We only use 5% of our brain, etc..

Douglas Fitts dfitts at u.washington.edu
Thu Jan 20 18:18:39 EST 1994


Comments within...

bill at nsma.arizona.edu (Bill Skaggs) writes:

>jonah at ugcs.caltech.edu (Jonah Michaud) writes:

>   If this is a faq I apoligize..  From the LA Times, Jan 11, _The Mystery of
>   Memory_, by Steve Emmons:

I don't think I've ever seen a FAQ for this group, but these questions 
occur periodically, yes.

>     "The brain is an unimaginable jumble of electrical circuits.  Each of 10

The brain does have cells that communicate directly by tight junctions, 
but to say the circuits are electrical is to bruise the analogy.  Much of 
the transmission is chemical either by direct synaptic transmission or 
by "volume transmission" or neuromodulation.  If you count the latter, 
the number of affected cells would be greatly enhanced.  From what to 
what I can't say.

>   billion brain cells connects with 50,000 others.  One square millimeter of
>   cortex, the crinkly surfaced dome of the brain, contains 80,000 brain cells,
>   making the cortex the most complex electronic circuit board on Earth.
>     This means the brain's memory storage capacity is effectively unlimited.
>   You'd need many more than one lifetime to fill it up."

>   Is this true?  How many cells would be needed [ . . . ]

>The numbers are pretty accurate, except that the 50,000 figure is a
>bit high -- 10,000 would be more like it.  But this certainly does not
>mean that the brain's memory capacity is "effectively unlimited".  A
>couple of years ago I took a shot at estimating the amount of memory a
>human brain can usefully store.  The number I came up with (after
>guessing values for a few parameters that have never been measured)
>was on the order of a terabyte (that is, one million megabytes).

>This may seem like a lot of memory, but let's look at it.  Suppose we
>wanted to keep a record of everything we see.  To be concrete, suppose
>we needed to store a 100K JPEG image every second.  (This is only a
>tiny fraction of the information that comes in from the retina.)  It's
>pretty straightforward to work out how long it would take to exhaust a
>terabyte of storage: the time is on the order of one year, even if
>storage only goes on during waking hours.

Seems to be straining a point to compare the human memory capacity with a
lossless compression algorithm.  Why not store that image in a
quasireliable 5K and forget about the next 30 sec or so of redundant data? 
Maybe your memory's better than mine, wouldn't be hard, but I tend to 
lose lots of detail.  In fact, I'd include the auditory and other senses 
along with that 5K snapshot.  It all seems to be encoded pretty efficiently.

...

>   Is the statement "we only use 5% of our brain" true?  And does it refer to
>   an untapped memory capacity?  Is it consistent with natural selection- is
>   there a reproductive advantage to having excess "brain" or memory capacity?
>   Or does natural selection simply have no say in brain capacity above that
>   needed for hunting/gathering?

This again..., but it's dropped from 10% to 5%!

>Nobody knows enough about how the human brain works to say what
>fraction of it we use, but I think there is a strong argument that

Speak for yourself :-)

>it's a lot more than 5%.  The argument is that humans suffer a much
>higher childbirth mortality rate than most other mammals, and the
>get rid of the redundant parts....

>The story I've heard (I'd appreciate any confirmation or denial) is
>that the 5% figure comes from Karl Lashley's studies, in which he
>lesioned the brains of rats, and observed how the lesions affected
>their performance in various tasks.  For cortical lesions, he found
>that they had to be very large to make the rat's unable to do the
>tasks.  The current consensus is that Lashley's results were correct
>but misleading, because he used tasks that could be done in many
>different ways.

Probably true about the source and interpretation.

This figure of 5 or 10 or 15% seems to fascinate lay scientists who 
imagine that "The Brain" then might hold vast untapped resources.  But 
keep in mind that the brain is more than the cortex, and much of the 
brain including the cortex is constantly active in the process of actually
*inhibiting* other parts of the brain to keep it quiet!  This includes 
much of the sensory and motor systems.  Does this bode well for untapping 
those resources?  Lay persons tend to forget that the brain does many 
things other than think.  Every beat of the heart and filing of the lungs 
is regulated.  The brain is constantly sampling the internal millieu to 
maintain a regulatory balance called homeostasis.  Much of this is 
subcortical, or even medullary or spinal (if you think of CNS instead of 
brain proper).  

Those who interpreted Lashley's studies as meaning we only use 5 or 10 or
15% of our brains seem to have neglected the fact that those huge chunks
of brain that he ablated and seemed to have little to do with the task at
hand might actually have had an active function in something else he wasn't
directly measuring at the time... 


Doug Fitts



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