According to the Young-Helmholtz theory color vision depends on pairs of opposing retinal processes:.1. The retina contains three kinds of color receptors. 2. Certain nerve cells in the brain respond to specific features of a stimulus. 3. The size of the difference threshold is proportional to the intensity of the stimulus. 4. The optic nerve processes top-down stimuli.

People who utilizes the progression principle will ensure their exercise program increases in frequency, intensity, time, and type. the changes need to happen gradually in order to ensure they make gains in their fitness program. a person who doesn't utilize the progression principle can still gain health benefits from exercising, but will not continue to improve in cardiovascular fitness.