You can train single layer neural nets. Not very useful, but they do exist. There are certain ANN architectures that relied on, essentially, classical conditio

You can train single layer neural nets. Not very useful, but they do exist. There are certain ANN architectures that relied on, essentially, classical conditioning based on Hobbian learning rules and variants thereof. Kohonen self-organizing maps are an example of that. Not that such historical systems are popular today, though.