Vision: From the Sun to consciousness
One of the the first humans saw the sun and thought it must be a big lamp. Then he felt that his skin is getting warm, he thought it is something dangerous. He runs to his wife and commanded her not to leave the cave. There was a big danger outside. They were never able to see reality as we do.
Human being perceives the world by 5 senses and one of them is vision. Our ability to see starts with a signal from the sun. There are up to 7 waves and we only see a small portion of them, called visible light. This light frequency is less than UV (ultraviolent) and more than IR (infrared) light. The wavelength that we can see is between 400–700 nm. When we see a cat, we see the light of the sun that is deflected from the cat’s body.
But there are more to wonder.
When the light passes from the cornea and lens, it reaches the retina. Lens just reverse the image and project it to the retina.
The retina is the innermost layer of the eye. It is composed of many types of neurons which convert light into a “language of brain” electrical signal. But it happens through complex processes.
There are cons, rods, and ganglion cell layers in the retina. The process of light energy conversion to electrical signal happens in several stages, i)light activates photosensitive chemicals ii)cascade of reactions turned into electrical signal iii)the electrical signal converted to chemical signal — ion flow. Transfer of ions is a transfer of information to the latter parts of the brain. Rods are responsible for night vision. Cons serve for colored vision. Both Rods and Cons contain opsins — chemicals that are sensitive to light. Cons have three types of opsins — Opsin R(Red), Opsin G(green), and Opsin B( blue), rods have rhodopsins. Opsin itself is a large protein and has 7 transmembrane domains. Lys296 residue connects opsin with retinal. Usually (when it is dark) retinal is a cis state (ground state), retinal changes its structure from cis state to trans retinal where it helps metarhodopsin II convert to metarhodopsin III. Conversion of metarhodopsin II to III takes roughly 4 minutes later than the first photon meets the retina. That is why when lights are on in the middle of the night, it takes time for us to see properly.
Visual representation of how rods and cons work together to send a signal to glutamate neurons is depicted below. (pic)
Important features of the retina:
- It does not fully charge off when the light source is cut. Instead, the retina converts light macroscopic electric currents. This is why we still see after lights are off even for a long time.
- The retina does not only receive and convert photon bombardment into the image, it also processes information. When we see visual illusions such as shaded and color square are the same, it is the retina that converts photon into something “meaningful”, before reaching the visual cortex.
When the information needs to be sent to long distances such as the thalamus, ganglion cells come into play, they are the only cell type that shows action potential.
Both the right and left visual field depicted on the retina of both eyes. This information is sent to Lateral Geniculate Nucleus where there are around 10⁶ neurons in each. Axons lie to the ipsilateral cortex (area 17) either directly or turning around the temporal lobe. The portion of the brain that axons branched from LNG and reach to Broadmann area 17 usually called Primary visual cortical area or V1.
V1-V5 neuron layers
V1 — usually called the primary visual cortex. It is presumed that the V1 pathway is a completely different pathway than the V2-V5 pathways. There is parvocellular motion (on the X-axis, horizontal) and magnocellular motion (Y-axis, vertical). Motion on that axis is mapped in different neuron networks on V1. Magnocellular pathways include motion, depth, and spatial information. The parvocellular pathway informs us about form, color, and intracellular disparity.
Each layer receives one part of the information and these layers are interconnected. For example, on the surface of the V4 network, information is received from the magnocellular pathway, whereas in the depth of this network, the parvocellular pathway is elucidated. Moreover, the timing of the information is also important. For example, roughly in the first 40ms V1 neurons can detect colors and spatial frequencies.
Usually, V1 receives information from LGN. There are “What” and “Where” pathways. THE “WHAT” pathway starts from V1 à V2 àV4 and to the inferior temporal cortex. This pathway is responsible for recognition and object representation. Some papers assume that long-term memory is also encoded by the Where pathway.
<p style=”font-size:22px” value=”<amp-fit-text layout=”fixed-height” min-font-size=”6" max-font-size=”72" height=”80">Another pathway is the “What” pathway starting from V1 à V2 àDorsomedial à V5 (Visual MT). This pathway is responsible for motion, object location and eye movement, and arms control. Many assume that this categorization is oversimplified because illusions such as the <a href=” https://www.youtube.com/watch?v=RfuRbOsubsc&ab_channel=LFernandes">Ebbinghaus illusion</a> distort our understanding. Another pathway is the “What” pathway starting from V1 à V2 àDorsomedial à V5 (Visual MT). This pathway is responsible for motion, object location and eye movement, and arms control. Many assume that this categorization is oversimplified because illusions such as the Ebbinghaus illusion distort our understanding.
The primary cortex is responsible for static and moving object processing and pattern recognition. V1 has 6 different layers and 4th layer of neurons receives the most amount of signal. 4Ca and 4Cb are responsible for magnocellular and parvocellular inputs respectively. V1 also has a well-defined spatial information pathway.
Visual area V2 has tight connections with V1 both feedback and feedforward. V2 area provides a complete map of the visual world, but small attentional modulation is observed. Orientation, spatial frequency, and color are tuned by the V2 area.
Visual area V3, color selective neurons are preliminary in the V3 area.
V4 area was tested experimentally of macaque monkeys. These neurons show strong attentional modulation, object orientation, color, and geometric shapes. It is known that women are not good at geometrical shapes and it might be related to the V4 area of the visual pathway.
V5 area is responsible for motion perception and eye movement-related actions. Processing of movements related to speed and direction. V5 area provides information to jump off the way when the car is rushing toward us, but stand solid when the car is another way around.
Information processing routes.
Currently, there are at least four pathways that cross the parietal lobe. The first pathway is for reaching, the second for grasping, others are awareness and when pathway. Communication between pathways was suggested and widely accepted. Integration of pathways is a highly accepted notion. For example, orientation and position information are integrated into V2 and these neurons are tuned both for orientation and position information.
Color information is one of the most complex ones. Color processing involves perception related to category, conscious, and preconscious level information. For conscious color perception, three-cone (red, blue, green) is a prerequisite.
Consciousness
After understanding how photons from the sun resulted in invaluable information in brain pathways that are related to vision. Several models are suggesting how we consciously SEE something. What is teleported to our consciousness, is it photons, is it an electrical signal from neurons (if so the model is far too complicated that we can never understand), or it is quantum teleportation of the image. For now, we should admit that we are in a country of fantasy where none of the suggestions can explain brain functioning.
REFERENCES:
The eye and Vision: An overview R.A. Armstrong et.al, 2019
Photons do collapse in the Retina not in the brain cortex: Evidence from Visual Illusion, 2011
On the usefulness of “what” and “where” pathways in vision, Edward H.F, 2011
Originally published at http://youngbme.wordpress.com on February 18, 2021.
