textfiles/science/merkle1.txt

How Many Bytes in Human Memory?
by Ralph C. Merkle
(appeared in Foresight Update No. 4, 1988)
(merkle.pa@xerox.com)


Today it is commonplace to compare the human brain to a 
computer, and the human mind to a program running on that 
computer.    Once seen as just a poetic metaphore, this viewpoint 
is now supported by most philosophers of human consciousness and 
most researchers in artificial intelligence.  If we take this view 
literally, then just as we can ask how many megabytes of RAM a PC 
has we should be able to ask how many megabytes (or gigabytes, or 
terabytes, or whatever)  of memory the human brain has.

Several approximations to this number have already appeared in the 
literature  based on 'hardware' considerations (though in the case 
of the human brain perhaps the term 'wetware' is more 
appropriate).  One estimate of 10**20 bits is actually an early 
estimate (by Von Neumann in 'The Computer and the Brain') of all 
the neural impulses conducted by the brain during a lifetime.  This 
number is almost certainly larger than the true answer.  Another 
method is to estimate the total number of synapses, and then 
presume that each synapse can hold a few bits.  Estimates of the 
number of synapses have been made in the range from 10**13 to 10**15 
-- with corresponding estimates of memory capacity.

A fundamental problem with these approaches is that they rely on 
rather poor estimates of the raw hardware in the system.   The 
brain is highly redundant and not well understood:  the mere fact 
that a great mass of synapses exists does not imply that they are 
in fact contributing to the memory capacity.  This makes the work 
of Thomas K. Landauer very interesting for he has entirely avoided 
this hardware guessing game by measuring the actual functional 
capacity of human memory directly ('How Much Do People 
Remember?  Some Estimates of the Quantity of Learned 
Information in Long-term Memory' in Cognitive Science 10, 477-
493, 1986).

Landauer works at Bell Communications Research -- closely 
affiliated with Bell Labs where the modern study of information 
theory was begun by C. E. Shannon to analyze the information 
carrying capacity of telephone lines (a subject of great interest to 
a telephone company).   Landauer naturally used these tools by 
viewing human memory as a novel 'telephone line' that carries 
information from the past to the future.  The capacity of this  <0B>3/<00>3
 
<EFBFBD>phone line' can be determined by measuring the information 
that goes in and the information that comes out -- the great power 
of modern information theory can be applied.

Landauer reviewed and quantitatively analyzed experiments by 
himself and others in which people were asked to read text; look at 
pictures; hear words, short passages of music, sentences and 
nonsense syllables.  After delays ranging from minutes to days the 
subjects were then tested to determine how much they had 
retained.  The tests were quite sensitive (they did not merely ask 
'What do you remember?') often using true/false or multiple choice 
questions, in which even a vague memory of the material would 
allow selection of the correct choice.  Often, the differential 
abilities of a group that had been exposed to the material and 
another group that had not been exposed to the material were used.  
The difference in the scores between the two groups was used to 
estimate the amount actually remembered (to control for the 
number of correct answers an intelligent human could guess 
without ever having seen the material).  Because experiments by 
many different experimenters were summarized and analyzed, the 
results of the analysis are fairly robust; they are insensitive to 
fine details or specific conditions of one or another experiment.  
Finally, the amount remembered was divided by the time alloted to 
memorization to determine the number of bits remembered per 
second.

The remarkable result of this work was that human beings 
remembered very nearly two bits per second under ALL the 
experimental conditions.  Visual, verbal, musical, or whatever --  
two bits per second.  Continued over a lifetime, this rate of 
memorization would produce somewhat over 10**9 bits, or a few 
hundred megabytes.

While this estimate is probably only accurate to within an order of 
magnitude, Landauer says 'We need answers at this level of 
accuracy to think about such questions as:  What sort of storage 
and retrieval capacities will computers need to mimic human 
performance?  What sort of physical unit should we expect to 
constitute the elements of information storage in the brain: 
molecular parts, synaptic junctions, whole cells, or cell-circuits?  
What kinds of coding and storage methods are reasonable to 
postulate for the neural support of human capabilities?  In 
modeling or mimicking human intelligence, what size of memory 
and what efficiencies of use should we imagine we are copying?  
How much would a robot need to know to match a person?'

What is interesting about Landauer's estimate is its small size.  
Perhaps more interesting is the trend -- from Von Neumann's early 
and very high estimate, to the high estimates based on rough 
synapse counts, to a better supported and more modest estimate 
based on information theoretic considerations.  While Landauer 
doesn't measure everything (he did not measure, for example, the 
bit rate in learning to ride a bicycle nor does his estimate even 
consider the size of 'working memory') his estimate of memory 
capacity suggests that the capabilities of the human brain are  <0B>3f<00>3
 
<EFBFBD>approachable than we had thought.  While this might come as 
a blow to our egos, it suggests that we could build a device with 
the skills and abilities of a human being with little more hardware 
than we now have -- if only we knew the correct way to organize 
that hardware.



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