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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 ?3/ ?3
?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 ?3f ?3
?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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