Clustered Regions by Each Neuron in Self Organizing Map (SOM)
When clustering an input, one should calculate the Euclidean distance to all neurons, then the input is assign to the cluster of the nearest neuron. 

In other words, for an input $v$ and a set of neurons $N$ (in your case $|N| = 16$ ) the cluster is:

$\text{Cluster}(v)=\arg \min_{i\in A}(||w_i-v||_2)$