1. A signal processing method for processing a plurality of input signals within a neuron unit, and for outputting an output signal which is indicative of a result of the processing, the signal processing method comprising: (a) carrying out a forward process within the neuron unit, the forward process including the steps of: a1) carrying out an operation on a plurality of weight functions, wherein each of the input signals corresponds to the weight functions; and a2) outputting a result of said operation as the output signal; and (b) carrying out a self-learning process within the neuron unit, the self-learning process including the substep of: b1) controlling the weight functions based on an error signal representing an error between (1) the output signal obtained by the forward process carried out in said step (a) and (2) a teaching signal; wherein: the error signal on which control of the weight functions is based in said step (b) substep b1) includes: e1) a first error signal component representing pulse densities; and e2) a second error signal component also representing pulse densities; the first and second error signal components take on values other than a negative value; the error between said output signal obtained by the forward process carried out in said step (a) and the teaching signal is equal to a difference between the first error signal component and the second error signal component; and the first error signal component and the second error signal component can simultaneously take on values other than the teaching signal and the output signal.