Basic Structure of the Human Eye & VA Measuring & Recording Week 1 PDF

**Basic Structure of the Human Eye I** **Visual Optics --** study of Optics which is directly related to the human eye and appliances used to correct vision. **Human eye --** transparent refracting media that brings images of external objects to a focus on a complex light sensitive membrane, retina....

**Basic Structure of the Human Eye I** **Visual Optics --** study of Optics which is directly related to the human eye and appliances used to correct vision. **Human eye --** transparent refracting media that brings images of external objects to a focus on a complex light sensitive membrane, retina. Eye acts as a transducer converting light energy into a neural signal (brain) to allow us to perceived visual environment. **Main Structures: Cornea --** transparent tissue covering front of eye, no blood vessels, has nerves. **Iris --** circular band of muscles, controls pupil size, pigmented. **Lens --** transparent tissue that bends light passing through eye, to focus light, it changes shape by bending. **Pupil --** aperture in centre of eye where light passes through. **Aqueous Humour --** clear, watery fluid found in anterior chamber of eye. **Choroid --** layer of blood vessels that nourish the eye, because of high melanocyte content it acts as light absorbing layer. **Sclera --** protect coating around posterior 5/6^th^ of eyeball. **Vitreous Humour --** clear, jelly-like fluid in back portion of eye, maintains shape of eye. **Retina --** layer of tissue on back portion of eye that contains cells responsive to light (photoreceptors). **Photoreceptors:** **Rods --** responsive in low light conditions. **Cones --** responsive to colour and in bright conditions. **Eyeball has 3 coats:** **1. Outer Fibrous Tunic** (formed by sclera and cornea which allows intraocular pressure to be maintained). **2. Middle Pigmented Vascular Coat** -- Uvea (choroid, ciliary body, iris). **3. Inner Retina** (posterior part is photosensitive). **3 Chambers:** **1. Anterior Chamber** -- contains aqueous humour, is bounded anteriorly by cornea and posteriorly by iris. **2. Posterior Chamber** -- lies between iris anteriorly and ciliary body and lens posteriorly. **3. Vitreous Chamber** -- contains vitreous humour. **Crystalline Lens --** optically clear biconvex structure suspended from ciliary body by the zonular fibres, change in shape facilitates accommodation (ability to adjust focus from distance to near). **Ciliary Body --** 2 regions, outer ciliary ring and inner ciliary crown bearing ciliary process, zonular fibres are attached to ciliary body and support crystalline lens. **Optic Disc --** lies medial to posterior pole and four branches of central artery radiate from it, at inner surface of anterior half of eye, retina is seen to thin abruptly at a toothed margin called ora serrata (transition between photosensitive and non-photosensitive/ciliary part of retina which is only 2 cells thick, non-photosensitive part of retina covers ciliary body). **Optic Nerve --** leaves eyeball 3mm medial to its posterior pole and follows orbital axis to reach optic canal at posterior part of medial orbital wall, covered by dense connective tissue sheath continuous posteriorly with dura mater and anterior with sclera. Eye is a body organ, biological photo sensor, data processor, an instrument of perception. **VA Measuring & Recording:** **Vision --** smallest detail we can see when not wearing spectacles/contact lenses. **Visual Acuity --** smallest detail we can see when wearing spectacles/contact lenses. **Levels of Visual Function:** **1. Light Perception:** Very basic function of being able to tell whether scene is light or dark. Examiner shines light into eye to see if patient notices light. Very gross measure and recorded as PL (perception of light) or NPL (no perception of light). **2. Detection (Discrimination):** Relates to eyes ability to detect a difference in retinal contrast. For something to be noticed there must be significant difference between it and everything else. Also used to measure vision in a variety of clinical and research situation, e.g. Teller acuity cards (black and white gratings) **3. Resolution:** Measures eyes ability to see detail. Resolution threshold -- smallest separation of detail that can be distinguished (usually expressed as visual angle or Snellen fraction). **4. Recognition (Form Vision & Recognition):** Ability to recognise shapes. Kay picture cards used to test children. **5. Relative Position:** Often tested using technique called Vernier acuity (tasks that require integration across retinal image e.g. stereopsis/depth perception). Vernier acuity is a measure of your ability to determine whether a line is to the left or right of another line. **6. Higher Functions:** Tasks related to perception of motion, colour, flicker and memory. This relates to tasks where the visual system stimulates other responses.