Sister blog of Physicists of the Caribbean in which I babble about non-astronomy stuff, because everyone needs a hobby

Tuesday, 2 May 2017

Blindsight : you don't know what you're aware of

This is seriously weird. Of course, it's well-known that the brain does a lot of extremely clever processing on the signals from your eyes, which is why there are so many possible optical illusions. But it can go much, much further than that. Your conscious mind isn't necessary for those visual signals - if part of your brain is damaged, you can still process the images unconsciously : vision without sight. The question then is, how much of this happens anyway ?

In 2008, Tamietto and Weiskrantz’s team put another blindsight patient through the most gruelling test yet. Unlike Daniel, he was blind across the whole of his visual field, and normally walked with a white cane. But the team took away his cane and then loaded a corridor with furniture that might potentially trip him up, before asking him make his way to the other side. “Despite saying he wasn’t able to see, we saw him shooting by on his very first attempt,” says Tamietto.

Importantly, the participant claimed that not only was he not aware of having seen anything; he was not even aware of having moved out of the way of the objects. He insisted he had just walked straight down the hallway. According to Beatrice de Gelder, who led the work, he was “at a loss to explain or even describe his actions”.

Only in very rare circumstances do they come close to being aware of what they are seeing. For instance, one subject was able to distinguish movement in fast, high-contrast films; he described it as being like “a black shadow moving against a completely black background” – a “sense of knowing” that there was something beyond. But even then, he could not describe the content itself, meaning that his experience lacked almost everything we would normally associate with vision.

Enter the mad scientists :

To test their ideas, scientists can use a form of non-invasive brain stimulation that disrupts different brain regions, in an attempt to induce a reversible form of blindsight in healthy participants. The technique is called “transcranial magnetic stimulation”, which uses a strong magnetic field to scramble the neural activity underneath the skull.

The experiment began with Allen placing a magnet over the back of my skull, just above V1. Next, he began applying the magnetic field for short intervals at increasing strengths. After Allen had found the right power, I sat in front of a computer screen, and he flashed up pictures of arrows for a split second: my job was to say whether they pointed left or right. The pictures were sometimes timed with the TMS signals causing the temporary blindness – and like Daniel in those original experiments, I often saw nothing and felt that I was guessing. Nevertheless, once I had finished, Allen told me that I had answered many more correctly than would be expected by chance alone, suggesting the TMS had succeeded in giving me blindsight.

[I'd also add that this has important epistemological consequences. We usually think of ourselves as "knowing our own minds", that if we think we're happy or sad then no-one can refute this but ourselves. But if we don't even know what we're sensing, then our self-knowledge can be much worse than we'd like to think.]


Substituting one's own reality

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