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The retina is an incredibly complex structure.  This entry will be about the larger structures involved in processing an image. To learn how an image is focused on the retina, check out anterior eye anatomy. If you want to learn about the individual cells of the retina, check out phototransduction (conversion of a light ray into a chemical message for the brain). For information on the utilization and regeneration of vitamin A in the retina, check out the biochemistry of the visual process. The goal of glasses, contact lenses or refractive surgery is to get images focused on the fovea of the macula. The macula is the general pigmented area surrounding and supporting the fovea. The fovea is the specific structure that processes all of the detailed (letters, print) visual information we see. The fovea is a small structure, seen on examination only as a small pinpoint of light. In this small space, the fovea packs in over 100,000 cones (red and green cones only, no blue). Macular de

For vision to be a continuous process the photoreceptors (rods and cones) must continually regenerate 11-cis retinal. Rods and cones have slightly different ways of doing this. WARNING: this is only for the extremely nerdy. If you aren't very interested in the biochemical processing of vitamin A in the retina, this entry probably isn't for you. For rods: A photon (light ray) is absorbed by pigment within the opsin in the outer segment of the photoreceptor cell. Inside the activated opsin, 11-cis retinal is converted to all-trans retinal and released to the disc of the inner segment of the photoreceptor. Within the disc, all-trans retinal binds phosphatidylethanolamine. This produces N-retinylidine-phosphatidyl-ethanolamine (NRPE). ATP binding cassette transporter carries the NRPE complex to the cell membrane of the disc. At the cell membrane the all-trans retinal portion of the NRPE complex is transported to the cytoplasm within the cell body of the photoreceptor. Insid

Light rays from an image are focused onto the retina by glasses or contacts and the anterior structures of the eye. Light rays are converted from a physical wave into a chemical and electrical message to be interpreted by the brain. This process is called phototransduction. In the sensory retina, there five main cell types and nine layers involved in this process. The sensory retina is an optically clear structure. For phototransduction to begin, an image must pass through the anterior blood supply and every layer of the sensory retina until the image finally reaches the photoreceptor layer. The photoreceptor layer is composed of rods and cones. There are roughly 120 million rods in the adult retina. These cells allow us to see the difference between light and dark. Rods are utilized for night vision and low lighting conditions. There are 6 to 7 million cones in the adult eye. Cones are only utilized for day or brighter conditions. These cells are specialize for color detection.

The main purpose of the front half of the eye is to focus a clear image on the retina. The next step in the process of vision is to convert that visual information (light rays) into a chemical message for the brain to process. However, before the image reaches the retina, it must first pass through a jelly-like substance called the vitreous. The vitreous fills most of the volume of the eye. It is mostly composed of water, but can also contain salt, sugar, a collagen called vitrosin, hyaluronic acid and other various proteins. When all these components are spaced out uniformly the vitreous remains clear and images pass on through to the retina undisturbed. With time more water concentrates in the lower portion of the eye. This leaves more proteins in contact with one another in the superior portion of the eye. The positive and negative charges of the molecular components of these proteins attract one another and result in the various proteins clustering together. This clustering cre

My favorite part of each eye exam is getting the slit lamp (microscope) out and studying the various structures of a patient's eyes. The eye is a structure with many beautiful features. Each component of the eye plays a role in capturing images from the external environment and preparing that visual information for its journey through the brain. The first structures examined are the eyelids and eyelashes. These structures work to protect the eye from debris in the air and reset the tear film with each blink. A major contributor to our natural tears, the meibomian glands, are located just behind the eyelashes. Our major concern when looking at the eyelid are various skin cancers (basal cell carcinoma, squamous cell carcinoma, sebaceous gland carcinoma). Past the eyelids the tear film is observed. The tear film lubricates and protects the front surface of the eye, the cornea, and plays a vital role in keeping images clear on the way to the retina. The tear film is composed of thr

Vision is a very complex task that begins in the eye and ends in visual cortex located in the back end of the brain. For visual information to get to the back of the head it must take a long and complex course. If there is an obstruction along the way, a scotoma results. A scotoma is a missing piece of the visual puzzle. Based on the size, shape and location of the missing vision we can predict the location of abnormal brain function. Normal peripheral vision consists of a 150 degree view for an individual eye and 180 degrees for both eyes working together. Peripheral vision can be roughly estimated by counting fingers at the edge of the visual field (confrontations) or more precisely with a small beam of light at random spots in the visual field (automated visual field testing. Different structures in the eye can influence peripheral vision testing. Cataracts or a corneal condition can dim vision and only allow patients to see brighter test points. Retinal defects produce a scotom

The pupil is the hole that allows images to enter the eye and determines the level of brightness inside the eye. The pupil can sharpen vision by constricting and blocking out peripheral image rays that cause a larger blur circle. This can be demonstrated by looking through a pinhole (1 to 1.5mm hole). During the day or in a bright environment the pupil constricts (miosis). At night, the pupil dilates to let more light into the eye. A larger pupil is the cause for most patients night driving problems. A larger pupil allows more peripheral image rays into the eye. A larger surface area of the cornea is required to focus images which exaggerates nearsightedness or astigmatism present. If cataracts are present, a larger pupil leads to more image ray scattering and bluriness. Pupil size can be affected by factors other than light. When reading up close the pupil constricts. The pupil is closely tied to the focusing system. This link is located anatomically at the ciliary ganglion. The c

Headaches are a common problem patients encounter. Sometimes a new glasses prescription can fix the problem; other times the problem can be more severe or complex. When headaches are located in the forehead, temples and/or around the eyes the headaches are likely due to changes in prescription. Focusing issues from excess computer use and/or reading can also cause headaches in this area. Anti-reflective coating and polarized sunglasses can prevent headaches caused by glare. Eye alignment should also be evaluated. Cluster headaches are felt in or behind the eye. These headaches can be caused by caffeine, stress, nitroglycerin and/or tobacco use. For some patients the headaches are caused by narrow drainage strucures within the eye which require laser treatment to open the drainage structures. For those with normal drainage, the source of pain can be ovetaction of the trigeminal nerve or hypothalamus. Prescription headache medications are often required for patients with normal drain

Double vision should be considered an ocular emergency. Double vision is assessed by the Eye Doctor with extraocular muscle (EOM) testing, cover test and phoria testing. During EOM testing the Eye Doctor moves a target up, down, and side to side. This test assesses the function of cranial nerves (3, 4, and 6) and the muscles they control. These are the muscles that move the eye in various directions. The cover test is useful for testing eye alignment by measuring the phoria and tropia. A phoria or heterophoria is the natural position of eye alignment. If the eyes are naturally misaligned horizontally or vertically the EOMs can strain to some degree to correct this and keep vision single. If double vision is present, then a tropia is present. For a tropia, eye misalignment is to large to be compensated for. Tropia is known as a strabismus or eye turn. For a new tropia or strabismus double vision results. The brain really dislikes double images and attempts to prevent double visio

We don't know whether the question has been asked for hundreds or thousands of years. We do know that it creates unneccesary anxiety for many patients. Luckily, there is new technology attempting to eliminate the need for this question. This article will discuss the process of refraction (1 or 2 test), how it can be simplified to reduce anxiety, and how new technology measures up in its attempt to make it easier and more accurate. Using the traditional methods of refraction I say "is 1 or 2 better" 150 to 600 times per day. Needless to say, I've put a lot of thought into making the test easier for patients and my vocal cords. I will compare the old fashioned methods with new technology available today. The traditional (old) process of refraction begins with gathering data. A lensometer is used to measure to correction in the current spectacle prescription. Visual acuity with the spectacles on can be used to estimate how far the prescription is off. Keratometry can

Hopefully your eye exam will find your vision to be 20/20, but what does that mean and how does it compare to everyone else out there? In the US, the ability to see visual detail (visual acuity) is described in a fraction. The first number indicates the distance to the visual target. This is 20 feet for the standard in the US and 6 meters for most other countries. Most exams rooms are not 20'. This is why it's common to see mirrors which lengthen the target distance in smaller exam rooms. The second number is the size of the target letter. For 20/20, the target letter is 8.87mm (0.35") tall by 8.87mm wide. Therefore, 20/20 is the ability to identify an 8.87mm character at 20'. Of course some letters are easier to identify than others. Letters with straight lines, such as an "E" are more readily recognized than circular letters "O". Straight lines provide more clues to the brain as it is putting the visual picture together. Some patients may no

Why do patients have to suffer through the air puff test? What is it measuring? Is there an easier way to get this measurement? For many patients the dreaded air puff test is the most dreaded part of the eye exam. You're left sitting there in suspense and the moment you relax - BAM! - you're blasted by a shot of air right in the eye. The Eye Doctor isn't trying to torture you. The air puff test is done to measure the pressure in the eye to see if there is risk of vision loss from glaucoma or vascular occlusion. Eye pressure is a measure of fluid pressure in the eye. Aqueous humor is the fluid that causes pressure to be high or low. If aqueous is over produced and/or not drained fast enough from the eye the result is a high eye pressure. Low eye pressure results from under production and/or over drainage of aqueous. Gonioscopy is done to examine the drainage structures in patients with abnormal eye pressure. Eye pressure is a necessary thing, but when the pressure is t

With every trip to Eye Doctor it seems there is more paperwork to complete. The goal of this article is to answer: Why is this information needed? How much information should I disclose? Is there any way to make the process faster or less inconvenient? Eye Doctors dislike additional paperwork just as much (if not more than) their patients. The information gathered does serve a purpose. First of all, a medical history is required to complete the legal definition of an eye exam. Also, for insurance to cover exam fees, personal information (social security number, date of birth) must be included. Contact information is utilized for tasks such as: reminding patients of appointments or inform patients that glasses and/or contacts are ready to be picked up. How much information should be disclosed? In reference to medical history. Be as thorough as possible. When the Eye Doctor knows of any current medical conditions they can examine the eyes more precisely for subtle signs that if t

Eyedork - providing the latest eye and vision research to encourage a more proactive approach in seeking vision care and related products. The purpose of this sight is to inform the user about all aspects of eyecare. Eye examinations and selection of materials (spectacles and contact lenses) can be confusing and overwhelming. We are here to help you understand the process and hopefully make it more enjoyable. All questions are welcome. You can also follow on Twitter/eyedork.