That Neuroscience Guy - The Neuroscience of Vision

Episode Date: June 25, 2025

In today's episode of That Neuroscience Guy, we start our "Senses" series by discussing the neuroscience behind vision. 

Hi, my name is Olav Krogolson and I'm a neuroscientist at the University of Victoria. And in my spare time, I'm that neuroscience guy. Welcome to the podcast. Okay, so for the next five episodes, by request, we're going gonna do an overview of the sensory systems. Now we have talked about them before here and there, but we're gonna dive specifically into each of the five senses, starting today with vision,

and just give you an overview from start to finish. And I'll point out some places where you can do a deep dive to learn more about specific parts of the visual system. So on today's podcast, how we see the visual sensory system. Well, it starts fairly simply. Light comes into the eye and it hits receptors at the back of the eye, literally photons of light. And these receptors, you probably know, called rods and cones.

So rods are primarily responsible for vision in low light, and they help out with peripheral vision, and they're also used in what we would call night vision. They are highly sensitive to light. They can detect even the most imperceptible amounts of light. Well, I guess they would be perceptible, but you get the idea. They're there just to see the presence of light. They don't contribute to color vision, all right? So rods function in a black and white world. They're found

throughout the retina, but they're a little bit denser in the periphery, which makes sense when you think about the fact that we don't need what rods do in central vision as much as we need it in the periphery. And they're way more numerous than cones. There's roughly 120 million rods in the human retina. So cones, what do cones do? Well, their big job is color vision and central vision. So central vision is you know the middle of our visual field and that's why it's so sharp and it works well in bright light and we see colors is the presence of cones. Now they're less sensitive to light than rods so they need more light to activate so they work better during daylight.

The cones themselves contain pigments that are sensitive to different wavelengths of light. So if you think back to elementary school, you remember Roy G. Biv. You remember that light follows a spectrum. Each color is a different wavelength of light. And the pigments in the cones allow us to see these different colors and perceive the range of colors that are present. They're particularly concentrated in the fovea, so the central part of the retina, and

there's less of them in the periphery, so the opposite of rods, and there's about six million cones in the human retina compared to the 120 million rods in the human retina. But at the million rods in the human retina. But at the end of the day these are receptors right? Light hits these receptors, photons, and they detect. So they're basically just hit by a stimulus and they fire. Now we'll get more on that when we get to V1, but in the meantime, what happens? So these neurons fire, because remember, these are receptors, so these are the dendrites of the neurons, and they fire. Now vision goes from the front of the brain, where the eyes are, to the back, but on the way during that journey, it goes through the midbrain, and it goes through a couple of

nuclei that do what we call midbrain processing. Now there's quite a few here, but I'm gonna mention some of the key ones because you might've heard of them. Probably the most important one is the superior colliculus and it plays a couple of roles. One is it helps you orient to visual stimuli. So it works with the attention system you orient to visual stimuli. So it works with the attention system

to orient to visual stimuli, and it helps coordinate eye and head movements. So it's receiving input from the retina, but it's also receiving higher brain region input, and it integrates these to allow us to track moving objects and respond to visual cues. Another midbrain region is the pre-tectal area. And basically it works best and helps us deal

with responses in ambient light conditions. So things like the pupillary light reflex, it receives direct input from the retina and it's being associated with following moving objects with the eyes as well, but the reflexive following of objects that move. There's the ocular motor nucleus, bet you can guess what this does. It basically is the nucleus that's responsible for controlling most of our eye movements. There is the Edinger Westfall nucleus.

It's near the ocular motor nucleus. And basically it helps shape the lens. All right, so the lens of the eye can actually change due to muscle contraction and that allows us to focus and play with the way we can see the world. And the last one I'll mention is the trochlear nucleus. Again, tied to eye movements. So the theme for the midbrain nuclei is eye movements. And it seems to play a specialized role with downward gaze and outward gaze. Now these nuclei are still being studied, but we know that the midbrain nuclei are responsible for tracking things, attention, and and orienting and jobs like this. So from the midbrain, visual information flows to the back of the head.

Alright, and the first place it comes is the primary visual cortex, area V1. Now it's important to realize what V1 actually is, is an accurate, what we call a retinotropic map of the world. Alright, so it's basically what, for every receptor in the eyes, there is a receptor in V1. So it's recreating what the eye see at the back of the brain. Now it plays a role in initial visual processing, detection of lines, color, basic features. And I want to make this really clear and I'll say this a few more times and I've said it before, vision is a build-up of information. Alright, what you get at the back of the eye in area V1 is just a two-dimensional

image of the world. It's a photograph in a sense, and it's a photograph that we keep updating, and we have to extract meaning from that photograph, and I've talked about that a lot. So what is V1 doing? Well, it's trying to figure out line orientations, colors, spatial frequency. Some of the neurons respond to specific orientations of lines, horizontal versus vertical. Some are sensitive to color.

And basically what V1 is doing is it's detecting these early features in the visual scene, and it's passing that information on to V2, the next area in visual processing. So it's creating this retinotropic map of the world, alright, and then it begins to extract information from that image. So what V2 gets, which is secondary visual cortex, it receives processed information from V1 and it basically starts to analyze more complex visual features. So it starts looking at the orientation of things, disparity, the depth of objects in the

picture, trying to determine what they are. Because remember it's a 2D photograph, so much like if you look at a photograph you can see cues and understand where things are in space. Well, V2 helps us establish that. All right. It plays an advanced role in color processing, integrating colors. It helps us basically distinguish between the foreground and the background. And it's the beginning of the recognizing of objects, basically based on their relationships in the scene

and their overall shape. It plays a role in continuity and proximity. Again, it's just trying to extract information and it begins this bottom-up building of information. Now from V to things start going all over the place if I'm being honest, but a couple of the other major early visual areas are V3. All right, and it's still an area of study, but it's basically tied heavily to form if we're talking about the ventral visual stream, which we've talked about before. And I may as well get into that right now.

There's a split in visual processing. There's a ventral visual stream, which we did an episode on, I think back in season one, but visual information flows through the inferior temporal cortex, and this is vision for perception. And then visual information also flows up through the parietal to the parietal cortex, which is the dorsal visual stream, which is vision for action, if you will.

And it's where things are in space. And in this case, V3 plays a role in motion because we have to perceive motion. So if you remember 2D picture, back of the eye, 2D picture area V1, we don't see in three dimensions. So we have to construct that three dimensional image, but we also don't see motion. So we have to perceive motion by objects getting bigger or smaller. So something coming towards us gets bigger, something getting smaller is moving away from us. In the ventral stream, V3 seems

to play a really big role in terms of color sensitivity and developing color even further, if you will, to round out the image. And it seems that V3 is a communications pathway between the two visual streams. When the model of the two visual streams first came out, it was thought that they were highly independent. But what we've realized is that's not really true. The two visual streams are constantly talking back and forth to help us build up our visual image of the world.

Okay, V4, color perception. This is where we get conscious awareness of color and object recognition. So basically, it's where we start saying, hey, that's red, that's blue, that's green. So that information is built up through V1, V2, V3, but V4, we get conscious awareness of color. Object recognition, basically for simple objects

and straightforward objects, squares, circles, things like this, it allows us to segment things within the visual display. V4 plays a role in attention. It allows us to orient to things, but it's important to realize it doesn't identify complex objects. I could do a whole season series on the visual stream,

but I'll give you an example of this. There is an advanced area of the ventral visual stream called the fusiform face area, which is just responsible for identifying faces. So there's a buildup of information in V1, then V2, then V3, then V4. And then eventually that information gets to the fusiform face area and you go, wow, that's a face and you, you add identification to it, maybe a name. Do you know the person or not?

Now, I said there was V5 as well. V5 is basically area MT. This is where we integrate and perceive motion, direction of objects, speed of objects. It helps us integrate all of this motion information so we can interact with the world. When you're catching a ball, for instance, a visual motor task, you need to... information from MT to be able to do that. And V5, actually, MT V5, also plays a role in helping us track movements, so it works with those midbrain nuclei.

There's a back and forth there. And V5 is conscious awareness, all right. This is when we know things are moving, we go, whoa. Now, once we leave these initial areas, there is probably, no one really knows. I've seen estimates anywhere from 40 to 140 other visual areas, but like I said not that long ago, the key thing is that there is a split. There's a flow of information

into the ventral stream, vision for perception, and we have a full episode just on the ventral stream. So if you want to know more about that, go back to that episode on the ventral stream and it flows up into the posterior parietal cortex, visual information, and that's the dorsal stream, or vision for action. All right, and we have another episode on that, so you can dig that out and dive into the dorsal stream. And last but not least,

there's this concept of top-down processing. We have an episode on that as well. Top-down processing in a nutshell is that as there's this build-up of visual information through the visual system, where you're going, okay, that's a line, that's a square, that's a couple of squares arranged together, oh, that's a house. At the exact same time, the prefrontal cortex is trying to figure out what that object is, and it's throwing out ideas. Hey, I think that might be a house.

Top-down processing, we have an episode on that as well. That is an overview of the entire visual system. Like I said, I used to teach a third-year course where we spent the entire semester on vision. I just tried to do it in less than 15 minutes. But don't forget, we have other episodes on vision, so hunt those ones down and you can truly understand the neuroscience of human visual processing.

That sensory episode one down will tackle another sensory processing part of the brain. Proprioception or the somatosensory cortex, touch in other words, will tackle that one next on the next episode Don't forget the website that neuroscience guy calm links to Etsy. We have lots of new merch up finally I'm so happy we can say that so buy something please if money goes to Students in the Kregel's in lab they appreciate it links to patreon. Thanks to those of you that support us

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hey, keep going love your podcast We just got to keep going but we need your support to do so We don't really need your money the grad students do I don't but your support is what keeps us going So send us ideas shout out a bit of love whatever it is Alright, that's it for this episode It's 15 minutes away from game seven of the NBA finals. And I hate to take sides, but go Pacers, go Indiana. My name is all of Krogolson and I'm that neuroscience guy.

I'll see you soon for other full episode of the podcast.