ConnectionismConnectionism is a movement in cognitive science which hopes to explain human intellectual abilities using artificial neural networks (also known as neural networks or neural nets). Neural networks are simplified models of the brain composed of large numbers of units (the analogs of neurons) together with weights that measure the strength of connections between the units. These weights model the effects of the synapses that link one neuron to another. Experiments on models of this kind have demonstrated an ability to learn such skills as face recognition, reading, and the detection of simple grammatical structure. Philosophers have become interested in connectionism because it promises to provide an alternative to the classical theory of the mind: the widely held view that the mind is something akin to a digital computer processing a symbolic language. Exactly how and to what extent the connectionist paradigm constitutes a challenge to classicism has been a matter of hot debate in recent years. 1. A Description of Neural Networks 2. Neural Network Learning and Backpropagation 3. Samples of What Neural Networks Can Do 4. Strengths and Weaknesses of Neural Network Models 5. Connectionist Representation 6. The Shape of the Controversy between Connectionists and Classicists 7. The Systematicity Debate 8. Connectionism and Semantic Similarity 9. Connectionism and the Elimination of Folk Psychology Bibliography Other Internet Resources Related Entries 1. A Description of Neural NetworksA neural network consists of large number of units joined together in a pattern of connections. Units in a net are usually segregated into three classes: input units, which receive information to be processed, output units where the results of the processing are found, and units in between called hidden units. If a neural net were to model the whole human nervous system, the input units would be analogous to the sensory neurons, the output units to the motor neurons, and the hidden units to all other neurons. Here is a simple illustration of a simple neural net: Each input unit has an activation value that represents some feature external to the net. An input unit sends its activation value to each of the hidden units to which it is connected. Each of these hidden units calculates its own activation value depending on the activation values it receives from the input units. This signal is then passed on to output units or to another layer of hidden units. Those hidden units compute their activation values in the same way, and send them along to their neighbors. Eventually the signal at the input units propagates all the way through the net to determine the activation values at all the output units. The pattern of activation set up by a net is determined by the weights, or strength of connections between the units. Weights may be both positive or negative. A negative weight represents the inhibition of the receiving unit by the activity of a sending unit. The activation value for each receiving unit is calculated according a simple activation function. Activation functions vary in detail, but they all conform to the same basic plan. The function sums together the contributions of all sending units, where the contribution of a unit is defined as the weight of the connection between the sending and receiving units times the sending units activation value. This sum is usually modified further, for example, by adjusting the activation sum to a value between 0 and 1 and/or by setting the activation to zero unless a threshold level for the sum is reached. Connectionists presume that cognitive functioning can be explained by collections of units that operate in this way. Since it is assumed that all the units calculate pretty much the same simple activation function, human intellectual accomplishments must depend primarily on the settings of the weights between the units. The kind of net illustrated above is called a feed forward net. Activation flows directly from inputs to hidden units and then on to the output units. More realistic models of the brain would include many layers of hidden units, and recurrent connections that send signals back from higher to lower levels. Such recurrence is necessary in order to explain such cognitive features as short term memory. In a feed forward net, repeated presentations of the same input produce the same output every time, but even the simplest organisms habituate to (or learn to ignore) repeated presentation of the same stimulus. Connectionists tend to avoid recurrent connections because little is understood about the general problem of training recurrent nets. However Elman (1991) and others have made some progress with simple recurrent nets, where the recurrence is tightly constrained. 2. Neural Network Learning and BackpropagationFinding the right set of weights to accomplish a given task is the central goal in connectionist research. Luckily, learn