In a nutshell: In lab tests human reaction times are longer for blue colours than for others. This research helps explain why: nerve signals for seeing red and green colours travel more rapidly from eye to brain than those for blue.

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The Big Picture:

Our brain is responsible for assigning colour to the images that our eyes see. It does this by bringing signals from three different colour receptors in the eyes together with streams of information about shape and movement.

Just to make the brain’s job that bit harder, blue nerve signals move more slowly from the eye to the brain than red and green signals, according to this research led by CIBF fellow Alexander Pietersen of the University of Sydney.

“We don’t notice that the brain processes blue colours slower,” says Pietersen. “The brain compensates for [the lag].”

Although colour assignment occurs mainly in the visual cortex, the outer region of the brain that runs its most complicated processing tasks, this team had showed previously that a deep-brain region, the thalamus, also plays a significant role.

In this study, direct recordings from the thalamus, showed that the processing of blue colour signals from the eyes started later than for other colours.

The time lag suggests that the blue signal may be transmitted by more primitive, slower nerve pathways, says team member CIBF chief investigator Paul Martin, also of the University of Sydney.

The finding fits with detailed lab tests that show that human reaction times are longer for blue colours. However, the team also found that red-green colour-blind marmosets process black, white and blue patterns at the same speed as marmosets with normal colour vision. That suggests that in the messy real world, where the brain processes floods of visual information, timing differences in colour processing are unlikely to affect performance.

“If we want to understand colour phenomena like that crazy blue dress, we need this basic knowledge” says Martin, referring to the optical illusion of a blue and black dress …or was it a white and gold dress… that went viral in 2015.

Next steps:
The key question posed by this research is how does the brain process latency — the timing discrepancies between multiple signals that must somehow be brought together into seamless consciousness? The Pietersen team is currently exploring interactions between the visual thalamus and visual cortex in search of an answer.

Pietersen, A. N., Cheong, S. K., Solomon, S. G., Tailby, C., & Martin, P. R. (2014). Temporal response properties of koniocellular (blue-on and blue-off) cells in marmoset lateral geniculate nucleus. Journal of neurophysiology, 112(6), 1421-1438.

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