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As light enters the eye, it reaches the photoreceptors at the "base" of the retina, which then pass that signal to the bipolar and ganglionic neurons -- the latter of which send the signal outside of the eye via their axons (collectively forming the optic nerve).

  • The exit point of the optic nerve is sometimes referred to as the "blind spot" because there are no photoreceptors present there and therefore no sensory information is gathered.

Now, I know photoreceptors exist everywhere else along the retina, so it's not surprising that we perceive vision from the otherwise broadly distributed photoreceptors.

However, my question: why do the blood vessels associated with the superficial vascular plexus (which exist between incoming light and the rest of the retina) not obstruct our vision?

  • More broadly, I guess of interest is: why none of the vascular plexuses (or cell structures of the bipolar and ganglionic neurons for that matter) obstruct our vision despite existing between the photoreceptors and incoming light?

enter image description here enter image description here

Sources: LEFT: Figure 1 from Zhongjie et al (2020) ; RIGHT: Figure 5 from Selvam et al (2018)


Fu, Z., Sun, Y., Cakir, B., Tomita, Y., Huang, S., Wang, Z., Liu, C.H., S Cho, S., Britton, W., S Kern, T. and Antonetti, D.A., 2020. Targeting neurovascular interaction in retinal disorders. International journal of molecular sciences, 21(4), p1503

Selvam, S., Kumar, T. and Fruttiger, M., 2018. Retinal vasculature development in health and disease. Progress in retinal and eye research, 63, pp.1-19.

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  • $\begingroup$ To follow-up for clarity: I understand the brain is essentially integrating the overlapping visual fields from both eyes to allow us not to regularly perceive the blind spot. However, it seems these cellular structures (i.e., vessels and nerve cells) must be so prevalent that I'm wondering by what mechanism (perhaps similar) that this more [structurally-]complex process is accomplished. $\endgroup$ Mar 30 at 17:43
  • $\begingroup$ Fun fact that many probably already know: Cephalopod eyes don't have a blind spot, having the retina in front of the blood vessels and optic nerve. When Bryan's answer mentions an "inverted" eye, it's as opposed to the "better engineered" cephalopod eye. $\endgroup$ Mar 31 at 9:17
  • $\begingroup$ I think there are experiments like this to make the blood vessels visible to you: aao.org/museum-education-healthy-vision/… $\endgroup$
    – sietschie
    Mar 31 at 11:35
  • $\begingroup$ youtu.be/oAaG34WfsC0?t=9254 has an interesting anecdote about this. This is from an interview of a neuroscientist/Stanford prof/skateboarder, who explains, among others, why we don't see those spiderweb shadows. $\endgroup$ Mar 31 at 15:16

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Avoid the fovea

Figure 2 from the same paper shows the distribution relative to the fovea:

As you can see, it's pretty much devoid of this superficial vasculature, so anything you are directly focusing on, say, text you read on a computer screen (or even a book!) is not impacted.

Figure 2

Receptive fields might be bigger than you think

Receptive field sizes for retinal ganglion cells in the primate retina are about 50-300 um, depending on eccentricity (distance from the fovea). Capillaries are going to be around the size of a red blood cell in diameter, so about 10 um; it seems like by the time you get to the far periphery, these vessels are mostly going to be quite small relative to receptive field size, and they are even a bit small in the vicinity of the fovea.

Tissue isn't that opaque

I'm mostly focusing on the RBC size themselves, because RBCs have a bit of pigment in them, but otherwise, tissue is overall quite transparent. If you've ever looked at an unstained tissue section less than 100 microns thick, you know that it doesn't look like much at all. If you've lost track of one in any volume of water, good luck finding it. For the same reason that RGCs being on the "wrong side" of the inverted vertebrate eye, this thickness of tissue just doesn't seem to be that big of a problem, and it doesn't seem that any affordances for this issue have evolved outside of the fovea in primates (whereas you can see in the figure above that there is a clear exclusion of these vessels from the fovea).

We perceive with our brain, not our eyes

The general idea of predictive coding models of the brain is that you have some generative model of the world that is constantly making predictions, and sensory organs merely provide evidence to update those models which is propagated as an error signal with respect to the original model; if everything is static and as predicted, nothing needs to be propagated in the brain to alter the perception. Much of what you think you are "seeing" at a given moment you aren't seeing at that moment at all, but merely "remembering" what you saw previously, and having not seen any evidence to the contrary, continue to "see" it there. When a person looks at an object, they do not typically look at one spot, but quickly saccade around to scan different parts of it and form a complete model of the object. It will escape attention until it moves or changes in some way.

These blood vessels are going to be quite static, and not provide much of a changing visual image, so there's nothing there for the brain to be interested in.

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    $\begingroup$ +1 for everything, but especially the "it's all in your mind" paragraph! $\endgroup$
    – AnoE
    Mar 31 at 8:13
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    $\begingroup$ "Tissue isn't that opaque" — but we do easily see the vessels in fundus photos, so they aren't that transparent either. And they are very noticeable in the Purkinje tree effect. $\endgroup$
    – Ruslan
    Mar 31 at 11:15
  • $\begingroup$ @Ruslan Those are much larger vessels than the bulk of them that are capillary-sized. $\endgroup$
    – Bryan Krause
    Mar 31 at 14:20
  • $\begingroup$ Thanks @Bryan! +1. 3 comments: (1) could you cite/link something about the primate fovea evolution you allude to [very interesting, by the way!]. (2) I've experienced thin cultures before and so can appreciate your comments about their near translucence. However, capillaries would still have continuous RBCs -- which are more red than completely transparent -- so I'd guess these would stand out slightly more than thin epithelia. (3) final paragraph about predictive coding is informative and interesting -- some of it was familiar, but it's been a long time since studying this stuff. Thanks :) $\endgroup$ Mar 31 at 16:05
  • $\begingroup$ @theforestecologist 1) I'm really just commenting on the structure visible in figure 2: it's clear that something particular has evolved around the fovea in primates to keep that area clear of vessels, and not elsewhere. I could perhaps comment on the evolutionary history of the fovea but it's a bit of a long story of convergent evolution. 2) Agreed, RBCs may stand out, I mostly meant this to defend my use of a RBC width as a reference point for the size relative to receptive fields, rather than using the vessel diameter. 3) Although I put this point last, it's probably most important. $\endgroup$
    – Bryan Krause
    Mar 31 at 16:12
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You CAN see your blood vessels!

Following-up from Bryan Krause's answer:

Bryan mentioned that the blood vessels are typically not visible partially because they are unchanging (i.e., static). In support of that, one could imagine that if you somehow make these structures appear more dynamic, then one might be able to visualize them.

The American Academy of Ophthalmology actually recommends trying an experiment to do just that: to "see" your retinal blood vessels!

https://www.aao.org/museum-education-healthy-vision/experiment-see-blood-vessels-in-your-eye

Paraphrasing their experiment:

  1. Darken a room.

  2. Hold black sheet of paper in front of face so that the paper fills your field of view.

  3. Hold a small flashlight 1.5 cm in front of one eye so it's aiming just below the center of the pupil.

  4. Move the lit flashlight slowly from side to side a short distance (~0.4cm) without letting your eye follow the motion of the light. Continue moving for ~20 seconds.

A tree-like pattern of blood vessels will become visible.

Why this works:

You can use a dim point of light to cast a shadow of the blood supply of your retina. This will allow you to see the blood supply of your retina, and even your blind spot... Your light is causing a shadow from the blood supply layer to the other layers underneath. As you move the point of light, the shadow moves, making it visible to you. Normally, there would be no shadow and your brain would ignore seeing the blood supply.

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    $\begingroup$ One of my friends experimented with this using a computer program: stare at one point, and the program flashes dim dots elsewhere. Press button when you see the dot. Very slow but the mapping he got corresponds very well with a retinal photo from opthomologist. $\endgroup$
    – jpa
    Apr 2 at 8:53

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