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How We See

How We See

Description:

This lesson will introduce you to the anatomy and physiology of the human visual system, its connection to the brain and how this system processes light.

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Tutorial

Source: Image of Head Anatomy, Creative Commons http://pt.wikipedia.org/wiki/Ficheiro:Head_lateral_mouth_anatomy.jpg Image of Electromagnetic Spectrum, Creative Commons http://en.wikipedia.org/wiki/File:EM_Spectrum_Properties_edit.svg

Video Transcription

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Hi, everyone. My name is Mario. And I'd like to welcome you to today's lesson on how we see.

So we'll introduce the anatomy and the physiology of the human visual system. It's connected to the brain. And how this system processes light.

So stick with it. It should be interesting. As always, feel free to stop, fast forward, and rewind at your own pace. So when when you're ready to go, let's jump in.

So we all know what eyes are, but not everyone knows how they function. The human eye is a complex sensory organ and it's designed to see light and color, which plays an important part in the visual communication process. And currently there's a lot that we do understand about this process, but it's still very incomplete, and there's plenty of visual research ongoing.

So we have this image here of side cut to the eye. And I'm going to point out some key areas here, primarily the retina and optic nerve. Now the retina is a thin transparent tissue at the back of the eye that contains light sensitive receptors called the rods and cones. And rods and cones look something like this.

Rods are the long, thin, light-sensitive parts of the retina that process night vision. And cones are the light-sensitive parts of the retina that process color and day vision. And there are a lot of these in the eyes, in the retina, anywhere from 120 million rods and 6 or 7 million cones.

So what happens is you see an image, and light passes through the eye lens and hits the retina. In the retina, with those rods and cones create nerve impulses and convert light into electrical signals, which travel through the brain through the optic nerve. And the optic nerve is the tissue that connects the retina to the visual cortex in the back of the brain, which I'll show you here in a minute.

If we look at this side cut here of a face, we have our eye from earlier and a nice view of the brain. So light hits the retina with our cones and rods, and that information travels through the optic nerve, which routes it to this junction, that's really difficult for me to pronounce called the lateral geniculate, I think, nucleus, which then routes to the lower back side of the brain called the visual cortex, which is the part of the brain which processes visual information from the retina communicated via network of nerve cells. Now, there's a lot of complexity to the way we process images and light. And it continues with electromagnetic radiation.

So you're thinking, well, wait a minute. How do we go from eyes to cortexes, and now radiation? Yeah, it's complicated. And electromagnetic radiation is another name for light, which is why we got here.

So I guess it's another name for light, the human eye is only sensitive to a portion of it known as visible light. So light travels in waves. And all electromagnetic radiation or light travels at the same speed, which refer to as the speed of light. And a wavelength is the measure of the distance from two consecutive wave crest or troughs, commonly measured in nanometers.

When we talk about light traveling in waves, we also talk about frequency, which is the number of waves passing a certain point per second, measured in Hertz. So you might be asking how or why is this all relevant? How does this all relate?

And light can be characterized by its wavelength. So for example, the human eye can see wavelengths somewhere between 400 and 900 nanometers, which is what we refer to as visible light within the electromagnetic spectrum. So light waves with a shorter wavelength and a higher frequency are beyond the human eye's visibility range. That'll be things like gamma, x-rays, and ultraviolet light, which you see on the right portion of the chart there. And likewise, infrared, microwaves, and radio waves have longer wavelengths and longer frequencies, and are also beyond what the human eye can see.

So the human eye has a very small region within the electromagnetic spectrum that it can visibly discern. And it's quite miraculous really what we're able to see. It's quite nuts.

Well, everyone, that actually ends our lesson for today. And that was a bit complex. But I hope we learned a bit more about how we see.

Our key terms for today were rods, cones, retina, visual cortex, optic nerve, a wavelength, frequency, and electromagnetic radiation. My name, again, is Mario. And I will see you next lesson.

TERMS TO KNOW
  • Rods

    The long, thin light sensitive parts of the retina that processes night vision.

  • Cones

    The light sensitive parts of the retina that processes color and day vision.

  • Retina

    A thin, transparent tissue at the back of the eye that contains light sensitive receptors called rods and cones.

  • Visual Cortex

    The part of the brain which processes visual information from the retina, communicated via a network of nerve cells.

  • Optic Nerve

    Tissue that connects the retina to the visual cortex in the back of the brain.

  • Wavelength

    The measure of the distance from two consecutive waves crests or troughs, common measured in nanometers.

  • Frequency

    The number of waves passing a certain point per second, measured in hertz.

  • Electromagnetic Radiation

    Another name for light; the human eye is only sensitive to a portion of it, known as visible light.