CRITICAL ANALYSIS OF SELF-SUPERVISION
This paper evaluated the performance of state-of-the-art self-supervised methods on learning weights of convolutional neural networks (CNNs) and on a per-layer basis. This paper also aims to figure out whether current self-supervision techniques can learn deep features from only one image.
The main goal of self-supervised learning is to take advantage of a vast amount of unlabeled data to train CNNs and find a generalized image representation. In self-supervised learning, unlabeled data generate ground truth labels per se by pretext tasks such as the Jigsaw puzzle task, and the rotation estimation. For example, in the rotation task, we have a picture of a bird without the label "bird". We rotate the bird image by 90 degrees clockwise and the CNN is trained in a way that to find the rotation axis, as it can be seen in the figure below.
In recent literature, several papers addressed self-supervised learning methods and learning from a single sample.
A BiGAN , or Bidirectional GAN, is simply a generative adversarial network plus an encoder. The generator maps latent samples to generated data and the encoder performs as the opposite of the generator. After training BiGAN, the encoder has learned to generate a rich image representation. In RotNet method , images are rotated and the CNN learns to figure out the direction. DeepCluster  alternates k-means clustering to learn stable feature representations under several image transformations.
Method & Experiment
In this paper, BiGAN, RotNet and DeepCluster are employed for training AlexNet in a self-supervised manner. To evaluate the impact of the size of the training set, they have compared the results of a million images in the ImageNet dataset with a million augmented images generated from only one single image. Various methods of data augmentation including cropping, rotation, scaling, contrast changes, and adding noise, have been used to generate the mentioned artificial dataset from one image.
With the intention of measuring the quality of deep features on a per-layer basis, a linear classifier is trained on top of each convolutional layer of AlexNet. Linear classifier probes are commonly used to monitor the features at every layer of a CNN, and are trained entirely independently of the CNN itself . Note that the main purpose of CNNs is to reach a linearly discriminable representation for images. Accordingly, linear probing technique aims to evaluate the training of each layer of a CNN and inspect how much information each of the layers learned. The same experiment has been done using the CIFAR10/100 dataset.
Figure 2 shows how well representations at each level are linearly separable. According to results, training the CNN with self-supervision methods can match the performance of fully supervised learning in the first two convolutional layers. It must be pointed out that only one single image with massive augmentation is utilized in this experiment.
The source code for the paper can be found here: https://github.com/yukimasano/linear-probes
This paper revealed that if a strong data-augmentation be employed, as little as a single image is sufficient for self-supervision techniques to learn the first few layers of popular CNNs. However, even the presence of millions of images are not enough for learning the deeper layers, and supervision might still be necessary. The results confirmed that the weights of the first layers of deep networks contain limited information about natural images. Accordingly, current unsupervised learning is only about augmentation, and we probably do not use the capacity of million images, yet.
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