New Chromaticity Diagram

MYOPIA & AMETROPIA
AS COMPLEX SYNDROMES


based on

PROCESSES IN BIOLOGICAL VISION

by JAMES T. FULTON


Last Update: July 2009    Rhodonine and Activa are trademarks

A CAUTION

There must be a distinct line between the medical and academic aspects of syndromes as serious as myopia. This material should not be considered medical advice. Subjects aware of this syndrome should speak with their doctor.

Students subject to rote testing based on the content of their textbooks are encouraged to review the Cautions Page before proceeding. Others may find the material controversial. However, the results speak for themselves.

INTRODUCTION

Myopia is a widely distributed syndrome lacking a well differentiated descriptive structure. Because of this, the term has acquired an umbrella role with respect to a group of related syndromes. Ametropia is a disease characterized by a geometric error (or equivalent in signaling space)affecting the spatial frequency performance of the eye. Myopia, also known as hypometropia, is the condition associated with near-sightedness. Its opposite condition is hypermetropia, or far-sightedness.

Ametropia involves both physiological and neurological aspects. Beyond elementary concepts, the neurological aspects of the syndrome have not been presented previously. They will be detailed below.

The occurrance of myopia can reach 20% among isolated populations, suggesting a strong familial or genetic cause. It appears it can also be induced through behavioral activity. However, in a broader sample, more than 90% of young adult humans are considered emmetropic, i. e. essentially free of geometric error of major significance.

Because of the techniques used within the accommodation servomechanism of the eye, the impact of hypometropia and hypermetropia are not symmetrical. Hypometropia introduces a considerably greater handicap to vision than does hypermetropia for those under 40 years of age. To a large extent, hypermetropia is ignored because of this situation.

This page is offered in conjunction with the parent site, PROCESSES IN BIOLOGICAL VISION. Additional information on the subject will be found there. [references in brackets refer to paragraph numbers in the main text which are accessible from the home page of the parent site. The first number is the Chapter number.]

This webpage will concentrate on the following areas.

  1. DEFINITIONS CRITICAL TO UNDERSTANDING THIS SYNDROME.
  2. A BRIEF REVIEW OF THE TYPES OF MYOPIA.
  3. A MODEL DESCRIBING THE SYNDROME OF MYOPIA.
  4. PROGRESSION OF THE AMETROPIAS WITH TIME
  5. BEHAVIORAL IMPACT ON MYOPIA
  6. INTRODUCTION OF SURGERY AS A CURE FOR MYOPIA
  7. COMMUNICATIONS WITH THE AUTHOR

The list of definitions has been physically moved to the end of this page. It may be accessed at any time to avoid misunderstanding and confusion.

A BRIEF REVIEW OF THE TYPES OF MYOPIA

Myopia can be divided into three distinct categories, refractive myopia, neural myopia and pathological myopia.

Refractory myopia

Clinicians have historically described myopia to their patients in terms of the axial length of the eyeball. This has been unfortunate and has frequently introduced anxiety. The highest correlate to axial diameter of the eyeball are the related transverse and sagittal diameters. A longer axial diameter is primarily indicative of a larger eye. There are two secondary correlates to axial diameter. One is the distance between the lens and the retina, the classical explanation for hypometropia based on geometry. Of equal significance is the correlation with the distance between the lens and the cornea. This myopia is due to an increase in the optical power (in diopters) of the optics. Thus myopia is caused about equally by an error in the optical power of the cornea/lens combination and by a displacement of the retina relative to the lens.

Pathological myopia

The common caricature of myopia as caused by a gross elongation of the eyeball originated with Heine in 1899. The typical elongation of the lens/retina distance in refractory myopia is less than one millimeter (or 4-5%). Distortions significantly larger than one millimeter (relative to the other diameters) are usually associated with pathological myopia due to other underlying diseases of the retina and sclera.

Neurological myopia (More appropriately neurological ametropia)

There are at least three forms of neurological myopia (more appropriately neurological ametropia). The most widely documented form is amblyopia (sometimes denoted as lazy eye). Less well known are errors familiar to anyone familiar with servomechanisms. The servomechanism used in accommodation is of the closed loop form. However, it is not of the balanced or symmetrical form. One error is a loss in amplification factor leading to poor overall performance. Another error common to such a servomechanism is an offset voltage that leads to inappropriate accommodation.
Amblyopia

Amblyopia is a neurological failure related to the pretectum of the midbrain. This morphological element is charged with the initial analysis of images presented to the foveola of the eye. It is tasked with performing a two-dimensional correlation of the spatial properties of the image. Normally, the spatial performance level is compatible with the overall performance of the nominal physiological optical system. If the correlation process is not accomplished satisfactorily, the subject is said to have the disease called amblyopia. The condition is not correctable with auxiliary optics. It is probably affected by training at a young age. It is probably irreversible beyond the age of 5-10. It may have a significan impact on the individuals reading ability relative to small type sizes.

Flashes of clear vision

Many myopes report flashes of clear vision while not wearing their normal glasses. This is achieved by closing the eyes, relaxing the level of accommodation, and then suddenly observing a pre-arranged scene at a distance. The scene briefly appears clearer than it does after about 400 milliseconds. This is the result of the servomechanism attempting to accommodate properly while it contains an internal offset (DC) error. In this situation, the accommodation system will continually attempt to optimize performance based on this error. The result is a continual error in best focus following an initial transient. Technically, the error could occur in either hypometropia or hypermetropia. However, it appears to be reported primarily in the myopia literature.

Some authors have attempted to associate this phenomenon with the operation of the iris, since changing the pupil diameter does affect the depth of focus of the eye. However, the time constants of the pupil are much slower than the fraction of a second normally reported in these flashes of clear vision.

Inadequate performance at low light or contrast level

There are a number of amplifiers within the servomechanism that controls accommodation. If the gain of any one of these amplifiers is inadequate, the result will be poorer than normal accommodation performance when observing low contrast or poorly lighted scenes. In fact, the performace of the servomechanism is directly related to the light level.

A MODEL DESCRIBING THE FUNCTIONAL CONDITIONS UNDERLYING
AMETROPIA AND MYOPIA

This section will present a simplified explanation for the major functional abnormalities associated with the clinical condition of Ametropia and more specifically Myopia. It is hoped that this material will aid future investigators, both clinical and research, define the loci of this syndrome more precisely. It will be impossible to explore all elements of the model on this site. However, the main site can be accessed and the author welcomes any comments, corrections or inquiries.

An OVERALL BLOCK DIAGRAM of the Human Visual system is available on this site. A simplified Block Diagram pertinent to the syndrome of Myopia is shown below. It omits many other visual functions not directly related to Myopia. The above reference provides a broader description of this diagram.

Simplifed Block Diagram of Retinal Signal Processing re focus

Beginning in the upper left, this figure shows image light passing through the cornea and the lens and falling upon the optical focal surface of the retina at the location marked. This surface is formed of an array of photoreceptor cells shown to the right of this surface. The signals from those cells are passed to a series of processing matrices.

The luminance matrix is of primary interest. It appears to contains two distinct portions. The majority of the signals received from the photoreceptors are processed into luminance signals that are sent to the magnocellular portion of the lateral geniculate nuclei (LGN) over path [S]. There are approximately 23,000 photoreceptors associated with the foveola, an area of the retina intersected by the line of fixation of the eye. The foveola is an area approximately 0.35 mm in diameter and representing a circle of 1.2 degrees in object space. For the signals from the photoreceptors of the foveola, an alternate path bypasses the matrix and sends the signals directly to the pretectum of the thalamus over path [S']. The thalamus is a part of the midbrain, not the cerebral cortex. The pretectum is responsible for extracting signals from [S']that specify the condition of focus of the image and of extracting the initial interpretation of what is imaged on the foveola of the retina. The focus related signals are passed to the superior colliculus, via one or more nuclei. The superior colliculus is responsible for preparing the command signal that is sent over the circuit marked "B" to the ciliary muscle that controls the lens.

The normal eye is able to focus on objects anywhere in object space between infinity and a location close to the eye that depends on age. To accomplish this, an automatic focus servomechanism is used in the visual system. This system is able to change the optical power of the physiological optics (primarily the cornea and the lens) by changing the physical form of the lens. As part of this change, the location of the lens along the optical axis is also changed slightly. The dynamics of the normal eye are shown in the following figure

Range of accommodation in normal eye.

The dashed horizontal line shows the nominal optical power of the eye in diopters when the ciliary muscle is relaxed. The open circles show the typical optical powers associated with the near points shown by the dotted line. The gray area shows the normal range of accommodation. The dotted line shows the accommodation range extending from about 7 cm (less than 3 inches) at nine years of age and 25 cm (10 inches) in the young adult to 2 meters at 60 years of age. [Rossi]

For most purposes, an accommodation range of 25 cm to infiinity is quite adequate. This only requires a change in optical power of about 4 diopters. Thus, the typical young person has an excess capability of about 10 diopters.

In the context of what is known about reading (see reading) and the available data (7.3.2), it appears that the autofocus function is accomplished in the first 500 milliseconds after a major saccade. During the study of a single scene element, it is likely that the eye does not attempt to refocus. Instead, it maintains its focus based on memory while awaiting the next major saccade. In the context of reading a scene element may be a line of text or a small figure.

The acuity achieved by the visual system is controlled by the quality of the physiolgical optics, the ability of the automatic focus servomechanism to maintain the appropriate focus condition, and the ability of the two-dimensional correlator of the pretectum to extract fine detail from the signals presented to it. The operation of the visual system with regard to acuity can be summarized using the following figure.

Visual acuity versus focus error.

The two straight lines meeting at 0.0 on the horizontal axis represent the quality of an image formed by an ideal optical system in the absence of any degradation. It is defined at the point of focus on the optical axis. This condition can not be achieved by any real optical system. Diffraction limits any real optical system to the value labeled the on-axis diffraction limit. The value of this limit depends on the numerical aperture of the optical system.

The center of the foveola of the human eye is not located on the optical axis of the eye. Therefore, its maximum performance is limited to the value labeled the foveal position limit. This is the nominal limit in performance of the eye if the neurological portion of the visual system were perfect. In practice, this is approximately the condition achieved by the best "young eyes." It is only achieved if the illumination level is adequate to fully constrict the aperture stop (colloquially known as the pupil) of the eye. When averaged over a larger population, the nominal "best acuity" is described as 20/20 (6/6 in metric units) and labeled the normal neurological limit. There is a disease (condition) that will be discussed below that further limits the performance of the system. In clinically described cases, this disease is known as amblyopia or lazy eye.

Importance of adequate light in vision

The importance of adequate light level cannot be stressed too much when discussing Myopia. The noise level associated with both the circuits of the eye and the quantum nature of light play a major part in limiting the performance of the neurological portion of the visual system. The performance of the visual system deteriorates rapidly outside of the photopic range (defined roughly as daylight prior to civil (not astronomical) sunset). It is near this light level that the pupil begins to dilate. This leads to significant additional aberrations in the physiological optics of the eye.

 

PROGRESSION OF THE AMETROPIAS WITH TIME

The eye develops in a haphazard manner during the first ten years, and particularly the first five years of life. Conditions of ametropia, including myopia, observed during this period are not indicative of the subjects long term visual performance. While it is important to correct myopia occurring at an early age to insure development of reading skills, it should not be assumed that this level of myopia will prevail. Beginning at ten, the ametropia of the eye usually stabilizes. Subsequently, it develops along a predictable time line unless other causal diseases appear.

The following figure illustrates the normal progression of two typical subjects. The solid line shows the optical performance of a myope with 3.0 diopters of basal myopia. The dotted line enclosed the optical performance of a typical hypermetrope with 1.5 diopters of basal hypermetropia. The basal error level is shown at the intersection of the horizontal lines with the scale on the right. The nominal accommodation range of the eye is shown by the height of the triangle above the basal level at a given age. The gray area shows the accommodation range required in everyday activity. The bottom of the shaded box represents objects at infinity. The top of the box represents objects at a distance of 25 cm (10 inches) from the eyes. The middle represents objects at 50 cm.

Progression of Ametropia with age.

Typical progression of Ametropia with age.

Typical errors in accommodation due to physiological refraction errors

The typical hypermetrope exhibits a basal accommodation error of -1.0 to -3.0 diopters. The typical myope exhibits a basal accommodation error of +1.0 to +3.0 diopters. These errors are correctable with glasses of the opposite power in diopters. These opposite numbers are those found on a prescription for far vision.

There is still room for argument but it appears the lens accommodation system degrades in performance with age due to the continual growth in volume of the lens of the eye. This introduces a resistance to changes in its shape that cannot be overcome by the limited strength of the ciliary muscle controlling the shape of the lens. There is little evidence that the ciliary muscle degrades during life, nor that it can be strengthened sufficiently to overcome the growth of the lens.

Looking first at the hypermetrope, note that in the presence of a significant basal accommodation error, the subject has more than sufficient accommodation range to meet the needs of daily life up to an age shown here as 28 years. At this point, he will encounter difficulty in reading at 25 cm. By holding the material slightly farther away, he will be able to read satisfactorily for a few more years. After that, reading glasses will become necessary. At an age of about 48, the average hypermetrope will begin having trouble seeing objects at long distances. His range above the basal accommodation level of -1.5 diopters no longer reaches the zero diopter level required to see far distant objects clearly.

The hypometrope (myope) faces a more difficult situation. Since the accommodation system is unilateral with respect to the basal accommodation level, his accomodation range does not include the zero diopters required to see objects at a distance clearly. For close work, his accommodation range is more than adequate. He can usually focus without difficulty on objects only a few inches from his eyes. In the case shown, to see at a distance, the myope will require glasses of -3 diopters for his entire life. Beginning at about 50, the subject will also need correction for short distance viewing.

Less common errors in accommodation due to neurological

There are two classes of neurological errors commonly encountered in vision. The first is amblyopia. This is a rare neurological and probably developmental problem of the pretectum, a portion of the thalamus in the (old) midbrain. It does not involve the cerebral cortex. However, the problem involves the circuits performing the initial interpretation of the scene prior to further processing in the cerebral cortex. This condition is frequently described as lazy eye. If callede anything in the vernacular, it is more appropriately named lazy midbrain.

The second, though still rare, disorder involves a problem in a second task of the pretectum. This is the creation of the signal that controls the lens of the eye. If this task is performed poorly, the subject can exhibit a condition similar to amblyopia, particularly at low scene illumination and contrast levels. This pseudo-amblyopia is due to an error in the amplification of the signal used to control the lens. It is associated with the Precision Optical System of the midbrain (previously designated the auxiliary optical system).

A second error in the second task introduces flashes of clear vision when a myopic subject first opens his eyes after attempting to relax his accommodation. Although he has attempted to accommodate to an infinite distance, his basal accommodation error actually optimizes the focus of the eye for a shorter distance. When he opens his eye, he briefly sees a sharp image of an object at the distance corresponding to his basal accommodation error. If there is an offset error in his Precision Optical System controlling the lens, the eye will proceed to accommodate to a different distance (and the image will lose focus. The initial clear image will be seen for 300 and 400 msec. This will be followed by the steady-state blurred image. There are no statistics available on the prevalence of this condition. The author would like to hear from anyone with this condition. (See address at bottom of the page)

 

BEHAVIORAL IMPACT ON MYOPIA

Over the years, charismatic speakers and authors have proposed methods of treating myopia based on concepts of the visual system that were incomplete. The efficacy of these treatments is yet to be demonstrated and many appear to be largely irrelevant to the condition.

It appears that training prior to the age of ten can have an effect on the neurological performance related to myopia. It appears less likely that training or behavior can impact refractory myopia except at the margin. Accommodation excercises are not likely to impact the progressive changes in the structural properies of the lens with age.

Many stratified studies have shown correlations between myopia and behavior in various population groups. However, they have generally not determined which is the cause and which is the effect. Does excessive participation in near work lead to myopia? Or, are people who are myopic drawn to near work? Do hunters achieve high visual acuity through participation in that activity? Or, are high acuity people drawn to hunting because of their unique capability?

 

LASIK AND CORNEAL KERATOTOMY SURGERY AS CURES FOR MYOPIA

There is now a significant record of success in the surgical cure of myopia due to refractive errors in the physiological optics of the eye. The results can be precise,immediate and appear to be long term (although the patient does not escape the progressive nature of presbyopia).

The procedure is not recommended for people under 18 since the growth of the individual elements of their physiological optics may not be complete.

The case of S. M. is illustrative. His myopia was near +8.0 in both eyes the day before surgery (he wore -8.0 correction lenses). He was a slight hypermetrope a few days after surgery. He did not require glasses for distant vision. Graphically, his results are compelling.

Progression of Ametropia with surgical intervention.

He is now 39 years of age and his basal level of accommodation error is not known. Following surgery, he required slight positive lenses for near work as expected of a hypermetrope his age.

Additional Details concerning Myopia

Additional support (at a technical level) for the findings presented on this page are available in Section 18.8.4 of Chapter 18 of the above treatise, PROCESSES IN BIOLOGICAL VISION. The dynamics of the visual system are detailed in Section 7.3.2. of Chapter 7. These Chapters can be downloaded from the documents page.

COMMENTS

Comments, corrections, etc. to the author by form, THE FASTEST WAY


Comments, corrections, etc. by routine E-MAIL

DEFINITIONS

It is important to be clear in the definitions used in discussing the syndrome (disease) of Myopia and the underlying functional condition(s).

Definition and functional description of Myopia

Origin: Gr. Myein + optos = to shut the eye. Based on the frequent squinting by individuals with this condition. This procedure increases the depth of focus of the eyes and reduces aberrations under some conditions.

Myopia is fundamentally an inadequacy in the ability of the visual system to focus on objects at significant distances from the eye while focusing adequately on objects close to the eye. The condition can be eye-specific but usually affects both eyes to a similar degree. In the above definition, it is assumed that the lighting available is within the photopic range of illumination. A colloguial name for Myopia is near-sightedness.

The Myopia syndrome involves the complex interplay of at least three underlying disorders.

1. The most obvious potential disorder involves a focal length of the physiological optics of the eye that is less than required by the physical dimensions of the eye. This condition is known as hypometropia. The opposite condition, the focal length exceeds the nominal dimension, is known as hypermetropia. 2. A second potential disorder relates to the failure or inadequacy of the automatic focus servomechanism of the eye. 3. A third potential disorder relates to a failure or inadequacy of the pretectum of the midbrain to extract the first order interpretations (the interps)related to the detailed imagery presented to the foveola of the eye. Each of these factors will be discussed below.

The scientific literature is beginning to distinguish between refractive myopia (#1 above) and neurological myopia (#2 & 3 above). The former can be readily corrected by auxiliary optic (spectacles). Until recently, neurological myopia has been overlooked in the literature. It will be shown that some neurological myopia has been treated (somewhat unknowingly)in the clinic in the past. In other situations, it has been associated with reading disorders. These factors will also be discussed below.

Related Definitions

Achromatopsia (with an s) is a complex medical condition, labeled a syndrome, consisting of a number of commonly observed underlying individual conditions. See above. One of these conditions is achromatopia (without the s) that gives the syndrome its name. Another is myopia, frequently serious enough to be labeled amblyopia

Acuity is the ability of the visual system to perceive fine geometrical detail. The visual system is optimized for black and white imagery. The acuity for colored imagery is reduced. Acuity describes a unique capability associated with the human foveola and only shared with a few other primates.

Amblyopia-A neurological condition, associated with the Precision Optical System of the midbrain, limiting the acuity of vision in a subject. Frequently described in optometry as uncorrectable myopia.

Autosomal dominant inheritance (AD)–Every generation is affected. Males and females are affected with equal frequency. The trait is transmitted only by an affected individual. Those without the trait do not transmit it.

Autosomal recessive inheritance (AR)–Only members of the same generation are affected. The trait is transmitted from asymptomatic carrier parents who each have one affected chromosome. Expression of the trait requires that both members of a chromosome be affected. Males and females are affected with equal frequency. Every child of an affected person is a carrier of the trait.

Electrostenolytic-Descriptive of an electrolytic process occurring on the surface of a cell membrane and capable of providing electrical energy to an electrical circuit.

Low Vision is vision uncorrectable to better than 20/70

Visual operating modes--There are four distinct physiological operating modes of the eye with respect to illumination. These modes are normally associated on a one-for-one basis with the illumination level of a scene. While this may be true for achromats and normals, it is not true for those with Achromatopsia. The four physiological levels are:

Hypertopic where the neural system of vision is electrolytically (electrically)overloaded. The subject loses acuity as well as color constancy in his perception. He also experiences physical pain. Photopic where the neural system of vision operates normally. Within this physiological condition, it exhibits a remarkable perception of color constancy and illumination constancy. The observable contrast is dynamic range limited by the internal signal processing system. Mesotopic where the neural system of vision is limited in its performance by the noise associated with the light striking the eyes and the performance is further reduced in the long wavelength spectral region as a function of the precise light level. Neither perceived illumination constancy or color constancy is maintained within this range. Perception of color is usually lost within this range. Presbyopia a condition where the eye has lost the ability to focus sharply on things up close. An age related progressive disorder. Most noticable under poor illumination and attempting to read small print. Scotopic where the neural system of vision is limited in its performance by the internal threshold of that system. Performance falls as a direct function of the external light level.

Primary references:

Elements of the following papers were used in formulating this page. However, not all of the ideas presented in these papers are supported here. This is particularly true of the statements made in the introductory paragraphs of these papers.

The Lighthouse International Organization for statistics and general information. Considers myopia to be of strictly refractive origin.

Rossi, B. (1957) Optics London: Addison-Wesley, Inc. pp 90-91 (only one line)

Grosvenor T. & Flom, M. (1991) Refractive Anomalies. Boston, MA: Butterworth-Heinemann. This book provides a significant amount of statistics on myopia.

An extensive list of references are provided in the appropriate Chapters of PROCESSES IN BIOLOGICAL VISION. The website for this material is given above.

The Laser eye surgery hub of the United Kingdom also provides a range of background material that may also by useful.