Module 1 Lesson 2 - 4

The Pathway to the Brain

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Refraction

As light enters the eye, the speed of light slows and the light rays are bent. This process is called refraction. Most refraction occurs when the light passes through the cornea. The remaining refraction occurs at the lens.

The lens has an interesting shape. It is curved on both surfaces, as you saw during the cow eye dissection lab. This shape is called biconvex. When light rays pass through a biconvex lens, they converge inside the eye and project the image on the retina.

However, the image projected on the retina is inverted and reversed. Why don't we see the world upside down and backwards?

When this information reaches the brain, it is processed so that we can perceive the image correctly right side up and left to right.

Did You Know?

Our brain is an amazing organ! To test how flexible our brain is, people were given special glasses that inverted the visual field before it reached the eye. Initially, everything looked upside down for everyone. After a few days, everyone could see everything correctly. The brain adapted to the new visual field and processed the information accordingly. When the special glasses were taken off, everyone needed a couple days to adjust to the original visual field.

Optic Chiasm

The information you receive from each eye is communicated through the optic nerve to the brain. The brain processes this information into one image without gaps or blind spots.

At the x-shaped optic chiasm,  some axons from each eye cross over to the opposite side of the brain while others continue to their respective side of the brain. The crossing over ensures that each half of the occipital lobe receives the same image or part of the visual field as viewed by each eye. The left visual field from both eyes will cross the optic chiasm to the right side of the brain, and the right visual field will be processed in the left side of the brain.

Depth Perception 

How do we perceive depth? You probably noticed that objects closer to you appear larger and any objects that are closer to you overlap the objects that are behind them.

In addition to this, we can also take information from where an image falls on the retina. Due to the distance between the two eyes, images will not fall on the same spot on the retina for both eyes. When an object is placed closer to us, its image will fall on the inner portion of the retina, toward the nose. For objects placed farther from us, their image will fall on the outer portion of the retina, toward the ears. By comparing where an image falls on the retina, we can determine its distance from us.

Accommodation 

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The ciliary muscles and suspensory ligaments are excellent examples of how the sympathetic and parasympathetic nervous systems work together to establish homeostasis in vision. By changing the shape of the lens they allow for light rays to be bent differently (refracted) allowing you to focus on the words of this sentence as you read it, or on the words on a poster you see on the far wall when you look up. This process is called accommodation.



Focusing on Distant Object:

1. Ciliary muscles relax
2. Tension on suspensory ligament 
3. Tension pulls on lens 
4. Lens flattens 
5. Focus on far objects
   

Focusing on Near Object:

1. Ciliary muscles contract
2. Tension on suspensory ligament relaxes
3. Less pull on lens
4. Lens bulges 
5. Focus on near objects

Watch and Listen

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Watch this section of Biologix-06 that deals with eye accommodation.