What are long short-term memory (LSTM) networks?

Long short-term memory (LSTM) networks are a specialized type of recurrent neural network (RNN) designed to handle sequential data while mitigating the vanishing gradient problem, a common issue in standard RNNs. Unlike traditional RNNs, which struggle to retain information over long sequences due to diminishing error signals during backpropagation, LSTMs use a gated architecture to control the flow of information. This makes them particularly effective for tasks requiring memory of past inputs, such as time series prediction, natural language processing, and speech recognition. LSTMs achieve this by maintaining a cell state—a persistent memory pathway—and regulating updates through mechanisms called gates.

The core innovation of LSTMs lies in their three gates: the input gate, forget gate, and output gate. Each gate uses a sigmoid function to produce values between 0 and 1, determining how much information is retained or discarded. The forget gate decides which parts of the cell state to erase, the input gate selects new information to add, and the output gate controls what is passed to the next time step. For example, in a text prediction task, the forget gate might discard outdated context (e.g., a topic shift in a sentence), while the input gate incorporates new words. The cell state acts as a conveyor belt, allowing gradients to flow unchanged over many steps, which preserves long-term dependencies. This structure enables LSTMs to learn patterns spanning hundreds of time steps, a feat impractical for vanilla RNNs.

LSTMs are widely applied in scenarios where context and timing are critical. In machine translation, they process input sentences and generate outputs by remembering key words from earlier in the sequence. For instance, translating “The cat, which ate the mouse, slept” requires retaining “cat” across the relative clause to correctly associate it with “slept.” Similarly, in speech recognition, LSTMs model the temporal dependencies between audio frames to transcribe spoken words accurately. Developers often implement LSTMs using frameworks like TensorFlow or PyTorch, where prebuilt layers simplify integration. While newer architectures like Transformers have gained traction, LSTMs remain relevant for tasks with moderate sequence lengths or limited training data, offering a balance of performance and computational efficiency. Their modular design also allows customization, such as stacking multiple LSTM layers or combining them with convolutional networks for hybrid models.