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Monday, March 22, 2010

Photos and long term memory

Why do I take photos of my kids or while traveling? The answer is so obvious it's a cliche: for the memories.

Remarkably, long term human memory capacity is probably only on the order of a gigabyte. Ask yourself what specific things you remember from the year 2000. How about your first day of class in college?

While the brain processes huge amounts of data (dominated by visual input), the amount of information retained is quite small -- no more than a few bits per second. When I ask you what your date's prom dress looked like, you are probably reconstructing it from conceptual primitives like "blue", "prom dress", or "velvet" :-) Do you think you could picture all of the girls you ever dated if you hadn't looked at their photographs over the years?

Looking at old photographs allows me to recapture and enhance otherwise elusive memories. Also, there's some selection bias in what one chooses to photograph: the memories triggered tend to be happy ones. Viva Espana! :-)

Human Memory: ... 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 (See "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 "telephone line" can be determined by measuring the information that goes in and the information that comes out, and then applying the great power of modern information theory.

Landauer reviewed and quantitatively analyzed experiments by himself and others in which people were asked to read text, look at pictures, and hear words, short passages of music, sentences, and nonsense syllables. After delays ranging from minutes to days the subjects were tested to determine how much they had retained. The tests were quite sensitive--they did not merely ask "What do you remember?" but often used 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 allotted 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.

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