Table of contents:
- Vision Anomalies:: main diseases of the eye
- Corneal abnormalities: astigmatism
- Retinal abnormalities
- Optic nerve abnormalities
- Convergence anomalies
- How does the human eye work? - Dynamic aspects of vision
- Convergence
- Sensitivity
- Balance Sensitivity - Resolution
- Vision abnormalities: The purkinje effect
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If we continuously notice serious difficulties in carrying out our daily activities, such as reading, writing, ironing, drawing, watching television, driving a car or working on a computer, it is very likely that we have a refractive visual defect, that is, a vision abnormality.
The main vision abnormalities are:
- Astigmatism
- Myopia
- Farsightedness
- Retinal diseases
- Optic nerve abnormalities
In this Psychology-Online article, we will list and define the main anomalies of vision under the theory of basic psychology.
You may also be interested in: The perception of color - Basic psychology Index- Vision Anomalies:: main diseases of the eye
- How does the human eye work? - Dynamic aspects of vision
- Vision abnormalities: The purkinje effect
Vision Anomalies:: main diseases of the eye
These optical abnormalities, no matter how small, cause significant discomfort that can become disproportionate if we compare it with the small refractive abnormality (myopia, hyperopia or astigmatism) that causes it.
Corneal abnormalities: astigmatism
A normal cornea has a constant radius of curvature. In astigmatism it has no equal. This causes parts of the object to focus well and others not. This problem can be treated, sometimes it is innate and sometimes it is due to mechanical injuries. There are other lesions or infections producing defects, such as scratch marks or encrustations.
Retinal abnormalities
- Detachment: holes, tears and separation of the retina as a result of a blow or eye disease.
- Retinopathy: bleeding, fluctuations in visual acuity caused by poor treatment of diabetes.
- Macular degeneration: reduced central vision, poor visual acuity, inability to discriminate colors.
- Pigmentary retinitis: This vision abnormality is characterized by progressive degeneration of the pigment layer of the retina, night blindness, and poor color discrimination.
Optic nerve abnormalities
The most frequent anomaly of the nerve is its destruction or deterioration caused by glaucoma, it is characterized by an abnormally large pressure in the posterior chamber of the eye. The most frequent cause is that the aqueous humor must be renewed, if the drainage channel is occluded the pressure rises.
Convergence anomalies
They are a product of the accommodation function performed by the lens. There are two important ones:
- Hyperopia, is characterized by a difficulty of accommodation with respect to objects located at a short distance from the individual. It does not bulge enough for the image to form on the retina. The hyperopic person can see far objects well but not near ones. It can also be due to the eye being too short.
- Myopia, consists of an excess of bulging. If the object is close, no problem. The problem is in the distant objects. It can also be understood that the eye is excessively long, in which case most objects would be blurred. It can be corrected by reducing the refractive power, placing a concave lens in front, which brings distant objects closer. You can also use a keratotomy or carve the lens to flatten it.
The intervention of the ocular media produces blurriness in the image. We could call this the point's spread function. The image is sharp in the center and as we zoom out in any direction it becomes more blurred. This means that after the previous processes there will be others that will correct this.
How does the human eye work? - Dynamic aspects of vision
The retina is the place that contains the photosensitive cells. This is where transduction occurs. It is composed of several layers of cells: photoreceptors, horizontal, bipolar, amacrine and ganglionic. Photoreceptors are of two types: rods and cones. The cones are large and not very sensitive to light. That is why they are the cells in charge of daytime or photopic vision.
There are several classes of cones, and each one would be characterized by being most sensitive to lights of a certain wavelength: 440 (blue), 530 (green) or 560 (red). This does not mean that they are not activated with other colors. There are about 8 million per eye that are concentrated mainly in the area of the retina that we call the fovea. Rods are much more sensitive to light than cones. They are responsible for scotopic or night vision. There is only one type and it is most sensitive to intermediate wavelengths. There are about 120 million per eye.
Convergence
It consists in that we have around 130 million receptors and around 10 million ganglion cells. Therefore a convergence must occur, several photoreceptors connect with a ganglion. This phenomenon raises several questions: convergence entails sensitivity, the ability to work in minimal light conditions. We also lose resolution.
Sensitivity
It is the ability to detect minute amounts of light. The way to ensure this is spatio-temporal integration. If we have an undetectable light point, this is added with other points that appear and that affect another photoreceptor. This is also temporary. These two additions are explained by the Laws of Ricco and Block respectively. They come to say that the absolute threshold of luminosity of the individual within certain limits is constant for the intensity and size of the luminous point (intensity * area or intensity * duration of the stimulus).
Balance Sensitivity - Resolution
There are two subsystems, one in charge of sensitivity and the other of resolution. There could be one system based on rods and the other on cones. There is evidence about the two systems:
- Adaptation to darkness: This is based on the fact that the sensitivity changes depending on the environment is more or less illuminated. The procedure measures the threshold over time. Two components are seen in the graph, which could indicate that each corresponds to a system. If we project to an area where there are only cones, the effect that would occur can only be due to these.
- If the range is deviated to 20º from the fovea, the curve that would be obtained would be due only to the rods. Dark adaptation has been applied to various fields: Reading, dyslexics seem to have problems with the parafovea. These subjects' baton system may not function properly.
- Children and some elderly people show different dark adaptation curves, the anomaly occurs in relation to canes. It is believed to be involved in seasonal affective disorder.
Vision abnormalities: The purkinje effect
We characterize this vision anomaly as follows:
- Both rods and cones are sensitive to lights of different wavelengths, but they are not sensitive in the same way.
- In low light, colors do not look the same as in high light.
- At night the only color that is well perceived is green.
- The quality of the color changes when the intensity changes.
- The visual system would solve the problem by having a double system: for high sensitivity and for resolution.
This article is merely informative, in Psychology-Online we do not have the power to make a diagnosis or recommend a treatment. We invite you to go to a psychologist to treat your particular case.
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