Just how plastic is the brain?

By guest blogger David Robson

A patient researchers call "M.M." had lived for 40 years as if he was viewing the world through frosted glass on a moonlit night. The reason? When he was just 3 years old, a chemical explosion destroyed one eye and left the other damaged beyond repair. Without a cornea to focus clear images on his retina, he could see no more than a vague perception of dark and bright.

Then, in the early 2000s, the seemingly unthinkable happened. Thanks to advances in stem cell research, doctors realised they could help regrow the delicate tissue around the white of the eye, allowing them to fit M.M. with a transplanted cornea from a donor.

From a surgical perspective, the operation was a success – but his doctors knew that patching up the eye was only half the battle. After decades of near blindness, could M.M.s brain learn how to appreciate his richer visual world? The answer, sadly, was no – but his experience tells us a lot about the way the brain changes as we age.

Our first few years are an intense learning experience. Rather than coming fully programmed to understand the world, the baby's brain has to learn how to make sense of the extraordinarily complex information hitting the retina. Consider how even a simple object, like a chair, looks completely different from different angles.

That's not to mention an even more complex skill like face recognition. We can recognise people even if they are wearing make-up or have a new hair cut – and we instantly pick up on characteristics such as gender, or someone's emotions, even when the faces are as radically different as Scarlett Johansson and John Prescott. When you are very young, you come to almost every scene anew, as if you are seeing the world for the first time, until you learn the patterns that help you recognise what things look like in different situations.

But this adaptability doesn't last forever. Childhood is thought to consist of "sensitive periods", after which the brain finds it more difficult to forge the necessary circuits for learning. M.M. was partly down that road when he lost his sight, and as an adult, it was unclear whether he would be able to retrace those steps and relearn how to see.

Testing M.M. soon after his operation (pdf), Ione Fine at the University of Washington and colleagues found that he could recognise simple two-dimensional shapes, and he could pick out motion and colour, but he struggled to extract more complex visual information. When shown pictures and photos, he struggled to name objects, recognise facial emotions (happy, sad or neutral) or label the gender of a person. He also had some difficulty with 3D perception – failing to recognise if rotated shapes were the same or different. Clearly, he had not kept the skills he was developing before his accident.

It was still possible that M.M. would relearn them, so the researchers waited a further ten years before retesting him – but in a recent paper for Psychological Science, Fine reports that his progress has been disappointing. In many tests, M.M's performance remains no better than the results straight after the operation. Overall, his accuracy was between 30 and 70 per cent – much less than the healthy controls, who rarely made a mistake in any of the tasks.

Differences between M.M. and controls were also seen in brain scans. Typically, the ventral visual cortex shows different patterns of activity depending on whether you are viewing a face, an object, or a scene – but M.M. didn't seem to have developed the necessary networks.

Congenitally blind people's brains often adapt to use information from other senses. The sounds of echoes from a cane, for instance, can help them estimate the size and position of objects – and this seems to be processed in the visual cortex, as if the information were coming from the eyes. Similarly, reading Braille seems to trigger the same regions most of us would activate when seeing a written word, and the areas for face recognition light up when a blind person feels someone's features.  At the moment, it's unclear whether these compensatory processes should help or hinder the return of vision after surgery. It will be interesting to test whether M.M. had developed any of these "cross-modal responses".

If anything, the new paper underlines the fact that we still understand very little about the brain's sensitive periods and its abilities to adapt and change as we age – its "plasticity". Although there are some surprising cases – such as a man who was born "stereoblind" (unable to see depth) only to develop normal vision after a trip to see the 3D film Hugo – patients like M.M. remind us that we shouldn't expect miracles. Fortunately, neuroscientists are exploring drugs that might make the brain more malleable – including the surprising finding that certain anti-depressants may re-open up the window for childlike learning.

While that is still a distant prospect, M.M. has learnt to make the best of his situation. "I have learned what works with vision and what doesn't, so I really don't challenge my vision much anymore," he told the researchers. "Where motion or colour might be clues, I use my vision. Where details might be required, like reading print or recognising who someone is, I use tactile and auditory techniques." Even if the brain's circuits themselves are not as flexible as they once were, we shouldn't underestimate the astonishing ability of humans to adapt to their circumstances.

Further reading

Huber, E., Webster, J., Brewer, A., MacLeod, D., Wandell, B., Boynton, G., Wade, A., & Fine, I. (2015). A Lack of Experience-Dependent Plasticity After More Than a Decade of Recovered Sight Psychological Science DOI: 10.1177/0956797614563957

About the author

Post written by David Robson (@d_a_robson) for the BPS Research Digest. David is BBC Future's feature writer. Image: Flickr/Jeff Ruane