Here is my confusion: we can see colored light of different wavelengths: form red to violet. To my understanding, these stimuli cause a confirmational change in the photoreceptors in our eyes and results in a STP that eventually leads to an "all or nothing" action potential that sends another signal, again an all or nothing action potential through the optic chiasm to the occipital lobe and we perceive the colors as we see them.

My question is *how does this signaling work? *; How can a minor stimulus, resulting in an "all or nothing" chain of action potentials be converted into something as specific as the vision of color?

Asked another way, how does variation in a confirmational change at a receptor that results in "all or nothing" signaling lead to specific signals being sent such as colored vision ?

PS: I dont know jack about sensory physiology


Short answer
Action potentials generated to different colors are indeed similar throughout the nervous system and do not encode color as such. Instead, the different color- sensitive cells in the retina are connected to different neurons and these color-specific signals are kept segregated up until the higher visual cortical areas.

Action potentials are indeed pretty similar throughout the nervous system. However, the color-sensitive sensory cells in the retina, called the cones, come in three flavors: red, green and blue. These colors form the RGB system just like in your LED TV and can together make all the available millions of colors. These three cones synapse ultimately onto color-specific secondary sensory neurons (Fig. 1).

Hence, R, G and B cones indeed generate identical action potentials in downstream neurons, the trick is they do so in different retinal ganglion cells, and at different firing rates depending on the intensity of light that particular cone is sensitive to. These different classes of retinal ganglion cells project onto different classes of neurons in the brainstem (lateral geniculate nucleus, or LGN) and ultimately onto different neurons in the higher cortical visual areas in the brain.

color system
Fig. 1. Different classes of cones synapse onto different classes of secondary sensory neurons in the retina. source: Discovery Eye Foundation

The reason why we can differentiate millions of colors can be explained by the Hering model of color vision (Fig. 2). Basically, the different cones converge pairwise onto opponent color-sensitive cells. The red-green opponent system, for example, operates by weighing the amount of red and green in the incoming signal. This weighting results in an analogue system that can code millions of colors along the red-green axis (Fig. 3).

Fig. 2. Hering model of color vision. source: Webvision

red-green color axis
Fig. 3. Red-green color axis. source: SO

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    $\begingroup$ Beat me to an answer - glad yours came up before I got too far in to mine. You might add something about the nature of communicating via action potentials: that although they are all-or-none, the rate of APs can encode intensity, and a brief mention of how photoreceptors themselves don't have action potentials but rather drive their targets to fire at different rates. $\endgroup$ – Bryan Krause Jul 26 '17 at 20:53
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    $\begingroup$ Yeah, I don't think it needs anything but a brief mention, but I disagree that rate coding doesn't apply to color vision. The relative rates of cells affected by different cone types is what gives a broad spectrum of possible colors. $\endgroup$ – Bryan Krause Jul 26 '17 at 20:58
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    $\begingroup$ Also related: biology.stackexchange.com/questions/52315/… though I don't think that question answers this one well, this one goes more toward the basic physiology. $\endgroup$ – Bryan Krause Jul 26 '17 at 21:00
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    $\begingroup$ @BryanKrause - I've added the Hering model. How's that? $\endgroup$ – AliceD Jul 26 '17 at 21:11
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    $\begingroup$ Added to one of your sentences, feel free to review and revert if you don't like it. Otherwise I think it's great; Fig 3 might not be necessary. Already gave you an upvote so here's an extra 5 Internet Points. $\endgroup$ – Bryan Krause Jul 26 '17 at 21:29

How can a minor stimulus, resulting in an "all or nothing" chain of action potentials be converted into something as specific as the vision of color?

Visual phototransduction is the one of answers to your question.

It is a process by which light is converted into electrical signals in the rod cells, cone cellsand photosensitive ganglion cells of the retinaof the eye. This cycle was elucidated by George Wald (1906-1997) for which he received the Nobel Prize in 1967. It is so called "Wald's Visual Cycle" after him.


The visual cycle is the biological conversion of a photon into an electrical signal in the retina. This process occurs via G-protein coupled receptors called opsins which contain the chromophore 11-cis retinal. 11-cis retinal is covalently linked to the opsin. When struck by a photon, 11-cis retinal undergoes photoisomerization to all-trans retinal which changes the conformation of the opsin leading to signal transduction cascades which causes closure of cyclic GMP-gated cation channel, and hyperpolarization of the photoreceptor cell.

Signal Transduction

In dark glutamate is continually being secreted at synapse between photoreceptors and bipolar cells.

In light

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1)A light photon interacts with the retinal in a photoreceptor cell. The retinal undergoes isomerisation, changing from the 11-cis to all-trans configuration.

2) Retinal no longer fits into the opsin binding site.

3)Opsin therefore undergoes a conformational change to metarhodopsin II.Metarhodopsin II is unstable and splits, yielding opsin and all-trans retinal.

4)The opsin activates the regulatory protein transducin.

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5) This causes transducin to dissociate from its bound GDP, and bind GTP, then the alpha subunit of transducin dissociates from the beta and gamma subunits, with the GTP still bound to the alpha subunit.

6) The alpha subunit-GTP complex activates phosphodiesterase or PDE.PDE breaks down cGMP to 5'-GMP.

7)This lowers the concentration of cGMP and therefore the sodium channels close.

8) Closure of the sodium channels causes hyperpolarization of the cell due to the ongoing efflux of potassium ions.

9)Hyperpolarization of the cell causes voltage-gated calcium channels to close.As the calcium level in the photoreceptor cell drops, the amount of the neurotransmitter glutamate that is released by the cell also drops.

10)A decrease in the amount of glutamate released by the photoreceptors causes depolarization of On center bipolar cells (rod and cone On bipolar cells) and hyperpolarization of cone off-center bipolar cells.

Impulse conduction As is explained by AliceD above via CN 2 using labelled line principle.

  • $\begingroup$ thanks anubhav!! I never knew they worked by hyperpolarization $\endgroup$ – Confusedbyeverything Jul 27 '17 at 15:49

How do our eyes detect light at different frequencies?

Different frequencies are recepted via different photoreceptors i.e. different pigments in cones. Tricolor mechanism being the most famous. Here, our eyes can detect three different colours Red, Green and Blue. These are mixed in appropriately for detection of different colours.

For eg: 99:42:0 will give you perception of orange color. Which means 99% Red 42% Green and 0% Blue.

enter image description here

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    $\begingroup$ Don't add another low-quality answer on top of an existing post - please write a single answer and edit it instead. $\endgroup$ – AliceD Jul 27 '17 at 11:56
  • $\begingroup$ @AliceD Madam, I knew your answer was perfect for question. If you had not written that, I would have had written same. But, when we look at question, what comes in mind at first glance is Tricolor mechanism. This is the only answer any examiner will write in response to question like How do our eyes detect light at different frequencies?. So, I gave the same, so that anyone who searches for this question on internet, shall find an answer, an answer which OP does not want. $\endgroup$ – Anubhav Goel Jul 27 '17 at 16:07
  • $\begingroup$ Firstly, you shouldn't write answers which OP doesn't want... You should write answers that target the question. Secondly, I'm a bloke 8-) $\endgroup$ – AliceD Jul 27 '17 at 22:37

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