[jcarreiro]

The "decoder" rat was not trained beforehand, unless I am badly misreading the article. The training process that occurred used the real-time signal from the "encoder" rat's brain. It simply took some time for the "decoder" to learn how to process the signal sent by the other rat.

[aidos]

I think he means trained as in, was taught that by pushing one of the levers it got food.

As you say, there was additional training to then handle the encoding / decoding.