Ask a Neuroscientist! – Seeing Colors

Ask a neuroscientist! is a new column where we answer your questions about neuroscience! Our first question comes from personal correspondence from Michael in Azusa, California.

Michael asks: “I recently read a news article about a woman who could see more colors of the spectrum than a normal person can. Can scientists see a difference in the brains of someone who can see more colors compared to a normal person? What about someone who is colorblind?”

That’s an interesting question, Michael! As far as I could tell, no one has done that experiment, so I’ll make a prediction based on what we do know.

First a little background: the condition you are referring to is called Tetrachromacy, that is, the ability to see light at four different wavelengths, which manifests in humans as the ability to see 100x as many colors as a normal person. In a normal human retina, there are two types of light-receptive cells -- cones, which are responsible for color vision, and rods. Each cone cell contains specialized proteins called opsins, each of which responds to a specific wavelength corresponding to red, green, or blue (making most of us trichromatic). The retina then translates these light waves into electrical pulses and chemical signals, which are then sent to an area of the brain called the visual cortex, where they give rise to the full spectrum of color that we observe and experience in the world around us. Birds, fish, and insects are naturally tetrachromatic, meaning they can perceive ultraviolet light waves in addition to red, green, and blue, and a tetrachromatic human would presumably be able to see the same. A colorblind individual, on the hand, typically is only lacking 1 of their 3 opsins (making them dichromatic), which can manifest in specific colorblindness (i.e. someone who can see green and blues, but not reds), or general colorblindness.

Both tetrachromatism and colorblindness are caused by mutations in the genes encoding opsins. In the case of colorblindness, these mutations result in a loss of function, whereas with tetrachromatism, there is a duplication of a gene, causing a gain of function. Both conditions are quite rare, with colorblindness affecting only 2% of the population, and the first official case of a human tetrachromat was only discovered in 2010.

In a paper published in the Journal of Vision in 2010, Gabriele Jordon and colleagues from Newcastle University, used a series of visual decision tasks to identify a woman who sees the world through a four-channel color filter. They presented volunteers with three colored circles, two of which were a single, pure color, whereas one was composed of a subtle mixture of both red and green. To a normal individual, all three circles appeared to be the same, but because of her extra cone, the tetrachromat was able to distinguish the red/green circle from the other two. So what does this mean for your question? While it is unlikely the brain of a tetrachromat or a colorblind person will look significantly different from that of a normal individual, there are ways that we could see a difference in brain function, and perhaps more importantly, this difference could give us clues as to how the brain processes color information.

So let’s design an experiment to address your question:

To differentiate between a person with normal vision and one who is color blind, we will place our volunteers in a functional magnetic resonance imaging, or fMRI, device, while subjecting them to a color vision test. Because the fMRI detects changes in brain activity, we will use it as a read out for whether or not a color is being perceived. For the color vision test, the volunteers will first be shown a series of paired colored images, similar to this:

When presented with Figure A., the visual cortex of both individuals should become active (which manifests on an fMRI as that brain region “lighting up”) because even though the normal individual perceives extra information in the form of color, the colorblind individual still perceives the shapes. However, when presented with Figure B., the normal individual perceives a change in the visual stimulus, which would then be captured by a change in activity in the visual cortex, whereas the colorblind individual wouldn’t perceive any change, and therefore there would be no change in the fMRI signal from their visual cortex. In this way, we should be able to correctly identify the colorblind individuals based solely on their brain scans.

We will then show the volunteers Figure C:

When presented with Figure C., even the colorblind individuals will see the number “2” emerge in the center of the screen, so again, the visual cortex of both individuals should be activated in a similar way. We can then compare the activation pattern seen after the presentation of Figure B (color and shape change) with the activation pattern seen after Figure C (only shape change), and in this way, be able to isolate color perception in the visual cortex.

To differentiate between a person with normal vision and a tetrachromat, we will again place our volunteers into an fMRI, only this time, we will ask them to complete a task similar to the one described by Gabriele Jordon and colleagues. The volunteers will first be shown three mono-chromatic circles (Figure 1), followed by a rapid switch to Figure 2, in which two of the circles are a single, pure color, and one circle becomes a subtle mixture of two colors:

When presented with Figure 1., the visual cortex of both individuals should look identical, however, because normal individuals cannot distinguish a red circle from a red/green circle (in the real task that is, the one in Figure 2 has been colored for demonsrative purposes), the when normal individuals are shown Figure 2., the activity in their visual cortex should not change. Because the tetrachromat can detect the change in color (and therefore perceive the stimulus as something new), then the activity in their visual cortex should increase. And so similar to our colorblind experiment, we should be able to correctly identify the tetrachromat individuals based solely on their brain scans.

Hope this answers your question, Michael! Thanks for writing in!

If you have a question for one of our neuroscientsist contributors, email Astra Bryant at stanfordneuro@gmail.com, or leave your question in the comment box below.