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Title: Data-Efficient Image Recognition with Contrastive Predictive Coding

Paper content:
Human observers can learn to recognize new categories of objects from a handful of examples, yet doing so with machine perception remains an open challenge.
We hypothesize that data-efficient recognition is enabled by representations which make the variability in natural signals more predictable, as suggested by recent perceptual evidence.
We therefore revisit and improve Contrastive Predictive Coding, a recently-proposed unsupervised learning framework, and arrive at a representation which enables generalization from small amounts of labeled data.
When provided with only 1% of ImageNet labels (i.e. 13 per class), this model retains a strong classification performance, 73% Top-5 accuracy, outperforming supervised networks by 28% (a 65% relative improvement) and state-of-the-art semi-supervised methods by 14%.
We also find this representation to serve as a useful substrate for object detection on the PASCAL-VOC 2007 dataset, approaching the performance of representations trained with a fully annotated ImageNet dataset.
ResNet trained on CPC ResNet trained on pixels With decreasing amounts of labeled data, supervised networks trained on pixels fail to generalize (red).
When trained on unsupervised representations learned with CPC, these networks retain a much higher accuracy in this low-data regime (blue).
Equivalently, the accuracy of supervised networks can be matched with significantly fewer labels.
Deep neural networks excel at perceptual tasks when labeled data are abundant, yet their performance degrades substantially when provided with limited supervision (Fig. 1, red ).
In contrast, humans and animals can quickly learn about new classes of objects from few examples (Landau et al., 1988; Markman, 1989) .
What accounts for this monumental difference in data-efficiency between biological and machine vision?
While highly-structured representations (e.g. as proposed by Lake et al., 2015) may improve data-efficiency, it remains unclear how to program explicit structures that capture the enormous complexity of real visual scenes like those in ImageNet (Russakovsky et al., 2015) .
An alternative hypothesis has proposed that intelligent systems need not be structured a priori, but can instead learn about the structure of the world in an unsupervised manner (Barlow, 1989; Hinton et al., 1999; LeCun et al., 2015) .
Choosing an appropriate training objective is an open problem, but a promising guiding principle has emerged recently: good representations should make the spatio-temporal variability in natural signals more predictable.
Indeed, human perceptual representations have been shown to linearize (or 'straighten') the temporal transformations found in natural videos, a property lacking from current supervised image recognition models (Hénaff et al., 2019) , and theories of both spatial and temporal predictability have succeeded in describing properties of early visual areas (Rao & Ballard, 1999; Palmer et al., 2015) .
In this work, we hypothesize that spatially predictable representations may allow artificial systems to benefit from human-like data-efficiency.
Contrastive Predictive Coding (CPC, van den Oord et al., 2018) is an unsupervised objective which learns such predictable representations.
CPC is a general technique that only requires in its definition that observations be ordered along e.g. temporal or spatial dimensions, and as such has been applied to a variety of different modalities including speech, natural language and images.
This generality, combined with the strong performance of its representations in downstream linear classification tasks, makes CPC a promising candidate for investigating the efficacy of predictable representations for data-efficient image recognition.
Our work makes the following contributions:
• We revisit CPC in terms of its architecture and training methodology, and arrive at a new implementation of CPC with dramatically-improved ability to linearly separate image classes (+17% Top-1 ImageNet classification accuracy).
• We then train deep networks on top of the resulting CPC representations using very few labeled images (e.g. 1% of the ImageNet dataset), and demonstrate test-time classification accuracy far above networks trained on raw pixels (73% Top-5 accuracy, a 28% absolute improvement), outperforming all other unsupervised representation learning methods (+15% Top-5 accuracy over the previous state-of-the-art ).
Surprisingly, this representation also surpasses supervised methods when given the entire ImageNet dataset (+1% Top-5 accuracy).
• We isolate the contributions of different components of the final model to such downstream tasks.
Interestingly, we find that linear classification accuracy is not always predictive of low-data classification accuracy, emphasizing the importance of this metric as a stand-alone benchmark for unsupervised learning.
• Finally, we assess the generality of CPC representations by transferring them to a new task and dataset: object detection on PASCAL-VOC 2007.
Consistent with the results from the previous section, we find CPC to give state-of-the-art performance in this setting.
We asked whether CPC could enable data-efficient image recognition, and found that it indeed greatly improves the accuracy of classifiers and object detectors when given small amounts of labeled data.
Surprisingly, CPC even improves results given ImageNet-scale labels.
Our results show that there is still room for improvement using relatively straightforward changes such as augmentation, optimization, and network architecture.
Furthermore, we found that the standard method for evaluating unsupervised representations-linear classification-is only partially predictive of efficient recognition performance, suggesting that further research should focus on efficient recognition as a standalone benchmark.
Overall, these results open the door toward research on problems where data is naturally limited, e.g. medical imaging or robotics.
image detection accuracy to other transfer methods.
The supervised baseline learns from the entire labeled ImageNet dataset and fine-tunes for PASCAL detection.
The second class of methods learns from the same unlabeled images before transferring.
All of these methods pre-train on the ImageNet dataset, except for DeeperCluster which learns from the larger, but uncurated, YFCC100M dataset (Thomee et al., 2015) .
All results are reported in terms of mean average precision (mAP).
† denotes methods implemented in this work.

assistant: text: Unsupervised representations learned with Contrastive Predictive Coding enable data-efficient image classification.