Patent Publication Number: US-5293459-A

Title: Neural integrated circuit comprising learning means

Description:
This is a continuation of application Ser. No. 07/452,858, filed Dec. 18, 1989, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a neural integrated circuit, comprising: 
     a memory M for synaptic coefficients Cij 
     a memory for neuron states 
     resolving means for determining states of output neurons I on the basis of states of input neurons j, 
     learning means for determining new synaptic coefficients Cij by performing local learning steps. 
     The invention also relates to a neural network system comprising a processing device for cooperation with such a neural circuit. 
     A circuit of this kind is known from the document &#34;A fully digital CMOS integrated feedback network including the learning process&#34;, M. Weinfeld, G. Dreyfus, A. Johannet, L. Personnaz, Neural Networks for computing, Snowbird, Utah, Apr. 6-9, 1988. 
     The cited document concerns an integrated circuit which executes resolving steps and learning steps on the basis of the Widrow-Hoff learning rule. In this circuit parallelism exists as regards the states of output neurons i. The circuit also has a mechanism for the circulation of data which is applied to the index i of the states of input neurons j. It follows therefrom that a particular treatment cannot be isolated to a given neuron. Thus, this circuit can operate with a limited number of learning rules only. 
     The problem faced by the invention is to realize a neural integrated circuit which is suitable for use in a neural network system comprising a processing device, is capable of operating at high speeds for simultaneously updating all synaptic coefficients of a given output neuron i, and is also capable of executing the bulk of the learning rules so that it can be used for solving the multitude of problems to be dealt with by neural network systems. 
     SUMMARY OF THE INVENTION 
     The solution to this problem consists in that for a given output neuron i the learning means simultaneously operate in parallel on each of the associated synaptic coefficients which are stored in the memory and which characterize the synapses which link the input neurons i converging towards an output neuron i; to this end, the learning means comprise: 
     means for imposing a learning function on parameters which are formed by the states Vj of neurons and by a modification parameter Si which is associated with each output neuron i in order to determine whether a modification is to be applied to each synaptic coefficient and to determine also the sign of this modification, 
     and incrementation/decrementation elements which determine in parallel the new synaptic coefficients Cij on the basis of the sign of the modifications and of a correction value Δi which is associated with the relevant output neuron i and which is common to all input neurons j which converge towards the relevant output neuron, where Δi is a correction value which relates either to the state of the input neurons i or to the state of the output neuron i or to a function which combines these two types of state and which is determined either by the circuit itself or is supplied from outside the circuit, for instance by an external processing device, and where Si represents either the state of an input neuron j or the state of the given output neuron i, or states which are functions of these two types of state and which are supplied from outside the circuit, for instance by the processing device. 
     During the &#34;resolving&#34; step, the neural circuit selects all synaptic coefficients Cii of a given output neuron i in the memory M, multiplies these coefficients by the states Vj of the input neurons j and forms the sum of all these products so that: ##EQU1## where N is the total number of input neurons influencing the output neuron i. 
     The synaptic coefficients Cij are output in parallel by the memory M and reach an incrementation/decrementation element whose output signal is applied to a calculation member which is formed, for example by multipliers and an adder tree. The output result of the circuit is applied to the processing device which performs a non-linear function on this result. This non-linear function may also be applied to the result by the calculation member itself, before it is applied to the processing device. The states of input neurons i to be treated may be supplied either by the processing device or by the environment. During the &#34;learning&#34; step, the incrementation/decrementation elements receive control signals so as to perform either an incrementation or a decrementation on the relevant synaptic coefficient or to leave it unmodified. These control signals originate from central signal operator means for performing the learning function by supplying each incrementation/decrementation element with a two-bit signal for encoding the preceding commands. 
     The learning functions take into account the states Vj of the input neurons j and a modification parameter Si which is associated with each output neuron i. The means for performing the learning function are formed by learning elements, the number of learning elements being equal to the number of incrementation/decrementation elements and hence to the number of synaptic coefficients Cij to be simultaneously treated. Each of these learning elements receives a state Vj and the modification parameter Si which may both be encoded on one or more bits. These learning elements may be formed either by logic AND-gates or exclusive-OR logic gates, or by a combination of gates performing a complex function. They may also be formed by multiple-input multiplexers which select either the logic value 1 or the logic value 0 or at least one bit of the neuron state Vj of the modification parameter Si, or their complement. 
     The incrementation/decrementation elements receive these control signals as well as a correction value Δi which is common to all elements and which is specific of the output neuron i treated at a given instant. Thus, each incrementation element independently increments the synaptic coefficient by a value Δi or decrements it by a value Δi or leaves it unmodified. This correction value Δi may be applied to the circuit by an external processing device. However, it may also be determined by the circuit itself in a correction circuit. This correction value Δi is preferably an integer in order to impart the required precision to the synaptic coefficient. 
     The incrementation/decrementation operations can be performed on several consecutive cycles in order to enable operation with neuron states encoded on several bits. Actually, each learning element can receive states Vj and a modification parameter Si encoded on several bits. By performing several consecutive cycles it is also possible to perform multiplication operations Vj.Si with an adequate choice of the values Δi supplied during each cycle. A selection member can impart to the modification parameter Si either the state of an input neuron i or the state of the given output neuron i or states which are a function of these two types of state and which are supplied for instance by an external processing device. 
     Each of the incrementation/decrementation elements may also have a storage function in order to free the synaptic coefficient memory for other tasks. 
     By way of example the invention will be described in detail hereinafter with reference to the accompanying drawing; therein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a diagram of the neural circuit in accordance with the invention, 
     FIG. 2 shows a diagram of another method of realizing learning functions. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The neural circuit which preferably comprises an integrated circuit 10 is connected to a processing device 50 in order to form a neural network system or neural net. Normally a second and further integrated neural circuit is also connected to form the neural net. The second and any further integrated neural circuits preferably contain the same elements as circuit 10. The memory 11 for synaptic coefficients Cij outputs in parallel the synaptic coefficients Cij (bus 19 1  to 19 N ) concerning a same given output neuron i. The synaptic coefficients reach incrementation/decrementation elements 12 which apply their coefficients to a calculation member 13. This member comprises, for example multipliers for performing the multiplications cij.vj and an adder tree for forming the sum of all multiplications. The result appears on the connection 14 for supply to the processing device 50 which performs a non-linear function on this result. This function may also be applied in the calculation member 13 itself. 
     The states Vj of input neurons j are stored in a neuron state memory 15 which receives these states from the processing device 50 or from the environment (connection 25) and which supplies these states to the calculation member 13. The neuron state memory 15 can receive correction states from the processing device 50 during the learning step. The control sign at generator means 16 for performing learning functions are formed by learning elements 16 1 , 16 2 , . . . 16 N  when there are N input neurons i to be treated. Each learning element receives a state Vi and the correction element Si supplied by a correction register 17. Each learning element supplies a two-bit data to each incrementation/decrementation element so that the latter elements perform either an incrementation or a decrementation or no operation at all. The modification register 17 stores the modification parameter Si which may have several origins. It may be either one of the states Vi of the input neurons i or a state Vi of the output neurons i or a function of these two types of state originating from the processing device 50. This selection is made through a selection member that connects either an output of processing device 50, or an output of calculation member 13, or an output of input neuron state memory 15 to modification register 17. 
     All incrementation/decrementation elements 12 also receive the same correction value hi which is used for modifying (or not) the old values Cii in order to determine new values Cii in accordance with the expression: 
     
         C.sub.ij (new)=C.sub.ij (old)+Δi·F(S.sub.i,V.sub.j) 
    
     This correction value Δi may originate either from the processing device 50 or from a correction circuit 18 which imposes a correction function G on the modification parameter Si. 
     The learning elements 16 1  . . . 16 N  are formed, for example either by logic AND-gates or exclusive-OR gates or a combination of gates performing a complex function. As is shown in FIG. 2, it is also possible to perform the learning function itself by a common circuit which supplies common commands to multiplexers which treat each of the states Vj of the input neurons. In this case the various learning elements 16 1  . . . 16 N  share a circuit 20. 
     For example, the learning element 16 1  of FIG. 1 is formed by multiplexers 21, 22 and 23 in FIG. 2. The multiplexer 23 reads a bit of the state V 1  in the memory 15. It successively delivers all bits of V 1  under the control of the command S. If V 1  is encoded on one bit, the multiplexer 23 is absent. The multiplexers 21 and 22 receive the state V 1  one bit after the other (connection 24). They are capable of selecting either the logic state 1 or the logic state zero or the state V 1  or the complementary state of V 1  (inverter 26). The multiplexers 21 and 22 are controlled by the circuit 20 which executes the selected learning function, for example AND-functions, exclusive-OR functions or complex functions. The circuit 20 is formed, for example, by a programmed logic array (PLA) or a RAM. The multiplexers 22 and 21 have distinct control signals and output the signals ±, respectively, for incrementing/decrementing and also the signals OP/NOP for determining whether the operation is to be performed or not. 
     The incrementation/decrementation elements may comprise means for avoiding overflow during the operations. For example, in the case of overflow the synaptic coefficient may assume either the maximum value or the minimum value permitted by the circuit. 
     The incrementation/decrementation elements may be arranged in parallel in order to operate on synaptic coefficients encoded on an accrued number of bits. To this end, each incrementation/decrementation element comprises an input for an input carry and an output for an output carry, enabling either reconfiguration of the circuit or the cascading of several circuits in order to extend the processing possibilities to an accrued number of neurons. 
     For example, a learning algorithm which can be carried out by the circuit in accordance with the invention is referred to as &#34;the minimum recovery of prototypes&#34; (on this subject, see W. Krauth and M. Mezard, J. Phys. A 20 (1987), L 745-L 752). The object is to realize an associative memory for which the network associates on its output the prototype presented to its input. This can be applied for the correction of errors. Learning is iterative and progresses independently along the output neurons i. 
     For a given output neuron i, a prototype ##EQU2## is presented (where μ is the index of the prototype and j is that of the neuron) to the input of the circuit which then operates in the resolving mode. 
     The circuit calculates: ##EQU3## 
     When all prototypes have been presented, the prototype having the index ν is chosen so that ##EQU4## is smaller than ##EQU5## for all μ≠ν. 
     The circuit then operates in the learning mode while modifying the synaptic coefficients relating to the output neuron i in accordance with the expression: ##EQU6## The modification parameter is thus given by: ##EQU7## and the increment is given by: ##EQU8## This procedure is interated until the sums ##EQU9## all exceed a criterion Z for all prototypes p and all output neurons i. 
     It is to be noted that this algorithm has an optimum effect for this type of network.