Seeing with your brain

BlindfoldBiological brains can do something that we’re still working on building into machines: rewire themselves to take advantage of the best information available. This is a well-known phenomenon, but it’s not widely understood just how radically different the rewiring or optimized wiring can be from what is considered ‘normal’.

Probably the best known example of this kind of neural plasticity is that, if a mammal is born with an eye that is defective or missing, the brain will only develop connections to the good eye. This makes sense because the other one isn’t delivering any useful information. The original work showing this was done by Hubel and Wiesel back in the ’60s.

More recently, Mark Bear and colleagues at the Howard Hughes Medical Institute (MIT) have showed that, in this kind of experiment, the normal eye actually improves in this case: it’s not just as good as it would have been without the other eye being defective, it’s better. Not only that, but in experiments where the animal is temporarily blinded in one eye (by having it kept shut, for example), when the second eye is opened and eventually becomes useful (more rewiring), the original good eye gets worse again.

From an engineering point of view, there are a few things that fall out of this. First, it means that the eyes can provide more individually than is necessarily passed on to the brain (i.e. that the connections to the brain are the bottleneck, not the eye itself). Second, the two eyes are sharing fixed resources, and the neurons choose to work with the best information available.

Pushing this even further, a paper by Roy Hamilton and colleagues at Harvard and the University of Valencia in Spain showed that, if a patient is completely blind, the bits of the brain normally connected to the eyes find other work to do. Their 63-year-old patient had suffered a stroke. When they looked at her brain using a magnetic resonance imager they found lesions in her occipital cortex: a part of the brain that deals with processing visual information. This should have been good news for a woman who had been blind since birth.

It wasn’t. It turns out that, in the absence of good visual information to work on, the brain had turned to touch instead, and the patient was no longer able to read braille…

It’s long been understood that those with sensory disabilities compensated for these by having their remaining senses enhanced. Now we know why.