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TTT++: When Does Self-Supervised Test-Time Training Fail or Thrive?

Test-time training (TTT) through self-supervised learning (SSL) is an emerging paradigm to tackle distributional shifts. Despite encouraging results, it remains unclear when this approach thrives or fails. In this work, we first provide an in-depth look at its limitations and show that TTT can possibly deteriorate, instead of improving, the test-time performance in the presence of severe distribution shifts. To address this issue, we introduce a test-time feature alignment strategy utilizing offline feature summarization and online moment matching, which regularizes adaptation without revisiting training data. We further scale this strategy in the online setting through batch-queue decoupling to enable robust moment estimates even with limited batch size. Given aligned feature distributions, we shed light on the strong potential of TTT by theoretically analyzing its performance post adaptation. This analysis motivates our use of more informative self-supervision in the form of contrastive learning. We empirically demonstrate that our modified version of test-time training, termed TTT++, outperforms state-of-the-art methods by a significant margin on multiple vision benchmarks. Our result indicates that exploiting extra information stored in a compact form, such as related SSL tasks and feature distribution moments, can be critical to the design of test-time algorithms.

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