Neurons are stimulated in various ways. There are some specialized neurons that convert the physical energy of our environment into a neural signal. These neurons are called receptors. The receptors of greatest interest to us in this web book are the ones found in the retina and are responsible for converting light energy to an electrochemical neural signal for vision.

We have noted elsewhere that when light (a photon) is absorbed by photopigments in the outersegment of our receptors it causes the photopigment to change its shape; a process called isomerization. When this isomerization occurs, an electrophysiological process is initiated that results in signals being sent through our retina and up into the brain. The end result is that we say that "we saw something." Here we will discuss how such signals travel along a neuron and transfer from one neuron to another.

It is necessary to use the concept referred to as "ions."

Let's see how the concept of ions applies to the atoms on both sides of a cell membrane. It is important to understand that cell membranes are semi-permeable.

Movement of charged particles across a membrane causes an electrical current. If one places an electrode connected to the positive pole of a voltmeter inside the neuron and another electrode, which is connected to the negative pole of a voltmeter, on the outside of the neuron the voltage difference between inside and outside of the neuron can be measured. This voltage will be very small and is measured in millivolts (one millivolt is 1/1000 th of a volt). When one measures the voltage across a neural membrane that has not been stimulated they will find -70 mV. This is called the resting membrane potential and is found because there are more negative ions inside the neuron than outside due to large ions in the cell that can't get out and a membrane "pump" that pushes most of the positive sodium ions out.

Let's assume that a neuron has been stimulated. An action potential is formed and travels down an axon headed toward the junction between this neuron and another called a synapse. A synapse is a small gap between two neurons. The presynaptic ending contains synaptic vesicles that contain transmitter chemicals. When an action potential reaches the presynaptic ending it causes some of these vesicles to bond to the presynaptic membrane and to spew its transmitter chemical into the synaptic cleft. It migrates across the cleft and is received by the postsynaptic receptors. There are two kinds of postsynaptic receptors: 1. Those which when they receive the transmitter chemical exhibit an excitatory postsynaptic potential which results in a neuron depolarizing. 2. Those which when they receive the transmitter chemical exhibit an inhibitory postsynaptic potential which results in a neuron hyperpolarizing.




The receptors are the sensitive elements that absorb light and start the electrophysiological process that sends visual signals to the brain.

The rods and cones are not evenly distributed across the retina.  Most of the cones are in the foves.  In the very center of the fovea there are very few if any rods.  Click on distribution to see how the receptors are distributed across the retina.

When light enters the eye some of it will eventually find its way to the outer segment of the receptors. The outer segments contain photopigment molecules. When light (more properly speaking, a photon) is absorbed by one of these photopigment molecules it undergoes a morphological (shape) change called isomerization.

There are four classes of receptors: 1. Rods, which are used to see at night or under very low illumination. They are very sensitive but color blind. Rods are not shown in this diagram of the retina. 2. L- receptors are ones which are most sensitive to long wavelength light. Long wavelengths are the ones which appear red to us. 3. M- receptors are most sensitive to middle wavelengths which appear green to us. 4. S- receptors are most sensitive to short wavelengths which appear blue to us.

People with normal color vision have L- sensitive, M- sensitive and S- sensitive receptors. People with color variant (sometimes called color defective, or inappropriately called color blind) vision are missing one or more of these receptors.

About 8% of the population has color variant vision; most of these are men.

Select on receptor to see how rods and cones appear when viewed through a light microscope and through an electron microscope.

The ends of the receptors are called the outer segments. These outer segments contain photopigments with known spectral absorbencies which when they absorb light, that has entered the eye, starts the neurophysiological process that eventually sends signals to our brain.



Receptive Fields

Understanding Mach bands and radiating lines, (the perception of bright and dark areas that physically do not exist in the stimulus) requires our understanding the concept of lateral inhibition and receptive fields. We will consider receptive fields first.

There are many millions of receptors in the retina. There is only about one million optic nerve fibers sending visual signals up to the high brain centers. Consequently, individual receptors do not have private lines up to the visual cortex. Rather, multiple receptors converge on to subsequent neural units on their way to the higher visual centers. This convergence results in a physiological concept known as receptive fields.

To put it another way, a receptive field is the receptor area which when stimulated results in a response of a particular sensory neuron. If you clicked on the receptive field diagram then you already have seen a cartoon of a hypothetical set of neurons which shows how this works. You can also jump to a more detailed explanation. You may have already read this explanation if you clicked to it from the receptive field diagram.

If you feel that you now understand the concept of receptive fields, we can proceed to use this idea to explain Mach bands and Radiating lines. Let's look again at Mach band diagram but this time with a receptive field overlay, or the Radiating line figure.


Phototherapy is the clinical approach to treating various ailments with the use of light.  Although this really has little or nothing to do with visual perception, over the years I have been approached to provide assistance to those who wanted information on this subject.  My reply for many years was that "I don't know anything about it."

In 1982/83 while on sabbatical leave I decided to do a critical review of the literature on phototherapy.  The results of this effort were published in COLOR RESEARCH AND APPLICATION. (Phototherapy using chromatic, white and ultraviolet light, vol. 9, Number 4, Winter 1984, pp. 195-205.)

Here I can give only a brief synopsis of this journal article published in 1984.

"The purposes of this article are to (1) convince the reader that the vast majority of color-therapy literature is ill-conceived and a potential danger to society; (2) review various research methodologies that would aid in the effective investigation of color therapy; (3) review the existing research  literature using color therapy; (4) review a selected research literature using white, and ultraviolet light for therapeutic purposes; this latter research, plus that involving phototherapy with jaundice, represents examples worthy of being followed by the general field of color therapy research; and   (5)review some basic research that shows the biological interaction of humans with light; this area of research suggests that the area of color therapy is worth further exploration if done by competent investigators." (p. 195)

In the main body of the journal article I present a table that shows the myriad of ailments ostensibly treated with various colored lights.  In this table I present the references  that show where these alleged successful treatments are reported.   The vast majority of this literature is not scientific and not published in peer reviewed sources.

A simplified conclusion of my journal article is as follows"

"The use of light as a therapeutic agent has a long history.  However, there is little or no scientific foundation for much of what has been published in the area of color therapy.  There is a good foundation for the treatment of neonatal jaundice using blue light.  Preliminary basic research  suggests that when used with photodynamic agents, some malignant tumors may be  treatable with white light.   Other studies suggest that psoriasis can be treated with ultraviolet light when the affected area is first  covered with anthracene.  Evidence is accumulating that show how the human body does interact with light at a hormonal, metabolic, and endocrine level.  I conclude that there has never been a better time for competent scientists to investigate whether there are additional effective color therapeutic procedures in addition to the use of blue light for jaundice."

I can not stress strongly enough that the article I wrote and from which I quote above is more than 15 years old.  A lot has happened since then  One of the treatments that has received a great deal of attention in the public media and I believe has a good scientific foundation is the treatment of Seasonal Affective Disorders (SAD) with white light.

Below I provide a number of links to websites I found on the WWW.     information on Seasonal Affective Disorders (SAD)     information on treating neonatal jaundice with blue light   information on the use of phototherapy and photochemotherapy   information on risk factor in phototherapy with neonatal jaundice   asks whether UVB used in phototherapy is carcinogenic  information on UVA and UVB phototherapy   discusses using 311 nm light for phototherapy discusses phototherapy in the treatment of SAD   Vanderbilt University Medical Center has a Phototherapy Center.

The above links are by no means a complete list.  But it may be enough to provide a start for those interested in pursing this subject.

Physiological Response To Color

In 1984 I published a journal article entitled "Physiological Response to Color: A Critical Review" in Color Research and Application, Vol. 9, Number 1, Spring, 1984, pp. 29-36.  It is impractical to reproduce the entire article on this website.  However, since I frequently receive queries on this topic I thought it worth while to provide the above reference and the conclusions at which I arrived in the critical review.


"Does color effect human behavior? Yes.  Do human beings respond physiologically to color?  Yes and perhaps.  There is no question that there are physiological responses to color.  If there were not, we would not see color.  But color vision was not the subject of this review.  The question was, are there nonvisual responses to color that can affect our behavior?  Clearly, there are some, for example, the effect of colored light on hyperbilirubinemia (jaundice).   It has been shown that red radiation is more conducive to producing epileptic seizures than blue light.  Indeed, it has been suggested that wearing spectacles that eliminate long wavelengths would be helpful in preventing epileptic seizures.  The research on alpha-wave suppression due to colored light  is more ambiguous.  A reasonable hypothesis is that these EEG (electroencepholographic) responses are cognitively mediated and are not direct responses.  Likewise, the galvanic skin response (GSR)to colored light is probably cognitively mediated; however, the evidence on this matter is far from conclusive.  It is known that the GSR  is mediated by sweating, which in turn is mediated by emotional arousing of activation responses.

The data on blood pressure, respiration, and heart rate are inconclusive.  There was only one investigation that used eyeblink frequency.  Gerard found the greatest frequency to red, less to white and least to blue.   These results bear replication.  Indeed, research on physiological responses to color has not been very active for several years.  There is much more basic research to be done so that more effective  use of color can be made in our environment.

The reader is cautioned that the above conclusions were reached in 1983 and may be out of date.  Although, I have not been made aware of any dramatic changes in the scientific literature.  Further, it is strongly advised that before running off with the above conclusions that one read the original article first.  If you are unable to find Color Research and Application in your library request it from the Inter-library Loan Department.

Physics of the Visual Stimulus

In order to get a fuller understanding of why we see things as we do, it helps to learn a little about the physics of light. Light is, after all, that which enters our eyes and causes us to see.

Light is electromagnetic energy. The electromagnetic spectrum is very large ranging from gamma rays to AM waves. The visible part of this range is very small ranging from about 400 nanometers to a little over 700 nanometers.

That light may come directly from a source like a light bulb and TV screen or it may be reflected light as comes from, say, a piece of paper or from a movie screen.

Light has been considered as energy packaged in particles (the particle theory) or in waves (the wave theory) Since it is easier to diagram the idea behind the wave theory I give an illustration of the wave nature of light.

Reflecting Surfaces

As can be seen in the surfaces demo, there are three major classes of surfaces from which light can be reflected.

One surface with which we are all familiar is a mirror. A beam of light that hits a mirrored surface at a given angle will be reflected off the mirror in exactly that the same angle. See middle panel of the demo. The jargon is that the angle of reflection = the angle of incidence.

When light hits a perfectly diffuse surface, (we often call such surfaces mat) it is reflected approximately equally in all directions. See the top panel of the demo .

Some surfaces appear neither mirror like (glossy) or perfectly diffuse (mat), they are what is frequently called semi-gloss. That is to say they exhibit highlights as well as mat areas. The bottom panel of the demo. illustrates the light reflection properties of such a surface.

The color reflected from a mirrored surface will depend only on the color of the light hitting the mirror. However, the color reflected from, say a piece of paper depends on the spectral reflectance of the paper as well as the spectral properties of the light. To see this idea graphically click on colored paper.

Transparent Surfaces

When light encounters certain materials, for example, glass and plastic, most of the light appears to pass through unhindered. These materials are called transparent. However, in fact, 100% of the light will not pass through them. Click on transparent media to see the various things that can happen to light in such materials. When light passes from one transparent medium to another it often under goes a process called refraction and obeys Snell's Law.

Polarized Light

You probably have, at one time or another, worn sunglasses that carried the Polaroid label. Polaroid is a company, probably, best known to the public for its instant cameras. However, the company got it start when Edwin Land discovered a new and inexpensive way to make polarizing filters.

If you see sunglasses that claim to be polarizing you can easily check them out for yourself. Take two pairs of sunglasses. Superimpose two of the lenses and then slowly rotate one with respect to the other. If the lenses are polarizing filters you should see the amount of light that passes through them change as a function of their relative angles. In one position almost very little will pass through (probably only some deep blue light*) and 90 degrees from this position you should see the maximum amount of light pass through. To understand this test for polarizing filters and how they work select an explanation.

The dark greenish filter we often seen in polarizing sunglasses is not the only means of polarizing light. Light can be polarized by scattering, reflection and absorption (Falk et al., 1986). Polaroid filters typically seen in sunglasses absorb one electrical field of polarized light while transmitting the orthogonal field.

For a more complete account of polarization see a very readable chapter entitled "Scattering and Polarization" by Falk, Brill and Stork (1986)

* This type of polarizing filter is not very effective for short wavelength lights. Therefore these wavelengths do not become very polarized and pass through the crossed filters even when other wave lengths don't.




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"Whilst part of what we perceive comes through our senses from the object before us, another part (and it may be the parger part) always comes out of our own mind." -- William James


     Most of us take vision for granted. We seem to do it so effortlessly; however, perceiving images, objects, color, and motion is a very complicated process.

     Take a moment to observe the world around you.  For example, if you tilt your head, the world doesn't tilt. If you shut one eye, you don't immediately lose depth perception. Look at what happens to color under varying types of illumination.  Move around an object: The shape you see changes, yet the object remains stable.  Look at some of the illusions on this web site. Even though you may intellectually know that you are being fooled, it does not stop the effect from continuing to trick you. This indicates a split between your perception of something and your conception of it. In many cases your higher order cognitive abilities can not influence your lower order perceptions. 

     Only in the last one hundred years, and especially in the last twenty years, have scientists started to make some progress in understanding vision and perception. Illusions can be a wonderful window into this process. And they are fun too! Because they combine both the element of joy with the element of surprise.

     The late, great physicist Richard Feynman wrote, "It's quite wonderful that we can 'see,' or figure it out so easily. Someone who's standing at my left can see somebody who's standing at my right - that is, the light can be going this way across, or that way across, or this way up, or that way down; it's a complete network. Some quantity is shaking about, in a combination of motions so elaborate and complicated the net result is to produce an influence which makes me see you, completely undisturbed by the fact that at the same time there are influences that represent the guy on my left side seeing the guy on my right side. The light's there bounces off this, and it bounces off that - all this is going on, and yet we can sort it out with this instrument, our eye."

     This is not the end of this wonderful process. Light waves enter your eye and then enter photoreceptive cells on your retina. The image that forms on your retina is flat, yet you perceive a world of shape, color, depth, and motion.

     How does our visual system recover three-dimensional information? This is an important question. Our retinal images, whether from a two-dimensional image or from the three-dimensional world, are flat representations on a curved surface. Yet, for the most part, we perceive an accurate world of depth, surfaces and objects.

     A closely related problem is that any one aspect of a visual scene is spatially ambiguous. There is an innate ambiguity in the retinal input (many to many mapping between objects and retinal images). In other words, for any given retinal image, there are an infinite variety of possible three-dimensional structures that can give rise to it. Our visual system, however, usually settles for the correct interpretation. When a mistake is made, an illusion occurs.

     The fact that we can recover accurate three-dimensional information from a visually ambiguous two-dimensional representation means that some very powerful constraints must be imposed on our interpretations of two-dimensional images.

     These constraints must also account for many illusions. In fact, illusions are a powerful and fun tool for revealing constraints that mediate vision and perception. In some cases, illusions take place because the constraints for interpreting an image are ambiguous. Your visual system can interpret the scene in more than one way. Even though the image on your retina remains constant, you never see an odd mixture of the two perceptions - it is always one or the other, although they may perceptually flip back and forth. Normally, this does not happen in the real world, as your visual system has evolved many different ways to resolve ambiguity. Visual perception is essentially an ambiguity-solving process. This process is called "inverse optics."

     The early visual process behaves intelligently, but mostly in a bottom-up fashion (separated from cognitive processes). The visual system is also highly adaptive, e.g., visual adaptation is not merely fatique. It should be understood that both evolution and learning contribute to visual capabilities.

     Artists have also been trying to understand how we perceive, and much of our understanding of vision comes from learning how artists manipulate images into meaningful and realistic scenes. Artists have always created illusions. That's their business.

     Artists and scientists over the years have experimented with these rules to produce illusions either by reducing the number of visual cues for interpreting images or by deliberately setting up situations where the rules come into conflict.

     We hope that you will have fun exploring both the scientific and artistic sides of this web site and see how both artists and scientists have used illusions to reveal the underlying process of the human mind.


Entire siteŠ1997 IllusionWorks, L.L.C.