A Complete Introduction to Continual Learning

April 05, 2023

Continual Learning (CL) focuses on developing models to learn new tasks while retaining information from previous tasks. CL is an important area of research as it addresses the real-world scenario where the data and tasks are constantly changing, and a model must adapt to these changes without forgetting previous knowledge.

In traditional machine learning, a model is trained on a fixed dataset and is expected to perform a single task. However, this approach becomes problematic when the data and tasks are dynamic and changing, as the model must be able to adapt and learn from new data over time. This is where CL comes into play, enabling the model to continuously learn and improve without forgetting previous knowledge.

One of the significant challenges in CL is the issue of catastrophic forgetting, where a model trained on multiple tasks needs to remember information learned from previous tasks when exposed to new data. Various techniques have been developed to overcome this challenge, including regularization and memory-augmented networks.

Regularization involves adding constraints to the learning process to prevent overfitting to new data. On the other hand, memory-augmented networks include memory components that store information from previous tasks and use this information to improve performance on new tasks.

The architecture of a model also plays an important role in its adaptability. Some models are designed to be more flexible and adaptable, with architectures that can more easily incorporate new information and knowledge. Modular architectures, for example, allow different components of the model to be trained and adapted independently, which increases flexibility in adapting to new tasks and data.

The availability of task-specific data also affects a model’s adaptability. Models that have access to large amounts of task-specific data are better able to adapt and learn, as they have more information to base their predictions on. This is why some models are trained on vast amounts of data in order to improve their adaptability and ability to generalize to new tasks.

In conclusion, CL is an important field in machine learning that addresses the challenge of training models that can continuously learn and adapt to new tasks and data without forgetting previous knowledge. The use of techniques such as regularization and memory-augmented networks, as well as the design of the model architecture, play an important role in the adaptability of CL models.

Use cases

Continual learning poses some inherent benefits that can serve the following use cases.

  • Anomaly detection – Continual learning can be particularly useful in anomaly detection scenarios where the distribution of the data changes over time and traditional machine learning algorithms may not be effective. The goal of this type of continual learning is to constantly monitor the normal behavior of a system to learn its standard operating levels, process, or data stream and detect anomalies by comparing new data to the learned normal behavior. To accomplish this, a continual learning algorithm is trained on a stream of data to learn the normal behavior of the system. As new data becomes available, it is compared to the learned normal behavior and deviations from the norm are flagged as anomalies. The continual learning algorithm can then be updated to incorporate the new data, allowing it to adapt to changes in the normal behavior over time.  For example, in the financial sector, the normal behavior of transactions can change over time as malicious actors develop new techniques to evade detection. Continual learning can help detect these evolving anomalies by updating the model to capture the changing behavior. In contrast to traditional machine learning algorithms, which are trained on a fixed dataset and assume that the data distribution does not change, continual learning algorithms can continuously adapt and improve over time.


  • Personalization – Another use case for continual learning is in personalized recommendation systems where the goal is to provide highly customized and up-to-date recommendations to users. By continuously learning a user’s preferences and behavior, a recommendation system can improve its ability to make accurate and relevant recommendations. To achieve this, a continual learning algorithm is trained on a user’s historical interaction data, such as purchase history or search queries, to learn their preferences and behavior patterns. As new data becomes available, the model updates its understanding of the user’s preferences and behavior. This updated information is then used to make more accurate and relevant recommendations to the user. For example, in an e-commerce system, the model might use a user’s purchase history to learn about their preferences for certain types of products or brands. Over time, as the user continues to make purchases, the model will continually update its understanding of their preferences and use this information to make personalized recommendations for other products or services that the user might be interested in. This type of continual learning allows the model to adapt to changing user preferences and provide increasingly accurate recommendations over time.


  • Forecasting – Continual learning can also be applied in the area of forecasting to continuously update predictions based on new data as it becomes available. This approach allows models to adapt to changes in the data distribution and maintain their accuracy over time, even when dealing with complex and dynamic data. In a forecasting system, a continual learning algorithm is trained on a stream of historical data, such as financial data or time-series data, to make predictions about future events. As new data becomes available, the model updates its understanding of the relationships in the data and uses this information to refine its predictions. For example, in a financial forecasting system, a continual learning algorithm can be trained on a stream of stock price data to predict future stock prices. As new data becomes available, the model can continuously update its understanding of market trends and use this information to refine its predictions about future stock prices. This ability to continuously adapt and improve, even in complex and dynamic data environments, makes continual learning a valuable tool for forecasting applications.

Biologically-inspired Continual Learning

Biologically-inspired methods in lifelong learning entail scientists leveraging the ways that the human brain acquires, stores and retrieves new information, and then adapting these concepts for machine learning. The goal is to create artificial neural network systems that learn and remember information in a more human-like way, while retaining the ability to generalize and make predictions.

One such approach is synaptic stabilizationization, which seeks to emulate the biological process of synaptic pruning in the human brain. This process is thought to play an important role in retaining memories and discarding irrelevant information. In the same vein, machine learning algorithms are being designed to eliminate the connections between neurons that are no longer useful to the network, freeing up resources for the learning of new information.

Another biologically-inspired approach is the brain-state-in-a-box technique. This aims to replicate the process of memory storage in the neocortex of the human brain. In this method, a memory buffer is used to store crucial information from past tasks, which can later be retrieved and used to avoid catastrophic forgetting. By incorporating these biologically-inspired methods into AI systems, researchers hope to improve the performance and memory capabilities of these systems in the future.

Multimodal-Multitask Learning

Another interesting aspect of continual learning is the development of models that can handle multiple tasks simultaneously. Multi-task learning is a type of machine learning that allows a model to learn several tasks at once, instead of learning each task individually. This approach is particularly useful in real-world applications where data is limited, and the model needs to make the most of available information. Multi-task learning can be combined with continual learning to allow AI systems to handle multiple tasks over time, with the model continuously adapting to new tasks while retaining previous knowledge.

The goal of multimodal multitask learning is to develop a model which is capable of effectively learning and performing multiple tasks simultaneously. This presents a significant challenge, as conventional deep learning models tend to perform well on a single task but face difficulties when faced with multiple, diverse tasks that involve different modalities.

Multi-modal multitask learning algorithms enhance the model’s capacity for learning multiple tasks while maintaining information from previous tasks. These algorithms commonly use regularization techniques such as incremental learning and class-incremental learning to prevent overfitting to any one task and promote information sharing between tasks.

One approach to multi-modal multitask learning involves using a shared feature representation, where the model learns a common representation of the input data that can be applied across all tasks. This allows the model to reuse knowledge acquired from previous tasks and learn new tasks more efficiently and quickly.

Another strategy is to use a separate feature representation for each task, where the model learns a task-specific representation optimized specifically for that task. This approach enables the model to concentrate on the specific requirements of each task and handle variations in data distribution and modality more effectively.

Techniques such as memory-augmented neural networks – in which a memory module is employed to store information from previous tasks – allow the model to retrieve and reuse this information when learning new tasks, leading to improved performance. This approach has been demonstrated to be effective in computer vision applications, where the model can learn to recognize objects from various categories and perform classification tasks. Additionally, the use of memory-augmented neural networks enables the model to leverage its prior knowledge and experience to improve its performance on new tasks.

Rehearsal buffer

Using a rehearsal buffer is a solution to address the issue of catastrophic forgetting in machine learning models by preserving a subset of past data in a memory buffer and mixing it with the current data during the model’s training process. This helps to reinforce the knowledge acquired from previous tasks and prevents it from being overwritten by new information.

Different methods can be used to select the data stored in the rehearsal buffer, including random sampling or based on metrics such as the accuracy of the model on previous tasks or the similarity of the data distribution to the current task. Using rehearsal buffers can not only prevent catastrophic forgetting but also improve the overall performance of the model. By combining current and past data, the model can better handle diverse data distributions and increase its ability to generalize to new data.

However, the success of this solution is largely dependent on the size of the buffer and may not be feasible in cases of data privacy concerns. Alternative solutions exist that involve task-specific components to avoid task interference, but these often have limitations such as assuming knowledge of the test task or requiring a significant number of parameters.

The purpose of the rehearsal buffer is to efficiently balance the preservation of information about all past tasks with the utilization of limited memory resources. The system only stores a small portion of examples from previous tasks and updates the buffer’s contents as it learns new tasks. This allows the system to maintain a compact representation of its past experiences and effectively avoid catastrophic forgetting.

Deep Generative Replay

An alternative approach to sequentially train deep neural networks without referring to past data. In the deep generative replay framework, the model retains previously acquired knowledge by the concurrent replay of generated pseudo-data. In particular, a deep generative mode is trainedl in the generative adversarial networks (GANs) framework [10] to mimic past data. Generated data are then paired with corresponding response from the past task solver to represent old tasks. Dubebd ‘the scholar model’, the generator-solver pair can produce fake data and desired target pairs as much as needed, and when presented with a new task, these produced pairs are interleaved with new data to update the generator and solver networks. As usch, a scholar model can both learn the new task without forgetting its own knowledge and teach other models with generated input-target pairs, even when the network configuration is different.

As deep generative replay supported by the scholar network retains the knowledge without revisiting actual past data, this framework can be employed to various practical situation involving privacy issues. Recent advances on training generative adversarial networks suggest that the trained models can reconstruct real data distribution in a wide range of domains.

Learning to Prompt

The limitations of rehearsal buffer methods in continual learning have led to the need for more effective and compact memory systems. To address this challenge, Learning to Prompt (L2P) is introduced as a novel approach. Instead of continually retraining the entire model for each task, L2P provides learnable task-specific “prompts” to guide pre-trained backbone models through sequential training with a pool of learnable prompt parameters.

L2P outperforms previous state-of-the-art methods and demonstrates strong performance across a range of benchmarks. Additionally, L2P is more memory efficient than traditional rehearsal-based methods. By using a single frozen backbone model and learning prompt parameters, L2P offers a new and promising approach to tackle the challenges of continual learning.

In L2P, the prompt pool acts as a task-specific selector that chooses relevant information from the fixed backbone model to solve each task. The prompt parameters are trained using a combination of supervised learning and reinforcement learning, allowing the framework to automatically adapt to new tasks while retaining task-specific knowledge. In this way, L2P eliminates the need for a rehearsal buffer and reduces the memory requirements for continual learning. Additionally, the ability to conditionally select information from a pre-trained backbone model also makes L2P suitable for deployment in real-world settings, where new tasks and data distributions may arise without prior knowledge. The result is a more flexible and scalable continual learning framework that outperforms existing methods while also being more memory efficient.

When encountering a new task, the system can utilize the information stored in the rehearsal buffer to initialize its parameters and make predictions based on past experiences. This enables the system to learn new tasks more efficiently and maintain knowledge acquired from previous tasks.

Prompt Pool and Instance-Wise Query

L2P utilizes a learnable prompt pool to dynamically select task-relevant prompts based on the input features. The pool is associated with learned keys that reduce cosine similarity loss between matched input query features and prompts. The query function then maps inputs to the top-N closest keys, and the associated prompt embeddings are fed to the model for prediction. The optimization of the prompt pool and the classification head is done via the cross-entropy loss.

Instance-wise query removes the need for prior knowledge of task identity or boundaries, making L2P suitable for task-agnostic continual learning. Input examples with similar features tend to select similar sets of prompts, and prompts that are frequently shared encode more generic knowledge while other prompts encode more task-specific knowledge. Additionally, prompts store high-level instructions and keep lower-level pre-trained representations frozen, reducing the risk of catastrophic forgetting.

Illustration of L2P in action: the method selects a subset of prompts from the key-value paired prompt pool through the instance-wise query mechanism, prepends the selected prompts to the input tokens, and feeds the extended tokens to the model for prediction.

L2P is a method for dynamically prompting a pre-trained model to learn tasks sequentially in the face of non-stationary data distributions. The approach involves maintaining a prompt pool, which are small learnable parameters, to instruct the model’s predictions and manage both task-invariant and task-specific knowledge while preserving model plasticity. Unlike traditional supervised learning which trains on i.i.d data, continual learning problems such as training a single model on different classification tasks presented sequentially. Ordinary methods that adapt the entire or partial model weights with a rehearsal buffer to counteract forgetting are replaced by L2P’s single backbone model and prompt pool that learn conditionally. The task-specific knowledge is stored inside the prompt pool, eliminating the need for a rehearsal buffer, and L2P selects and updates prompts from the pool in an instance-wise manner, eliminating the need for task identity at test time.

Residual Continual Learning

Transfer learning methods such as finetuning  utilize source task knowledge and to boost training for target tasks. As transfer learning methods consider only target task performance during training, most of source task performance is lost as a side effect of catastrophic forgetting.

Recent studies on continual learning do not use the source data directly, but they often refer to parts of the information about source data, for example in forms of generative adversarial networks, which somewhat dilutes the original purpose of continual learning. Second, the size of a network should not increase. Without this condition, a network can be expanded while keeping the entire original network.

Residual-learning-like reparameterization allows continual learning, and a simple decay loss controls the trade-off between source and target performance. No information about source tasks is needed, except the original source network. The size of a network does not increase at all for inference (except last task-specific linear classifiers).The proposed method can be applied to general CNNs including Batch Normalization (BN) (Ioffe and Szegedy 2015) layers in a natural way. 

Continual Learning for the Future

Another exciting aspect of continual learning is the development of methods that allow artificial intelligence systems to continually learn in an unsupervised manner. Traditional supervised learning algorithms rely on labeled training data to learn new tasks, but in many real-world applications, the data is often unlabeled or incomplete. Unsupervised continual learning methods aim to overcome this limitation by allowing AI systems to learn new representations from unstructured or raw data without explicit supervision. As we saw above, generative models, such as Variational Autoencoders (VAEs) and Generative Adversarial Networks (GANs), have shown promising results in unsupervised continual learning by generating new data from existing representations.

The integration of continual learning with other areas of machine learning is another exciting aspect that holds immense potential. For instance, meta-learning, which is the process of learning to learn, can be combined with continual learning to allow AI systems to quickly adapt to new tasks by leveraging previous experience. Another area of integration is reinforcement learning, where AI systems can be trained to continually learn new skills in an environment by maximizing a reward signal. In such systems, continual learning algorithms can be used to avoid catastrophic forgetting, allowing the AI system to build on previous experiences and knowledge as it encounters new challenges.

Innovations in continual learning have the potential to greatly impact both the development of machine learning and its real-world applications. In the field of machine learning, continual learning has the potential to dramatically improve the performance of AI models and reduce the need for frequent retraining. By allowing AI models to continually learn and adapt to new data and tasks, researchers can create more robust and flexible AI systems that can operate in dynamic environments and handle evolving data distributions. This will enable machine learning systems to maintain their accuracy over time and avoid catastrophic forgetting, a major challenge in traditional AI training.

In the business world, innovations in continual learning have the potential to revolutionize the way organizations use AI to solve complex problems. For example, by incorporating continual learning into their AI systems, businesses can use AI to continuously monitor and analyze large amounts of data, providing real-time insights and making informed decisions based on evolving conditions. This can be particularly valuable in industries such as finance, healthcare, and retail, where large amounts of data must be analyzed and acted upon quickly. Additionally, continual learning can help organizations personalize their AI systems to meet the unique needs of their customers and employees, providing a more tailored and effective experience. Overall, the impact of continual learning on business will be significant, allowing organizations to stay ahead of the competition and achieve their goals more efficiently and effectively.

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