“The theory is that stimuli have individual dedicated neural paths in the early stages of visual processing, and that repetitive stimulation of only one or a few channels causes a physiological imbalance that alters perception.” This is a common explanation of the neurological processes that occur when an individual experiences a physiological optical illusion. It means that our visual cortex uses several neural paths (structures of neurons that send information to the brain) and optical illusions occur when only a few of these neural paths undergo stimulation from an external source. This theory explains physiological illusions as a product of the aforementioned stimulation of certain neural paths.

There are many different types of stimulation that lead to physiological illusions. These include color, movement, tilt, and brightness. Illusions could be the result of rapidly alternating patterns, changes in color and brightness through flickering or due to the so called “blind-spot.” The blind-spot is an area of the eye which has no photoreceptors because the space is occupied by the optic nerve. These blind spots are on opposite sides of the visual field. The “blind-spot illusion”* occurs when the eye focuses on a certain point in space. When the eye is at a certain angle, the view is blocked by the blind spot because there are no photoreceptors to perceive that particular area of space. Therefore, the brain simply “fills” that space, usually with the same color that it perceives at that moment.

Another type of illusions caused by color and contrast are called “developed illusions.” These occur because the “cone” nerve cells that are located in the center of your retina become tired. These cones make us perceive color and when looking at complementary colors. For example, the cells become tired and the brain confuses colors. This illusion effect is also strengthened by the “rod” nerve cells that are sensitive to brightness. These cells are located at the edges of the retina which means that it’s possible to perceive something from the corner of the eye, but to see it clearly the retina needs to be centered on whatever it wishes to perceive. An example of a “developed illusion” is the “gray oval illusion”*.

Physiological illusions base themselves on our visual system’s physical constraints. Illusions are caused by physical limitations (blind-spot) and natural phenomena we have no control over (color, brightness, etc.). Because they base themselves physical aspects of the world, it is possible to experience these in real life. This means that the visual system does not always tell the whole truth. Don’t trust your eyes!