Probability density function for each observational class in the J, H, and K_s band. These maps highlight the regions on the SOM in which the probability of finding a specific class of YSO is highest. The five panels from left to right show the probabilities per neuron for each class, starting with the youngest on the left and ending with the oldest, most evolved class to the right. The contours in each panel enclose the area within which 25%, 50%, and 75% of all YSOs of a given class are located within the map.