This is a section of the web book that I have never developed. Fortunately, there is a document on the WWW by Dr. Giordano Beretta of the Hewlett-Packard Company that has an  excellent Glossary.  The name of the  document is Understanding Color and the Glossary starts on page 75. Understanding Color is more easily viewed if you choose the No Frames option shown on the home page of this web book.


The adjustment of the eye to obtain maximal sharpness of the retinal image for an object at which one is looking. One of the important changes involves is in the shape of the eye lens. Click on lens to see diagrams on this topic.

Action Potential

An action potential is the result of a brief, localized change in the resting membrane potential. When this change of resting potential is measured it can be visualized on an oscilloscope. Action potentials are sometimes referred to as spikes or spike potentials.


Contralateral means opposite side. So, for example, if a particular layer of the LGN, in the left hemisphere, receives input from the contralateral eye that means it receives input from the right eye.


The resting potential of a neuron is typically about -70 millivolts. This means that the inside of the neuron is about 70 millivolts more negative than the outside. Under this state the neuron is said to be polarized. When the neuron is stimulated and an action potential results the inside of the neuron briefly becomes positive relative to the outside of the neuron. When this occurs the neuron is said to have been depolarized.

Hue, Saturation and Brightness

Hue is most closely related to the wavelength of the stimulus. Different colors have different hues.

Saturation is related to how much white content is in the stimulus. Monochromatic hues are very highly saturated. The least saturated color is white. So for example a pink would be less saturated than a spectral red but more saturated than white.

Brightness relates to the amount of light coming from a source or being reflected from an object. Strictly speaking the amount of reflected light is called lightness.

Select Demo to see hue, saturation and brightness demonstrated


If you will look at the representation of an action potential you'll note that after the potential becomes positive it drops rapidly and for a brief time is more negative than it was at its resting potential. The increased negativity is called being hyperpolarized.


Ipsilateral means the same side. So when it is said that the left half of each retina projects to the ipsilateral cortex it means that they project to the left cortex.

Unique hues

Unique hues could also be called psychologically pure. For example, a unique green has no yellow or blue content. A unique yellow has no green or red content. Four unique hues have been identified: red, green, yellow and blue. There are monochromatic wavelengths that produce unique green, yellow and blue. Unique red, however, can only be produced by mixing a little blue light with even the longest wavelengths. A wavelength of 700 nm for instance contains a small yellow content. When blue light is mixed with 700 nm the yellow is cancelled and a unique red is produced that contains no yellowishness or blueness.

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