The eye is a sense organ possessed by many species in a variety of phyla that allow for some sort of sight to occur in organisms. The eye is composed of many parts, all which play a role in the signal which eyes send to the brain. The sclera is the white portion of the eye which is used for protection and the point at which muscles can connect so the eye may rotate. The transparent layer on the front of the eye is known as the cornea which bends light and is covered by a thin layer of cells referred to as the corneal epithelium which protect the cornea from friction i.e. rubbing your eyes. Behind the cornea, is an anterior chamber filled with fluid known as the aqueous humor which is located in front of the lens and the iris. The lens, similar to the cornea, bends the light even more and inverts the incoming image but it can also change it’s shape depending on how close or near an object may be, and the iris is made of two different muscles which control the size of the pupil by expanding and contracting. The posterior of the chamber is made up of the vitreous humor which is a thick liquid made up of water, salt and the protein albumin that helps give the eye structure and suspend the lens so it remains in place (Structure of the eye). The back of the eye is covered in the retina which is a structure made of multiple kinds of cells which include, ganglion cells, bipolar cells, horizontal cells, and photoreceptors. The photoreceptors are located furthest back, directly in-front of the pigment epithelium. The role of the pigment epithelium is to reconstitute the structure of the light which has been bleached on its way in (Wessley College,2013). The light is then absorbed by photoreceptors which convert it to a neural signal by sending it to the brain from fibers connected to the posterior end of the eye known as the optic nerve.
Light absorbed by in the retina by the photoreceptors is the base for all that we see, and those photoreceptors are divided into two different cell types known as rods and cones. There are roughly 100 million rod cells located in the retina compared to about 6 million cone cells. Rod cells are responsible for light absorption settings with low light, this is known as scotopic vision. Rod cells can not detect color so we often only see grey in dark environments as rods only attempt to see very small amounts of light bouncing off objects showing their shape. Cones are used in well lit environments conditions where we see what’s known as photopic vision (Rods and Cones, cis.rit.edu). Color absorption occurs in cone cells by three different types of cones red, green, and blue. Each cone absorbs a different range of light frequencies (blue/short= 445nm, green/medium= 508nm, red/long= 565nm) and combinations of multiple light frequencies can lead the human brain to perceive different colors, such as yellow when red and green waves enter the retina. In the retina, there is a small impression known as the fovea is where visual acuity is at it’s highest and where the greatest density of cones (primarily red and green) are located and the lowest density of rods are located (Purves, Distribution, 2001). While cones and rods are both photoreceptors that have similar functions, they each have different neuronal pathways which end in their own specific responses.
Rods and cones are each unique photoreceptor cells which have the ability to absorb different light frequencies and convert those wave into what we know as vision. Cones interpret specific light waves, which differs depending on the cone (R,G,B), in a very similar fashion as rods with a few slight changes, such as the change of opsin groups. Because of this, I will only be focusing on what happens in rod cells. The neuronal pathway for rods begins with three main molecules known as rhodopsin (an opsin group bound to an 11-cis-retinal group), a transducin molecule which holds GDP molecules, and a phosphodiesterase molecule which has two alpha subunits attached. These molecules stay as is in the dark, but when a photon of light is introduced a series of reactions take place. First, the 11-cis-retinal molecule will isomerize to an all-trans-retinal molecule, this happens with the introduction of a photon which promotes the molecule to a higher energy state (Casiday, 2000). This allows for the molecule to leave the opsin protein which it was bound to which frees the binding site of opsin. The free binding site on the opsin then acts as a catalyst with transducin, converting hundreds GDP molecules that are bound in transducin into GTP molecules which are then released from transducin. Once released, GTP binds to and removes one of the alpha subunits attached to phosphodiesterase, this happens again with another molecule of GTP so that phosphodiesterase is free of alpha subunits. The phosphodiesterase then breaks a carbon oxygen bond in cyclic GMP which converts the molecule to GMP causing cGMP gated sodium (Na+) channels to close. This causes the cell to hyperpolarize and lower the release of the neurotransmitter glutamate (Skjeltorp, 2006) at the synapse which sends an electric impulse to the brain from that specific rod or group of rods by means of retinal ganglia cells ().
When signals from cones and rods are sent to the brain, they first must go through the optic chiasm. The optic chiasm is the structure where the left temporal visual field (contralateral eye) and right nasal visual field (ipsilateral eye) are sent to the right lateral geniculate nucleus (LGN) and the right visual cortex. The opposite is sent to the left (LGN) and left visual cortex, which receive signals from the right temporal visual field (contralateral eye) and left nasal visual field (ipsilateral eye). The LGN is a structure in the brain with six layers of cells both the right and left LGN, 80% of the nerves from the retinal ganglion cells are terminated. By the numbering in the LGN image below, signals from the contralateral eye are sent to layers 1, 4, and 6 while signals from the ipsilateral eye are sent to 2, 3, and 5. Layers 1 and 2 of the LGN are known as the magnocellular layer, also known as M cells, as they contain the larger nerves in the LGN. Layers 3 through 6 are known as the parvocellular layers, aka P cells (Dubuc,B). When a signal is introduced, M cells fire a stimulus that lasts a very short time period and do not include color opponency. P cells on the other hand exhibit color opponency as the firing of their neurotransmitters are dependant on the wavelength of light introduced to the cells. P cells also fire stimuli that last much longer than M cells, this could mean P cells contribute more to recognition than M cells (Visual Pathways, Brown.edu).
Image from: Department of Psychology, David Heeger, New York University
Axons from the LGN are sent to a few different subdivisions of the visual cortex, located in the occipital lobe. Most of these axons are sent to the subdivision known as V1, commonly referred to as the primary visual cortex. There are two primary visual cortex, one for each hemisphere, which receive signals from the LGN that is located in the same hemisphere. Each visual cortex is retinotopy, this means they have a map of the neurons of the retinal areas that the specific cortex covers (Heeger, 2006). The map of the right visual cortex shows where light is being introduced into the left visual field and the left visual cortex shows where light is being introduced into the right visual field, each of these are inverted. The visual cortex contains receptive cell groups known as simple, complex, and end stopped cell receptive fields. More information on these cells can be found in my blog post about David Hubel, but to summarize, these cell groups all have excitatory and inhibitory fields that respond to specific contours of light that possess specific orientation which trigger responses which can be sent to all cortical lobes in the brain which each have their own purpose.
Visual memories are a stimuli created by axons entering the temporal lobe from the visual cortex. Visual memories are important for us in understanding what objects are and how we are able to use them. Also, long term visual memories such as facial recognition, body language, and location recognition are useful and dramatically impact our every day lives(Temporal Lobe). The frontal lobe’s purpose with vision is primarily visual motor skills. These motor skills activate head and limb movements and reactions as well as understanding spatial perception (Schall, 2015). The parietal lobe is the primary sensory location in the brain and it receives and manages the information of senses including vision. Visuospatial navigation and reasoning primarily occur in the parietal lobe, this allows us to comprehend what we are seeing such as directions on a map, understanding size, shape, and orientation of objects, and also create numerical relationships based on the objects we are looking at (Parital Lobe). With damage to any of these lobes, vision may be affected and one could lose a characteristic of the most crucial sense humans posses.
Casiday, Rachel, and Regina Frey. “‘I Have Seen the Light!” Vision and Light-Induced Molecular Changes.” Vision and Light-Induced Molecular Changes, Washington University, 2000, Available from: www.chemistry.wustl.edu/~edudev/LabTutorials/Vision/Vision.html.
Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. Anatomical Distribution of Rods and Cones. Available from: https://www.ncbi.nlm.nih.gov/books/NBK10848/
A. Skjeltorp and A. V. Belushkin, “Dynamics of Complex Interconnected Systems: Networks and Bioprocesses.” Dynamics of Complex Interconnected Systems: Networks and Bioprocesses, Springer, 2006, p. 91.