How are light rays converted to electronic impulses?

Question

...and how are those impulses processed in the brain?

Answer 1

As much as I respect secretsauce I really can’t agree with him here: misoma5’s answer is not excellent. In fact, it is absolutely riddled with mistakes.

An additional problem here is that vision is a very complicated process and so to correct the errors, I will have to write a very long answer – which probably no one will read. Oh well.

Before I start with that, though, there are a couple of misconceptions in your question as well:
1. Physicists tell us that light can be thought of as being made up of particles (known as photons) or waves. In the case of your sense of sight, the relevant way that light acts is as a particle, a photon, not as a ‘ray’.

2. The mammalian nervous system is not ‘electronic’. That is, it does not use electrons for energy or to transfer information. Your nervous system is ‘electrochemical’. The ‘electro-‘ part means that your nerve cells use ions like sodium (Na+), potassium (K+), calcium (Ca++), and chloride (Cl-) to activate and inhibit processes inside the cells. The ‘chemical’ part means that your brain uses molecules called neurotransmitters to carry signals between cells.

So, back to the eye …

Your eye is very much like a camera: there is a focusing lens that directs the light onto a light sensitive surface that records an image. In your eye, the structures that detect light are the photoreceptors of the retina. These are the cone and rod cells. They form two separate systems for seeing: the cones are active in bright light and the rods are active in dim light. Cones detect color, and so we see colors during the day, but rods do not detect color, so in dim light everything is shades of gray (If you see colors at night, it is because there is enough light for your cones to be active).

The process of vision begins when a rod or cone absorbs a photon of light. This happens when the photon is captured by the photopigment of the photoreceptor. The photopigment is a protein, known as an opsin, plus a form of vitamin A. To see color, we have three different kinds of cones, each with their own opsin: red opsin, green opsin, and blue opsin. Rods only have one and it is simply called opsin; it is the complete photopigment in rods, the opsin protein + the vitamin A, that is known as ‘rhodopsin’. In all photoreceptors of all vertebrate eyes, the form of vitamin A is the same (11-cis retinaldehyde), and it is the part of the molecule that actually absorbs the light.

When a photon hits the photopigment, the vitamin A changes shape. This causes the whole photopigment to change shape and initiates what is known as the visual cascade: the activated photopigment stimulates a protein known as a G-protein (AKA transducin) which stimulates a protein known as phosphodiesterase (NOT guanylate cyclase!). The phosphodiesterase breaks down cyclic GMP (it does NOT increase it!). This causes an ion channel on the membrane of the cell to close (NOT open!). When the channel closes, the cell hyperpolarizes (NOT depolarizes!), and this causes a decrease in the amount of neurotransmitter released by the photoreceptor cell (NOT an increase!).

So far, all of this happens only in the first cell that detects the light. Notice that all it did was sense a photon. The photoreceptor itself only senses this tiny part of the visual field. The whole rest of the scene is constructed out of this information by combing the inputs from about 100,000,000 photoreceptor cells/eye in a very complicated, and still not entirely understood, process of neural processing. The retina probably contains over 100 kinds of nerve cells that act to start the process of assembling a visual scene from the individual photons that have been detected. See an answer I gave here about two of the kinds of cells involved:
http://answers.yahoo.com/question/index;_ylt=As4qCZUIX1Irxi.jlb1jKVPty6IX?qid=20070429032546AAmStWS&show=7#profile-info-6p6JUBH6aa

Anyways, the photoreceptor sends its information to a bipolar cell, and this connects to a ganglion cell. The G-cell is the cell that sends an axon out of the eye along the optic nerve to the brain. The first stop is the lateral geniculate nucleus (NOT the medial geniculate = this is used for hearing!). For vision to make sense, it is very important that the relationships of the various parts of the visual scene be maintained, and so the geniculate nucleus is organized such that the incoming axons from the retina form a precise ‘retinotopic’ map that keeps the information organized in the same relative positions as the photons were detected in the retina.

From the lateral geniculate, the information is passed to the visual cortex at the back of the brain. Again, a retinotopic map is set up during development so that the information is a coherent representation of the visual field. The central retina is represented at the very back part of the brain.

As both of the other answers described, there is a process, which starts in the retina and which is continued in the brain, that takes the visual information and finds it’s characteristics. The information is passed up through levels of computing to detect edges, movement, shapes, colors, and so on. Eventually, the hierarchy of computing assembles all the pieces together into a full picture: the edges and shapes become recognized as a face, which you then recognize as your Mom, and the background becomes organized into a room she is standing in, which you then recognize as your living room. The highest levels of computation and assembly are the ones that allow recognition of things that you personally identify with ... first it is edges, then features, then a face, then a face that matters a lot to you.

Francis Crick, of Watson and Crick fame, wrote a whole book about this called “The Astonishing Hypothesis”, if you are really interested in learning a lot about this.

All of this is accomplished through the movement of ions across cell membranes and the diffusion of neurotransmitters from pre-synaptic to post-synaptic cells. Incredible!

Incidentally, the retina is an outgrowth of the brain. It is actually the only part of the central nervous system that can be seen from outside the body = when an ophthalmologist is looking through your eye pupil at your retina, she is actually looking at part of your brain!

A final difficulty about answering a question like this is that it is tough to know what level to answer at … hopefully I haven’t been too technical!

Good luck!

Answer 2

Damn. Misoma stole my thunder with his great answer (vision is my other big passion in biology along with evolution).

But Misoma is correct. Photopigments are protein molecules that literally change *shape* when they absorb a photon (packet of light) of a certain wavelength. This is what starts the chain reaction that causes a nerve impulse to travel down the photoreceptor (the cell in the retina that contains the photopigment) to the nerve cells in the next layers of the retina.

Some of these impulses from the many photoreceptors are processed right in the retina. A single nerve cell in the retina may be connected to multiple photoreceptors ... some causing a firing, and some *inhibiting* the firing of that nerve cell. For example, a nerve cell may receive a signal from photoreceptor A, B, and C that each have different photorpigment tuned to react to red, green, and blue photons respectively. However in order for this nerve cell to fire it needs a signal from A and B, but this signal is "cancelled" (inhibited) if it also gets a signal from C. This means that this nerve cell is detecting the presence of light that has red and green wavelengths but a lack of blue wavelengths ... the sensation we call "yellow."

This kind of "sorting" occurs in the retina (basic detection of colors, edges, movement, etc.) ... what happens in the visual cortex is essentially the same concept, but multiplied by millions of layers. Multiple already-"sorted" nerve impulses acting as inputs (some of them inhibitory), to other nerve cells that produce more "sorted" impulses for more nerve cells. So that "yellow" impulse gets combined with the right "shape" impulses and the right "context" impulses which produce the right memory association impulses to produce the thought "I see the object I recognize from previous experience as a banana."

Answer 3

There are cells in the retina of your eye called rods and cones. They both respond to photon stimulation in the visible range. There are proteins called rhodopsin that undergo a conformational change in response to light stimulation. These proteins when hit by light, activate guanylate cyclase creating cyclic GMP which acts as an intracellular second messenger. This triggers depolarization of the neurons in the optic nerve that sends the signals to the medial geniculate body and to the occipital lobe where the visual cortex is located. There the signals undergo processing at several levels tp allow you to see. There are processing cells that for example, will recognize when straight objects are moving within your visual field. When these objects pass, certain processing cells will fire if the linear object is vertical, others will fire if the object is in a horizontal position, and still others will fire if the object is at an angle. There are groups of cells that will recognize certain shapes while others will recognize motion and still others recognize specific colors, or brightness.