Learning sparse features can lead to overfitting in neural networks

TL;DR

This paper investigates how learning sparse features in neural networks can sometimes cause overfitting, especially when the target function is smooth or constant, contrasting with the benefits of feature learning in certain settings.

Contribution

It provides a theoretical explanation for when feature learning can harm generalization, highlighting the role of sparsity and its impact on smoothness and overfitting.

Findings

Sparse feature learning can worsen generalization in certain tasks.

Methods avoiding feature learning may outperform learned representations in some cases.

Empirical evidence shows learned features can lead to less smooth, overfitting predictors.

Abstract

It is widely believed that the success of deep networks lies in their ability to learn a meaningful representation of the features of the data. Yet, understanding when and how this feature learning improves performance remains a challenge: for example, it is beneficial for modern architectures trained to classify images, whereas it is detrimental for fully-connected networks trained for the same task on the same data. Here we propose an explanation for this puzzle, by showing that feature learning can perform worse than lazy training (via random feature kernel or the NTK) as the former can lead to a sparser neural representation. Although sparsity is known to be essential for learning anisotropic data, it is detrimental when the target function is constant or smooth along certain directions of input space. We illustrate this phenomenon in two settings: (i) regression of Gaussian random functions on the d-dimensional unit sphere and (ii) classification of benchmark datasets of images. For (i), we compute the scaling of the generalization error with number of training points, and show that methods that do not learn features generalize better, even when the dimension of the input space is large. For (ii), we show empirically that learning features can indeed lead to sparse and thereby less smooth representations of the image predictors. This fact is plausibly responsible for deteriorating the performance, which is known to be correlated with smoothness along diffeomorphisms.