Pretraining is an initial, typically unsupervised stage that learns useful representations before supervised fine-tuning for a downstream task. In deep belief networks, each layer first learns from the data distribution, preparing weights that can support classification, regression, image recognition, or speech recognition. section 8.13 section 8.9.1
A layer-wise pretraining workflow proceeds as follows:
- Train the first restricted Boltzmann machine with unlabeled data to capture low-level features. section 8.9.1
- Use the learned representation as input to the next restricted Boltzmann machine, which learns increasingly abstract features. section 8.9.1
- Train each layer separately with contrastive divergence, using Gibbs sampling to approximate the log-likelihood gradient. section 8.9.1
- Fine-tune the complete network with backpropagation and labeled data to reduce prediction error. section 8.9.1
Image or speech recognition
A pretrained deep belief network can learn basic patterns in early layers and more complex features in later layers before supervised fine-tuning for recognition. section 8.13 section 8.9.1













