Wednesday, June 25, 2008

A little bit of basic ocular anatomy… The Retina.

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As I wrote in the first post of this series about “Basic Ocular Anatomy”, the retina (10) is a layer of the eye that deserves one blog post just to itself.

It is the most important layer of the eye, because it contributes to the first part of the Visual Information Processing that we receive: the formation of the image, which will be processed later on.

I am going to show the retina from different points of view, in order for it to be easier to understand. Its importance is appropriate to its complexity.

In one hand, in order to understand how the eye works, you must think about it as an “old picture camera” (that is, with film ;-)); just as the image is expressed in the camera film, it is expressed in the eye retina.

In the other hand, you must also think that the eye behaves as a “dark chamber”; this consists for example, of a dark box which has a little hole in one of its walls (as the pupil in the eye). In opposite wall (as the retina in the eye) an inverted image is made from the external objects.

One eye without any problem of refraction (myopia, hyperopia or astigmatism), that looks at distance (more than fifteen feet), is at a state of rest. The iris (2) behaves as a camera diaphragm, whose contraction controls the quantity of light that gets into it through the pupil (3); in this situation, the lens (8), the other dynamic part of the eye, is at rest. Therefore, the human eye does not have to strain when it looks at distance.

When one person looks at near, if these dynamic parts of the eye do not modify their state, he will see blurred. It is the same thing that happens if the lens of a camera does not modify its focusing in order to clear a close object (after focusing something that was far): the picture will be blurred.
In the eye, in order to clear a close object, the thickness of the crystalline lens (8) varies. This mechanism is called accommodation, but I will write about this later.

When we look distant something as well as close something, this has to get to the film inside of a camera, or to the retina in the eye, in order to achieve a clear image.

Once you have understood how the eye works, now I am going to show you how the light gets into it, and it reaches the retina (10):

The light thrown by one object converges on the cornea (1), passes through the pupil (3), and gets to the crystalline lens (8). At this point, the image is inverted (according to the optics laws). If there is no reaction in the lens, the image that reaches the retina(10) can be more or less blurred, depending on the distance to the object (considering that this is not further than fifteen feet, as I explained before). That way if the image is blurred, this information gets to the brain and it answers by sending a command to the crystalline lens in order to modify its shape and therefore, allowing the image to get to the retina, thus creating a clear image. This action happens automatically and the process is very fast; it is similar to the autofocus process of a picture camera, which focuses automatically on what it detects in the central framing of the viewfinder.
So, after the light passes through the gelatinous vitreous humor (9), it will reach the end of the route in the eye: the retina (10); in this point the image is clear (I will explain later which are the cases where the image is not clear and why).

But unlike a picture camera, the image does not just stay in the retina. This eye’s innermost nervous layer is the one responsible for converting the photons of the light that it receives into nervous signals that can be relayed to the brain; there it interprets them and gives them a right meaning. Therefore, the brain is the part responsible for “developing the film”, that is, interpreting it (knowing what the object is, what it means to us, what emotion causes, how to answer before it, and so on).

In order to achieve this transformation, the retina (10) is composed of five kinds of nervous cells, which collect all luminous information. They are not just responsible for making the image clear, but they also extract the basic information of the object about its color, its shape, its orientation, its movement, transmitting it to the brain. The human eye transmits visual data to the brain at about the same speed as two computers can share data.

These cells are placed in the retina in different layers, but as I do not want to make it very complicated, I am just going to point out three of them:

The light passes through all retina nervous layers to reach the outermost one: the one composed by photoreceptors (called rods and cones) (A); there the incoming light is reflected and passes through the retina again in the opposite direction, transforming this light into nervous impulses and transmitting the neurological information through different layers of nervous cells to reach the last one: ganglion cells (C); in this layer their thin axons bundle together in order to get out the eye and to the Optic Nerve (11) and send the impulses to the brain, thus starting, the VISUAL PATH.

There are two subtype of photoreceptors: rods and cones (A) distributed by the whole retina, where each one has a certain function and location.

- Neither of them exists in the point of the ganglion cells exit (Blind Spot -12-).

- Cone cells are found mainly in the central area of the retina, because they are the cells responsible for the details (the clarity, the shape and the color) of the object. This central area of the retina is where the eye “machine” leads the image of an object, in order to see it clear and with 20/20 of vision. This central area is the macula (14) ( and its central point with higher vision in the retina is the fovea (13) ( As these cells are the ones responsible to catch the details, they work better with well-light conditions. So, some activities where these cells are used are for instance, when reading or writing.

- Rod cells, in the other hand, are found mainly in the peripheral area. As we get away from the macula (14), the quantity of cones decreases and the quantity of rods increases. In the peripheral area the information about the clarity or the color is not so important, but detecting the orientation or the movement of the object that we look at. Therefore, these cells are stimulated with low-light conditions. Also, these cells are very sensitive to changes in contrast even at low-light level.

These photoreceptors (A) at the same time, stimulate certain ganglion cells (C); that is, each type of photoreceptor stimulates one type of ganglion cell, so each cell takes a certain information; both information elements travel parallelly to different areas of the brain, and once there, all information is mixed. The brain gives the meaning of the world surrounding us: where the object is, what it is, how big is, what color it is, how far it is,… It takes all information from the world and puts it together, in order to find similarities and differences, compare, discriminate and so on.

Consequently, in one hand, CONES (A) send information to PARVO ganglion cells (C), which take the information about the shape, the color and the detail, that is, what the object is; they help us identify and take out a meaning; help to see that object clearly (the Visual Acuity) and they work better if the object is stopped.

In the other hand, RODS (A) send information to MAGNO ganglion cells (C), which take information about the movement, the space and the orientation; that is, they report on the movement direction, its speed, calculate distances, where the object is, where I am and three-dimensionality. They help us move ourselves inside a dark room avoiding that we hit ourselves with other objects, or avoiding that we crash with the things that we do not look at directly (for example, with the doorframe when we go through it without looking at it), and so on.

As you can see, we do not just “see” the objects, or rather, we do not just see them clear or blurred; what a retina catches from an object, that image, is not only a picture, goes along with much more information and it all begins to be processed in the retina. But the rest of this complex but amazing processing of visual information that happens in the brain, will be explained it in the next post…

A little bit of basic ocular anatomy… Eye or Ocular Globe
A little bit of basic ocular anatomy… What is the eye surrounded by?

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