How the retina works

Discussion in 'Optometry Archives' started by Wil Hadden, Jul 1, 2004.

  1. Wil Hadden

    Wil Hadden Guest


    Sorry for the bit of cross-posting, I've had a couple of problems working out the best
    place for this post.

    I've done quite a lot of googling to find out how the retina works in simplistic terms
    to no avail.

    I have been working at Carver Mead's Silicon Retina in May 91's Scientific American and
    have worked out so far:

    Light hits the cones and rods, and passed on to the horizonal cells.
    The horizontal cells smooths or spacially averages the signal from the rods and cones,
    there is also some feedback to them.
    The rods and cones along with the horizontal cells send a signal to the bipolar cells
    through the triad synapse, which basically takes the difference between them.

    So far the effect of this basically detects edges and motion from the original image. Is
    this correct so far?

    Now I can't find much information on the amacrine and ganglion cells, except that the
    amacrine cells perform a similar role to the horizontal cells and the ganglion act as a
    conduit to the lateral geniculate in the thalamus.

    Could someone kindly expand on that a bit or point me to a site that can explain it in
    layman terms?

    Any help appreciated.

    Wil Hadden, Jul 1, 2004
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  2. I suggest a decent med school textbook like Kandel and Schwartz. It
    may help you beyond a simplistic understanding.

    Kalman Rubinson, Jul 1, 2004
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  3. Wil Hadden

    Wil Hadden Guest

    I probably should get a book like that.

    What I'm trying to do is make a simulation of essentially the essence of how the various
    layers work, and then to later build on the output of that work.

    I'm currently at the stage of researching whether my plans make ant sense!
    Basically I don't want to be spiralling off doing research on retinas at least until I
    know there's merit in building a simplistic model.
    To that end I currently need an overview of the various layers.

    Wil Hadden, Jul 1, 2004
  4. Wil Hadden

    r norman Guest

    What you really need is a book like "The Retina: An Approachable Part
    of the Brain" by John Dowling, Harvard U Press, 1987. That work is,
    in fact, referenced in Mahowald and Mead's "Silicon Retina" paper.
    (Incidentally, it is not polite to ignore coauthors when citing work!"

    R. Masland has a brief introduction to "The Functional Architecture of
    the Retina" in the Dec. 1986 Scientific American.

    Just about every book you can find on neurobiology or neurophysiology
    or neuroscience or physiological psychology should have a large
    section, perhaps a whole chapter, on retinal information processing.
    these books are really essential to get a firm understanding of the
    anatomy and physiology behind the information processing.

    This seems to be a nice web site on the retina
    r norman, Jul 1, 2004
  5. Wil Hadden

    Wil Hadden Guest

    *blush* sorry, was being lazy!
    Thanks for the leads, off down to the library for me then!

    Wil Hadden, Jul 1, 2004
  6. Wil Hadden

    r norman Guest

    In that case, what you need is far more than the introductory type
    things I outlined in another response.

    If you want your model to be anything other than simplistic, you need
    to understand what real neurons do, as opposed to "cartoon" neurons
    used in most "neural" models. These are quite fine for research in
    information processing, even for research in visual information or
    image processing. They are not at all suitable for research in how
    the eye actually works.

    You need to understand the details of signal conduction down branching
    dendrites, about transmitter release by graded signals, about synaptic
    plasticity. You need to understand the details of microcircuit
    anatomy of the retina. You need to understand the details of
    neurophysiology of the retina derived from electrophysiological

    In short, you need to go through some hefty neuroscience text as just
    a start and then work your way into the primary literature. I
    strongly recommend
    Kandel, Schwartz, and Jessel,
    Principles of Neural Science
    McGraw-Hill, 2000

    Squire et al.
    Fundamental Neuroscience, 2nd ed
    Academic Press 2002

    Synaptic Organization of the Brain, 5th Ed
    Oxford U. Press, 2003

    You also must read
    Biophysics of Computation: Information Processing in Single Neurons
    Oxford U Press, 1999
    r norman, Jul 1, 2004
  7. I think you need more than an overview; you need an intimate
    understanding of the processing of each of the interacting components.
    By this, I mean that a 'simplistic' definition by cell or by layer
    will not permit you to design a useful model.

    Kalman Rubinson, Jul 1, 2004
  8. Wil Hadden

    Wil Hadden Guest


    What I'm after is something a lot more akin to Mahowald and Mead's Silicon Retina except
    done in software.
    Would I still need to do into such detail?

    Wil Hadden, Jul 1, 2004
  9. Wil Hadden

    Wil Hadden Guest

    I think you could be right, it might be time to have a rethink!

    Wil Hadden, Jul 1, 2004
  10. Wil Hadden

    r norman Guest

    That seems to be different from what you described earlier.

    Do you want to model the real retina or the "Silicon retina"? There
    is a big difference. To model the real retina, you must know really
    how the cells work. To model the silicon retina, all you have to know
    is how that works. And there, I can't help you.
    r norman, Jul 1, 2004
  11. FWIW, I'm not sure you need to go into each layer, so much as the
    effective output of the ganglion cells and center-surround antagonism.
    It depends on why you want to model the retina in the first place.

    Even more pertinent to your work might be Marr, "Vision", Freeman, 1982.
    In all your searches, add the phrase "lateral inhibition"-- its my
    favorite network architecture. It retina, it serves as a spatial
    differentiator, but in brainstem and spinal cord is serves a temporal
    integrator. Lateral inhibition-- is there anything it can't do?

    Scott Seidman, Jul 1, 2004
  12. Wil Hadden

    Wil Hadden Guest

    You are correct, I am really after something more aligned to the silicon retina, but I
    also want to understand it's design in relation to a real retina and to try to understand
    the myriad of amacrine cells. To be honest though my research will not be going in to PhD

    I have to say though I went though the site you suggested and it is extremely good, there
    is a pdf download that gives a fantastic overview of the retina and is exactly the sort of
    thing I was after.

    I also have a collection of papers about neurons on order it includes "The Functional
    Architecture of the Retina" in the Dec. 1986 Scientific American, though I just found out
    I think you can get those old papers from the sciam site, anyway definitely worth a read.

    All in all thanks for your responses, they have definitely helped me refine what I am
    looking for.

    Wil Hadden, Jul 2, 2004
  13. Wil Hadden

    Wil Hadden Guest

    That is exactly what I am trying to model, I have more of an interest in edge detection
    than in the make up of the retina in a physiological sense.

    Do you know of any models I could look at?

    Wil Hadden, Jul 2, 2004
  14. Wil Hadden

    Wil Hadden Guest

    Lateral inhibition looks promising I must say. It resembles a self organising map in
    neural networks and seems to be very useful!
    I'm presuming that the horizontal and amacrine cells exhibit lateral inhibition, so
    there's no getting away from it!

    Wil Hadden, Jul 2, 2004
  15. Lateral inhibition is the mechanism for the whole center-surround
    antagonism operation of the retina. If you think about it, it not only
    enhances edge detection, it increases the dynamic range of the entire
    retina. The horizontal cells implement lateral inhibition at the
    photoreceptor level, and the amacrine cells at the ganglion cell level.
    The bipolar cells essentially define whether a ganglion cell is on-
    center/off-surround, or off-center/on-surround.

    An interesting tidbit-- the light has to pass through the ganglion,
    bipolar, amacrine, and horizontal cells, as well as the cell body of the
    photoreceptors, before hitting the light sensitive parts of the
    photoreceptor. At the fovea, this isn't as true-- the upper layers are
    sort of pushed aside

    Scott Seidman, Jul 2, 2004
  16. Wil Hadden

    Wil Hadden Guest

    I thought this happens thought feedback from the horizontla cells and from the
    photoreceptors themsleves.
    When this happens is it not just a relative shift in sensitivity?

    I picked up on the the light passing though all the layers as well, what is the relevance
    of this not happening at the fovea?
    Or to be more precise, do you know why light passes through all the layers first?

    Wil Hadden, Jul 2, 2004
  17. The fovea does our highest acuity work, so the most direct path for light
    seems to have evolved there. As for why light passes through all the other
    layers, noones come up with a good reason for that. Maybe it has some
    protective function for the receptors, maybe the retina is mechanically
    more stable that way, maybe its just an evolutionary quick. FWIW, I seem
    to recall being told that the octopus retina is not layered this way, but I
    can't vouch for the info

    Scott Seidman, Jul 2, 2004
  18. Wil Hadden

    r norman Guest

    Lateral inhibition doesn't just change the sensitivity, it is what is
    primarily responsible for edge detection, contrast enhancement, etc.
    The center-surround organization is just another way to describe
    lateral inhibition. Yes, it involves connections through the
    horizontal cells.

    I don't think the retinal layers are different in the fovea -- light
    still does pass through all the neural cell layers. What is different
    is that in the rest of the retina light also has to dodge the bundles
    of axons and the blood vessels supplying the retina. These tend to
    skirt around the fovea.

    Why the eye is built that way is another story. One idea is that it
    is just an example of "incredibly stupid" design, in opposition to the
    "intelligent design" notion. It probably was simply a bad design
    "choice" early in the evolution of the vertebrate eye that was frozen
    in place by succeeding evolutionary steps. It certainly does work
    well enough, so no big deal. Cephalopod molluscs (squid and octopus)
    have complex camera eyes with layered retinas that are built the
    "right way", with the photoreceptors on the surface so they are the
    first cells that light hits.
    r norman, Jul 2, 2004
  19. At the fovea, the bipolar cells go off at an angle insted of coming
    straight off from the receptor, essentially getting out of the way. Kandel
    and Schwarz have a pretty good picture of it

    Scott Seidman, Jul 2, 2004
  20. Wil Hadden

    Wil Hadden Guest

    It would seem odd that light has to pass thought all the other layers first, you would
    imagine that light is being muted going thought those other layers. Seeing as we a so
    extremely dependant on sight this would be a very evolutionary big disadvantage.
    Wil Hadden, Jul 2, 2004
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