[YZF]

What are you asking here?

The brain contains/is the model. It is trained by a range of inputs and by definition it generalizes outside those inputs.

If you're asking how does the brain minimize out-of-sample error? It does that by the virtue that it's model isn't too complex for the training set, just like what you do in machine learning. If the brain had a model that was too complex it would overfit and poorly generalize just like machine learning would do with a too complex of a model...