What can't the human brain do?

March 27, 2011 9:10 AM

What are some limits of the human brain? I think everyone knows that people can't keep more than 7 items in short term memory, and I was just reading an article here that mentions limits to the theory of mind (A knows that B thinks that C thinks.. etc). What are some other things that it is just impossible to think about or remember or to visualize? I'd prefer actual studies and hard data to anecdotes and folk wisdom, but I'll take either.
posted by empath to science & nature (24 answers total)

You're terrible at statistics; at formal math; at intuitive physics.

That is to say, our mental models of these things work only within the limited domain of the human EEA, providing wrong answers outside those limits.
posted by orthogonality at 9:19 AM on March 27, 2011 [2 favorites]

Humans are terrible at judging risk. In particular, we overemphasize unusual risks over common risks (eg "I might be in a plane crash" vs "I might be in a car crash" - the former is more often a notable fear, while the latter is far more likely).
posted by rmd1023 at 9:21 AM on March 27, 2011 [2 favorites]

I remember reading somewhere once that it's hard for the average person to quickly assess quantities over 9. Nine objects can be quickly sorted into three groups of three (three is, I guess, really easy to resolve), but any more than that becomes "several" or "many".

And wherever I read that, I think it also said that geniuses can insta-count much higher numbers. Like the Rain Man bit with the 246 toothpicks.

I don't remember where I read about it, but hopefully someone else knows. I've actually been meaning to ask a similar question to yours for a while, so thanks!
posted by phunniemee at 9:23 AM on March 27, 2011

So, actually that 7 item short-term limit is a bit antiquated. Currently thinking is more like 4 items, lots of data out there on this one. This goes past attention or memory, you can actually only quickly count (subitize) 4 or less items, otherwise you have to just estimate. There's plenty of studies on how brain damage can affect your ability to remember/visualize things. Two big ones are the literature on hemispatial neglect, where you can't pay attention to, even in memory, things on one side of space (usually the right. Note this is a simplification of this disorder) and prosopagnosia (face blindness). But if you want normal people. There are also a ton of visual illusions that show times in where your visual system is not working correctly.

Lots of decision making errors too, like the risk rmd1023 is talking about, too.
posted by katers890 at 9:24 AM on March 27, 2011 [1 favorite]

There are "grammatical" sentences that are hard to understand as a result of their syntactic complexity. Example given here, showing embedding, is "The mouse the cat the dog scared chased ran away", which I can barely even remember, let alone parse quickly.
posted by knile at 9:30 AM on March 27, 2011 [2 favorites]

Probability, infinity, and large numbers.

For example: Your computer is doing 3 billion things a second. When the light from the screen hits your eyes, you computer has already done at least 5 things since those photons were launched from the screen towards your face - and this neglects that amount of time it takes to instruct the monitor to display a certain value, and the amount of time it takes for your brain to process the incoming light. When you take those things into account, it gets even sillier.
posted by pmb at 9:57 AM on March 27, 2011

Humans (at least, non-infant humans) are terrible at doing anything naïvely/objectively. That is, all judgments and sensory inputs are based on prior knowledge. The simplest examples of this are "top-down" processes in vision. You fill in information that you expect to be there, and when your expectations don't match the raw input (e.g. optical illusions) funky things happen. Put simply, everyday vision (or any sensory input) is NOT a raw stream of input from the world around you, but a mix of the world and your prior knowledge of how the world is put together.
posted by supercres at 9:59 AM on March 27, 2011 [1 favorite]

So, actually that 7 item short-term limit is a bit antiquated ... you can actually only quickly count (subitize) 4 or less items.

Shert term memory is different from subitizing. The seven item limit applies to short-term verbal memory. There are other types of short term memory.

There's been some interesting research on short term visual memory. Vision actually consists of a very rapid series of saccades, brief, split-second focuses on parts of your visual field. Your short term memory for these is very limited. Holistic visual perception of the world around you is very dependent on the world being reasonably stable and behaving in a fairly predictable way, and the ability to detect motion in periferal vision outside of the area of immediate focus. A lot slight of hand tricks take advantage of this limitation.

The seven-item limit of short term verbal memory applies to words or phrases (referred to in cognitive psych by the elegant word "chunks"). People can understand, and repeat back verbatim on one hearing, statements that are a lot longer than seven words, because sequences of words that can be understood as a phrase can be reduced into a single phrase, clearing "space" for more chunks. But we can't remember lists of unconnected words longer than seven items on one hearing without using mnemonic tricks,

Presumably, since short term verbal memory is used pars sentences as they're being spoken or read, this would place a practical constraint on how complicated sentences can be and still be understood, although there is no grammatical limit on, say, how many dependent clauses a sentence can have.

(The last bit about short-term memory and comprehensibility is just me talking. I'm not aware of any research on this, though there might well be some. I just think it would fun to study.)
posted by nangar at 10:52 AM on March 27, 2011

is used pars = is used to parse
posted by nangar at 10:55 AM on March 27, 2011

Short term memory is a very different from subitizing, but they both show approximately* 4 item limits. Yes, the 7 item limit came from verbal short-term memory, while the 4 item limit came from visual short-term memory, and I almost noted that, but there's been plenty of people saying that even that was a consequence of grouping, which is almost impossible to prevent in any task, making capacity limits difficult to measure, that I left it out for conciseness sake. If you're up for a long read, try Cowan 2001.

*depending on the task difficult, or not depending on whether you like the fixed slot or flexible resource hypothesis
posted by katers890 at 11:04 AM on March 27, 2011 [1 favorite]

Keeping time.
posted by phrontist at 11:33 AM on March 27, 2011

OK. katers890, that makes sense. Yeah, I'm up for a long read. I'll take a look at that.

And the fuzziness of the limits is interesting. I was taught the slot model (in early 90's), but why is the capacity of subitizing about three or four instead of one or the other? I'm remembering one of my professors from college, a cognitive psychologist, who said, trying to explain that theoretical constructs involve analogies and metaphors that can be useful, but also counterproductive, something like 'We have reasonably good understanding of how short-term verbal memory works, but we don't have seven little boxes in our heads.' Yeah, maybe something's wrong with the model.

(He was a really good professor, and a lot what he said has stuck in my head. Thanks for reminding me of him.)
posted by nangar at 11:51 AM on March 27, 2011

I think one of the best quotes about this I've heard goes something like this: "If the human mind was simple enough for us to understand, we'd be too simple to understand it."
posted by Chocolate Pickle at 11:59 AM on March 27, 2011 [4 favorites]

Holding more than one factor constant while varying others - most people can only reason about one factor while holding the other constant, and once you get beyond 2 it becomes increasing intractable for the brain.
posted by monocot at 12:10 PM on March 27, 2011

You are also functionally blind when your eye makes a saccade (as mentioned above) and your brain "fills in" what it expected to be there during the movement so you don't experience gaps in your visual experience. Even when you move your eyes around the world appears stable because the brain predicts and recalibrates your fixation point when you move your eyes.

Try poking/moving your eye gently instead - the environment then appears to move because the brain is not getting the feedback it needs to maintain the semblance of stability.
posted by monocot at 12:16 PM on March 27, 2011

(attempting not to derail, but respond to nangar). The fuzziness of the limits comes from a couple different places, 1) different people have different limits, there's some evidence that things like sports or video games can increase your limits as they train you to split your focus, and 2) we don't really know what is causing these limits and how to best describe them. The fixed slot theory has been in place for a long time (at least since 1997), but many people question it these days (not to mention many have difficulty replicating the work that showed that), and have proposed either mixed models (where you have an upper limit of 4, but if the task is hard or the stimuli complex you will be able to attend to/remember less), or resource based models (which are critiqued for being too general) where you have resources that must be split when we pay attention to mulitple things, and the amount of the resources that each item needs, will change how many things you can attend to. Lots of work is being done now on this both behaviorally and with neuroimaging (ERPs, fMRI, etc)

(also, glad I could remind you of a good professor, that makes me feel warm and fuzzy inside).
posted by katers890 at 12:25 PM on March 27, 2011

This list of cognitive biases from wikipedia lists all the ways found (so far) that brains are bad at thinking.
posted by nooneyouknow at 12:31 PM on March 27, 2011 [1 favorite]

The human brain/eye is absolute rubbish at estimating heights much beyond ten feet. Particularly when we're talking about heights large enough to merit the term "altitude."

Seriously, next time you're downtown, just try guessing how tall, in feet, a particular building is. You'll be lucky to get within 25%. It's even worse if you're trying to estimate how far from the ground you are. I mean, you can get a pretty good sense of whether or not jumping is going to suck, but beyond that, all bets are off. Even experienced pilots will tell you that without instruments, there's really no way to tell how high up you are.

We're a lot better at estimating horizontal distance, as we're a lot more used to traveling in that direction, but even that can be pretty dicey.
posted by valkyryn at 12:43 PM on March 27, 2011

Dunbar's number (roughly 150) is the theoretical number of social relationships our brain can handle. Anthropologist Robin Dunbar derived this number after observing primates and conducting ethnographic studies of hunter-gather societies.

From the Wikip:
"Dunbar's surveys of village and tribe sizes also appeared to approximate this predicted value, including 150 as the estimated size of a neolithic farming village; 150 as the splitting point of Hutterite settlements; 200 as the upper bound on the number of academics in a discipline's sub-specialization; 150 as the basic unit size of professional armies in Roman antiquity and in modern times since the 16th century; and notions of appropriate company size."
posted by Eumachia L F at 1:58 PM on March 27, 2011 [1 favorite]

Imagine more than 3 dimensions of space and one of time.
posted by Corvid at 4:37 PM on March 27, 2011 [2 favorites]

What Corvid said. I'm working on classifying EEG data. One way to look at this is that I have a bunch of points located somewhere in a space. These points represent segments EEG recordings from people. There are two types of points, and I'm trying to find a plane that separates the points, such that all the points of one type go on one side, while the rest go on the other side. The last detail is that this space has 413000 dimensions. There is absolutely no way for the human brain to visualize this, even if the problem is very real, and just as mathematically well defined as it is in 3-dimensional world we have evolved to deal with.
posted by cheerleaders_to_your_funeral at 5:00 PM on March 27, 2011

Most people slowly forget things (Wikipedia: Forgetting Curve).
Most people also remember things incorrectly (SMBC comic, mildly NSFW).
posted by sninctown at 6:41 PM on March 27, 2011 [1 favorite]

Spin your left foot in a clockwise direction. Spin your right hand in a counterclockwise direction. Most can do it just fine.

Now try it again: spin your left foot clockwise, but spin your left hand counterclockwise. Usually you can do it, but it take a ton of concentration. I'm sure there's a fancy term for the above.
posted by Mister Fabulous at 10:19 PM on March 27, 2011 [1 favorite]

Similar to Mister Fabulous, and more hilarious, try rotating your foot counterclockwise while visualizing drawing the number six, easy right? Now try rotating your foot clockwise while imaging drawing that six. Bet you can't do it! It's just like Mister Fabulous's hand/foot thing, but you don't have to even be using your hand and it isn't necessarily related to the lateralization of motor functions (ie, left hemisphere controlling right side of the body), because it can work with both feet.
posted by katers890 at 7:03 PM on March 28, 2011