The brain basis of blindsight

A monkey with a missing primary visual cortex can still see... but how?
08 July 2019

Interview with 

Kristine Krug, University of Oxford

Monkey brain missing visual cortex

Brain scan of a monkey that could still see but lacked a primary visual cortex


It’s often said that nature reveals her inner workings through her mistakes. What Kristine Krug discovered recently is no exception, and could give us new insights into what “blindsight” might really be, as she explains to Chris Smith…

Kristine - We studied the structure and the function of a monkey brain in an animal where a big part was naturally missing, and we looked at how the rest of it was working in order to support all the things the animal could still do which we didn't expect it to be able to.

Chris - How did you come upon this animal in the first place? Was it just by chance?

Kristine - It was actually a chance discovery. The animal was being trained for a different study, looking at big visual images and making decisions about it, and it had to just touch a big coloured object on the screen for a reward. And all her cage mates could do it, but that animal couldn't. And eventually we gave it a brain scan and found actually that the back third of the brain just wasn't there on both sides.

Chris - Why not?

Kristine - They think it was just probably born without this part of the brain.

Chris - And if you map the missing bits onto what we know about the structure of the monkey brain, which bits are missing?

Kristine -So it's at the back of the brain, which in monkeys and humans has basically a complete map of the visual world. All the information from the eyes comes in there and is then distributed further into other parts of the of the brain that does the processing for perception, and decision making, and behaviour.

Chris - We refer to that in a human as the primary visual cortex, don't we. It's the first jumping-off point for the visual information coming in from the brain, and we're comfortable that if we damage that - with a stroke for example - patients say they can't see anything once they lose that bit of brain. So how do you reconcile that with the fact that your monkey appeared to, at least some of the time, be behaving normally and be able to see?

Kristine - It's true, so in humans who lose that part of the brain late in life they are virtually blind, though they have some remaining vision called blindsight in some cases. So they can point to very bright moving spots of light even though they say they're not aware of them. And we think what happened in the monkey is probably an extension of this. There are other pathways that go from a relay in the middle of the brain to these higher areas, bypassing what is usually the major gateway. We think there was enough there for the rest of the brain to develop normally and getting a more extensive version of that blindsight which you can see in patients. And what was quite intriguing is when you offer normally a monkey a treat, the monkey will just come to the front of the cage and carefully look at the treat and reach for it and grasp it. That animal would start running past it as if to put the treat into motion and grab it out of the corner of its eyes.

Chris - One would infer from what you're saying then, that the monkey moved in order to make the object appear to be moving because then the motion decoding bits of the visual system that were intact in that animal could then register the presence of the object and provide information to the conscious brain so it knew what it was interacting with.

Kristine - Yeah. So all the behaviour pointed to that. And when we then put the animal in the scanner we could see that the network of brain areas that talk to each other when we or the monkeys see motion, they still talk to each other normally despite a major part of this network - the major input to it - missing.

Chris - Monkeys, like humans, are also heavily driven by faces aren't they. They are good at recognising other individuals and that goes on in the temporal lobe of the brain, a very different part of the brain. Was your monkey capable of recognising its colleagues?

Kristine - We don't know behaviourally, though the animal showed no problems with its cage mates. They interacted normally. But what we did do is we looked at these specialist bits in the brain that code for faces. And to be frank we didn't expect to find anything, because it needs very detailed fine vision for which we thought you need this primal visual cortex, a major gateway. But when you showed the animal faces of other monkeys, these very same face patches lit up in the temporal cortex as they did in typical monkeys. And we think that points to a separate input directly in these face areas from emotional centers in the brain being fed directly from the eyes.

Chris - Our existing view of how the visual system farms out the information that comes in is that everything ends up in the the first visual area of the back of the head and then from there it's parcellated off into these other centres that process things like colour, like movement, like faces. Is the finding from this monkey suggesting then that actually, that spreading out or that distribution of the information could be happening a lot sooner, a lot further upstream in the process, and that's why it's able to do this? Or do you think that this animal has just adapted to the fact that half its brain is missing, and so it's able to do these things because it's had to?

Kristine - We always knew there are small pathways that spread out earlier. So it is still the case that through the primary visual cortex 90-95% of visual information ends up in the brain network that supports our visual behaviour, but we knew about the other pathways. But we thought they weren't to that extent capable of supporting the rest of the network to function, and that's that's clearly been the case in that animal. We looked for strengthening of known connections of these smaller pathways and couldn't find any evidence, though we haven't looked at that exhaustively. But that's one avenue. It just could simply's clearly not becoming stronger as a structure, these connections, but it seems like they become more efficient to drive the rest of the visual brain.


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