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Science Forum

  1. 15 Jul '16 18:02 / 16 edits
    In this wiki link;

    https://en.wikipedia.org/wiki/Rod_cell

    there is a diagram for the graph for wavelength responses for rod and cone cells;

    https://en.wikipedia.org/wiki/Rod_cell#/media/File:Cone-response.svg

    But I am confused by the left hand side of the shape of 3 of the 4 curves on that graph because the 3 for the rod cells and the green and red cone cells don't appear to tend towards the horizontal axis as you look leftward but appear to tend to some arbitrary none zero height above the horizontal axis and then mysteriously just suddenly discontinues at 400nm ! Why don't they show how the curve continuous beyond that point? And note it specifically says "wavelength responses" at the bottom of that graph, not merely what wavelengths the cells absorb irrespective of how they respond to those wavelengths.
    Can anyone give an explanation?
    Is the graph simply nonsense i.e. wrong?

    Not only that, but the curve for the red cone cell responses, after peaking at 564nm, at you continuously move leftwards from that, after reaching a local minimum, it starts to rise again thus implying there are not one but two local maximums (the left one not shown; but why not!? ) to that curve which is supposed to be the response curve for those so there are not just one but TWO such wavelengths for max red cone cell responses? Does that make biological sense?
    I suppose that doable local max to the red curve at least would explain why, as you go to the shorter wavelengths of blue, the blue starts to look violet as if you mix a dash of red to blue it makes it appear violet. But I find it strange that I have never heard of this as the explanation anywhere in my studies.

    Also, according to that graph, the peak for blue cone cells is 420nm. But that doesn't match at all with what this link below says is its peak:

    http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html
    "...The "blue" cones are identified by the peak of their light response curve at about 445 nm..."

    I would say 420nm is way off 445nm -that's a 25nm difference that I don't see how we can ignore. Which link is right and which is wrong?
  2. 15 Jul '16 22:20
    Do a Google image search for "cone cell response chart"
  3. 15 Jul '16 22:22
    Note the chart here:
    https://en.wikipedia.org/wiki/Tetrachromacy
  4. 15 Jul '16 22:30
    The complications start when you find out that the way the signals are sent to the brain have a big impact:

    http://www.ledsmagazine.com/articles/2012/05/understand-color-science-to-maximize-success-with-leds-magazine.html

    Luminance for example is based on L and M but not S.
  5. 16 Jul '16 06:33
    Originally posted by twhitehead
    Note the chart here:
    https://en.wikipedia.org/wiki/Tetrachromacy
    Arr thanks for that.
    That seems to answer at least one of my questions: assuming birds have the same red cone sensitivity of light as humans, there really is two local maxs to that response curve! And the lower max to the left explains why we see violet light as not blue but violet light!
  6. 16 Jul '16 07:23 / 1 edit
    Originally posted by humy
    And the lower max to the left explains why we see violet light as not blue but violet light!
    I think you would need to study the subject more before you can be sure of that. How the signal is processed then sent to the brain is a very important factor here. I am not certain but the second peak may or may not be real. It may be entirely a function of signal processing.
    In addition, I believe the blue peak is actually a lot lower in reality but gets amplified by signal processing.
  7. Standard member DeepThought
    Losing the Thread
    16 Jul '16 13:07
    Originally posted by humy
    Arr thanks for that.
    That seems to answer at least one of my questions: assuming birds have the same red cone sensitivity of light as humans, there really is two local maxs to that response curve! And the lower max to the left explains why we see violet light as not blue but violet light!
    I'm wary of this conclusion, the apparent double peak could easily be noise. You might want to look at the paper that this graph is taken from [1], which you can read for free online. The eye came from a patient at Moorfields Eye Hospital who had a malignant tumour. The eye was removed at 10:30 am and dissection started at 2:30 pm at Sussex University. Whether healthy eyes in the body will behave like this is an unresolvable question. However, the real problem for your statement about multiple local maxima is to do with the baseline behaviour of the microspectrograph. The central figure and the shape of the peak around it are good (i.e. well clear of the baseline behaviour of the instrument), but you should see the graphs on page 504 (the article starts on p.501 of the journal), the signal to noise ratio is getting pretty small and is not far from the baseline response of the instrument. The baseline behaviour of the instrument also shows this upswing. So the apparent upswing looks to me like an artifact of the equipment and nothing to do with the actual behaviour of the rods and cones.

    [1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1279132/?page=4

    This link takes you straight to the page where the raw data is plotted, the graphs Wiki copied are on p 505.
  8. 16 Jul '16 16:07
    Its a bit more readable as a pdf
    https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1279132/pdf/jphysiol00783-0492.pdf

    The experimenters admit a lot of uncertainty so if this one article is indeed the sole source of the Wikipedia information then it is not to be overly trusted especially with regards to the possible double peak.
  9. Standard member DeepThought
    Losing the Thread
    16 Jul '16 20:05
    Originally posted by twhitehead
    Its a bit more readable as a pdf
    https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1279132/pdf/jphysiol00783-0492.pdf

    The experimenters admit a lot of uncertainty so if this one article is indeed the sole source of the Wikipedia information then it is not to be overly trusted especially with regards to the possible double peak.
    I think the central figures are good, with the caveats they make about blue cells. The central figures for the red and green receptors are consistent with data from rhesus monkeys and with data from previous studies on human tissue. The blue cell data shows discrepancies, they point out that they had higher absorbency but state that there is no immediate explanation. So, apart from some question marks concerning their results for blue cone cells, I think their main results are fine, but I'd be very wary about trying to draw conclusions about the properties of these cells far away from their central points of light sensitivity, their data isn't great there.