How can I get the MSE over the training period when using a self-organizing map?

I'd like to know whether the network is still improving with additional training. I've seen in the documentation that tr should have field "perf" which contains performance at each epoch throughout the training, but my tr doesn't have that field, I'm guessing since SOM is an unsupervised method.

net = selforgmap([4,4],100,4,'gridtop','linkdist');

net_init = net;

net.trainParam.epochs = 1e4;

[net, tr] = train(net,Data);