes through the bipolar cells before it actually gets to the rods.

OK so you might be wondering, “well if all of these rods narrow down to a single cell here that’s going to then go to the optic nerve and out to the brain how do we get different information from this, right? If it can only sort of fire this cell or not?”

Remember neurons fire on this all-or-none principle, they either fire or they don’t, and actually that’s part of your answer there. The way that neurons send different messages is by changing their firing rate. They don’t change how they fire, they change how often they fire. So that’s what retinal ganglion cells do to tell us about different patterns of light on the receptive field. So some patterns of light are going to cause the retinal ganglion cell to fire more rapidly than other patterns.

So we can break these receptive fields up into two main types. Some of these receptive fields, let’s imagine one here, some of these really like it, they fire most intensely, when it’s dark on the outside of the receptive field but there’s a spot of light in the middle. When that happens these cells fire very intensely. So that type of receptive field will be called an “on-center” receptive field.

In contrast we could have a receptive field here where it responds most intensely when it’s dark in the center of the receptive field but light on the edges and that would be an off center receptive field. So what happens is these situations actually cause the retinal ganglion cell to fire more intensely than all light or all dark situations and that tells us something about what’s happening. It’s essentially saying that this is more interesting because light on all of the rods, it’s like, “OK, I mean I tell the brain that there’s light here” but when this happens or this happens it’s like
“wow, there’s something interesting, I’m going to fire a lot”.

Now you might wonder, “how does this firing rate actually get changed by these different patterns?” This is something called lateral inhibition. So you don’t have to understand all of the details of this, it’s a bit complicated, but you should understand what this term means. Lateral inhibition, so lateral means “side” and inhibit is to stop something, and the idea here is that cells can inhibit their neighbors, they can change the firing rate based on what’s happening to the cells next to them. And the way that this actually happens in the retina involves other cells that you don’t really need to go into for an introductory course.

So there’s other cells called horizontal cells and they can influence the firing of the retinal ganglion cell and that depends on the pattern that we see. But the point is that we have this process of lateral inhibition where neighbors influence each other and that causes different firing rates for different patterns of light.

Ok, so why did all this matter? Why would our eyes be set up this way, why don’t we just see what each rod sees? Why do we need to have this process of organizing the information?

And it is organizing it. In some sense we can say that perception already occurs at the retina. We’re already organizing the information, we don’t get every bit of what’s happening to each rod.

So why is this? Well, the answer is that it enhances contrast. It exaggerates differences. And it exaggerates contrast so that means that when we have, like on this whiteboard, when I have just a small white spot there surrounded by darkness this is gonna cause, when that falls on that receptive field, it’s going to cause intense firing. And that’s essentially telling “this is very interesting”, it actually makes this white spot
look brighter than it actually is. Compared to if you just look at all white you get the message that there light there but light surrounded by darkness is like “pay attention to that light here”.

Just like when I move the cursor around here, you can follow it more easily because of this enhanced contrast, because of lateral inhibition. A great demonstration of this is called Mach bands.

These are named after this Austrian physicist named Ernst Mach. He’s also the guy who we have the name for speed relative to the speed of sound. So we talk about jets and things like at Mach 1 or Mach 2. You can see from his beard he obviously doesn’t use the Gilette Mach 3 razor, I don’t know if that’s actually named after him or not, directly. Doesn’t really make sense, I think it would be kind of dangerous to shave that fast, but that’s Ernst Mach and here we have these Mach Bands.

So what’s happening with these Mach Bands? These are a great illusion to demonstrate lateral inhibition because what’s happening is that each of these bars is a solid uniform color. But we don’t perceive it that way.

When you look at it, you might feel like this bar, for instance, it looks like it’s a little bit lighter on this side and a little bit darker on this side.

Why is that happening? The reason that’s happening is because of lateral inhibition because when you get to that point here on this line where there’s contrast, what’s happening is some of those receptive fields have this different pattern, they have a light in some area and dark in the other and they find this very interesting and they fire more rapidly That enhances the contrast vs. in the middle it’s all the same color, it’s kind of boring, they fire but not all that intensely. Then you get over here and then “oh wow there’s contrast again!” they emphasize it.

They want us to notice contrast. So we get this effect where along here it makes this side look a little bit darker compared to the light and thus this side looks a little bit lighter than it actually is. Then when we get to the other side of the bar, this side looks a little darker than it actually is, this side looks a little bit lighter than it actually is.

This tells us “hey, something is changing here”, there’s contrast and it exaggerates this. And there’s really no way for us not to see that. If you cut out one of these bars you could easily see that it was a solid color but in this situation we automatically see this illusion.

That’s because it’s happening at our retina, it’s happening because of this lateral inhibition. OK so that’s receptive fields, rods and cones, and this idea of lateral inhibition, on-center and off-center receptive fields and this is to exaggerate the contrast that we see.

OK, so I hope you found this helpful, if so, please like the video and subscribe to the channel for more. Thanks for watching!