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Title: Downsampling leads to Image Memorization in Convolutional Autoencoders

Paper content:
Memorization of data in deep neural networks has become a subject of significant research interest.
In this paper, we link memorization of  images in deep convolutional autoencoders to downsampling through strided convolution.
To analyze this mechanism in a simpler setting, we train linear convolutional autoencoders and show that linear combinations of training data are stored as eigenvectors in the linear operator corresponding to the network when downsampling is used.
On the other hand, networks without downsampling do not memorize training data.
We provide further evidence that the same effect happens in nonlinear networks.
Moreover, downsampling in nonlinear networks causes the model to not only memorize just linear combinations of images, but individual training images.
Since convolutional autoencoder components are building blocks of deep convolutional networks, we envision that our findings will shed light on the important phenomenon of memorization in over-parameterized deep networks.
As deep convolutional neural networks (CNNs) become ubiquitous in computer vision due to their applicability and strong performance on a range of tasks BID6 , recent work has begun analyzing the memorization properties of such networks in classification.
For example, BID19 show that popular CNNs can achieve almost zero training error on randomly labeled datasets, indicating that CNNs have the capacity to "memorize" large training data sets.
BID0 and BID15 build on the experiments from BID19 to better understand and evaluate the extent to which CNNs memorize training data.
BID0 show that CNNs, when trained on large datasets, are able to learn patterns from realistic data before memorizing training images.
BID15 present experiments on "membership inference" (i.e. determining whether an image was used during training) and conclude that modern architectures are capable of "remember[ing] a large number of images and distinguish [ing] them from unseen images".Although
the above methods analyze memorization in the classification setting, they do not provide a mechanism through which memorization of training data occurs. We here
present downsampling as one mechanism by which deep CNNs memorize specific training images. We will
focus our study on the memorization properties of linear and nonlinear fully convolutional autoencoders. The architectures
we use (such as U-Net, BID14 ) are commonly employed in imageto-image tasks, see e.g. BID17 . However, we will
use these architectures only in the autoencoding framework. We primarily focus
on autoencoders BID1 for the following reasons: (1) components of convolutional autoencoders are building blocks of many CNNs; and (2) layerwise pre-training using autoencoders is a technique to initialize individual layers of CNNs to improve training BID3 , BID4 ). It is important to
note that there are many potential solutions to the autoencoding problem when using over-parameterized autoencoders. In particular, in
the linear case, these models may range from learning the (full rank) identity function (which has 0 error in the autoencoding task) to low rank solutions where each training example corresponds to an eigenvector with eigenvalue 1. Thus, understanding
how autoencoders learn is of interest in order to gain insights into how deep CNNs memorize training data.Figures 1a and 1b provide two examples of memorization: A typical U-Net architecture (the same as e.g. used in BID17 for large hole impainting) when trained on a single image "memorizes" the training image in the sense that for any input, the output always contains the training image (even if the input is random noise or an arbitrary white square). This paper provides
a mechanism for this phenomenon.The outline is as follows: After introducing some notation in Section 2, we will show in Section 3 that memorization is tightly coupled with downsampling and also occurs in the simpler setting of linear autoencoding CNNs. In the linear setting
, the neural network corresponds to matrix multiplication. In Section 4, we show
how to extract this matrix representation and we provide our main conjecture, namely that linear combinations of the training images are stored as eigenvectors of this matrix, whose rank is given by the dimension of the span of the training set. We also provide strong
evidence for this conjecture on 2 × 2 images.In Section 5, we analyze the eigenvalue decay and show in various examples that using downsampling linear CNNs, linear combinations of the training examples are stored as eigenvectors with eigenvalues close to 1. Finally, we return to
the nonlinear setting in Section 6, providing evidence that memorization is an even stronger phenomenon in nonlinear networks, since the actual training images (in contrast to linear combinations of training images) are memorized. We end with a short discussion
in Section 7.
This paper identified downsampling as a mechanism through which linear CNNs memorize training images.
We demonstrated that downsampling convolutional autoencoders memorize training images in both the linear and nonlinear setting.
In particular, we showed that it is not just the dimensionality reduction of downsampling that causes these models to learn point maps by demonstrating that a downsampling CNN architecture with the capacity to learn the identity function still prefers the point map.
In the linear case, this preference for low-rank over the equally valid high-rank solutions is highly suggestive of similar phenomena observed in problems such as matrix completion (e.g., Gunasekar et al.) .In
the non-linear case, memorization in downsampling networks is manifested even more strikingly with nearly arbitrary input images being mapped to output images that are visually identifiable as one of the training images. While
the exact mechanism still needs to be explored, this is reminiscent of FastICA in Independent Component Analysis BID10 or more general non-linear eigen-problems BID2 , where every "eigenvector" for certain iterative maps has its own basin of attraction. On the
other hand, non-downsampling auto-encoders do not memorize the training data and consistently learn a "high rank" map, similar to the identity map, at least visually.We conjecture that our findings will help to shed light on the strong generalization properties of downsampling networks for image classification and recognition tasks. Indeed
, if downsampling networks memorize images or linear combinations of images, when trained on large datasets, they may be capable of learning representations within the space of all realisitic images instead of learning the standard full rank basis.We conclude with a mention of further areas of exploration spurred on by our work. We still
need to understand why downsampling forces the network to learn low rank solutions even when the network has the capacity to learn the identity. This requires
developing a better grasp of optimization and initialization, starting with linear autoencoders and proceeding to the non-linear settings. Finally, we need
to explore connections between our conjecture and the manifold hypothesis to better understand the space of realistic images.

assistant: text: We identify downsampling as a mechansim for memorization in convolutional autoencoders.