A summary of how light enters the eye and is processed by the sensory system. ** note: this content is not formatted as an easy read. Vision perception is complex stuff 😉
This post describes the process by which information enters the eye and gets turned into component features of the visual patterns in the occipital cortex. The first step of this process is receiving information from the external environment. Light gets into the eye. It passes through the cornea, aqueous humor, the pupil of the iris, lens, and vitreous humor, on its way to the retina. The light ray is diffused and refracted as it passes through each part of the eye. This helps to focus the image and filter away some of the unneeded photons from the light ray.
The retina is where seeing begins and sensation happens. The image “seen” by the retina is a flipped version of the image in the environment. The retina contains approximately 90 million rods and 4-5 million cone neurons that react to the photons in light. The rod neurons are highly concentrated in the peripheral of the eye and are almost absent from the center. These neurons are good at detecting movement in black and white scale in low-light conditions. In contrast, the cone neurons are heavily concentrated in the center of the eye and are good at detecting color and details (ex. identifying objects, reading) in high light conditions. There are three different photopigment receptors for cone neurons. Each cone will be receptive to a different wavelength of light. The different neurons are blue sensitive S-cones, green sensitive M-cones, and red sensitive L-cones.
Retinal information processing is completed by 5 main cells: photoreceptors, horizontal cells, bipolar cells, amacrine cells, and ganglion cells.
The first cell in the process are the photoreceptors. Light transduction by the rod and cone neurons are similar. For a rod, transduction begins with the photon of light hitting the outer segment of the rod neuron where it is absorbed by a molecule of rhodopsin. Then the energy is transferred to the chromosome portion of the visual pigment molecule during photo activation. It then is subjected to hyperpolarization to balance electrical current between the inner and outer segment of the rod. From here, photoreceptors communicate with the bipolar cells through synaptic terminals with graded potentials (as opposed to binary.)
Then a process of lateral inhibition takes place through the horizontal and amacrine cells. Lateral inhibition allows the signals that reach the ganglion cells to be based on differences in activation between nearby photoreceptors. This helps the visual system filter out noise from the photoreceptors. If every active photoreceptor was sent down the optic nerve of the eye for further processing it would overwhelm the visual system.
After initial processing and filtering the signals arrive at the ganglion cells which are the final layer of retinal processing. There are multiple types of ganglion cells but the primary two are P and M ganglion cells. P ganglion cells comprise approximately 70% of all ganglion cells and are the small-cells that are important for processing contrast. M ganglion cells comprise approximately 8-10% of all ganglion cells and are the large-cells that are important for processing how an image changes with time. These cells receive information from the intermediate retinal neurons, process the information further, and send it on to the brain. Ganglion cells have center-surround receptive fields that are sensitive to spots of light. There are both ON-center and OFF-center cells. As the names suggest, these cells either fire or inhibit the firing of action potential based on where the light hits its receptive field. This mechanism for processing further helps to filter the information coming into the visual system.
During visual processing within the eye 4 mechanisms exist to adapt to the high differences of illumination of different environments. The eye can adapt to light and dark environments by adjusting pupil size, regenerating photopigments, making use of the duplex structure of the retinal neurons, and most importantly by the ganglion cell’s reliance on patterns of illumination instead of overall light received within the visual system.
After the ganglion cells, information is sent through the optic nerve, optic chiasm, and optic tract to the lateral geniculate nucleus (LGN) of the midbrain. At this point, processing has transitioned from the eye ball to the brain. It has also transitioned from processing spots of light to processing stripes (edges, grates, etc.) of light.
The LGN is made up of two parts, one in each hemisphere of the brain. Within each hemisphere, the LGN is comprised of 6 layers. The bottom two layers are the magnocellular layers. These layers are large and respond to the M ganglion cells. Like the M ganglion cells, they process large, fast-moving objects. The top four layers are the parvocellular layers. These layers are small and respond to the P ganglion cells. Like the P ganglion cells, they process details of stationary objects. The topographical mapping of the LGN is highly structured. Each cell responds to either the right or the left eye but not both. The left LGN will process information from the left side of both eyes and the right LGN will process information from the right side of both eyes. The LGN is a stop from the retina to modulate input from the eyes.
Information from the LGN is then sent to the striate cortex. The striate cortex is also known as the primary visual cortex, V1, or area 17 and is located in the occipital lobe in the posterior section of the brain. Information from LGN magnocellular axons is sent to layer 4Cα and information from LGN parvocellular axons is sent to layer 4Cβ. Like the LGN, the striate cortex has distinct topographical mapping. Within this mapping, stimuli from the center of the visual field is processed in a larger cortical area than stimuli from the peripheral of the visual field. This priority of cerebral space given to the fovea area is known as cortical magnification. Cortical magnification allows us to conserve the amount of cortical space used for visual processing by only performing complex processing to a specific set of stimuli. Because of the human eye’s ability to move quickly, this system works nicely. One drawback of this design is that it does cause a bottleneck for processing for peripheral stimuli which leads to decreased acuity and visual crowding.
The striate cortex functions like a set of mini-computers. Each one is responsible for characteristics of the stripes such as width, orientation, and color for a small section of the visual field. This information is processed and then sent on to other parts of the brain for higher level processing such as location determination and object recognition.