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A new kind of vision

How does our brain do the amazing things it does, and how hard-wired are these functions? Could the part of the brain processing hearing manage sight if necessary? That isn't a question that has the rest of us all a-twitter, but it was a regular dilemma for Dr Mriganka Sur, the Fairchild Professor of Neuroscience and head of the Department of Brain and Cognitive Sciences at the Massachusetts Institute of Technology.

Sur always knew that parts of the brain could take over areas idling due to impoverished input. But he wanted to know if he could himself rewire a brain; if he could, say, make the part that manages hearing take over the visual department.

So, in newborn ferrets, he removed the sound inputs going to the part that processes sound -- the auditory cortex, which lies on the side of the head. Now the nerves from the ear don't streak straight for the auditory cortex; they go through an area close to the centre of the brain, the thalamus. Inputs from the eyes too pass through the thalamus as the animal develops. This is where a short circuit is most likely.

Sure enough, in the absence of competing sound input, the auditory thalamus attracts nerve fibers from the eyes and sends this information to the auditory cortex. Now the auditory and visual cortices together manage vision, the inexperienced former naturally not as well as the latter. Sur found that the ferrets had an auditory cortex that was wired in a manner similar to the visual cortex, and that they could see even with their auditory cortex.

Now that Sur had good evidence that the nature of inputs could indeed define function, he wondered -- and still does -- whether this feature, termed 'plasticity' in neuroscientific jargon, could be used to help repair brains or help recovery of function.

Here, in an interview with P Rajendran, he discusses his work and its possible implications:

When you began this experiment, what were you looking at?

We had two questions in mind: One question was, how do different areas of the cortex come to be what they are? So, for example, the visual cortex is at the back of the brain, the auditory cortex in the middle, and so forth.

I wanted to know how does the visual cortex become visual, how does the auditory cortex become auditory, how does the somatosensory cortex -- or the cortex that deals with the sense of touch -- become somatosensory and so forth. Is it all written in the genes or is there some interaction that genes have with the environment that make specific cortical areas work the way they do?

The second issue we had in mind was: If we can understand this, then maybe we can help brains repair themselves or help recovery of function if there is loss or damage to the brain. That interests me a great deal.

So you were essentially looking at the nature vs nature aspect of brain function?

That is correct. Or brain development.

You were working on the early stages of development.

Yes, I worked on the development, and plasticity, of the cerebral cortex. How a part of the cortex functions as visual, or auditory, or whatever, has much to do with how it develops to be visual or auditory.

So how exactly did you go about it?

I decided many years ago that one way to approach this question would be to present an area of the cortex that never gets a certain kind of input with a novel kind of input and see what the cortex does with this novel input.

Hence, I decided many years ago to rewire the brain, make vision and visual input go to a different part of the brain. It turns out that I could rewire it to the auditory part of the brain.

So I could ask this question: What does the putative auditory cortex do when it receives visual input? Does it remain an auditory cortex or does it become a visual cortex?

How did you keep out auditory stimuli?

The way we do the experiment has to do with many things that have been known for a while as to how the brain develop, how the brain gets wired up early on.

So inputs from the ears -- from the cochlea, which converts (transduces) sound into electrical impulses -- is taken into the brain via nerve fibres that form the auditory nerve and gets relayed into a structure deep in the brain called the thalamus. There is an auditory part of the thalamus that gets input from the ear.

We remove these inputs in newborn ferrets. The reason we use ferrets is that their brain is in a rudimentary stage at birth.

Similarly, the visual input from the eye go via the optic nerve to the visual thalamus. The optic nerve sweeps by the auditory thalamus and then enters the visual thalamus.

When you remove the auditory inputs to the auditory thalamus, it (the auditory thalamus) doesn't just sit there, it actually alters its molecular make up, it up-regulates (increases the supply of) some molecules, down-regulates (reduces the supply of) certain others.

It attracts the optic tract fibres into it. That's how visual input enters the auditory thalamus and makes connections there.

Normally, if the brain area is not in use, the areas surrounding it, take over that, isn't it?

That is often the case. In our case, the nerve itself sprouted inputs into the auditory thalamus. That's how visual fibres from the eye end up innervating the auditory thalamus.

Now, the auditory thalamus projects to the auditory cortex. In our animals, it is these same wires that project from the auditory thalamus to the auditory cortex that are driven by vision rather than by audition.

This is a subtle but very important point because it makes the case that the cortex is being driven differently, via a different pattern of electrical activity.

When you hear me speak, there is a spatial and temporal pattern of activity that is made up in the auditory nerve that enters your brain. When you look at an object there is a very different spatial and temporal pattern of activity.

All that the cortex knows about the outside world -- visual or auditory -- is the spatial and temporal pattern of activity in its input wires.

In our experiment, it is the same wires that go from the auditory thalamus to the auditory cortex, but they are being driven by a visual pattern of activity rather than by an auditory pattern.

In the visual cortex there are four or five different areas which specify contrast, movement etc. How much does the auditory cortex replicate these specialized areas?

The visual cortex has a primary visual cortex that gets inputs from the visual part of the thalamus and the other cortical areas that get inputs from the primary visual cortex.

One-third to half our brain is devoted directly or indirectly to vision because vision is so important to us. There are 30 odd visual areas in the cortex. They all get input from the primary visual cortex.

It is the primary visual cortex that is the main recipient of input from below. It transforms that input in new and interesting ways, thus forming the underpinnings of vision.

In our experiment, we have simply asked, does the primary auditory cortex -- which is now the equivalent of the primary visual cortex in these rewired brains -- work like the visual cortex in transforming its input in some fundamental ways?

Besides electrical activity, isn't there a chemical component too in nerve growth?

Yes, but the molecules or chemicals are involved in the routing of input and projections and fibre pathways from one source to its target. That's why I pointed out that the reason visual input -- or optic fibres -- enter the auditory thalamus is because the molecular constituents of the auditory thalamus have been altered.

Do you also remove the visual cortex here?

No, we didn't remove the visual cortex. We left it in place. What we have in these animals is a duplex visual pathway -- the retina to the visual thalamus to the visual cortex and the retina to the auditory thalamus to the auditory cortex.

But how do you judge whether it is seeing through the visual cortex or the auditory cortex?

We have two papers in Nature. One of them has to do with how the connectivity of the cortex, the wiring between neurons, changes as a result of vision. And the other has to do with how these animals use these novel rewired projections to see. The details are somewhat complicated, but the experiments were well-designed and we were satisfied with the answer.

'If you can construct tissue that recapitulates lost circuitry, you could possibly repair damage'

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