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