Scientists study the effect of the color red on brain waves

Red has a signal and warning effect. How is this color specificity reflected in the brain?

Scientists at the Ernst Strüngmann Institute for Neurosciences have now investigated whether red triggers brain waves more than other colors.

A red traffic light makes us stop. Because of their color, we immediately recognize ripe cherries on a tree. A signal and warning effect is attributed to the color red. But is this also reflected in the brain? Researchers at the Ernst Strüngmann Institute (ESI) for neuroscience have now investigated this question. They wanted to know if red triggers brain waves more than other colors.

The new research focuses on the early visual cortex, also known as V1. It is the largest visual area in the brain and the first to receive information from the retina. When this area is stimulated by strong and spatially homogeneous images, brain waves (oscillations) appear at a specific frequency called the gamma band (30-80 Hz). But not all images produce this effect to the same extent. The study by Benjamin J. Stauch, Alina Peter, Isabelle Ehrlich, Zora Nolte and ESI Director Pascal Fries was published in the journal earlier this year eLife.

Brain anatomy illustration.

Color is difficult to define

“Recently, a lot of research has been done to find out which specific input drives gamma waves,” explains Benjamin J. Stauch, first author of the study. “Colored areas seem to be a visual input. Especially when they are red. The researchers interpreted this to mean that red is evolutionarily a peculiarity of the visual system because, for example, fruits are often red.”

But how can the effect of color be scientifically proven? Or refuted? Finally, it is difficult to objectively define a color, and it is equally difficult to compare colors between different studies. Each computer monitor represents a color differently, so red on one screen is not the same as on another. In addition, there are a variety of ways to define color: based on a single monitor, perceptual judgments, or based on what their input does to the human retina.

Colors activate photoreceptor cells

Humans perceive color when visual cells, the so-called cones, are activated in the retina. They respond to light stimuli by converting them into electrical signals, which are then sent to the brain. In order to recognize colors, we need several types of cones. Each type is particularly sensitive to a specific range of wavelengths: red (L-cone), green (M-cone) or blue (S-cone). The brain then compares how strongly the respective cones have reacted and derives a color impression from this.

It works similarly for all people. So it would be possible to define colors objectively by measuring how much they activate the different retinal cones. Scientific studies with macaques have shown that the visual system of early primates has two color axes based on these cones: the LM axis compares red to green, and the S (L+M) axis is yellow to violet. “We believe that a color coordinate system based on these two axes is the right way to define colors when researchers want to study the strength of gamma oscillations. She defines colors according to how strongly and in what way they activate the early visual system,” says Benjamin J. Stauch. Because previous work on color-related gamma oscillations was mostly done with small samples from a few primates or human participants, but cone activation spectra can vary genetically from individual to individual, he and his team wanted to measure a larger sample of individuals (N = 30).

Red and green have the same effect

Benjamin J. Stauch and his team investigated whether the color red is special and whether this color causes stronger gamma oscillations than green with comparable color intensity (ie cone contrast). And a side question was: Can color-induced gamma oscillations also be detected using magnetoencephalography, a method for measuring the magnetic activities of the brain?

They conclude that the color red is not particularly strong in terms of the strength of the gamma oscillations it induces. Rather, red and green produce equally strong gamma oscillations in the early visual cortex at the same absolute LM cone contrast. Furthermore, with careful handling, color-induced gamma waves can be measured in human magnetoencephalography, so future research could follow the 3Rs principles for animal experiments (Reduce, Replace, Refine) by using humans instead of non-human primates.

Colors that only activate the S-cone (blue) appear to elicit weak neural responses in the early visual cortex in general. To some extent, this is to be expected given that the S cone is rarer, evolutionarily older, and more sluggish in the primate retina.

Development of visual prostheses

The results of this study, led by ESI scientists who understand how the early human visual cortex encodes images, could one day be used to design visual prostheses. These prostheses may attempt to activate the visual cortex to induce vision-like perceptual effects in people with damaged retinas. However, this goal is still a long way off. Before doing so, much more needs to be understood about the specific responses of the visual cortex to visual input.

Reference: “Human Visual Gamma for Color Stimuli” by Benjamin J. Stauch, Alina Peter, Isabelle Ehrlich, Zora Nolte and Pascal Fries, May 9, 2022, eLife.
DOI: 10.7554/eLife.75897