As mentioned previously, the gradual evolution of photoreceptors, from a few cells to an entire eye, is simplistic to the point of irrelevance. Vision almost certainly doesn’t occur in the retina, but in the brain. Here are several other obstacles to the evolution of the eyes, focused on motion perception.
1. Motion Blur – Vision is in constant motion. Visual objects move, your eyes move, your head moves, your torso moves, your whole body moves. Move a camera half as much, and without high fps, you’ll see almost constant blur. Our eyes solves this problem through a suppression mechanism, keeping vision clear. This mechanism essentially blocks vision while our eyes shift. It also uses the vestibular system to stabilize the eyes as your head moves. Obviously, without this suppression and stabilization, you can’t see what you’re doing when franticly escaping danger.
2. External Motion – If there are two dots in front of you, and you stare at the left one which is stationary while the right one moves, the retinal image produced is identical to a scenario in which right one is stationary and your eyes track the left one as it moves. In other words, you need a system outside the retina, which can detect if the objects are moving, or your eyes are moving. One simple method is to use information from the muscles around the eyes. Without that information you wouldn’t know if a predator is coming towards you, or you are coming towards the predator.
3. Motion in Retina – Both of the above examples assume we can detect motion already. However, photoreceptors in the retina cannot detect motion. The idea that the retina can gradually increase in size, slowly cupping, until it forms a compact eye, and ta-da! you have vision, is fundamentally flawed. You need a network dedicated to the perception of motion. Such networks can be found in area V1 and MT of the brain. The system is complicated, but to simplify, imagine there are two photoreceptors, in different locations of the retina. As an image travels across the retina, it activates one receptor first, and then the next. Both of these receptors sends information to a third neuron, that fires when both receptors are activated at the same time. Obviously they can’t activate at the same time if something is moving. So these receptors have a delay system. Depending on how long the delay is, an object traveling at a specific speed, activates one receptor, then the delayed signal reaches the third neuron by the time the object activates the second receptor. Notice that these neurons can only detect objects moving at a specific speed and direction. If the object is moving faster or slower, it won’t be detected. Without an entire system of these neurons, dedicated to motion perception, an animal cannot detect motion.
4. In case you don’t know, damage to the MT portion of the brain leaves people motion blind. They see the world as independent still images. To show how problematic it is to have vision without this system, consider that people with motion blindness can’t tell which direction a car is moving or how fast. If they’re not careful they can easily be killed. Many people with motion blindness need to compensate with practice, by focusing on the sound.
I only have two questions: How did our ancestors survive before the perception of motion evolved; and how did the perception of motion evolve in the first place?
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Many of you like to tell me I don't know how evolution works. That's a beautiful statement, because the easiest people to correct, and the easiest questions to answer, are those asked by people who don't know how something works. Much harder, are the questions asked by people who do know how the thing works. So don't just tell me I don't know how evolution works, show me by answering the question. Also, if you don't know the answer, don't complain to me about it. Go find out, or move on with your life.
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