The one about Vision
A human brain is the most complex thing in the known universe. Almost every single one of its stories from how it creates and decodes sounds to how it organizes and stores memories is worth learning. How our brain converts light photons to nerve impulses and how that translates to the world we see and understand is my favorite neuroscience story. So in this blog post, I hope to unravel this very complex circuitry into a language we all can understand.
Our brain works just like a computer. It gathers data from its various sensory centers like retina in eye, cochlea in ear, taste buds in tongue, touch/pain/temperature receptors in skin etc. This data is converted into nerve impulses(equivalent of converting data into binary). These nerve impulses travel down neurons in form of action potentials to its various targets. Different brain areas process different parts of each data. For example, thalamus in midbrain is the sensory processing center. All sensory data get sorted here and then relayed to other areas for analyzing of data. Once analyzed, the outcome is propagated via action potentials to either the sense organ so it can better orient itself to gather more data or to the prefrontal cortex for higher order conscious processing. All circuits give each other feedbacks and there are hundreds of pathways dedicated to each sense and there’s a lot of overlap in these pathways. So, some auditory information will get processed by vision pathways and vice versa. All sensory pathways are also connected to memory pathways in Hippocampus as well as the emotional pathways in Amygdala.
The human experience of vision involves three steps: (1) the transformation of a pattern of light to electrical impulses in the eyes, (2) the transmission of these electrical impulses from the eyes to the brain along the optic nerves, and (3) the decoding and assembly of these electrical impulses into visual sensations experienced in the brain.
Let’s begin our journey with light itself. Light is one of the most pervasive elements in our world. No matter where you are on Earth’s surface, the sun rises and sets in a predictable 24-hour cycle that brings with it periods of light and dark. Light is a form of energy, packaged in units called photons, and almost all living things are equipped to detect and respond to it in some fashion. In one form or another, creatures as varied as animals, plants and tiny microorganisms have photoreceptors — Simple or Complex structures that are stimulated by light energy. In green plants, photoreceptors capture sunlight and immediately harness it to make plant’s food. An earthworm has photoreceptors all over its body, so its simple nervous system can respond to light even though the worm has no eyes. More complex animals, however, do have eyes of one type or another. Eyes provide what vision requires — An organized array of photoreceptors that are linked to brain centers capable of receiving and interpreting the patterns of nerve impulses those photoreceptors generate.
When you look at a chair, you are not actually seeing a chair. You are seeing a bunch of photons that have reflected off of the chair. In the process of reflecting off of the chair, these photons have been arranged in a pattern that resembles the chair. When the photons strike your retina, your photoreceptors detect this pattern and send it to your brain. In this way, your brain thinks it’s looking at a chair when it’s really looking at a bunch of photons arranged in a chair pattern.
Let’s now look at how our own eyes gather and processes the information they receive from photons.
Photons reflected off of objects get bent by the cornea and then passed through the pupil which is literally a hole in the eye. The size of the pupil is controlled by the iris and ciliary muscles. The iris that surrounds the pupil contains muscles that control the size of the pupil. When confronted with low light conditions, the iris expands the pupil as wide as possible. This dilation lets as much light as possible into the eye so that sensitivity is enhanced. The pupil’s contribution to dark adaptation takes only a few seconds to a minute to be completed.
Once light enters through the pupil, it gets bent again by lens (though cornea does the most bending) to ensure they are focused onto the retina. How much light is bent depends on if it’s coming from a far away or nearby object. It’s in the retina that these photons are sorted and converted to nerve impulses to be analyzed by our brain. These nerve impulses are sent to the brain via the Optic nerve. The area that contains the optic nerve has no photoreceptors. This is our blindspot. Technically, we should see this large ugly black spot wherever we look. But our brain fills in the hole by papering it over, by averaging it out. This means that part of our vision is actually fake, generated by our subconscious minds to deceive us.
Also, we only see the center of our field of vision, called the fovea, with clarity. Fovea is a tiny pit in the retina containing the highest number of photoreceptors. The peripheral part of our vision (around the fovea) is blurry, in order to save energy. But since fovea is so small, our eyes dart around constantly to capture as much information as possible. This rapid, jiggling motion of our eyes is called saccades. All this is done subconsciously, giving us the false impression that our field of vision is clear and focused.
Our eyes also fool is into thinking we can see depth. The retinas of our eyes are two dimensional, but because we have two eyes separated by a few inches, the left and right brain merge these two images, giving us the false sense of third dimension. For more distant objects, we can judge how far an object is by observing how they move when we move our head. This is called Parallax.
Our eyebrows, eyelids and eyelashes have important functions as well. They help sheild the eyes from intense light, perspiration and foreign objects. The lining of the eyelid secretes lubricating fluid over the eyeball.
In order to sense and register any signal you need a receptor. In our case the photoreceptors responsible for receiving and transmitting information gathered from photons are called rods and cones. These are laid all across the retina with more rods only in the periphery and cones concentrated mostly in the center and the fovea. Rods help with low light, night vision and cones help with sharper, brighter, colored vision.
Color information is detected in the eye by having three different types of cone cells that each have a different range of color sensitivity. One of the types has a sensitivity range centered on red, another type has a range centered on green, and another type has a range centered on blue. The eye can see almost all of the colors in the visible spectrum by comparing the relative activation of these three different types of cone cells. For instance, when you look at a yellow tulip, yellow photons stream into your eye and hit your red, green, and blue cone cells. Only the red and green cone cells are triggered by the yellow photons, and your brain interprets red plus green as yellow. In contrast to cone cells, there is only one type of rod cell, and so the rod cells can only detect brightness and not color. The rod cells are primarily used in low lighting conditions.
Cone cells contain rhodopsin, which is one of many light-sensitive chemicals. Rhodopsin is very sensitive to light and is the primary chemical used by the cones when seeing in low light conditions. The problem is that rhodopsin is so sensitive to light that under normal light levels, the light deforms and deactivates (photobleaches) this chemical. Most of the day, when we are walking around in normal light, the rhodopsin in our eyes is deactivated. Upon exposure to darkness, the rhodopsin is able to regenerate and reactivate, becoming sensitive again to light and improving our night vision. But this regeneration process takes time
Rod cells in our eyes are responsible for black and white vision. They are the heavy-hitters when it comes to vision in low light conditions. The rods in our eyes achieve this great night vision through several mechanisms:
1. Like cones, rod cells contain rhodopsin, the chemical that is highly sensitive to light. In fact, rod cells rely more heavily on rhodopsin than cone cells, leading each single rod cell to be about 100 to 1000 times as sensitive as a single cone cell once fully adapted.
2. There are far more rods on the retina (100 million) than there are cones (5 million).
3. Several rods all connect to the same output signal (the same interneuron). This fact allows lower levels of light to be detected at the cost of image resolution.
4. Rods respond slowly to light (they collect light over long time periods). This slow response means that lower levels of light can be detected at the cost of sensing rapid changes in time.
Let’s look into how these photoreceptors convert photons into electrical signals. We will zoom into a rod to see how this happens. The mechanism is very similar in cones. This image here shows one complete rod cell.
The outer segment of the rod has hundreds of optic discs stacked on top of one another. There is a zoomed-in image of one disk showing several red dots. These dots are pigments called rhodopsin. There is also zoomed-in image of rhodopsin. It’s composed of 7 transmembrane proteins(shown in green). There is a molecule nested in the middle called 11-cis-retinal.This is the molecule that photon interacts with first.
The rods themselves relay signals to bipolar cells which pass it on to ganglion cells. Ganglion cells propagate signals further via action potentials through optic tract to brain. Let’s zoom into how this happens.
When a photon hits the 11-cis retinal hidden inside the rhodopsin pigments that are present on optic discs of rods, enough energy is transferred to the molecule to change its structure to 11-trans-retinal. This structural change triggers a cascade of signal transduction that travels down the hundreds of optic discs eventually reaching the bipolar cell. The actual signal propagation is made possible by sodium channels. The inside of all cells has a negative resting membrane potential. Ions like sodiums are used to make the membrane potential more positive(depolarization) which triggers an electric potential to propagate downwards.
One important thing to understand is that our retina is inverted. This means that photon first goes through the ganglion and bipolar cells before hitting the photopigments in rods and cones. Rods and cones propagate the signal received from the photons to horizontal cells which in pass it to bipolar cells and eventually to ganglion cells. Ganglion cells share their axons with the optic tract. These Retinal Ganglion Cells (RGCs) are the bottleneck through which all visual impressions flow from retina to brain. Optical signals are generated by photons that bombard the retina of the eye. Neurons in the retina collect and process these impressions. While doing so, the retina focuses on the important details: Is there contrast or color? Are there small or large objects? Is something moving? Once these details are filtered out, retinal ganglion cells (RGCs) send them to the brain, where they are translated into a specific behavior. Specific RGC types sends different details to different regions of the brain.
Our visual field for each eye overlaps like the diagram here. Each eye has two wiring — nasal side and temporal side. Only the nasal side wiring from each eye crosses over to the other side of the brain. And the temporal wiring from both side ends up on the same side of brain. This ensures that the entire right visual field from both left and right eye go to left side of brain and the entire left visual field from both eye go to right brain for processing. The crossing over happens at the optic chiasm. From here most axons go to LGN in thalamus and then on to the visual cortex. Few axons coming from the optic tract instead of going to LGN synapse at the suprachiasmatic nucleus of hypothalamus. The information about light intensity that the nucleus receives helps it mantain the biological clock. Few other axons synapse at the Superior colliculus which sends feedback to muscles of the eye and guides its movement based on the stimulus. Together the suprachiasmatic nucleus and the superior colliculus form the intrinsic pathway that most animals have. The evolved sorting of data in LGN and higher order processing in visual cortex is only present in more evolved species.
Majority of the initial processing takes place at the Lateral Geniculate Nucleus(LGN) and pulvinar nucleus of the thalamus. The LGN has six layers — two receive data from rods and other four from cones. Here most of the sorting of information based on color, shape, and movement happens. The six layers radiate these info to 6 specific layers of the visual cortex. The LGN as well as the visual cortex has the entire map of the retina.
The visual cortex itself is very very complicated with many layers within layers. We know of 8 layers so far (V1-V8). Here data is further refined and categorized and analyzed in the different cortical layers. Information is first received from LGN and analyzed in V1 which is just like a screen. This layer creates a pattern on the back of your brain very similar in shape and form to the original image. The image bears a striking resemblance to the original, except that the very center of your eye, the fovea, occupies a much larger area in V1 (since the fovea has the highest concentration of neurons). The image cast on V1 is therefore not a perfect replica of the landscape but is distorted, with the central region of the image taking up most of the space. The information gets further passed on to other areas layers. V2 contains neurons that compare images coming from each eye. V3 processes information about distance to the object, using shadows and other information from both eyes. Colors are processed in V4. V5 uses different circuits to process information about motion.
From the occipital lobe, the information is sent to the prefrontal cortex, where you finally “see” the image.
So let’s follow a path of a photon from the moment it leaves an object. I am able to see and recognize an apple because all this is happening:
- Photons reflected off the apple enter my pupil and get focused on the retina
- Some photons coming from the edge and background of the apple get processed by the rods but most red photons reflected off the apple itself get processed by cones
- The photons hit the 11-cis-retinal inside the rhodopsins of photoreceptors and converts it to 11-trans-retinal
- The structural change of the molecule leads to a transduction cascade via closing to sodium channels. This leads to a change in electrical potential inside cells
- This information is propagated downwards to horizontal cells →bipolar cells →ganglion cells →optic tract
- Information coming from the nasal sided of visual field from both eyes get crossed over to the other side of the brain. The temporal information stays on the same side
- Some data goes to suprachiasmatic nucleus of hypothalamus so our body can maintain the biological clock.
- Some data goes to superior colliculus of midbrain so feedback can be sent to eye muscles
- Majority of the data is sent to LGN and pulvinar nucleus of thalamus where it’s all sorted based on color, shape, and movement
- Data is then relayed to visual cortex 1
- Then onto visual cortex 2, 3, 4, 5 and many other areas for further higher order analyzing
- Data is compared to memory bank
- Brain recognizes the object as an apple
- I see the apple
The most amazing part is how all this feels like an instant.
Different visual conditions are a result of where in the pathway a defect happens. For example, visual agnosia results form a defect in the “what” pathway of your visual cortex. This pathway helps you recognize objects. In this condition a person loses his ability to recognize things. So, an apple may look like a book. But if the person was to hold the apple, it would feel like an apple and not a book.
There is so much more detail at the cellular and molecular level of exactly how the processing happens but that may be too much science jargon for this blog post. For now, hope you found this post informative and interesting. :)
