There is more to vision than meets the eye! For example, did you know that a visually impaired person can learn to see with their tongue? Have you heard of blindsight? Visual aspects of neuroscience continually amaze students of psychology, and vision is one of the most important aspects within the neuroscience field. Research and findings in vision are never ending.
What Exactly is the Eye - Behind the Scenes in Sight
The eye is one of the most complex yet amazing organs of the body, and highly detailed in its function to deliver sight. In a most basic summary, the eye is like a camera, it takes in a visual, and sends this vision to the back, ‘the retina’, and this retina processes the information and sends it through to the optic nerve which sends it straight to the brain for further processing.
Important Parts of the Eye
Light is the necessary element for vision, it can be either natural lighting or even light reflecting or illuminating from an object. Without light, vision could not exist.
This light will enter the eye through a transparent protective covering called the cornea. The cornea will bend and refract the light rays for access to the lens which sits behind the cornea.
This lens will subtly adjust its shape to accommodate for the light rays. The iris, which provides the color of the eyes will also control how much light can enter. The pupil in the centre of the iris is a small opening which will dilate to allow for more light to enter, or contract, restricting too much light from entering. This is a noticeable change that can be experimented with if you look closely in a mirror under very dim light, and then abruptly turning on a bright light.
The retina, at the back of the eye, is actually an extension of the brain and receives the light image upside down and then perceives it as right side up. The retina contains millions of visual receptors called rods and cones. Rods are long and narrow while cones are named for their shape and are concentrated in the central part of the retina called the fovea and focuses on detail and color. The rods, alternatively, are concentrated on the periphery of the eye and responsible for peripheral vision as well as an ability to see in dim light. This is something which cones are incapable of doing.
The visual receptors of rods and cones differ in their sensitivity to wave lengths, which provide rods and cones their individualized functions. Rods cannot distinguish between different wavelengths and thus unable to define colour, while cones are highly susceptible to the wavelengths, and therefore, provide great detailed colour vision.
The light waves that travel to the eye are characterized by three specific dimensions.
- Hue – Dependent upon the wavelength of the light and characterizes different colors. Shorter wavelengths are perceived as the ‘cooler’ colors such as blues and purples, and longer wavelengths are perceived as warmer colors such as oranges and reds.
- Brightness – Dependent upon the amount of light reflected or illuminated from an object. This brightness of color corresponds to the amplitude (height) of the wave. Alternatively, dark adaptation is when the visual receptors become maximally sensitive to dim light and the rods adapt slower than cones providing a more accurate image in this dim light.
- Saturation – This refers to the vividness of the colour which is dependent on the width of the wavelength. One single wavelength will result in the most vivid and saturated color.
Ganglion cells are neurons within the retina that receive information, and its axons (extensions) construct the optic nerve which travels to various parts of the brain for processing. At the location where the optic nerve joins the brain there is an optic disc, and it is here that a blind spot exists, and one that everyone has.
The Blind Spot
The optic disc does not contain photo receptors, and furthermore, this location is where the optic nerve exits the eye and retinal blood vessels enter, blocking any light on surrounding photo receptors. This results in a blind region within both eyes. It is difficult to notice, but the blind spot can be discovered with a visual experiment such as a picture of two objects on paper with a specific distance between them. Very slowly move this paper away from your face, and then slowly bring it back until you notice one object disappear. If you notice this, you can hold the paper in that exact position and the object will have virtually disappeared!
Seeing is Not Believing – Lateral Inhibition
Interestingly enough, what we believe we see, is not necessarily so. According to studies, the brain will utilize past experiences and knowledge to construct the complete picture. For example, if you are looking at a yellow wall, the visual messages sent through your optic nerve are only a fraction of what your brain will perceive. The brain will receive data on the actual outline of the wall, and then it will complete the image itself by filling in the colour. This is called lateral inhibition and is best explained as the ability of firing neurons to inhibit neighboring neurons. Only the most stimulated neurons will respond, which then will provide a far greater contrast and resolution of the image. If the brain processed all the information that could come in from all the firing neurons, the optic nerve would need to be much larger!
Visual Disturbances and Blindness
As you can imagine, the eye is very intricately designed and any abnormalities can result in visual disturbances. Here are a few visual disorders:
- Strabismus – An imbalance of intraocular muscles of both eyes resulting in each eye pointing at a different direction.
- Glaucoma – Increasing intraocular pressure compromising the shape of the eye and permanently damaging the location of where the optic nerve meets the brain causing loss of vision. It is possible to be treated if caught in time.
- Detached Retina – Fluid from the damaged area will cause further detachment increasing risk of vision loss.
- Macular Degeneration – Gradual loss of central vision and frequently related to age. The macula is a central spot on the retina important for central vision. This disease causes degeneration of the macula resulting in highly compromised central vision. A healthy diet and refraining from smoking can reduce the risk of this disease, or slow the process.
- Color Blindness – Frequently genetic, this mutation is carried on the X-chromosome and therefore more common in men. There are three types of cone cells each detecting a different wavelength and color, such as blue, green and red. Damage to any of these cones can result in color blindness. Variations of color blindness exist:
- Protanomaly – (abnormal red cone cells) colors of images are not as bright with reds, oranges and yellows appearing more like green.
- Protanopia – (complete void of red cone cells) red appears as black, oranges, yellows and greens appear as yellow.
- Deuteranomaly – (abnormal green cone cells) yellows and greens appear more red, and violet is difficult to distinguish from blue.
- Deuteranopia – (complete void of green cone cells) reds appear brown, while yellows and greens appear beige.
- Tritanomaly and Tritanopia – blue and yellow color blindness
- Cone Monochromacy and Red Monochromacy – Total color blindness, a very rare disorder with only black, white and gray vision.
- Diabetic eye disease or Diabetic Retinopathy - A disturbance in retinal blood vessels that cause a leak in fluids. This can affect various parts of the eye from the retina or optic nerve to the macula.
- Complete Blindness – This refers to an absolute void of vision including light. Causes can range from injuries, accidents, diabetes, glaucoma, macular degeneration, blocked blood vessels in the retina and complications at birth.
A strange phenomenon that has been known to exist in a blind individual called blindsight occurs as an evolutionary reflex and can offer protection from danger. For example; it is possible for a blind individual to have this unexpected reflex to catch a ball or other object thrown their way. This reflex is very precise as well. completely reflexive, and not planned or even understood. Scientists believe that a certain amount of visual information can bypass one perceptive part of the brain for another that is involved in responding to movement.
Another phenomenon or phenomenal discovery is a recently designed device that allows the blind to learn vision with their tongues. As we now know, light is the catalyst to vision, and there is now a tool that helps the blind experience vision with the nerves on the surface of their tongue, which then sends light signals to the brain. Brainport is a pair of sunglasses with a small digital video camera set in the centre of the glasses.
The data from the camera is transmitted to a hand held device that converts the information or data to an electrical pulse, which is sent to the tongue via a straw type, or lollipop , device. These electrical pulses correspond to pixels such as white, which promotes the strongest response, while black pixels promote no response.
The sensation felt is like that of ‘pop rocks’ on the tongue or ‘champagne bubbles’. This information is then sent to the brain and the user will learn how to read these signals utilizing it for depth perception with the sensations of pictures being painted on the tongue via the ‘bubbles’.
Vision being the most researched sense of all the senses, scientists are continuously researching and inventing new discoveries and data in hopes of contributing to the fascinating topic of sight. This is done in great effort to aid in even further blind research.
- Wade, Carole; Tavris, Carol; Saucier, Deborah; Elias, Lorin; (2007). Psychology, Second Canadian Edition. Pearson
- Pinel, J. P. (2011). Biopsychology (8th ed.). Boston, MA: Pearson.
- Bear, Mark F. Connors, Barry W. Paradiso, Michael A.; (2007). Neuroscience Exploring the Brain. Baltimore, MD. Lippincott, Williams & Wilkins