Where does Superman put his glasses?
This year, for the first time in my life, I thought to myself “I’m getting old”.
I don’t understand how to use an iPhone, Fresh-Prince of Bel Air has a spot on Nick-At-Nite, and I’m increasingly impressed with the amount of fiber in Corn Flakes.
The other day, while I was perusing through the Hustler for coupons, I realized that I was having a hard time reading the newspaper. The vestigial “whipper-snapper” in me denied any visual deficit and refused to put on my glasses. However, I have finally faced the fact that I need to wear glasses more often, and now carry them around all the time. I first noticed visual issues during classes in high school that had a blackboard far from where I sat. I was diagnosed with Astigmatism, a condition where the cornea is misshapen and light is distorted at at the retina. I wore my glasses during class and take them off right after. I was convinced that my vision was not horrible and that I would only need them to see far distances. Furthermore, I was worried if I wore my glasses all the time, I would become dependent on them and my vision would get worse.
This year, however, I have noticed that I have kept my glasses on for most of the day. I believe this is not because glasses have made my astigmatism worse, but rather that it has made the muscles that control my lenses weaker. Analogous to a patient who wears a sling for a broken collar bone, the body part that is being assisted atrophies due to lack of activity. The muscles around the lens flatten and thicken the lens, in the process of accommodation, to focus light onto the surface of retina.
Additionally, because the muscles around my lens are getting weaker, it actually is more strenuous to read without glasses. I notice while I study late at night, that I actually have more energy when using glasses then when I am “eyeballing” it. Thus, while I squint to see objects far away, when objects are close, my intra-ocular muscles are working overtime, and the strain is less noticeable. Maybe, my astigmatism has always given me problems reading objects that are close, but, in my youth, my muscles were able to compensate. In classrooms, I noticed immediately that my vision was poor because I would squint (using facial muscles) to read the board. Squinting is much more apparent than overworking your ciliary muscles. Only now, as I start receiving my AARP periodicals, are my muscles becoming stiffened with protein accumulation, making lens accommodation more fatiguing.
The Smiley Face Cortex
This past week within the heap of subservient undergraduate tasks that I perform in my research lab, I had the chance to explore an interesting component of the visual system. I work under Dr. Wallace, who is an authority on the subject of multisensory integration within the visual and auditory systems. The project that I am assigned to is trying to discover the differences in multisensory binding within varying stages of development. The general findings show that at younger ages humans will perceive a ‘simultaneous’ event when auditory and visual information are separated over a larger temporal window than in adults. That is a ball can have a sound played after it hits the floor, and because infants have slower neurological processes (less myelin) and less visual experience, a baby will perceive that the sound was generated by the ball hitting the floor.
Look back at the three sentences of ramble that I just wrote, I realize that this seems like a verbose description of a very basic feature of our visual and auditory integration. However, deficiencies within this ability can seriously impair normal interactions and are associated with disorders such as Dyslexia and Autism.
My task was to help design a multisensory experiment to optimize attention among infant subjects by using certain stimuli. Previous studies show that children are more receptive to circles, the color red, and faces. An interesting study done by Teresa Farroni at Duke University, found that among faces, babies specifically prefer dark regions under the eyebrows and nose. During this experiment they tested the infant’s response to normal faces, a face with inverted colors, upside down faces, and crude smiley faces with normal or inverted coloring.
I found this very interesting because this is one way the brain compensates for the visual deficiencies of babies. Because they have slower conductions and less experience in terms of social/ inter-personal interaction, infants inherit a disposition/preference for this basic outline of the human face. This simple shading pattern (essentially a smiley face) represents the two eyes and the mouth. This pattern is recognized by our visual system before our brain is fully developed and before we have socially learned the shape of the face.
Retinal Ganglion Cells and the Circle of Life
The visual system has always intrigued me especially in the context of predator-prey evolution. From varying visual acuities within predators and prey to the anatomical position of eyes, the system is highly evolved in order to meet the needs of the organism. One visual component, the retinal ganglion cells, is present in both predators and prey as it allows for contrast and object border information.
This interesting system entails two distinct receptive fields: the center and the surround. Within these receptive fields, two different bipolar cells, on-center and off-center cells, exist. On-center ganglion cells are stimulated maximally when light is shown on the center portion of the receptive field. The on-center cell signal is weakened if light is also shown on the surround (a ring around the center field). If both sets of photoreceptors (center and surround) are not exposed to light or only the surround is stimulated, the On-center cells will be inhibited entirely. The Off-center cell response is exactly opposite, firing only when stimulated in the surround and weakened when a light stimulus is present in the center of the receptive field. In addition to a qualitative signal (fire or not fire), the Off-cells relay high and low frequency firing depending on exposure to light in the surround (high) or the surround and center (low). Thus a gradient of responses are given from these cells, depending on whether the segment each cell is responsible for is exposed to light. The Off-center cells are linked to the surrounding ring of the receptive field and thus will fire rapidly when light is shown on the surround, minimally when light is shown on both surround and center, but will not fire if only the center is exposed. The On-center cells fire in a similar fashion.
While this system seems convoluted, these cells serve an immensely important purpose. The center-surround arrangement allows the human eye to perceive distinct borders around objects and, more importantly to detect contrast or quick movements within the visual environment. Because each receptive field is small and is coded to a minute fraction of our entire scope of vision, the retinal ganglion cells are able to make excitatory or inhibitory decisions about each “pixel” of our visual input. Using an example of a bird flying across a bright background of sky; when the light signal from the bird crosses from surround to center, the cell firing will change from firing in the off-center to firing minimally in both on and off-center and finally to firing only in the on-center cells. Thus, within just one receptive field, of the many present in our system, we have a pattern of cell firing that codes to a certain movement. This benefits prey species who need to detect the slightest movements in their visual field (which is increased by placing eyes on the sides of their head), and also predators who need to have supreme acuity in identifying and measuring the distance of objects (prey).
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