Independant fine/small controlled eye movement must be normal

Discussion in 'Eye-Care' started by andrewedwardjudd, Apr 18, 2005.

  1. I want to try and work out why people seem to disagree with this idea.
    Its not obvious to me why they disagree.

    There appears to be a concensus view on this list that eye muscle
    control is from a single central point.

    Dr Judy>> Eye muscle motor control is from a single central point,
    there may be damage in the motor nerve pathways to each eye, but the
    control is central.

    Andrew>>> What do you mean by a central point? A single tiny point
    without the ability to differenitate and independantly control the
    eyes? That cannot be true.

    Dr Judy>>Sorry, you are wrong. Check any neuroanatomy and
    neurophysiology text. The motor control of the two eyes is not
    independent. If it were, we would see double.

    Lets look at some observations:

    1. Human eyes have the ability to point precisely to an object by a
    process called fovealisation. Each object seen is precisely (or
    should be) on the centre of the fovea. To be aware of the gap between
    the bars of a 20/20 letter E and move from one bar to the other
    requires an eye movement of one minute of one degree. Such examples of
    precise eye movement are typical for a person with good vision - even
    the moon seen from earth only occupies 0.52 Degrees of arc.

    2. Fovealisation has to take account of what is seen on the retina
    *and* what is desired to be seen on the retina.

    3. Saccades are generally defined as being conjugate or exactly
    similar. Vergence movements can be unequal. Different brain areas
    seem to generate the two movements.

    4. If you place a string on your nose 40 feet long you see a double
    image of the string for the length of the string or until the limit of
    your ability to separate out the strings. Therefore you see double
    from the near point to optical infinity, and can by choice decide which
    part of the double image to resolve into a single image so that the
    string forms a cross at that point. There is no reflex which forces
    you to decide where to see the cross. Each fusion movement is by
    conscious choice of what is attended to.

    5. If the string position is moved so that it is held at the centre
    point of the left eye then for all string positions where a cross is
    seen (and assuming the string does not sag) only the right eye moves to
    target the string.

    So when people say "Eye muscle motor control is from a single central
    point" Baring in mind all of the above points, what exactly are they
    meaning?

    Andrew
     
    andrewedwardjudd, Apr 18, 2005
    #1
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  2. andrewedwardjudd

    g.gatti Guest

    Dear Andrew,
    how do you explain that an ex -23 D myopic person can read easily well
    the license plates of cars (9cm) at 15 meters?

    What is your opinion?
     
    g.gatti, Apr 18, 2005
    #2
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  3. I think they mean that voluntary oculomotor innervation origniates at a
    single location within the central nervsous system (as opposed to the
    autonomic nervous system, which might have some overlapping, fine-tuning
    feedback loops, depending on the task at hand, so it may be a bit of a
    simplification).

    w.stacy, o.d.
     
    William Stacy, Apr 18, 2005
    #3
  4. andrewedwardjudd

    Neil Brooks Guest

    I'm guessing the license plates must be painted on a billboard, and
    their numbers are something like 150 feet tall.

    That fits. Yep. Uh-huh.
     
    Neil Brooks, Apr 19, 2005
    #4
  5. andrewedwardjudd

    RM Guest

    I'm not sure what any of your points numbered 1-5 have to do with whether or
    whether not there is a single CNS "point" where eye movements are
    controlled.

    In my opinion, and I am not an expert in neuroophthalmology or eye movement
    control, a more correct statement might be that there is very tight
    coordination of eye movements.
     
    RM, Apr 19, 2005
    #5
  6. andrewedwardjudd

    Guest Guest

    Andrew, then buy some books on this subject.
    This newsgroup is not an education site.

    For instance:

    "An introduction to the visual system" Martin J. Tovée ISBN 0-521-48339-5

    ''Neuro-Ophthalmology Review Manual'' Lanning B. Kline Frank J.Bajandas.
    ISBN 1-55642-470-1

    If you don't agree with the authors then please tell them what's incorrect
    and all off us shall buy the revised books.
     
    Guest, Apr 19, 2005
    #6
  7. andrewedwardjudd

    Dr Judy Guest

    No reflex controls what object a person chooses to attend. Once the object
    is chosen, and attention is directed there, then the autonomic process
    begins and both eyes look at the chosen object and it is seen clear and
    single and on the fovea of both eyes. The two eye move in conjunction, to
    the same place and all, non attended objects are seen slightly blurred and
    double as they are not on the fovea of either eye.

    In your example, try putting some beads every four inches along the string.
    You will see only the bead to which you have directed your attention as
    single. If you think that the eyes can move independently then direct your
    right eye to one bead and your left eye to another bead, ie put one bead on
    the fovea of one eye and the other bead on the fovea of the other eye so
    that both beads are clear and seen as overlapping each other as opposed to
    horizontally separated. I think you will find you cannot do this.
    That both eyes move to put the object of regard on the fovea. Depending
    upon the starting location, the eyes may move differently, but they are
    moving together, to the same point and controlled by the same cortical area.

    Dr Judy
     
    Dr Judy, Apr 19, 2005
    #7
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