Seeking the Source of Blindsight

Remember Donald Rumsfeld's chestnut about the "known unknowns" and "unknown unknowns?" Neuroscientists can add one more category to that list: unknown knowns, things we know without even realizing that we know them.

Or, things we see without realizing that we see them. Patients with a rare condition called blindsight are blind due to damage to the primary visual cortex, the part of the brain that consciously processes information from the retina. But when scientists probe deeper, it turns out that these patients do have access to some visual information after all--they just aren't conscious of it.

Take a patient called TN. First, a stroke hit TN's left visual cortex. Then, barely a month later, another stroke took out TN's right visual cortex, blinding him entirely. But researchers had a suspicion that TN might have blindsight. To test this hypothesis, they asked TN to walk down a hallway without his cane. Though TN didn't know it, the hallway had been strewn with obstacles that should have tripped him up. Watch what happened.

Did you watch it? Here's the gist: TN successfully navigated the path, weaving his way around an overhead projector, a tripod, a few reams of paper, and various other items (apparently) pinched from someone's desk. Asked how he'd made it from point A to point B, TN reported no memory of avoiding any obstacles; as far as he was concerned, he'd just walked a straight line.

Now, researchers are pinpointing where blindsight "lives" in the brain. A new study, published last month in Nature, points to a structure called the LGN, or lateral geniculate nucleus. When neuroscientists used a chemical to shut down the LGN in the brains of two monkeys with damaged primary visual cortexes, the animals' blindsight stopped working. It could be that information from the eyes is normally routed through the LGN as well as the primary visual cortex, with the LGN providing the kind of rapid responses ("Hey, don't trip on that!") that we can't wait around to process consciously. On the other hand, the LGN might be picking up the slack in brain-damaged patients but play little role in healthy brains. It's too early to say.

Researchers hope that stroke patients could one day learn to consciously access the visual information captured by blindsight. Of course, this only stands a chance if other parts of the brain, and the eyes themselves, are healthy. And blindsight will never provide the kind of vision that, um, sightsight does. It registers shapes, motion, and the orientation of objects, but can't pick up small details. It can make out the emotions expressed in human faces, but not the gender or identity of those faces.

Doctors will face another hurdle, too: Convincing patients to trust their blindsight. As Beatrice de Gelder, one of the scientists working with TN, wrote in Scientific American, even TN won't be leaving his cane at home any time soon.

What else do we know that we don't know we know?

User Comments:

My grandpa drove a taxi for forty years before losing his sight to glaucoma. He knew the city like the back of his hand. What I didn't discover until after he lost his sight was that the city was mapped out in his mind. He'd lost his sight just as I was starting to drive myself. I always called him for directions. Before there was Mapquest, there was Grandpa.

The interesting thing is that as the city continued to change, buildings razed and new buildings erected, the map in his mind changed as well. I doubt he was aware of it consciously. But he listened to talk radio and the news, and as projects were discussed and the public made aware of changes, his mind incorporated all of the new information into his map. So, years after he went blind, he could still give you accurate directions, complete with updated landmarks. He would tell you what used to be there and what was there now - updating the maps in my mind. It was amazing. I was never lost listening to his directions. I don't know what you call that, but whatever it was, he had it.

I have recently completed a position paper that illuminates in detail the actual location, in space, where I have come to believe the human mind actually exists; after a half century of observation, examination, and consultation, as a classroom teacher, academic advisor, pastoral counselor, and ER crisis counselor, I have concluded that our image filled minds are generated as complex mental holograms by our equally complex functioning brains; my sense is that the human mind is not to be found in the structure of the brain, but rather in its function; I suggest that the working brain is generating an energy stream which is projecting sensory images into a mental imaging chamber comprised of the structural energy of the magnetic field that surrounds each of our physical bodies;
My blog is at: http://cid-

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