Does Optimal Source Task Performance Imply Optimal Pre-training for a Target Task?

Fine-tuning of pre-trained deep nets is commonly used to improve accuracies and training times for neural nets. It is generally assumed that pre-training a net for optimal source task performance best prepares it for fine-tuning to learn an arbitrary target task. This is generally not true. Stopping source task training, prior to optimal performance, can create a pre-trained net better suited for fine-tuning to learn a new task. We perform several experiments demonstrating this effect, as well as the influence of the amount of training and of learning rate. Additionally, our results indicate that this reflects a general loss of learning ability that even extends to relearning the source task.