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Caveat lector: This blog is where I try out new ideas. I will often be wrong, but that's the point.

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9.5.11

We are all inattentive superheroes

One of the more interesting parts about being a neuroscientist is when I'm suddenly struck by how absolutely weird our brains are.

And I'm not even talking about the super trippy stuff like free-will; even the mundane things are really mind-boggling.

For example: what does it mean to experience the world around us? We "see" things because photons that manage to pass through the inside-out design of our retinal cells cause a molecular change in the photoreceptors such that 11-cis-retinal isomerizes... etc.

(Tons of way overly detailed biology cut from here...)

When we "hear" things, the sound pressure waveform hits the tympanic membrane (eardrum) and ultimately causes the basilar membrane in your cochlea to vibrate. The basilar membrane is stiffer at one end (the basal end) and less stiff at the other end (the apical end). This fact was observed by Georg von Békésy (and earned him the 1961 Nobel Prize in Physiology or Medicine).

Here's a gratuitous, yet cool, video of this frequency decomposition in action:



Okay, great, so we know a ton of the basic biology and cellular mechanisms of the signal transduction mechanisms of our sensory apparatus.

But damn if I'm still not amazed by the actual experience of sensation.

Even beyond the philosophical wonder of passively sampling our outside environment in a shared, meaningful fashion is the ridiculous sensitivity of our senses.

We're used to thinking of our senses as being pretty shite: we can't see as well as eagles, we can't hear as well as bats, and we can't smell as well as dogs.

Or so we're used to thinking.

It turns out that humans can, in fact, detect as few as 2 photons entering the retina. Two. As in, one-plus-one.

It is often said that, under ideal conditions, a young, healthy person can see a candle flame from 30 miles away. That's like being able to see a candle in Times Square from Stamford, Connecticut. Or seeing a candle in Candlestick Park from Napa Valley.

Similarly, it appears that the limits to our threshold of hearing may actually be Brownian motion. That means that we can almost hear the random movements of atoms.

We can also smell as few as 30 molecules of certain substances.

I mean, we're talking serious Daredevil-level detection here!

(Frank Miller. Daredevil(c) Marvel Comics via Drawing Files)

These facts suggest that we all have some level of what we'd normally think of as "super human" sensory abilities already.

But what the hell? If I can supposedly see a candle from 30 miles away, why do I still crack my frakkin' shin on the coffee table when it's only slightly dark in my living room?

Well, for one thing, attention plays a very important role. For example, consider the very famous visual attention experiment below:



How can we see TWO PHOTONS, but miss THAT!?

The easy hypothesis? Attention.

You see, in the experiments testing the physical limits of the human sensory systems, the subjects involved are dedicating a lot of attention to the one sense being tested, almost certainly at the exclusion of the other senses.

I think we all have a pretty intuitive grasp of this. And sometimes our intuitive corrections are pretty damn funny. If you watch people's behavior carefully you'll notice some strange behaviors that we do.

Have you ever been driving around, trying to find a particular address, and then turn down the radio as you get close to where you think your destination is? Why would you turn down the radio when what you're doing is looking for an address? Seems pretty silly.

Do you close your eyes when you're trying to do calculations in your head? Why?

I think the answer to these questions is because we're trying to reduce sources of noise to maximize the amount of attention we can pay to the task at hand. The sounds from the radio capture your attention, making it hard to visually search for the address numbers on the house you're trying to find. Visual distractions in our surroundings may prevent us from maximally focusing our attention internally when trying to do hard math problems in our heads.

It strikes me that the experiments on the physical limits of our perception are probably also related to the adage that if you lose one sense, your remaining senses get heightened. This is a pretty common saying, but is it really true that if I became blind that I'd suddenly gain super-human hearing?

In a series of Nature studies published in the 1990s, it was shown that blind subjects reading via Braille actually use their visual cortex when reading by touch. This was demonstrated not only using brain imaging (PET, in this case), but also more causally via disruption of the visual cortex via TMS, a technique that can safely and reversibly disrupt the ability of a small region of the brain to process stimuli.

(See my basic primer on brain imaging techniques for a little more detail.)

When TMS was used to disrupt the visual cortex of blind subjects, their ability to read Braille characters dropped!

Anecdotally, the impressive sensory adaptations by blind people can be seen in two particularly striking subjects.

The first is a blind boy who was able to navigate so well via echolocating his own clicking sounds that he could ride a skateboard.



The second example is of another blind young man who was able to play video games by sound alone. The guy could track the sounds in a game and use them to play through to completion.

(That second story, by the way, would make for an amazing addition to the Serious Games Summit...)

In my own research I've tried to identify which regions of the brain are critical for attention and working memory, to understand how cognitive functions reorganize after brain damage, and to provide a physiological basis for how sensory and cognitive systems could interact.

In my future research I will further examine how sensory and cognitive systems interact and interrelate in more detail.

For now, sometimes the only thing I can do is sit back and marvel at how amazing it is that this three pounds of fat and water in my head does anything at all.
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(No matter how much beer I throw at it!)

ResearchBlogging.org
Bialek, W. (1987). Physical Limits to Sensation and Perception Annual Review of Biophysics and Biophysical Chemistry, 16 (1), 455-478 DOI: 10.1146/annurev.bb.16.060187.002323
Sadato N, Pascual-Leone A, Grafman J, Ibañez V, Deiber MP, Dold G, & Hallett M (1996). Activation of the primary visual cortex by Braille reading in blind subjects. Nature, 380 (6574), 526-8 PMID: 8606771
Cohen LG, Celnik P, Pascual-Leone A, Corwell B, Falz L, Dambrosia J, Honda M, Sadato N, Gerloff C, Catalá MD, & Hallett M (1997). Functional relevance of cross-modal plasticity in blind humans. Nature, 389 (6647), 180-3 PMID: 9296495