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Title: Joint autoencoders: a flexible meta-learning framework

Paper content:
The incorporation of prior knowledge into learning is essential in achieving good performance based on small noisy samples.
Such knowledge is often incorporated through the availability of related data arising from domains and tasks similar to the one of current interest.
Ideally one would like to allow both the data for the current task and for previous related tasks to self-organize the learning system in such a way that commonalities and differences between the tasks are learned in a data-driven fashion.
We develop a framework for learning multiple tasks simultaneously, based on sharing features that are common to all tasks, achieved through the use of a modular deep feedforward neural network consisting of shared branches, dealing with the common features of all tasks, and private branches, learning the specific unique aspects of each task.
Once an appropriate weight sharing architecture has been established, learning takes place through standard algorithms for feedforward networks, e.g., stochastic gradient descent and its variations.
The method deals with meta-learning (such as domain adaptation, transfer and multi-task learning) in a unified fashion, and can easily deal with data arising from different types of sources.
Numerical experiments demonstrate the effectiveness of learning in domain adaptation and transfer learning setups, and provide evidence for the flexible and task-oriented representations arising in the network.
A major goal of inductive learning is the selection of a rule that generalizes well based on a finite set of examples.
It is well-known ( BID11 ) that inductive learning is impossible unless some regularity assumptions are made about the world.
Such assumptions, by their nature, go beyond the data, and are based on prior knowledge achieved through previous interactions with 'similar' problems.
Following its early origins ( BID1 BID19 ), the incorporation of prior knowledge into learning has become a major effort recently, and is gaining increasing success by relying on the rich representational flexibility available through current deep learning schemes BID3 .
Various aspects of prior knowledge are captured in different settings of meta-learning, such as learning-to-learn, domain adaptation, transfer learning, multi-task learning, etc. (e.g., ).
In this work, we consider the setup of multi-task learning, first formalized in BID1 , where a set of tasks is available for learning, and the objective is to extract knowledge from a subset of tasks in order to facilitate learning of other, related, tasks.
Within the framework of representation learning, the core idea is that of shared representations, allowing a given task to benefit from what has been learned from other tasks, since the shared aspects of the representation are based on more information BID24 .We
consider both unsupervised and semi-supervised learning setups. In
the former setting we have several related datasets, arising from possibly different domains, and aim to compress each dataset based on features that are shared between the datasets, and on features that are unique to each problem. Neither
the shared nor the individual features are given apriori, but are learned using a deep neural network architecture within an autoencoding scheme. While such
a joint representation could, in principle, serve as a basis for supervised learning, it has become increasingly evident that representations should contain some information about the output (label) identity in order to perform well, and that using pre-training based on unlabeled data is not always advantageous (e.g., chap. 15 in ). However, since
unlabeled data is far more abundant than labeled data, much useful information can be gained from it. We therefore propose
a joint encoding-classification scheme where both labeled and unlabeled data are used for the multiple tasks, so that internal representations found reflect both types of data, but are learned simultaneously.The main contributions of this work are: (i) A generic and flexible
modular setup for combining unsupervised, supervised and transfer learning. (ii) Efficient end-to-end
transfer learning using mostly unsupervised data (i.e., very few labeled examples are required for successful transfer learning). (iii) Explicit extraction
of task-specific and shared representations.
We presented a general scheme for incorporating prior knowledge within deep feedforward neural networks for domain adaptation, multi-task and transfer learning problems.
The approach is general and flexible, operates in an end-to-end setting, and enables the system to self-organize to solve tasks based on prior or concomitant exposure to similar tasks, requiring standard gradient based optimization for learning.
The basic idea of the approach is the sharing of representations for aspects which are common to all domains/tasks while maintaining private branches for task-specific features.
The method is applicable to data from multiple sources and types, and has the advantage of being able to share weights at arbitrary network levels, enabling abstract levels of sharing.We demonstrated the efficacy of our approach on several domain adaptation and transfer learning problems, and provided intuition about the meaning of the representations in various branches.
In a broader context, it is well known that the imposition of structural constraints on neural networks, usually based on prior domain knowledge, can significantly enhance their performance.
The prime example of this is, of course, the convolutional neural network.
Our work can be viewed within that general philosophy, showing that improved functionality can be attained by the modular prior structures imposed on the system, while maintaining simple learning rules.

assistant: text: The paper focuses on learning common features from multiple domains data and ends up with a general architecture for multi-task, semi-supervised and transfer learning