The effect of the color red on brain waves – Neuroscience News

Summary: The color red is not particularly strong in terms of the strength of the gamma oscillations it produces in the brain.

Source: IT I

Red traffic lights force drivers to stop. The color red has a signal and warning effect. 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 study, titled “Human Visual Gamma for Color Stimuli,” is published in the journal eLife.

The research by Benjamin J. Stauch, Alina Peter, Isabelle Ehrlich, Zora Nolte and ESI Director Pascal Fries 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) occur at a specific frequency called the gamma band (30–80 Hz). But not all images produce this effect to the same extent.

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.

He and his team wanted to measure a larger sample of individuals (N = 30) because previous work on color-related gamma oscillations was mostly done with small samples from a few primates or human participants, and cone activation spectra can vary genetically from individual to individual,

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).

They also pursued a side question: Can color-induced gamma oscillations also be detected using magnetoencephalography (MEG), 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, color-induced gamma waves can be measured in human MEG with careful handling, 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.

The results of this study, led by ESI scientists, help to understand how the early human visual cortex encodes images and 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.

See also

Much more needs to be understood about the specific responses of the visual cortex to visual input.

About this news from visual neuroscientific research

Author: press office
Source: IT I
Contact: Press Office – ESI
Picture: Image is credited to ESI/C. Kernberger

Original research: Open access.
“Human visual gamma for color stimuli” by Benjamin J Stauch et al. eLife

abstract

Human visual gamma for color stimuli

Strong gamma band oscillations in the early primate visual cortex can be induced by homogeneous color surfaces (Peter et al., 2019; Shirhatti and Ray, 2018). Particularly strong gamma oscillations have been reported for red stimuli compared to other hues.

However, precortical color processing and the resulting strength of input at V1 were often not fully controlled. Therefore, stronger responses to red could be due to differences in V1 input strength.

We presented stimuli with equal levels of brightness and cone contrast in a color coordinate system based on responses from the lateral geniculate nucleus, the main input source for region V1. With these stimuli, we recorded a magnetoencephalography in 30 human participants.

We found gamma oscillations in the early visual cortex that, contrary to previous reports, did not differ between red and green stimuli with equal LM cone contrast.

Notably, blue stimuli with contrast solely on the S-cone axis induced very weak gamma responses, as well as smaller event-related fields and poorer change detection performance.

The strength of human color gamma responses to stimuli on the LM axis was well explained by LM cone contrast and showed no clear red shift when LM cone contrast was properly compensated.