Top 6 Most Useful Attention Mechanism In NLP Explained And When To Use Them

by | Jan 12, 2023 | Artificial Intelligence, Machine Learning, Natural Language Processing

Numerous tasks in natural language processing (NLP) depend heavily on an attention mechanism. When the data is being processed, they allow the model to focus on only certain input elements, such as specific words or phrases.

This is important for NLP tasks because the input data, often given as sentences or paragraphs, can be long and complicated, making it hard for the model to figure out the most critical information.

A model computes attention weights for each position in the input after first representing the input as a collection of query, key, and value vectors. This is how attention mechanisms operate. These attention weights indicate the importance of each position in the input for the task at hand. The output of the attention mechanism for the current position is then represented by a weighted sum that is computed using the attention weights to weigh the input.

Numerous NLP tasks, such as machine translation, language modelling, text summarisation, and question answering, have used attention mechanisms. They have been particularly effective in tasks like machine translation, where the model must comprehend the relationships between words in various languages. Additionally, they have been incorporated into transformers and their variants.

What is the attention mechanism?

The attention mechanism allows neural networks to concentrate on particular input components. It enables the model to selectively focus on the input portions that are most pertinent to the task at hand and to weigh the relative importance of various input components. Attention mechanisms have been applied in multiple natural language processing tasks, including language modelling and machine translation, as well as in other fields like computer vision.

The attention mechanism allows neural networks to concentrate on particular input components in NLP

The attention mechanism allows neural networks to concentrate on particular input components.

Attention mechanism in NLP example

The attention used in machine translation is one illustration of an attention mechanism. The attention mechanism in a machine translation model enables the model to concentrate on particular elements of the source sentence when decoding the target sentence.

For example, consider the following English sentence that needs to be translated into Spanish:

“The cat sat on the mat.”

When the model is translating the word “gato” (the Spanish word for “cat”), it needs to know which noun in the English sentence it refers to. The attention mechanism allows the model to focus on the word “cat” in the English sentence and use it as the reference to translate “gato”.

The attention mechanism works by first encoding the source sentence into a set of hidden states and then computing a set of attention weights for each word in the target sentence to show how vital each hidden state was in producing the current word. The hidden states are then weighed using the attention weights to create a weighted sum used as the input to the current word’s decoder.

In this example, the model will have high weights for the hidden states associated with the word “cat” to decode “gato,” enabling the model to concentrate on that particular section of the source sentence.

What is self-attention?

Self-attention, also known as intra-attention, is a type of attention mechanism in which the model attends to different positions of the input sequence by comparing each position with all other positions. Self-attention allows the model to weigh the importance of various elements in the input sequence and selectively focus on the parts of the input that are most relevant to the task at hand. It’s primarily used in models like a transformer and their variants. In a transformer, the self-attention mechanism is used to process the input sequence, allowing the model to learn relationships between the different elements of the input without explicitly being told which parts are related.

Attention vs self-attention

Attention and self-attention are related but distinct concepts.

When a model processes input, the term “attention” refers to letting it concentrate on particular portions of the input. It enables the model to selectively focus on the input portions that are most pertinent to the task at hand and to weigh the relative importance of various input components. Numerous tasks, including computer vision, language modelling, and machine translation, have used attention mechanisms.

On the other hand, a particular kind of attention mechanism is self-attention. By comparing each position to all the different positions, the model pays attention to various parts in the input sequence using this attention mechanism.

Self-attention enables the model to selectively focus on the input portions that are most pertinent to the task at hand and weigh the relative importance of various components in the input sequence. Most often, it is utilised in models like transformers and their variations.

So attention is a general idea, and self-attention is one specific implementation of attention.

There are multiple types of attention mechanisms; we will discuss the different options now.

What are the different types of attention mechanism in NLP?

1. Scaled dot-product attention

Scaled dot-product attention is a self-attention mechanism used to calculate the input sequence’s attention weights for each position. It is the most commonly used type of attention in transformer architecture.

The attention mechanism first represents the input sequence as queries, keys, and value matrices. The query matrix shows the current position in the input sequence, the key matrix shows all the other positions in the input sequence, and the value matrix shows the information that should be output for each position in the input sequence.

The attention weights for each position are calculated by taking the dot product of the query matrix and the key matrix and then dividing by the square root of the dimension of the key matrix. This is done so that the dot product stays manageable and makes the numbers stable.

The attention weights are then used to weigh the value matrix and figure out the output of the attention mechanism for the current position, which is a weighted sum.

In transformer architecture and its variations, the scaled dot-product attention is used for the model to pay attention to different points in the input sequence by comparing them to all the other points. The attention weights are then used to weigh the hidden states and compute a weighted sum, which is used as the input to the decoder for the current word.

2. Multi-head attention

Multi-head attention is a variant of scaled dot-product attention where multiple attention heads are used in parallel. Each attention head learns to attend to a different representation of the input.

In multi-head attention, the input is first transformed into multiple different representations (also called heads) using linear transformations. These representations are then used to compute numerous sets of attention weights, one for each head. The attention weights are then used to weigh the input and calculate a weighted sum, which is the output of the attention mechanism for the current position.

The idea behind using multiple attention heads is that each head can focus on a different aspect of the input. By using multiple heads, the model can learn to attend to different types of information in the input, and the final output is a concatenation of all the weighted sum from each head.

The multi-head attention mechanism is used in transformer architecture and its variants to improve the model’s ability to attend to different parts of the input and learn multiple representations of the input.

3. Additive attention

Additive attention is a type of attention mechanism similar to scaled dot-product attention, but the attention weight is calculated differently. Instead of taking the dot product of the query and key matrices and scaling the result, additive attention computes the similarity measure between the query and key vectors. Then it applies a feed-forward neural network to produce attention weights.

The process starts by representing the input sequence as a query, key, and value vector. Then the attention weights are computed by taking the dot product of the query vector with the key vector and passing it through a feed-forward neural network, which produces a scalar value that is used as the attention weight. This attention weight is then used to weigh the value vector and compute a weighted sum, which represents the output of the attention mechanism for the current position.

One of the main advantages of additive attention is that it allows the model to learn a non-linear similarity measure between the query and key vectors. This can be useful in cases where the dot product similarity measure is inappropriate.

Additive attention is less widely used than scaled dot-product and multi-head attention. Still, it has been used in some architectures and tasks, especially in some conversational models, and it has shown to be helpful in specific scenarios.

4. Location-based attention

Location-based attention is a type of attention mechanism that allows the model to focus on specific input regions using a convolutional neural network (CNN) to learn a set of attention weights.

In location-based attention, the input is first passed through a CNN to produce a set of feature maps.

These feature maps represent the input at different positions and scales. Then, the feature maps compute a set of attention weights for each position. Finally, the attention weights are calculated by applying a 1×1 convolution on the feature maps, which produces a scalar value for each position.

These scalar values are then used as attention weights. The attention weights are then used to weigh the feature maps and compute a weighted sum, which represents the output of the attention mechanism for the current position.

The main advantage of location-based attention is that it allows the model to focus on specific regions of the input rather than just individual positions in the input sequence. This can be useful in tasks such as image captioning, where the model needs to focus on specific regions of an image when generating a caption.

Location-based attention has been used in some architectures and tasks, particularly in computer vision, and it has proven helpful in some scenarios.

5. Co-Attention

Co-attention is a type of attention mechanism used when there are multiple inputs, and it allows the model to learn the relationship between the different inputs. It’s primarily used in visual question answering, image, and video captioning. The model needs to understand the relationship between the various inputs to decide.

In co-attention, the model simultaneously attends to both the visual and textual input, and the attention weights are computed for each input independently. These attention weights are then used to weigh the input and calculate a weighted sum, representing the output of the attention mechanism for the current position.

The model uses the attention weights to identify essential parts of the visual input and then uses that information to understand the text input.

Co-attention can be divided into two types: hard co-attention and soft co-attention. Hard co-attention is when the model makes a hard decision about which input is essential for each position, while soft co-attention is when the model makes a soft decision.

Co-attention mechanisms are helpful in tasks where the model needs to understand the relationship between multiple inputs. It’s been used in some architectures and functions, especially in computer vision and natural language processing, and it has shown to be helpful in specific scenarios.

6. Attention with external memory

Attention with external memory refers to a type of attention mechanism that allows the model to incorporate an external memory or knowledge base to improve decision-making.

In this type of attention, the model uses an external memory bank to store and retrieve information that is relevant to the task at hand. The model uses attention weights to focus on specific parts of the external memory, similar to how it focuses on particular aspects of the input.

There are several ways to implement attention with external memory. Still, one common approach is to use a memory-augmented neural network, such as the Neural Turing Machine (NTM) or the Differentiable Neural Computer (DNC). In addition to a neural network, these architectures also use an outside memory bank to do the job.

The external memory bank can store information essential to the task, like previous inputs, and the attention mechanism is used to get the data from the memory bank.

Attention with external memory has been used in many tasks, such as language modelling, machine translation, and question answering. It is helpful when the model needs external knowledge to make a decision.

Conclusion – Attention Mechanism in NLP

An attention mechanism has become crucial in many natural language processing (NLP) tasks, such as machine translation, language modelling, text summarisation, and question-answering. Attention mechanisms let the model pay attention to certain input parts and learn how vital different elements are.

Several attention mechanisms have been developed in recent years, each with strengths and weaknesses. Some of the most common types of attention mechanism used in NLP are:

  • Scaled dot-product attention: This is the most common type of self-attention, and it is used in transformer architecture. It takes the dot product of the query, key, and value matrices and scaling the dot product by the square root of the dimension of the key matrix.
  • Multi-head attention: This is a variant of scaled dot-product attention where multiple attention heads are used in parallel. Each attention head learns to attend to a different representation of the input.
  • Additive attention: This attention mechanism is similar to scaled dot-product attention, but the attention weight is calculated differently. It computes the similarity measure between the query and key vectors and then applies a feed-forward neural network to produce attention weights.
  • Location-based attention: this type of attention allows the model to focus on specific input regions using a convolutional neural network to learn a set of attention weights.
  • Co-Attention: This attention mechanism is used when there are multiple inputs, allowing the model to learn the relationship between the different inputs.
  • Attention with external memory: This attention mechanism allows the model to incorporate an external memory or knowledge base to improve decision-making.

Attention mechanism have worked well in many NLP tasks, and they are likely to continue to be important in developing new NLP models in the future.

About the Author

Neri Van Otten

Neri Van Otten

Neri Van Otten is the founder of Spot Intelligence, a machine learning engineer with over 12 years of experience specialising in Natural Language Processing (NLP) and deep learning innovation. Dedicated to making your projects succeed.

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