How the Accommodation System Works

Discussion in 'Optometry Archives' started by Otis Brown, Aug 31, 2004.

  1. Otis Brown

    Otis Brown Guest

    Dear Frends,

    Jan suggested that I needed to clarify the definition
    of accommodation. This should answer his request
    for that conceptual model.

    To clarify. The accommodation system detects
    "micro-blur" on the surface of the retina. (Outside
    the dead-band). At that point, the lens refractive-state
    is changed to ellimate the micro-blur, and clear the
    image on the retina. (This action is seen with
    an infra-red optometer -- and is below our threshold
    of preception.) This system has a "time delay" of
    about 1/3 second, and a time-constant of about 1/3 second.
    For a step-change in visual-enviroment, the response
    is about 2/3 second.

    The SECOND SYSTEM, controls the refractive state
    of the eye to this accommodation SIGNAL. This second
    system has a time-constant of 100 days.

    If the average-visual environment is made
    highly negative, the refractive status (measured
    with a trial-lens kit) will move from a positive
    value to a negative value. When this is accomplished,
    the natural eye will see "blur" at distance
    because the accommodation system "hits the stops".
    I would call this "macro-blur" and you do preceive
    this effect.

    No defect or "error" is part of this process.
    It is simply a designed-in characterstic of
    the natural eye.

    Please enjoy this heuristic approach to
    problem solving.

    Best,

    Otis
    Engineer


    ********************



    Paper 28

    HOW ACCOMMODATION WORKS

    TONIC ACCOMMODATION AS PART OF THE NATURAL EYE'S CONTROL SYSTEM


    By Otis Brown



    "The significant problems we have cannot be solved at the same
    level of thinking we were at when we created them."

    Albert Einstein



    BLANK FIELD ACCOMMODATION

    The earlier chapters point to the need for a more complete
    model of the normal eye's behavior. In this chapter we will bring
    all the system components together.

    The normal eye needs an accurate lens control system. The
    existence of optical dead-band, or depth-of-field, causes a
    non-linearity in the operation of an automatically focused camera.
    The lens muscles display a noise signal which produces a
    back-and-forth motion in the lens across the optical dead-band.
    The computer developed in this chapter predicts the eye's focal
    status as a function of depth-of-focus, perturbations in the
    muscles, and the eye's environment.


    TONIC ACCOMMODATION

    The focal status of the lens is controlled by blur, as long
    as the retina can sense blur. When the eye is in darkness, the
    lens is driven to a neurological "standby", or tonic accommodation
    position. This conceptual model yields focal status predictions
    for tonic accommodation as a function of time and the individual's
    visual environment.


    EYE RESOLUTION

    A camera or eye can be limited in resolution by diffraction
    effects caused by a small aperture, or by granularity effects due
    to the response characteristics of the film or retina. Less than
    an aperture of about 2 mm, the normal human eye's resolution is
    limited by diffraction effects.

    In daylight, the human eye has a resolution of about 1 to 2
    minutes of arc. In darkness, the human eye can resolve stars that
    are separated by between 3 to 5 minutes of arc.


    DEPTH-OF-FOCUS

    Because of granularity effects of the retina, we can change
    the focal power of the eye's lens without changing the sharpness
    of the image formed on the retina. This range of focal status
    values is called the depth-of-focus. Specific values are
    calculated by the following equation: (Encyclopedia Britannica,
    Optics, 1970)

    FG = d * I / a
    Where:

    FG = Depth-of-focus I = focal length

    d = Blur circle a = aperture diameter


    The depth of focus is inversely proportional to the diameter
    of the aperture. With an aperture of 8 mm, the human eye has a
    depth-of-focus of +/- .15 diopters. In sunlight the iris will
    have an aperture of approximately 2 mm, which gives us a
    calculated value for depth-of-focus of +/- .6 diopters.


    A BLOCK DIAGRAM OF THE ACCOMMODATION SYSTEM

    The accommodation system is shown within the dotted lines in
    Figures 1 and 2. Figure 2 shows the analog computer for
    accommodation.

    The light rays that pass through the lens ultimately produce
    blur on the retina. The blur is sensed; this then generates a
    neurological signal which changes the focal status of the lens.
    The lens is the summing point for the input signal versus the
    feedback signal. This is the essential definition of a
    servo-control system.


    ANALOG COMPUTER IMPLEMENTATION

    The visual environment is simulated by making one volt equal
    to one diopter. The accommodation system is implemented by using
    four operational amplifiers.


    1. The error detector amplifier compares the feedback signal
    (from the retina) with the input visual (environment). An
    output signal is produced which is proportional to the image
    blur.

    2. The dead-band operational amplifier duplicates the retina's
    inability to sense blur while the image is within the
    dead-band.

    3. Motor Simulation. This amplifier changes the neurological
    signal into movement by controlling muscles and ligaments
    attached to the eye's lens.

    4. Optical stop simulation. This is implemented by use of a
    buffer amplifier whose output state (voltage) is proportional
    to the focal state of the eye. This limit simulates the
    measurement of the optical state of the eye.


    DEAD-BAND SIMULATION

    A silicon diode has a knee of 0.6 volts. By using two
    back-to-back diodes in conjunction with an operational amplifier,
    we can simulate the effect of dead-band in a servo system. With
    this simulation there will be no output from the amplifier until
    the input exceeds +/- .6 volts.


    THE LENS PLANT RESPONSE

    Previous studies have shown that the plant (lens support
    muscles and ligaments) has a 1/(TAU s + 1) transfer function. We
    are using an operational amplifier with a gain of ten, and a
    time-constant of 1/3 second to simulate the muscle-ligament
    dynamics.


    NOISE IN ELECTRONIC SERVO SYSTEMS

    There are numerous sources of noise in a mechanical servo
    system. The development of these random perturbations are due to
    thermal effects, noise in the amplifier, and back-lash in the
    gears. This mechanical system produces a signal which is
    continually perturbated about the desired, or command signal.
    This is called control system jitter.

    The output position of a noisy servo system can be described
    by a statistical distribution in terms of frequency spectrum,
    mean, and root- mean-square values. Systems are normally designed
    to minimize the effect of noise. Paradoxically, a designer can
    intentionally use the existing noise in a system to produce a
    scanning motion in the lens and thereby minimize the effect of
    dead-band on the output of a control system.

    Noise will always exist in a physiological control system,
    and the output position of these systems can be measured as random
    perturbations around the mean of the command signal. If the input
    visual environment is constant, the output signal will be
    exclusively due to the effects of measurement errors and internal
    noise in the system.


    NOISE SIMULATION

    Dr. A. Suzumura has determined that the noise in the
    muscles has a characteristic frequency spectrum of from 1/4 to 4
    hertz. (1) The output of the analog computer shows a noise
    signature which is close to the dynamic noise characteristics of
    the human eye.


    THE SYSTEM'S DYNAMIC RESPONSE

    Blur control can only occur as noise drives the lens beyond
    the threshold of the optical dead-band. A recording of the eye's
    focal status is shown in Figure 3. The eye's focal status never
    rests and must be described by a statistical-distribution, with
    the skirts extending beyond the edge of the optical dead-band, as
    show on Figure 4.

    The focal status of the lens is equal to the diopter value of
    the environment. The standard deviation of this distribution will
    be perhaps +/- 0.6 diopters. Recent measurements, made by Dr. R.
    Yamaji using an infrared optometer, have shown servo noise
    response characteristics of the human eye which are similar to the
    voltage output of the analog computer. (2)


    A BLOCK DIAGRAM FOR DYNAMIC LONG-TERM CONTROL

    Based on Dr. Francis Young's experimental data, we show a
    block diagram that obtains its control from the accommodation
    signal. (3) Since the stop-to-stop lens travel is ultimately
    limited by the optical state of the eye, the block diagram shows
    the long-term system controlling the physical stop position of the
    lens of the eye, as shown in Figure 1.


    MEASURING THE EYE'S FOCAL STATUS

    The limiting effect caused by the eye's optical state (or
    length) can be simulated in the analog computer by using an
    operational amplifier with a diode in the feedback path, as shown
    in Figure 2. As long as the lens voltage is above the reference
    value (linear region), the output will be a replica of the input.
    When the input exceeds the reference value, the output will remain
    clamped at the reference voltage, thus simulating the measurement
    of the optical length of the eye.


    THE TONIC ACCOMMODATION SYSTEM

    When the eye is in darkness, the retina is unable to sense
    blur. The eye (accommodation system) will then select an
    alternate source for the reference signal. Under tonic
    accommodation control the lens is not part of the feedback loop.
    The status of the tonic control can be determined by measuring the
    focal status of the eye with an infrared optometer. A block
    diagram of the tonic accommodation control system is shown in
    Figure 5.

    CONCLUSIONS

    This model for the normal eye's focal status was constructed
    on the principle of using the smallest number of physiologically
    justifiable elements that still give a reasonably precise fit to
    the experimental data (as the data concerns the normal eye's
    behavior). The model presented is probably the simplest model
    that can be built that can accurately describe the behavior of the
    tonic and long-term focal control characteristics of the eye.


    REFERENCES

    1. Suzumura, A., "ACCOMMODATION IN MYOPIA", Proceedings of the
    Second International Conference on Myopia, Prefecture Hall,
    Yokohama, Japan, pp. 55-67, May 1980

    2. Yamaji, R., "TREATMENT OF ACQUIRED MYOPIA (PSEUDOMYOPIA)",
    ibid., pp. 91-100

    3. Young, F., "THE EFFECT OF RESTRICTED VISUAL SPACE ON THE
    PRIMATE EYE", American Journal of Ophthalmology, Vol. 52,
    Part II, Nov. 1961
     
    Otis Brown, Aug 31, 2004
    #1
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  2. Otis Brown

    Guest Guest

    The above is never asked or suggest Otis.

    However the following must be true:

    The ''dynamic (human) eye" vapouring by Otis, is, until Otis proves I'm
    wrong, just an hypermetropic eye.

    Saves lots of time and explains nearly everything in Otis statements so far.

    Free to Marcus Porcius Cato: ''Ceterum censeo Carthaginem esse delendam"

    I declare that Otis idea about preventing myopia in humans must be proved by
    Otis himself or be destroyed.

    Jan (normally Dutch spoken)
     
    Guest, Aug 31, 2004
    #2
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  3. Otis Brown

    LarryDoc Guest

    What? Do you just make these things up out of thin air?

    Inquiring minds would like to know why you choose to waste time with
    this rubbish. Do you not have anything better to do with your so-called
    life?

    --LB
     
    LarryDoc, Sep 1, 2004
    #3
  4. Otis Brown

    Otis Brown Guest

    Dear Larry,

    If you don't like scientific truth concerning
    the behavior of the accommodation system -- don't
    read the paper.

    You know, Rishi had you "pegged" correctly.

    While I would not call you a "jerk", a "stuffed shirt",
    and intellectually blind, he certainly has his point.

    Do me a favor -- don't study scientific truth -- if you
    can not accept the consequences.

    No wonder you are "practicing" the method put in
    place 400 years ago. You have your head buried
    in the sand my friend.


    Best,

    Otis


    ******
     
    Otis Brown, Sep 1, 2004
    #4
  5. What physical parameter changes back and forth so easily? Axial length?
    At least Mr. Otis has demonstrated that your so much claimed Depht of
    field effect in myopes is an idiotic thing that explains nothing.

    Just half a diopter.

    Nothing much for -5, -10 or -20 patients.

    There must be something elsewhere.

    If the calculations of Otis are true, they are amusing!
     
    Rishi Giovanni Gatti, Sep 1, 2004
    #5
  6. Otis Brown

    Otis Brown Guest

    Re:> What? Do you just make these things up out
    of thin air? Larry-Doc


    Otis> The SECOND SYSTEM, controls the refractive state
    of the eye to this accommodation SIGNAL. This second
    system has a time-constant of 100 days.


    Dear Larry-Doc,

    If you actually looked at objective, honest-to-God scientific
    data your would find this out. Sadly, your head is
    deeply buried in your own BS, so I doubt that you will
    learn anything at all.

    Hoever, you do maintain the "majority opinion" that
    NOTHING has ANY effect on the refractive status
    of the natural eye. You are tragically wrong
    on that point.

    The natural eye was and is, a dynamic and
    sophisticated system. Only your shop-practice
    theory of 140 years ago denies what is
    obvious to a coniderable number of MDs
    and ODs on this subject.

    Here is the sceond-opinion by Dr. Steve Leung
    who does support true-prevention with a
    plus lens.

    Enjoy,

    Otis
    Engineer

    *****

    LEADERSHIP FOR CHANGE

    "A true leader has the confidence to stand alone, the courage
    to make tough decisions, and the compassion to listen to the needs
    of others. He does not set out to be a leader, but becomes one by
    the quality of his actions and the integrity of his intent. In
    the end leaders are much like eagles...they don't flock, you
    find them one at a time."


    From: Dr. Steve Leung OD

    Dear Myopic Folk,

    Subject: An optometrist's personal experience or MY AWAKENING

    I am a practicing optometrist working in the field of optics
    for more than 16 years. During these years I faced an excessive
    high rate of children developing nearsightedness (myopia).
    It is very hard to resist the obvious need to use a minus lens
    (compensating lens) for these children.

    I deeply appreciate that we all value clear distant vision
    for life. Achieving this goal would be of great value for all of
    us.

    Everyday, a great many people are developing the vision
    problems of nearsightedness, farsightedness, aged vision -- as well
    as crossed and lazy eyes. These people come to my office amd require
    immediate vision correction. They all need glasses.

    Among the visual problems, the case of myopia correction
    bothers me greatly. It is a dilemma and tragedy of using a "correcting"
    lenses, when in fact these glasses eventually become a crutch for life.

    In the early years of practice, I was not aware the long-term
    bad effect that a minus lens has on the eye.

    This is because neither the curriculum textbooks nor the
    professors pointed out the ultimate side effect that a minus lens
    has on the eye -- during my many years of doing course work in optometry.

    After graduation, I practiced the full scope of optometry,
    from refraction to fundus eye examination, and vision correction
    by optical means. But once these means are removed, the vision is
    neither improved nor restored.

    The minus lens is merely an aid to vision, i.e., compensation
    by external means.

    In the majority of cases, naked-eye vision gets worse with
    the traditional minus lens correction.

    The children will need stronger power glasses in the following
    years. It is a matter of treating the symptom -- but does not
    achieve an effective cure.

    I have been mulling over in my mind -- to think about
    alternative and better methods to manage myopic eyes, because I
    also am nearsighted.

    With my accumulating experience, I am well aware that
    constant wearing of minus lens glasses are harmful especially the
    full power ones. However, there is no choice but to use a minus
    lens if the child cannot see well in his class.

    At times, the best that I can do is to emphasize that the use
    of (minus lens) glasses be restricted to chalk board, and always
    must be removed after class. This is the first step in goal of
    avoiding the glasses' side effect.

    But being myopic is unfortunate and inconvenient. At times
    it seems that none of us can escape the use of a minus lenses to
    restore clear vision.

    I have been driven into deep thinking about a way of, "how to
    restore clear vision from myopic and how to maintain distant
    vision for life."

    As a father and an optometrist, I felt a strong commitment to
    protect my own child's vision. It was because my child
    (age 4) in her curiosity asked me, "Dad, why do you always wear
    glasses? Why are the kids I play with in school wearing glasses?"

    Her statement had a serious impact on me, and I woke up to
    the fact that a child should not be fitted with minus lens
    glasses -- if there are means to doing so.

    Why? The earlier age you begin wearing the minus lens, the
    faster vision deteriorates. The minus lens can make vision worse
    all by itself! Many scientists, engineers and health workers
    have formed this opinion -- that the minus lens is definitely harmful to
    young kid's long-term vision.

    Because I was sensitive to both the requirement to use the
    minus lens, but also understood the secondary effect I began much
    broader research into the subject matter. This included the
    judgment of engineers and scientists (and some ophthalmologists)
    who "object" to the use of the minus lens.

    Fortunately, I met several enthusiastic engineers, physicists
    and scientists via internet in 2001 by chance. They provided
    excellent postings in their web sites where I got a deep insight
    about the development and management of child's acquired myopia --
    to include the potential of preventing it in the first place by
    wise use of a (preventive) plus lens. They are Donald Rehm, Otis Brown,
    James Arthur, Dr. Stirling Colgate and Alex Eulenberg.

    In fact, researchers such as Dr. Jacob Raphaelson and Dr.
    Francis Young had conducted pioneering work to determine the
    cause, effect, and remedy for myopia acquired in school. As early
    as 1904, Dr. Jacob Raphaelson had used the plus (convex) lens to
    effectively cure a child's myopia. Further, Dr. Francis Young
    has revealed the true cause of acquired myopia with his large
    number of insightful experiments and scientific publications in
    the 1960s. All the above mentioned scientists advocate that
    preventative measures be instituted to help children avoid getting
    into myopia in the first place.

    In view of their spirit and fortitude, I felt that I bore a
    responsibility as an optometrist if I did nothing to assist in the
    prevention of myopia.

    I regret that I became part of the system that was put in place
    long time ago -- and that this system has not changed in any
    significant detail since its inception.

    My goal is to look to the future and begin preventive methods
    which can be effective for the child who is on the threshold of myopia.
    Today, I make it clear that my mission and task is to try my best
    to discuss the alternate opinion on the therapeutic use of the
    plus lens -- instead of the compensatory use of minus lens.
    I do everything in my power to explain the long-term effect that
    the minus lens has on the eye's refractive status, and I encourage
    parents to review this issue for themselves.

    I have supported several hundred children with the plus lens
    since 2001. The long term effect of the lens is developing, and
    results will become better as the use becomes more complete.
    Most of the children retain their current refractive (focal)
    status and few of them achieved significant vision improvement.
    Although it is unusual, there have been several cases of complete vision
    recovery! I also felt that making this commitment is a matter of my
    personal integrity, and is necessarily part of my work and career.


    Steve Leung
    Optometrist
    Hong Kong,SAR
    May 2003

    _________________

    ATTITUDE

    The greatest discovery of any generation is that a human
    being can alter his life by altering his attitude.
     
    Otis Brown, Sep 1, 2004
    #6
  7. Otis Brown

    Otis Brown Guest

    Dear Mike,

    Subject: Who is lying?


    Then Mike you tell me. The majority opinion AGREES that the
    natural eye is dyanamic -- and its refractive status
    will move in a negative direction when you place
    a minus len on it.

    Yes or no! Up or down.

    For the past 3 months you have directly denied the
    above. Now are you changing your mind?

    Who is not telling the truth about the natural
    eye as a dynamic control system?

    It is true -- one of us has an inaccurate understanding
    of the behavior of the natural eye. But I do not
    call you a "lier" because your understanding is
    so shallow.

    Best,

    Otis
    Engineer

    cc: People how can be honest in an objective,
    scientific sense -- to include animal experiments!

    *******************
     
    Otis Brown, Sep 2, 2004
    #7
  8. Otis Brown

    Guest Guest

    "Mike Tyner" <> schreef in bericht

    Read my lips Mike, Otis starts a new thread and leave this one.

    Free to Marcus Porcius Cato: ''Ceterum censeo Carthaginem esse delendam"

    I declare that Otis idea about preventing myopia in humans must be
    destroyed.

    Jan (normally Dutch spoken)
     
    Guest, Sep 2, 2004
    #8
  9. Do you have one case where the glasses were NOT changed at least one time?
     
    Rishi Giovanni Gatti, Sep 2, 2004
    #9
  10. Otis Brown

    Otis Brown Guest

    Dear Friend,

    I agree that once you begin wearing the minus lens, the
    downward rate it -1/2 diopter per year.

    You insist that it is IMPOSSIBLE to prevent this
    -1/2 diopter per year -- and I say that it is
    possible to prevent.

    If the person concerned with it, uses the plus (when
    his vision is at 20/40 to 20/50, and clears his vision
    to better-than the DMV standard, and does this
    every time he sees his vision below this standard,
    then he has NO REQUIREMENT for the use of a minus lens,

    AND THE SUBJECT IS MOOT.

    If the person is not successful in the above context
    and understanding -- then of course the use
    of the minus lens must follow.

    I hope this clarifies that point.

    Best,

    Otis
     
    Otis Brown, Sep 7, 2004
    #10
  11. Otis Brown

    LarryDoc Guest

    Well there you go again, inventing statistics to go along with your
    unprovable theories. Or just quoting averages and medians and numbers
    that don't relate to real people. The rate of increasing myopia is not
    linear, not consistent and has nothing to do whether or not a corrective
    lens is worn.
    Then show us just one scientific study, on real human beings, to support
    that. Of course you wont because there isn't one. You've been asked,
    over and over again. You can't do it. You just make things up.
    So using your absurd recommendation, the person is going to get behind
    the wheel of a car and risk injury to self and others. Nice going,
    Otis. You just keep on making stuff up.
    So finally you ARE recommending minus lenses so that a person might
    safely drive and obey the law!
    You've made it abundantly clear that you don't know a damn thing about
    vision science. And, seeing as this is a newsgroup forum entitled
    "science and medicine of vision----sci.med.vision" perhaps it's time you
    unsubscribe and take your rhetoric and unprovable nonsense somewhere
    else.

    There's simply no place for you here. Got it?

    --
    Dr. Larry Bickford, O.D.
    Family Practice Eye Health & Vision Care

    The Eyecare Connection
    http://www.eyecarecontacts.com
     
    LarryDoc, Sep 7, 2004
    #11
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