Amblyopia or lazy eye is when one eye can't see as well as the other without underlining eye disease. When I mean one eye sees worst I mean one eye is 20/20 and the other is at best 20/25. What causes it is normally due to refractive error between the eyes, one eye's glasses prescription is stronger than the other. The other major thing is called deprivation amblyopia. Deprivation is a little more complicated it normally can be caused by disease or eye problems like cataracts or ptosis (droopy lid) but after the problem is fixed the vision doesn't fully come back to 20/20. This is where Colin Blakemore's study on kittens comes into play. What Colin's study found out was that if you patch an eye of a kitten for 3 months once you remove the patch the kitten cannot see out of that eye. The brain has limited resources to develop fully. So the brain won't use resources to develop something that is not working. He also did something else in his study he had the kittens grow up only being able to horizontal lines for 3 months. After those 3 months, the kitten behaviors were measured. The kitten lost a lot of normal reflexes a 3-month kitten would have. The kitten was not able to see the movement of an object/react to the movement. After months of being in a normal environment, the reflexes return the kitten could see movement. Even after the months of the kitten being in the real world, the kitten couldn't see were vertical lines especially moving vertical lines. The cat never gain the ability to see vertical lines because the brain never saw them in that critical period when it was developing that part of the brain.
So how does this apply to amblyopia in deprivation ambloypia? It may seem obvious, it may not, but when growing up one eye could see because the other was covered by disease so it never developed the ability to see. There is only a limited number of resources to develop the brain why waste them when something doesn't work is basically what the brain does. In refractive amblyopia, one eye is covered by its glasses prescription. If they are hyperopic (farsighted) they have to use near vision to see. If one eye is emmetropic (not needing eyeglasses prescription) the other eye that is hyperopic (farsighted) amblyopia can occur because the body is lazy and does not want to work harder than it has to, to see. The near vision is never being activated enough for this hyperopic(farsighted) eye to see clearly so it is derived from seeing. This is the most common glasses prescription in someone that is amblyopic this is why it is important for kids to get eye exams because this kid can see well but only out of one eye and this means losing out of things like depth perception and always worried about an injury or disease happening to his good eye. Another form of refractive amblyopia is myopic (nearsightedness). This is a lot less common because myopia means they can see up close but not in the distance. Most of our world nowadays is up close but if they are very myopic(nearsightedness) then it can happen.
The last type of refractive amblyopia is Merdian amblyopia and this is caused by astigmatism. This is a little more complicated to explain because you really need to know what astigmatism is. Some people know it is because the front surface of your eye (cornea) is misshaped but let's explain what that does optically. This astigmatism makes one image in the real world form 2 images in the back of the eye. One of the images is in focus the other image is out of focus. Let me explain using the letter E. If you have against the rule astigmatism (like Dr. Chappel FYI this is where on the cornea astigmatism is) that means you can see the horizontal lines of the E but vertical lines are out of focus/can't see. Does that sound like Colin Blakemore's study on kittens because that is basically what happens. The worst part is this kind of amblyopia can easily happen on both eyes. Where the person who is amblyopic can only see horizontal lines and has a very hard time seeing the vertical lines even with correction. This is why people with astigmatism see things like shadows on images, lights that seem like streaks instead of circles, and/or sometimes see two images instead of one (normally with lights). I hope you found this educational and help answer any question you have on amblyopia.