How Does Vision Work and What is Different with Astigmatism?
The brain and the eye are connected and dependent on each other to see (Kellogg Eye Center, 2015). Light transfer is responsible for what is seen (Kellogg Eye Center, 2015). Light enters the cornea and passes through to the lens; these two parts act together to focus rays of light onto the retina where light is absorbed by light-sensing nerve cells called rods and cones and changed into electrical impulses that travel the optic nerve to the brain (Kellogg Eye Center, 2015). The iris and pupil control the flow of light rays into the eye; at night, the pupils enlarge to let in more light (Kellogg Eye Center, 2015). The lens of the eye is responsible for focus and may need the help of corrective lenses (Kellogg Eye Center, 2015).
Irregularly shaped corneas or lenses cause astigmatism (Kivi & Boskey, 2015). When the lens has an irregular shape it alters the way light refracts to the retina (Kivi & Boskey, 2015). This leads to blurry or distorted vision (Kivi & Boskey, 2015). There is more than one type of astigmatism (Kivi & Boskey, 2015). Lenticular astigmatism is caused by an irregularly shaped lenses while corneal astigmatism is caused by an irregularly shaped cornea(Kivi & Boskey, 2015).
Anatomy of an Eye
The eye is made up of many parts. An irregularity in any of those parts could result in vision problems.
Rods and Cones
Rods and cones are verve cells found in the retina.They are named for their shapes.
Where does light go?
Light from an object is refracted by the cornea. It then goes to the lens for more bending and focusing. Then it is sharply focused by the macula of the retina.
Anatomy of an Eye
Perception of Vision and How it is Processed
There are two visual perception theories. The bottom-up theory is considered to be a direct theory (Intaitė, Noreika, Šoliūnas, & Falter, 2013).
Bottom-up processing is considered to be data-driven and comes with the belief that perception starts with the stimulus which is processed on its path from the retina to the visual cortex (Intaitė et al., 2013). In bottom-up processing perception is direct and does not rely on hypothesis testing (Intaitė et al., 2013). It is believed that there is no need for objects to be processed because sensation is perception (Intaitė et al., 2013). Perception information is only analyzed from one direction which gets more complex as it travels through the visual system (Intaitė et al., 2013).
This theory has three main components:
1. Optic Flow Patterns;
2. Invariant Features; and
Top-down processing is considered to be contextual and uses pattern recognition for understanding (Intaitė et al., 2013). Context clues from prior experiences aid in perception (Intaitė et al., 2013). Inferences are made, along with guesses, to guess at what is seen, based on prior knowledge and experiences (Intaitė et al., 2013). In this theory of perception, it is believed that a lot more information reaches the eye than reaches the brain so the brain guesses what is being seen based on memory (Intaitė et al., 2013). A stimulus is taken in by sensory receptors and then added to stored memories about the environment that have come from personal experiences (Intaitė et al., 2013). At times, incorrect guesses are made and optical illusions fool the brain.
The whole is different than the sum of its parts; this is the basis of the Gestalt Approach.
The Gestalt approach is based on several principles that explain how perception works.
*Principle of Similarity (similar items will usually be grouped together)
*Principle of Pragnanz (organization reduced reality to the simplest form possible)
*Principle of Common Region (objects located near each other are usually grouped together)
*Principle of Continuity ("points when connected result in straight or smoothly curving lines are seen as belonging together and the lines tend to be seen in such ways as to follow the smoothest path" (Goldstein, 2014, p.102))
*Principle of Common Fate (objects moving in the same direction are usually grouped together)
*Principle of Uniform Connectedness ("a connected region of the same visual properties such as lightness, color, texture, or motion is perceived as a single unit" (Goldstein, 2014, p. 104))
Treatment Options for Astigmatism
Not all astigmatism will need to be treated but there are many treatment options available if needed. The simplest treatment is glasses or contacts (Kivi & Boskey, 2015). There is also a treatment called Orthokeratology (Ortho-K) (). This treatment uses inflexible contacts that are designed to correct the abnormal curve of the cornea (Kivi & Boskey, 2015). These lenses must be continued to see the benefits (Kivi & Boskey, 2015). Surgery is another option and if the astigmatism is severe, this may be the best option. It is done using lasers to reshape the cornea and if it is successful, the correction is permanent (Kivi & Boskey, 2015). There are several different types including LASIK, PRK and RK and none come without risk (Kivi & Boskey, 2015).
Goldstein, E. B. (2014). Sensation and perception (9th ed.). Belmont, CA: Wadsworth.
Intaitė, M., Noreika, V., Šoliūnas, A., & Falter, C. M. (2013). Interaction of bottom-up and top-down processes in the perception of ambiguous figures. Vision Research, 89, 24-31. doi:10.1016/j.visres.2013.06.011
Kellogg Eye Center. (2015). The Eye. Retrieved April 06, 2016, from http://www.kellogg.umich.edu/patientcare/conditions/anatomy.html
Kivi, R., & Boskey, E. (2015, November 25). Astigmatism. Retrieved April 06, 2016, from http://www.healthline.com/health/astigmatism#Overview1