Now You See It, Now You Don’t
This highly complex system is designed to detect light energy and transduce it into electrical information that is sent to the visual cortex of the brain for perception. Interestingly, light input in the right eye is perceived in the left part of the brain and vice versa. This crossing of the information and the slightly offset position of the eyes allows the brain to perceive different dimensions, including depth perception.
Anatomy of the Eye
The eyes are organs that capture and focus light energy on the back portion of the eyeball where special visual perception cells exist. Structures of the eye include:
· the retina, which contains the photoreceptor cells
· the sclera, which forms the outer covering of the eye
· the cornea (clear portion of the eye), situated just behind the sclera, which allows photons of light to pass into the eye
· the iris, found under the cornea, a spherical diaphragm that can open or close to regulate the amount of light that passes through
· the pupil, an opening in the iris, which is black because the back of the eye is dark
· the lens, which is suspended behind the iris and positioned immediately behind the pupil, and which focuses the light that enters the eye
· the ciliary muscle, which keeps the lens in place and can apply tension to the lens to stretch it or allow it to contract in order to change the focal length of the light entering the eye (in order to shift between seeing objects up close and far away)
· the vitreous humor, which is the gelatinous material that fills the large chamber of the eyeball through which light passes after it enters the eye and is focused by the lens
Retina
The back of the eyeball has a layer of photoreceptor cells called the retina. Light passes through several layers of neurons and interconnecting cells before the photon impacts and excites a photoreceptor cell buried in the deeper parts of the retina. The only part of the retina deeper than the photoreceptors is a layer of pigmented epithelial cells that absorbs light and prevents its reflection to reduce extraneous light and improve fine detail.
How do some animals see at night?
Nocturnal animals have a reflective pigment in their epithelium that reflects light directly back onto the photoreceptor, thus stimulating it multiple times with a single photon and improving vision in low light.
Photoreceptors
These cells, found in the deep retina, contain a photo pigment that, when activated by a photon of light, change shape and lead to a signal transduction cascade, ultimately generating an electrical signal in a neuron that signals the visual cortex of the brain.
One type of photoreceptor is the rod cell. These cells are shaped much like a comb for your hair. Rows of folded membranes that resemble the teeth of the comb contain the photo pigment rhodopsin, which, when activated, leads to the change in the shape of the larger molecule where it is attached (opsin). An essential component of this photo pigment complex is a molecule called retinal, which is related to vitamin A. Rods are the more sensitive of the two types of photoreceptors and are responsible for vision in low light, which many have referred to as black-and-white vision.
Why do people say eating carrots improves your eyesight?
Carrots are a rich source of vitamin A, which is part of the photo pigment complex.
Cones are the second photoreceptor and are responsible for color vision in the retina. These cells also have folds of photo pigment, but are shorter than the rods and have a tapering or cone shape, hence the name. Red, green, and blue cones detect different wavelengths of light.
Which cones are stimulated and to what degree determines the color of light that is signaled and perceived. This is not unlike your television screen or computer monitor where these three same colors are used to blend into any visual color in the spectrum. While cones are much less abundant than rods in the retina in general, in the fovea of the retina (the central part of the retina responsible for sharp central vision), cones greatly outnumber rods.