1.3. HOW TO TELL A HUMAN FROM A COMPUTER
Are Computers like Human Brains?
The impressive contributions of computers during World War II made them earn the eponym of Giant Brains. This was the start of a long history of attributing human-like qualities to computers. Some people were struck by a superficial similarity between the switching circuits that made up computers and the cells of the human (and animal) nervous system.
The nervous system (including the brain) consists of cells, neurons, that have an elongated part (axon) and connect to other neurons through structures called synapses. Under certain conditions a neuron becomes "excited" transmitting an electric wave called action potential. When displayed on an oscilloscope the wave has the form of a spike that has a fixed size for a given preparation. The fact that a neuron can have only one of two states (quiet or transmitting a spike of fixed size) seemed to make neurons similar to the switching circuits of a computer that can be ON or OFF.
There are two things wrong with such an analogy. The first is that while the spike of the action potential has a fixed amplitude a neuron may fire several of them and the time between spikes seems to be an important carrier of information. Therefore neurons are not binary devices.
The most serious fallacy is ignoring the fact that the important thing in animal brains is how such elements are connected. There are billions of them in the human brain and their connectivity is the result of millions of years of evolution. In order to build a computer able to simulate the human brain it is not enough to start with basic blocks that may be similar in the two systems. We have to figure out how they are interconnected. To put it in another way, we have to figure out how to write programs that perform some of the operations that human brains do.
The fact that we have been successful in programming machines to solve mathematical problems tells us nothing about the prospect of replicating human intelligence. Mathematics is a human invention that came very late in our evolutionary history so our brains have not adapted to mathematical tasks. It is quite a different story with tasks such as recognizing faces and facial expressions.
The pioneers of computing were well aware of the issue. John von Neumann (whom we mentioned in Section 1.1) wrote a short monograph, The Computer and the Brain. He wrote it in the hospital while fighting the cancer that eventually took his life. In the book "he observed that the basic computing hardware of the brain indicated a different methodology than the one used in developing the computer". FIND REFERENCE
The Turing Test and CAPTCHA
Turing was also interested in the comparison of mechanical to human thinking and he proposed a test to determine whether a machine exhibits human level intelligence. A person sits on a teletype and types questions that are sent to someone in another room who then replies. The questioner tries to determine whether the responder is a human or a computer. If it is a computer and the questioner thinks the responder is a human, then the computer passes the Turing test. No machine has ever passed the Turing test but the concept found a modern practical application.
Many web services want to make sure that user accounts are set up only for people and not for computers. (Computer programs that visit web sites are called web-bots, a contraction of web robot.) So how can they tell apart a web-bot from a human? Simply, by asking the prospective user to pass a Turing test! If the user passes the test, the web service assumes that the user is human, otherwise the service rejects the user as a web-bot. Figure 1.3.1 shows the screen that a user encountered when he/she tries to e-mail a link to a news story of BBC (British Broadcasting Corporation). The company's news web site allows users to send links to articles to their friends but the company does not want computers to do that. So BBC displays some text (the numerals in red) and asks you to read it and type what it says.
That particular text is displayed in a way that it highly unlikely that a machine could read it. (Machines can read text printed in one of the standard fonts.) That test is call CAPTCHA. (See Figure 1.3.2 for an explanation of the acronym.) For people who visually impaired the BBC site offers an alternative CAPTCHA using sound. You activate that by clicking on the button marked "Listen".
 |
| |
Figure 1.3.1: A screen containing
CAPTCHA |
Figure 1.3.2 |
There are other types of CAPTCHA besides those using distorted text and we will devote Chapter X to them.