How we see

What happens after a photon of light enters the eye?

In the end, the reason that we have a visual experience is that our brain has got some pattern of activity that associates with the inputs from the periphery (the outside part of our body).
But we can have a visual experience without any input from the periphery too - for example, when we dream we see things that aren’t coming through our eyes.
Are dreams memories?
- They may reflect our visual experience, not necessarily specific visual memories (but they can be).
The bigger point is that the experience of seeing is a brain phenomenon.
Under normal circumstances, we see the world because we’re looking at it using our eyes. And thus, when we are looking at the exterior world, what the retina is telling the brain is what matters.
Ganglion cells - neurons that are the key cells for communicating between the eye and brain. The eye is like a camera - detecting the initial image, doing some initial processing, and sending the signal to the brain. Then, at the level of the cortex is where we have our conscious visual experience.
There are other parts of the brain that get visual input as well doing other things with that input

Color Vision

Light is just a form of electromagnetic radiation - vibrating and oscillating.
- Photons are certainly moving through space.
- One way of thinking about light: photons as particles. Another way is to think of it as a wave like a radio wave. Both ways are acceptable.
Radio waves have frequencies and certain frequencies in the electromagnetic spectrum can be detected by neurons in the retina which convert that into electrical signals - the things that we see. Further, there are different wavelengths within the light that can be seen by the see. Those different wavelengths are decoded by the nervous system to lead to our experience of color as different neurons are tuned to the different wavelengths of light.
How does a single photon of light eventually lead to the perception of one color vs another? (e.g. red vs blue vs green)
- Imagine that in the first layer of the retina (where the transformation from electromagnetic radiation to neural/electrical signals occurs) that you have different kinds of sensitive cells that are making different molecules within themselves for the sole purpose of absorbing photos (the first step in the process of seeing)
- There are 5 proteins (photopigments or photoreceptor proteins) like this that we need to think about in the typical retina. But for seeing color, we care about 3 of them, each absorbing light with a certain preferred frequency. Then, the nervous system keeps track of those signals, and compares and contrasts them to extract some understanding of the wavelength composition of light.
- E.g. just by looking at a landscape it must be late in the day as things are looking golden. It is a function of our absorbing all the light that’s coming from the world and interpreting it with our brain because of the different composition of the light reaching our eyes.
Is one person’s perception of a certain color (e.g. red) the same as another person’s perception of that color?
- A deep philosophical question, and is not likely to be answered ever.
- The biological mechanisms for seeing color seem to be very similar between individuals - whether other human beings or even other animals. Thus, the physiological process seems to be the same.
- But at the level of perception, understanding, or experience, it is hard to say anything.
The difference in vision between humans and other animals
- There are 3 types of cones (given the way color vision works, there has to be 3 different signals). Cones are one of the 5 photopigments mentioned above.
- 2 other types of photopigments (out of the 3 that were mentioned above to be relevant to our discussion out of the 5 that exist):
  - Rod cell: One for dim light vision (when we are walking around on a moonless night and seeing things with very low light)
  - Melanopsin pigment (ganglion cells, will be described later)
- Human beings have 3 cone types and we see colors that stem from that. In most mammals (like dogs, cats, etc.), there are only 2 cone types that limit the kind of vision they can have in the domain of wavelength/colors.
- Thus, a dog sees the world like a particular type of color-blind human as instead of 3 channels to compare and contrast, they only have 2 channels which makes it difficult to figure out which wavelength you’re looking at.
How bad really is color blindness?
- It can be really bad in certain circumstances but is a fairly modest visual limitation. E.g. not being able to read acutely (e.g. fine print) can be much more damaging.

Ganglion cells and their importance