1. A system for selecting a signal processing program to be executed by a signal processor in a hearing aid and comprising a single layer of neurons each having one or more neighbours and being arranged in a memory device, and each neuron comprising a neuron vector referring to a specific acoustic situation and a neuron label referring to signal processing program associated with said specific acoustic situation, and comprising a sensor adapted to detect an external acoustic situation and to define a sensor vector associated with said external acoustic situation, wherein a processor is adapted to calculate a vector difference between said sensor vector and each of said neuron vectors, to identify that neuron providing the smallest vector difference, and to select a signal processing program referred to by a neuron label of that neuron.