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If you close your eyes, you can often see visual static, where individual pixel-like things are much more visible than with the more smooth, crisp images one gets with open eyes. This led me to wonder a few things:

  1. Is there a name for this unit, like "pixel"?

  2. What is the biological correspondent? A single rod or cone or something?

  3. Are the individual pixel-like things making up the static noise the same underlying mechanism (and size) as the smallest perceptible units of vision one gets when actually looking at something? I suppose this is related to visual acuity. I've read that the cause of static is suspected to be excitation of neurons, but that's not what I'm asking.

Thank you.

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    $\begingroup$ The analogy between the human visual system and electronic screens does not go very far. While an electronic display can be described as the sum of individual pixels, what the human eye sees cannot be described as the 'sum' of inputs from all the rods and cones. The inputs from one cell interact with those from other cells at various levels: retina, lateral geniculate nucleus, visual cortex. Plus, there is interaction with neurons from other brain areas. This processing creates a complex 'image' very different from that of an electronic screen. $\endgroup$
    – Adhish
    Commented Jun 28, 2020 at 9:11

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The human eye has two types of sensors that become stimulated when wavelengths of visible light (photons) hit the pigments in these sensors. They are known as 'rods' and 'cones'. Rods detect the brightness of light, cones detect the color. There are three different cones in the human eye: red, green and blue. Each cone reacts to the corresponding waves of light. When they are stimulated due to their corresponding wavelengths, they send a signal that travels through the optic nerve to the visual cortex at the occipital lobe.

individual cones don't have different levels of stimulation and always send the same frequency, it's the lack of the stimulation of other cone types that causes certain colors to appear.

in short:

  1. signal or photon of the appropriate wavelength I suppose.
  2. rods for brightness, cones for color.
  3. probably not since the cones are microscopic and tightly packed, too close and too small to see what each cone is literally receiving. The moving static you're seeing is called phosphenes and is comprised of the inherent electrical charges the retina produces both during resting state and with the eyes open. they're just easier to notice when your eyes are closed. Try covering your eyes and you may get static just like when you close your eyes.

The electrical charges get sent to the visual cortex. that's where your brain processes the image. Here's an diagram of how the rods and cones send their electrical signals to the optic nerve (the optic nerve is the [4] label).

Here's a diagram of how those signals travel to the visual cortex:

enter image description here

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  • $\begingroup$ Thank you. So a phosphene is a single pixel-like light? What would be the path from this electrical charge to a person's awareness of the light? The charge activates a single rod (or very small clump) and then travels down a nerve to the visual cortex or something? And each rod would have its own nerve line? $\endgroup$
    – Somatic Custard
    Commented Jul 2, 2020 at 15:35
  • $\begingroup$ "So phosphene is a single pixel-like light?" It's a phenomena that causes your rods and cones to still send signals even when the eyes are closed, the rods and cones (or arguably the gangion cells) are the real 'pixels'. I made edits to the answer so that you see the pathways. Multiple rods can cones can be joined to one bipolar cell and multiple bipolar cells can join to one ganglion cell meaning that multiple rods and cones can send a signal through one pathway. to the visual cortex. $\endgroup$
    – Ark Lomas
    Commented Jul 2, 2020 at 18:20
  • $\begingroup$ ganglion cells could be argued as the pixels since they send the signals to the visual cortex. The rods and cones just depict what the pixel will display (e.g. brightness and/or color of each pixel) $\endgroup$
    – Ark Lomas
    Commented Jul 2, 2020 at 18:23
  • $\begingroup$ Thank you. This clears things up for me. A very informative answer! $\endgroup$
    – Somatic Custard
    Commented Jul 4, 2020 at 12:59

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