As we saw in the previous chapter, training simple Recurrent Neural Networks (RNNs) presents significant hurdles, most notably the vanishing and exploding gradient problems. These issues stem from the repeated application of the same weight matrix and activation function across time steps during backpropagation. Consequently, simple RNNs struggle to capture relationships between elements that are far apart in a sequence, limiting their effectiveness on tasks requiring long-term context.

Long Short-Term Memory (LSTM) networks were specifically developed to address these limitations. Their primary advantage lies in their sophisticated internal architecture, centered around the cell state and gating mechanisms, which allows them to regulate information flow much more effectively than simple RNNs.

Here’s why LSTMs generally outperform simple RNNs, especially when dealing with long sequences:

Combating Vanishing and Exploding Gradients: The core innovation of the LSTM is the cell state ( $C_t$ ), often visualized as a conveyor belt running through the entire chain of LSTM cells. Information can be added to or removed from the cell state via carefully regulated gates. Crucially, the cell state update involves element-wise addition and multiplication operations controlled by the forget gate ( $f_t$ ) and input gate ( $i_t$ ).

Recall the cell state update:
$C_t = f_t \odot C_{t-1} + i_t \odot \tilde{C}_t$
The additive interaction ( $+$ ) between the selectively forgotten past state ( $f_t \odot C_{t-1}$ ) and the new candidate information ( $i_t \odot \tilde{C}_t$ ) means that gradients flowing back through the cell state are less likely to vanish or explode compared to the repeated matrix multiplications found in simple RNNs. The gates, using sigmoid activations ( $\sigma$ ) outputting values between 0 and 1, act like controllers, deciding how much of the old state to forget and how much of the new information to add. This structure provides a more stable path for gradient propagation over many time steps.
Learning Long-Range Dependencies: Because the cell state allows information to flow largely unchanged unless explicitly modified by the gates, LSTMs can maintain context over extended durations. The forget gate learns to identify and discard irrelevant information from the past, while the input gate learns to recognize and store important new information. This selective memory capability is fundamental for tasks where understanding context from many steps prior is necessary. For example, in processing the sentence "I grew up in France... I speak fluent French," an LSTM can potentially carry the information about "France" across several intermediate words to correctly understand the context for "French." A simple RNN might lose this context due to diminishing gradients.
Explicit Control Over Memory: Unlike simple RNNs where the hidden state tries to implicitly capture all relevant past information through a single transformation, LSTMs explicitly decouple the memory (cell state $C_t$ ) from the output computation (hidden state $h_t$ ). The output gate ( $o_t$ ) determines which parts of the cell state are relevant for the current prediction or the hidden state passed to the next time step. This provides finer control over what information is used and when.

A comparison illustrating the difference in information flow between a simple RNN cell and an LSTM cell. The LSTM introduces a separate cell state path with gated, additive updates, helping preserve information over time.

In summary, the architectural enhancements of LSTMs, primarily the cell state and the gating mechanisms (forget, input, output gates), provide significant advantages over simple RNNs:

They mitigate the vanishing and exploding gradient problems, enabling more stable training.
They possess the ability to selectively remember relevant information and forget irrelevant details over long sequences.
They offer more precise control over the memory content and the information passed to the output.

These characteristics make LSTMs a powerful tool for a wide range of sequence modeling tasks, from natural language processing to time series analysis, where capturing long-term patterns is often essential for good performance.