Fuzzy Cognitive Maps (FCM) may be defined as Recurrent Neural Networks that allow causal reasoning. According to the transformation function used for updating the activation value of concepts they can be characterized as discrete or continuous. It is remarkable that FCM having discrete neurons never exhibit chaotic states, but this premise cannot be guaranteed for FCM having continuous concepts. On the other hand, complex Sigmoid FCM resulting from experts or learning algorithms often show chaotic or cyclic patterns, therefore leading to confusing interpretation of the investigated system. The first contribution of this paper is focused on explaining why most studies on FCM stability are not applicable to FCM used on classification or decision-making tasks. Next we describe a non-direct learning methodology based on Swarm Intelligence for improving the system stability once the causal weight estimation is done. The objective here is to find a specific threshold function for each map neuron simulating an external stimulus, instead of using the same transformation function for all concepts. At the end, we can compute more stable maps, so better consistency in hidden patterns is achieved.
G. Nápoles et al. / How to improve the convergence on sigmoid FCM?decision-making tasks, risk analysis, prediction, text categorization, pattern recognition, management, and classification. However, estimating parameters that characterize the whole system (e.g. the causal weight matrix) may be tedious for humans, leading to inefficient models. In order to increase the reliability of FCM-based models several learning algorithms for tuning such parameters have been introduced [10], although most of them are focused on computing causal relations between map concepts.On the other hand, existing approaches suppose that FCM are closed systems and they do not consider external influences, while other factors such as the FCM stability are frequently ignored. As far as known, there is no existence of any learning method for enhancing the system stability once the system causality is established. For example, let us suppose a FCM resulting from experts where causal connections may be partially modified (e.g. we know the direction of causalities and an approximation of their values that should be preserved). Can we expect lineal stability in the final map? If not, how to improve the stability of the system without affecting causal connections estimated by experts?In the literature a few researches concerning FCM convergence have been proposed. For instance, Kosko [4] developed an analytic method based on Liapounov functions for reaching stable solutions on Feedback Standard Additive Models (SAM -which share several of the FCM characteristics). Unfortunately, Kosko concluded that such conditions cannot be extended to FCM due to the large number of feedback links involved in FCM-based models. More recently, other analytical methods were introduced (see Section 3), but we believe that such approaches are mostly useful for stabilizing FCM used in...