A hollow, fluid-filled sphere, the eye is our organ of sight. We only see 1/6th of the eye, most of it lying posterior to the skull. Made of three layers: the fibrous tunic, the vascular tunic, and the nervous tunic, the nervous tunic gives the eye its function. The retina, which detects light, lies in the nervous tunic. Blood supplies the eye through the choroid, which lies in the vascular tunic. The sclera, lying in the fibrous tunic, holds the eye together.
Light enters the eye through the cornea, a transparent portion of the fibrous tunic. The ciliary body opens and closes the iris, controlling the amount of light entering the eye. When light hits the retina, that light is translated into a message of vision, which is sent to the brain.
Photoreceptor cells, called rods and cones makes up the retina. Of the two, rods are the most sensitive to light, being able to detect light and transmit that reception to the brain in very dim light. Cones can separate light, telling the brain what color an object is. But cones need much more light to do this.
The Vocabulary of Sensory Impairments
To be able to understand fellow students or friends with sensory impairments, you need to understand the terminology used to describe these disabilities. The following terms are some you will encounter in your dealings with people who have sensory impairments and the specialists who work with them.
Types of vision problems:
Disorders of the eye
Myopia – nearsightedness
Hyperopia – farsightedness
Astigmatism – refractions / not focused
Disorders of the eye muscles
Strabismus – 2 images received by the brain, possible cause for a nonfunctional eye
Nystagmus – involuntary movements of they eye that interfere with bringing objects into focus
Disorders of the cornea, iris, and lens
Glaucoma – fluid Aniridla – extremely sensitive to light
Cataract – a cloudy film over the lens of the eye
Disorders of the retina
Diabetic retinopathy – changes in the eye’s blood vessels caused by diabetes
Macular degeneration – damage to a small area near the center of the retina; difficulty in reading and writing
Retinopathy of prematurity (ROP) – excess oxygen
Retinitis pigmentosa -detachment of the retina interrupts transmission of visual info to the brain
Refractive disorders: The way the eye focuses light is impaired, as in myopia (near-sightedness), hyperopia (far-sightedness), and astigmatism (blurred vision).
Muscle disorders: The ability to control eye movements is impaired, as in strabismus (crossed eyes).
Receptive disorders: The ability to receive and process signals from light is impaired, as in reti nal detachment caused by glaucoma or a blow to the eye.
Mixed losses: A combination of conductive and sensorineural impairments.
Seeing More Than The Sum Of The Parts
The eye and brain break up the visual world into various aspects, such as color, form, motion, and depth. These pieces of the picture are interpreted in a complex network of processing centers. To form a coherent picture of the world, the eye-brain takes signals from the retinas, relays them through the lateral geniculate bodies, and then passes them on to areas Vl and V2 (revealed in the inset, which shows the brain from beneath). These areas engage in a dialogue with centers farther along the pathway-some of which are still unidentified. Sensory Filtering: The Visible Spectrum As you’ll learn when we discuss light, our eyes have evolved to process electromagnetic radiation. However, the range of electromagnetic radiation that falls within the visible spectrum is only a small part of all electromagnetic radiation. So we can see wavelengths between about 400 and 700 nanometers (all the colours of a rainbow), but we can’t see Infrared or Ultraviolet. Not all creatures evolved with the same visual systems though. Depending on what cues would be most useful for a particular animal, it may evolve different capabilities. For example, although we cannot see Infrared light, some animals can. This picture shows a face photographed with film that is sensitive to infrared light. Looking at the picture, you can imagine how different your world would look if our visual system processed different sorts of information.
Sensory Filtering: The Visible Spectrum
Our eyes have evolved to process a small portion of electromagnetic radiation, which runs from sound waves to UV, and up to Gamma Rays. Our eyes only detect a sliver of the entire range of electromagnetic radiation. We call that sliver the visible spectrum. So we can see wavelengths between about 400 and 700 nanometers (all the colours of a rainbow), but we can’t see Infrared or Ultraviolet.