By Chris Jung
A month ago I finished reading East of Eden, perhaps Steinbeck’s most acclaimed novel. I loved so many aspects of the multi-generational story - the colorful characters, biblical allusions, overarching themes of loneliness and humanity - but while writing this post, I recalled a certain quote spoken to the novel’s villain, Cathy Trask.
“I seem to know that there's a part of you missing. Some men can't see the colour green, but they may never know they can't. I think you are only part of a human. I can't do anything about that. But I wonder whether you ever feel that something invisible is all around you. It would be horrible if you knew it was there and couldn't see or feel it. That would be horrible.”
Steinbeck raises very interesting questions about perception. But unfortunately, there are really no sounds, images, tastes, or smells in the real world, and nothing “invisible” that Cathy can’t see. The color green does not really exist - humans merely created ways to classify different forms of energy due to the fascinating ability of our sense organs to transduce signals, or convert forms of energy. The visual system is one of the most complex senses, involving 30% of your cerebral cortex. As with many scientific experiments, research is conducted on animal models for ethical purposes: fruit flies have been the major model for studying the conversion of electromagnetic radiation , or EMR, to electrical energy, with mammals such as cats and monkeys studying higher-level visual processing.
Light passes through the cornea, and enters through the pupil to the lens, which bends the light to focus on the retina on the inner surface of your eyeball. The iris regulates how much light can enter our eyes by contracting or dilating the pupil.
The retina has three general types of neurons organized in layers: photoreceptors, ganglion cells, and interneurons (also called bipolar cells).
Photoreceptors are neurons that turn light into electrical signals and include light-sensitive rods, color-sensitive cones, and the lesser-known photosensitive retinal ganglion cells. More specifically, the light-sensitive rods and cones are located in the outermost layer of the retina, which means that light passes through the aforementioned ganglion cells and interneurons before reaching the photoreceptors, where they are converted into action potentials. It is through these cells that the excitation of particles that we call “light” can be transmitted into actual impulses and neural circuits that we associate with certain particles.
There are ~125 million photoreceptors in each human eye. Rods are extremely sensitive and allow you to see in even very dim light, due to proteins called rhodopsins. Cones, named after the shape of their outer segment, are color sensitive due to pigments called photopsins and also allow us to pick up very fine detail. There are three types of cones (Long, Medium, and Short) that are sensitive to different wavelengths of light that correspond to red, green, or blue: these sensitivities overlap, which allow humans to see the familiar range of colors of the rainbow.
There are two very important regions that are centered in the retina: the fovea (1.5 mm in diameter) and the macula (5 mm). The fovea is a small, pitted area where there are many cones (red and green) to resolve very small detail. The macula (or macula lutea; Latin for “yellow spot”), which is composed of many cones, is the area immediately around the fovea; it is critical for reading and driving. The yellow color of the macula comes from the lutein and zeaxanthin, which can be obtained through diet. In fact, daily supplementation with these carotenoids (my dad takes lutein soft gels and swears by them - just make sure to take them with meals, as carotenoids are fat-soluble compounds) have been shown to prevent macular degeneration, the leading cause of blindness in citizens of developed countries (older than 55 years old). . The yellow pigment acts as natural “sunglasses” for your eye to filter out the extremely damaging blue light and UV light. Both structures are crucial for high-resolution color vision.
In the macula and fovea, vision acuity is high because each ganglion cell leading to the optic nerve only receives input from a few photoreceptors. (I like to imagine that each ganglion can focus on a few cells at a time, resulting in higher resolution). However, the edges of the retina are responsible for your more blurry peripheral vision, because a ganglion cell will receive input from many photoreceptors (multitasking = less precise).
This is where the concept of receptive fields, the portion of visual space that corresponds to one ganglion cell, come in: ganglion cells with a smaller receptive field will have a higher acuity.
The receptive fields of ganglion cells “tile” the retina, painting a complete 2D picture of our surroundings. The receptive field is activated when light hits the center, and inhibited when it hits the “donut” around the center. If light hits the entire receptive field, the response is very weak. This is called “center-surround antagonism”, and allows us to perceive contrast.
The axons of the ganglion cells exit through a small opening in the back of the eye and a layer of astrocytes (or glial nutritional support cells) and form the optic nerve (cranial nerve II). There are no photoreceptors in this opening, which forms a blind spot in each eye.
The optic nerve converges at a junction called the optic chiasm. The nerve fibers that correspond to the left visual field will (mostly) continue on to the left cerebral hemisphere, while signals corresponding to the right visual field travel through the nerve fibers to the right cerebral hemisphere. These fibers reach the lateral geniculate nucleus (LGN), located in the thalamus, and to the primary visual cortex at the back of the occipital lobe.
The primary visual cortex is a sheet of neural tissue that is packed with neurons in several layers: the middle layer is similar to the retina, with receptive fields that relay information from the nerve fibers from the thalamus. The layers above and below this layer register this information through more complex receptive fields, detecting edges and bars. The further along the impulses are relayed in this pathway, the more specific the processing becomes, recognizing faces or specific objects.
The ventral stream, or the “what” stream, is thought to integrate information that helps you recognize what a certain object is. The ventral stream is thought to be composed of neurons that run from your primary visual cortex to the temporal lobe on the sides of your cerebrum. On the contrary, the dorsal stream, or the “where” stream, processes spatial information and ends in the parietal lobes. For example, if you see a car on the road, your ventral stream would integrate the car’s color and shape with other information, such as sounds and memory, to recognize it is a car belonging to a certain person. Your dorsal system would process the location and motion of the car subconsciously - maybe you are walking across the street and the car is going way too fast - you may start running without thinking. But the more modern view is that there is crosstalk between the ventral and dorsal stream that creates our conscious visual experience, not a black-and-white division of functions.
Luckily, most of us have “binocular vision”, or can see with two eyes, which means we can perceive three dimensions due to the difference in angle that each eye observes the same object. Of course, this only works if the visual fields of each eye overlaps - which is not always the case. People with strabismus (crossed eyes) can not perceive depth. If strabismus is not treated before the age of 8, children favor one eye over the other (as they cannot merge images from their eyes) - which often leads to loss of vision in the underused eye. For this reason, prophylaxis is encouraged early on in a child’s life - through surgery, specialized contact lenses, or vision therapy.
People who are color-blind have abnormalities in pigments of their cones. There are many types of color-blindness: red-green, blue-yellow, and complete color blindness, each classified into its own subtypes. The most commonly known form is deuteranomaly, an autosomal-dominant trait found mostly in males, where green and yellow seem redder and blue and violet look the same. This may have been what Steinbeck was talking about in the quoted text. People with achromatopsia have no pigments in their cones and therefore cannot see color.
Pigments in rods and cones - rhodopsins and photopsins, respectively - are getting a surge of attention due to the emerging field of optogenetics. By genetically engineering channelrhodopsins, special voltage-gated ion channels activated by wavelengths of light (instead of bio messengers), scientists can control the depolarization of certain cells - like cardiac pacemakers or neurons. Channelrhodopsins are naturally found in our eyes, but this new technology has a wide variety of applications from treating arrhythmias and mind control!
There is obviously so much more to talk about, but I think that is enough for one post! Any subject that you choose to go into will be endlessly complex. I hoped you learned something new and will consider reading East of Eden!
Sources:
Brain Facts: a Primer on the Brain and Nervous System. Society for Neuroscience, 2012.
Hobbs, Ronald P, and Paul S Bernstein. “Nutrient Supplementation for Age-Related Macular Degeneration, Cataract, and Dry Eye.” Journal of Ophthalmic & Vision Research, Medknow Publications & Media Pvt Ltd, 2014, www.ncbi.nlm.nih.gov/pmc/articles/PMC4329711/.
Steinbeck, John. East of Eden. New York: Penguin Books, 1992.