The phantom touch illusion

It seems like every time researchers look further into our sensory-perceptive systems, we have another hole punched in our certainty that what we think we're perceiving is actually real.

We've looked at optical illusions -- and the fact that dogs fall for 'em, too.  We've considered two kinds of auditory illusions, the postdictive effect and the McGurk effect.  Sometimes we see patterns of motion in still objects -- and illusory "impossible" motion that our brains just can't figure out.  A rather simple protocol convinced test subjects their hands had turned to stone.  Stimulating a particular clump of neurons in the brain made patients see the doctor's face as melting.  We can even be tricked into feeling like we're controlling a second body, that just happens to be invisible.

As eminent astrophysicist Neil deGrasse Tyson put it, "The human brain is rife with ways of getting it wrong."  Honestly, at this point it's a wonder we trust anything we perceive -- and yet you still hear people say "I saw it with my own eyes" as if that somehow carried any weight at all.  Add to that all the problems with the reliability of memory, and you have to ask why eyewitness accounts are still considered the gold standard of evidence.

If you needed more proof of this, take a look at some research that came out last week from Ruhr-Universität Bochum into what happens when a person watches a virtual-reality avatar of their own body.  Participants were suited up in VR gear, and after a period of acclimation -- during which they got used to their avatar's arms and hands moving as their own did -- they were instructed to use a virtual representation of a stick to touch their avatar's hand.  Nearly all of the subjects reported feeling a sensation of touch, or at least a tingling, at the spot the virtual stick appeared to touch.

[Image licensed under the Creative Commons Samuel Zeller samuelzeller, VR (Unsplash VK284NKoAVU), CC0 1.0]

The researchers decided to check and see if the sensation occurred simply by drawing awareness to the hand, so they did the same thing only using a virtual laser pointer -- and no feeling of touch occurred.

Apparently all it took was convincing the subjects they were being touched to stimulate the sensation itself.

"The phantom touch illusion also occurs when the subjects touched parts of their bodies that were not visible in virtual reality," said study co-author Marita Metzler.  "This suggests that human perception and body sensation are not only based on vision, but on a complex combination of many sensory perceptions and the internal representation of our body."

The whole thing brings to mind a conversation I had with an acquaintance, a Ph.D. in philosophy, some years ago about the impossibility of proving materialism.  I'd always considered myself a hard-nosed materialist, but her stance was that no one could prove the external world was real.  I shot back with a snarky, "Well, that works until someone throws a rock at your head.  Hard to deny the rock isn't real after that."  She patiently responded, "No.  What is real are the sensations you experience -- the shock, the pain, the adrenaline rush.  Possibly a period of loss of consciousness.  You're still locked inside your own skull, and the only thing you have access to are your own thoughts and feelings.  Those are all you can be certain are real experiences -- and even those might well be false or misleading."

Well, it was a fair knockout (pun intended), and I still haven't really come up with a rejoinder.  Not that this is surprising; philosophers have been discussing the whole materialism vs. idealism thing for centuries, and haven't really settled it to anyone's satisfaction.  And since the time of that argument, I've found more and more evidence that we experience through our sensory-perceptive apparatus only the barest fraction of what's out there -- what neuroscientist David Eagleman calls our umwelt -- and even that part, we see inaccurately.

Kind of humbling, isn't it?  Think about that next time someone starts acting so all-fired certain about their own perceptions, memories, experiences, and opinions.  The more you know, they more you should realize that none of us should be sure of anything.

But after all, doubt isn't a bad place to start.  I'll end as I did yesterday, with a quote from the brilliant physicist Richard Feynman: "The first principle is that you must not fool yourself; and you are the easiest person to fool."

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Walls in our minds

One of the biggest unsolved mysteries of science is how the brain encodes what we know, think about, and experience.

For many types of information, we know where in the brain they are stored.  But in what form, and how we retrieve it, is still not known.  As I tell my neuroscience students: think of something simple, like your middle name, the name of your first pet, the name of the first president of the United States.  Now: where in your mind was that information before I asked you to remember it?

Pretty bizarre to consider, isn't it?

Interpretation of sensory input is as mysterious as memory.  When I look around my office, where I'm currently writing this, I can recognize all sorts of objects -- my CD collection, books, the masks hanging on the wall, a wine glass hand-made by my son, an antique typewriter my wife got me for my last birthday.  But... how?  What's being projected onto my retina is just a bunch of splotches of light, shadow, and color, and my brain has to take that chaos and somehow make sense of it.  How can we tell where the edge of an object is?  How do we recognize things -- and know them to be the same objects if seen from a different angle, meaning the pattern of colors thrown onto your retina is completely changed?

Interestingly, there are some people who can't do this.  In apperceptive visual agnosia, usually caused by damage to the visual cortex of the brain, people are cognitively normal in every respect except that they can't recognize anything they see.  It all looks like a random moving kaleidoscope of colors.  Because in other respects they're normal, they can remember what they're told and respond appropriately -- if someone said, "Hey, you see that blob of blue and tan and brown over there?  That's a person, and he's named Gordon," a sufferer from apperceptive visual agnosia would be able to say, "Oh, hi, Gordon," and have a normal conversation with me.  But if I stood up (changing the shape of the blob of color) or changed my shirt, or made any other sort of alteration to what he's seeing, he'd no longer recognize me, not only as a particular human, but as human at all.  Because they're perfectly intelligent, he might be able to reason that since Gordon was over there a few seconds ago, and there's a different blob of blue and tan and brown nearby, that's probably Gordon, too, but it wouldn't be because he actually recognized me.  It would be a logical inference, not visual interpretation.

An interesting piece was added to the puzzle last week with a paper in Neuron that came from some research at Columbia University led by neuroscientist Nikolaus Kriegeskorte.  He and his team were investigating how we perceive walls -- how we know where the edges and barriers are in our environment, a pretty critical skill for spatial navigation.  By showing participants images with walls and other barriers and allowing them to navigate the space virtually, and using fMRI and magnetoencephalography (MEG) neuroimaging, they were able to narrow down where we do edge and obstacle processing to the "occipital place area" (OPA), one of the visual processing centers.

[Image licensed under the Creative Commons Pawel Wozniak, Brick wall close-up view, CC BY-SA 3.0]

"Vision gives us an almost instant sense where we are in space, and in particular of the geometry of the surfaces -- the ground, the walls -- which constrain our movement," Kriegeskorte said.  "It feels effortless, but it requires the coordinated activity of multiple brain regions.  How neurons work together to give us this sense of our surroundings has remained mysterious.  With this study, we are a step closer to solving that puzzle...  Previous studies had shown that OPA neurons encode scenes, rather than isolated objects.  But we did not yet understand what aspect of the scenes this region's millions of neurons encoded...  We would like to put these [data] together and build computer vision systems that are more like our own brains, systems that have specialized machinery like what we observe here in the human brain for rapidly sensing the geometry of the environment."

Once again, we have an idea of where our perception of barriers is housed, but not so much information about how it's stored or accessed.  How, when we see a wall -- especially, as in this experiment, a two-dimensional representation of a wall -- do we recognize it as such, and not just as a smear of colors, lines, and angles?  As Kriegeskorte said, we're a step closer, which is fantastic -- but still a long way away from solving the puzzle of perception.

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When the brilliant British neurologist and author Oliver Sacks died in August of 2015, he was working on a collection of essays that delved into some of the deepest issues scientists consider: evolution, creativity, memory, time, and experience.  A year and a half ago, that collection was published under the title The River of Consciousness, and in it he explores those weighty topics with his characteristic humor, insight, and self-deprecating humility.

Those of us who were captivated by earlier works such as The Man Who Mistook His Wife for a Hat, Musicophilia, Awakenings, and Everything in its Place will be thrilled by this book -- the last thoughts of one of the best thinkers of our time.

[Note:  If you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]