Writing the story below forced me yet again to contemplate the wonderful mysteries of vision.
Over the years, any number of scientists have reminded me that the visual world we experience is really constructed by our brain. Sure, sure, the eyes are vital, but the visual input that comes via the retina is only the raw material for what we experience as vision.
Optical illusions are an example of this. The research I describe in the story below is another.
As we age, the ability of our eyes to detect colours diminishes. But our brains compensate, to make sure the world retains the technicolor glory we’re used to.
Our lenses yellow, the retina starts failing…and the brain pumps up the colour to make sure the world keeps looking the same to us, even if the signal from our eyes changes.
Somehow, all our brains do this. But why, you ask? And how? Good questions. Read on and all will become clear, or not.
Colour a constant throughout ageing
It’s well known that our colour vision declines with age. Gradual yellowing of the lenses cuts out light in the blue range of the spectrum, while colour-sensing cone receptors on our retinas slowly lose sensitivity.
“Our ability to discriminate small colour differences declines as we age, there is no doubt about that,” says neuroscientist Sophie Wuerger from the Department of Psychological Sciences, University of Liverpool.
But she has found our brains apparently compensate for at least some of these physical frailties. Her results are published online this week in the journal PLoS One.
Wuerger explored the colour perception of 185 people aged between 18 and 75 years with normal colour vision, an unusually large and diverse group for a study of this kind.
First, she used well-known data on how the lens changes with age to predict the light signal that would be sent to the brain by the volunteers’ retinas.
She then asked the participants to undertake a variety of tests that required them to select patches of colour representing pure red, green, yellow, or blue, under different lighting conditions.
The idea was to compare the predicted physiological changes in the eye with the participants’ actual experience of colours.
“That’s the surprising bit. If you look just at the lens, it should introduce significant colour changes in older people, but we observed that … most of the time we have a very constant perception and it doesn’t change with age,” says Wuerger.
The only age-related effects detected in the study were small changes that became apparent for green hues viewed under daylight.
In other words, although the colour signal being sent from the eye was changing significantly with age, the perception of colour was almost constant regardless of how old the study subject was.
This suggests that somewhere between the retina and the conscious perception of colour, the brain must recalibrate itself, she says.
“Something must be happening to change neural connections to maintain constant colour appearance,” Wuerger says.
Exactly how this happens was not part of this study, but Wuerger offers one possible explanation.
“You could think our brain might be using some external standard like the blue sky or sunlight as a reference. There are things in the environment that don’t change and we could use them to recalibrate our visual system.”
One useful clue about the mechanisms involved came from the fact that age did not affect all aspects of the visual system equally. While 18 year olds and 75 year olds were equally good at picking pure red or green and so on, older people were less able to distinguish between subtly different colours, particularly in the bluish range.
Because the recalibration doesn’t affect all our colour vision abilities, Wuerger concludes the adjustment isn’t likely to be taking place in the retina.
“I think that suggests that it must be happening later in the visual processing pathway, closer to the brain. We don’t have any proof of that but the experiments taken together suggest it’s … a kind of plasticity in the adult brain.”
The next question might be why the brain performs this recalibration. What benefit is there in ensuring our perception of colours remains constant? For now, answering that question requires entering the realm of speculation.
Perhaps it has to do with a need to communicate colours effectively when describing objects, Wuerger ventures. “After all, to communicate colour meaningfully,” she says with a chuckle, “we all need to be – so to speak – on the same wavelength.”