Alan Turing was conceived in Chhatrapur, Orissa, India.[3] His father, Julius Mathison Turing, was a member of the Indian Civil Service. Julius and wife Sara (née Stoney; 1881 – 1976, daughter of Edward Waller Stoney, chief engineer of the Madras Railways) wanted Alan to be brought up in England, so they returned to Maida Vale,[4] London, where Alan Turing was born on 23 June 1912, as recorded by a blue plaque on the outside of the building, now the Colonnade Hotel.[5][6] He had an elder brother, John. His father's civil service commission was still active, and during Turing's childhood years his parents travelled between Guildford, England and India, leaving their two sons to stay with friends in Hastings in England.[7] Very early in life, Turing showed signs of the genius he was to display more prominently later.[8]

His parents enrolled him at St Michael's, a day school, at the age of six. The headmistress recognised his talent early on, as did many of his subsequent educators. In 1926, at the age of 14, he went on to Sherborne School, a famous and expensive public school in Dorset. His first day of term coincided with the General Strike in Britain, but so determined was he to attend his first day that he rode his bicycle unaccompanied more than 60 miles (97 km) from Southampton to school, stopping overnight at an inn.[9]

The computer room at King's is now named after Turing, who became a student there in 1931 and a Fellow in 1935

Turing's natural inclination toward mathematics and science did not earn him respect with some of the teachers at Sherborne, whose definition of education placed more emphasis on the classics. His headmaster wrote to his parents: "I hope he will not fall between two schools. If he is to stay at public school, he must aim at becoming educated. If he is to be solely a Scientific Specialist, he is wasting his time at a public school".[10]

Despite this, Turing continued to show remarkable ability in the studies he loved, solving advanced problems in 1927 without having even studied elementary calculus. In 1928, aged 16, Turing encountered Albert Einstein's work; not only did he grasp it, but he extrapolated Einstein's questioning of Newton's laws of motion from a text in which this was never made explicit.[11]

Turing's hopes and ambitions at school were raised by the close friendship he developed with a slightly older fellow student, Christopher Morcom, who was Turing's first love interest. Morcom died suddenly only a few weeks into their last term at Sherborne, from complications of bovine tuberculosis, contracted after drinking infected cow's milk as a boy.[12] Turing's religious faith was shattered and he became an atheist. He adopted the conviction that all phenomena, including the workings of the human brain, must be materialistic.

In 1947-48 Turing theorized that a network of initially randomly connected artificial neurons--a Turing Net--could be "trained" (his word) to perform a given task by means of a process that renders certain neural pathways effective and others ineffective. Turing foresaw the procedure--now in common use by connectionists--of simulating the neurons and their interconnections within an ordinary digital computer (just as engineers create virtual models of aircraft wings and skyscrapers).

However, Turing's own research on neural networks was carried out shortly before the first stored-program electronic computers became available. It was not until 1954 (the year of Turing's death) that Belmont Farley and Wesley Clark, working at MIT, succeeded in running the first computer simulations of small neural networks. Farley and Clark were able to train networks containing at most 128 neurons to recognise simple patterns (using essentially the training procedure described above). In addition, they discovered that the random destruction of up to 10% of the neurons in a trained network does not affect the network's performance at its task--a feature that is reminiscent of the brain's ability to tolerate limited damage inflicted by surgery, an accident, or disease.

Turing gave quite possibly the earliest public lecture (London, 1947) to menion computer intelligence, saying, “What we want is a machine that can learn from experience,” and that the “possibility of letting the machine alter its own instructions provides the mechanism for this.” In 1948 he introduced many of the central concepts of AI in a report entitled “Intelligent Machinery.” However, Turing did not publish this paper, and many of his ideas were later reinvented by others. For instance, one of Turing’s original ideas was to train a network of artificial neurons to perform specific tasks, an approach described in the section Connectionism.

Modern Connectionism

Connectionism is the emerging science of computing with networks of artificial neurons.

A natural neural network. The Golgi method of staining brain tissue renders the neurons and their interconnecting fibres visible in silhouette.

At the present stage of development of connectionism, reserachers simulate neurons and their interconnections using an ordinary digital computer (just as an engineer may use a computer to simulate an aircraft wing or a weather analyst to simulate a storm system). A training algorithm that runs on the computer adjusts the connections between the neurons, honing the network into a special-purpose machine dedicated to performing some particular task.

In a vivid demonstration of connectionism's potential, James McClelland and David Rumelhart have trained a network of 920 neurons to form the past tenses of English verbs.

Each of the 460 neurons in the input layer is connected to each of the 460 neurons in the output layer

Root forms of verbs--such as come, look and sleep--were presented to a layer of input neurons (in a suitably encoded form). The supervisory system running on the computer observed the difference between the actual response of the layer of output neurons and the desired response--came, say--and mechanically adjusted the connections throughout the network in a way that gave the network a slight push in the direction of the correct response. About 400 different verbs were presented one by one to the network and the connections were adjusted after each presentation. If this whole procedure is repeated often enough using the same verbs--and it took almost 200 repetitions--the connections come to accommodate the differing needs of all the verbs in the training set. Once this stage is reached, the network will correctly form the past tense of unfamiliar verbs as well as of the original verbs. For example, when presented for the first time with guard it responded guarded, with weep wept, with cling clung, and with drip dripped (notice the double 'p'). (Sometimes, though, the peculiarities of English were too much for the network and it formed squawked from squat, shipped from shape, and membled from mail.)

Modern connectionists look back on Frank Rosenblatt as the founder of their approach. Rosenblatt published the first of many papers on connectionism in 1957. Few realize that Alan Turing wrote a blueprint for much of the connectionist project as early as 1948, in a little-known paper entitled 'Intelligent Machinery'. Written while Turing was working for the National Physical Laboratory in London, the paper did not meet with his employers' approval. Sir Charles Darwin, the rather headmasterly director of the Laboratory, called it a 'schoolboy essay' and wrote to Turing complaining about its 'smudgy' appearance. In reality this far-sighted paper was the first manifesto of Artificial Intelligence, but sadly Turing never published it. In it he not only set out the fundamentals of connectionism but also brilliantly introduced many of the concepts that were later to become central in AI, in some cases after re-invention by others.

Turing's B-Type Neural Networks

Turing introduced a type of neural network that he called a 'B-type unorganised machine', consisting of artificial neurons, depicted below as circles, and connection-modifiers, depicted as boxes. A B-type machine may contain any number of neurons connected together in any pattern, but subject always to the restriction that each neuron-to-neuron connection passes through a connection-modifier. (What Turing called an 'A-type unorganised machine' is simply a B-type without the connection-modifiers. Without the ingenious connection-modifier, A-type machines cannot be trained.)

Two neurons from a B-type network. The red and green fibres on
the connection-modifier enable training by an external agent.

Training a B-Type Network

A connection-modifier has two training fibres (coloured green and red in the diagram). Applying a pulse to the green training fibre sets the box to pass its input--either 0 or 1--straight out again. This is pass mode. In pass mode, the box's output is identical to its input. The effect of a pulse on the red fibre is to place the modifier in interrupt mode. In this mode, the output of the box is always 1, no matter what its input. While it is in interrupt mode, the modifier destroys all information attempting to pass along the connection to which it is attached. Once set, a connection-modifier will maintain its function unless it receives a pulse on the other training fibre. The presence of these modifiers enables a B-type unorganised machine to be trained, by means of what Turing called 'appropriate interference, mimicking education'. Turing theorized that 'the cortex of an infant is an unorganised machine, which can be organised by suitable interfering training'.

How Turing's Model Neurons Work

Each neuron has two input fibres, and the output of a neuron is a simple logical function of its two inputs. Every neuron in the network performs the same logical operation, called 'nand'.

Nand is defined by the following table:

The output of a connection-modifier in interrupt mode is always 1. So if one of the neuron's input connections passes via a modifier in interrupt mode, the neuron's output is simply the opposite (or 'boolean negation') of whatever comes in along the second input fibre. For example, the first two lines of the table show what happens if INPUT-1 is connected to a modifier in interrupt mode. In this case the output from the neuron is the opposite of INPUT-2.

Turing chose nand as the basic operation of his model neurons because every other logical (or boolean) operation can be carried out by groups of nand-neurons. Turing showed that even the connection-modifier itself can be built out of nand-neurons. So each B-type network consists of nothing more than nand-neurons and their connecting fibres. This is about the simplest possible model of the cortex.

Turing wished to investigate more complex models of the cortex as well. He longed to do what modern connectionists are able to do: simulate a neural network and its training regimen using an ordinary digital computer. He would, he said, 'allow the whole system to run for an appreciable period, and then break in as a kind of "inspector of schools" and see what progress had been made'. But his own research on neural networks was carried out shortly before the first general-purpose electronic computers were up and running and he used only paper and pencil. Thereafter he turned his attention to related research in what is now called Artificial Life. It was not until 1954, the year of Turing's death, that B.G. Farley and W.A. Clark succeeded in running the first computer simulation of a small neural network, at MIT.

Two Examples of B-Type Networks

When both the connection-modifiers in this tiny B-type network are in pass mode, the network behaves as shown in the table on the left. That is to say, the network computes what logicians call the inclusive disjunction of the inputs. However, if the lower connection-modifier is switched to interrupt mode, the network behaves as shown in the table on the right. In this case, the output takes the same value as A, no matter what the value of B. If both modifiers are switched to interrupt mode, the network's output is always 0.

The second example of a small B-type network is Turing's own and is from page 11 of his 1948 paper 'Intelligent Machinery'. Turing describes the example as 'chosen at random'. Can you work out how this network behaves? (You may like to refer to the discussion on pages 9-11 of 'On Alan Turing's Anticipation of Connectionism' by Jack Copeland and Diane Proudfoot (from Synthese, vol. 108 (1996) pp. 361-377).)

An example of a larger B-type network is given later in this article.

Making and Breaking Connections

In 1958, Rosenblatt defined connectionism as the theory that 'stored information takes the form of new connections, or transmission channels in the nervous system, or the creation of conditions which are functionally equivalent to new connections'. The destruction of existing connections can be functionally equivalent to the creation of new connections. A network for performing a specific task may be produced by taking a network with more connections than are needed and selectively destroying some of them. Both processes, destruction and creation, are employed in the training of a B-type. In its initial state, the network that is to be trained has a large number of random inter-neural connections, and the modifiers on these connections are also set randomly, some in pass mode and some in interrupt mode. Unwanted connections are destroyed by switching their attached modifiers to interrupt mode. The output of the neuron immediately upstream of the modifier then no longer finds its way along the connection to the neuron on the downstream end. Conversely, changing the setting of the modifier on an initially interrupted connection to pass mode is in effect to create a new connection. This selective culling and enlivening of connections hones the initially random network into one organised for a given task.

Turing discovered that a large enough B-type neural network can be configured (via its connection-modifiers) in such a way that it itself becomes a general-purpose computer.

B-Types and the Brain

A large number of the output fibres of a neuron in the brain may be connected to the neuron's own input fibres, either directly or via some intervening chain of neurons. Neuroscientists have long stressed the importance and ubiquity of feedback within the brain. For example, the brain uses feedback to help us focus our attention on certain perceptions to the exclusion of others. Stefan Treue and John Maunsell have recently shown that when a monkey has its attention directed to one of several independently moving dots on a computer screen, feedback returns from neurons in the higher cortex to neurons in the lower cortical areas where motion is identified. This feedback serves to inhibit the activity of neurons that are firing in response to the motions of non-attended dots. However, despite its importance in the brain, feedback is seldom employed in modern connectionist networks. In contrast, the neurons in a B-type network interconnect very freely and, like a brain, a large network will typically be awash with feedback.

Below is a network of the sort typically studied by modern connectionists. Notice the regular, layered structure and the absence of any feedback. Information moves unidirectionally through the net from layer to layer.

A conventional neural network

In contrast, the neurons in a B-type neural network interconnect freely and a large B-type may be awash with feedback:

Part of a large initially random B-type network

Two Examples of B-Type Networks

When both the connection-modifiers in this tiny B-type network are in pass mode, the network behaves as shown in the table on the left. That is to say, the network computes what logicians call the inclusive disjunction of the inputs. However, if the lower connection-modifier is switched to interrupt mode, the network behaves as shown in the table on the right. In this case, the output takes the same value as A, no matter what the value of B. If both modifiers are switched to interrupt mode, the network's output is always 0.

The second example of a small B-type network is Turing's own and is from page 11 of his 1948 paper 'Intelligent Machinery'. Turing describes the example as 'chosen at random'. Can you work out how this network behaves? (You may like to refer to the discussion on pages 9-11 of 'On Alan Turing's Anticipation of Connectionism' by Jack Copeland and Diane Proudfoot (from Synthese, vol. 108 (1996) pp. 361-377).)

An example of a larger B-type network is given later in this article.