ory synapse to be weighted by the negative weight; a two-input exclusive OR circuit for providing an XOR signal at an output terminal of said exclusive OR circuit, said XOR signal being a logical 1 when one of said two inputs is a logical 1 and the other one of said two inputs is a logical 0, and said XOR signal being a logical 0 when both of said two inputs are logical 1s or both are logical 0s,