  Contrastive learning is a paradigm for learning representations from
unlabelled data that has been highly successful for image and text data.
Several recent works have examined contrastive losses to claim that contrastive
models effectively learn spectral embeddings, while few works show relations
between (wide) contrastive models and kernel principal component analysis
(PCA). However, it is not known if trained contrastive models indeed correspond
to kernel methods or PCA. In this work, we analyze the training dynamics of
two-layer contrastive models, with non-linear activation, and answer when these
models are close to PCA or kernel methods. It is well known in the supervised
setting that neural networks are equivalent to neural tangent kernel (NTK)
machines, and that the NTK of infinitely wide networks remains constant during
training. We provide the first convergence results of NTK for contrastive
losses, and present a nuanced picture: NTK of wide networks remains almost
constant for cosine similarity based contrastive losses, but not for losses
based on dot product similarity. We further study the training dynamics of
contrastive models with orthogonality constraints on output layer, which is
implicitly assumed in works relating contrastive learning to spectral
embedding. Our deviation bounds suggest that representations learned by
contrastive models are close to the principal components of a certain matrix
computed from random features. We empirically show that our theoretical results
possibly hold beyond two-layer networks.
