A deep belief network (DBN) is a generative neural-network model composed of multiple layers of stochastic binary latent variables. Each layer is a restricted Boltzmann machine (RBM): its visible units represent observed data, while its hidden units learn features. section 8.9 section 8.9.1
Train a DBN through two connected phases:
- Pretrain the RBMs layer by layer without labeled data. The first layer captures lower-level features, while later layers learn increasingly abstract representations. section 8.9.1
- Use contrastive divergence to approximate each layer’s log-likelihood gradient through Gibbs sampling, adjusting weights to model the input distribution. section 8.9.1
- Fine-tune the complete network with labeled data and backpropagation, minimizing prediction error for a supervised task such as classification. section 8.9.1
This staged approach can learn complex representations for image or speech recognition and organize unlabeled data. Computational expense, overfitting, vanishing gradients, and intricate fine-tuning can limit performance. section 8.13
Recognition pipeline
Feed unlabeled inputs into successive RBMs to learn increasingly abstract features, then fine-tune the network with labeled examples for classification, image recognition, or speech recognition. section 8.9.1 section 8.13
