Convolutional Neural Networks are not invariant to translation, but they can learn to be

Valerio Biscione, Jeffrey Bowers

Keywords: Invariance, Convolutional Networks, Translation, Internal Representations

Abstract:

When seeing a new object, humans can immediately recognize it across different retinal locations: we say that the internal object representation is invariant to translation. It is commonly believed that Convolutional Neural Networks (CNNs) are architecturally invariant to translation thanks to the convolution and/or pooling operations they are endowed with. In fact, several works have found that these networks systematically fail to recognise new objects on untrained locations. In this work we show how, even though CNNs are not 'architecturally invariant' to translation, they can indeed 'learn' to be invariant to translation. We verified that this can be achieved by pretraining on ImageNet, and we found that it is also possible with much simpler datasets in which the items are fully translated across the input canvas. Significantly, simply training everywhere on the canvas was not enough. We investigated how this pretraining affected the internal network representations, finding that the invariance was almost always acquired, even though it was some times disrupted by further training due to catastrophic forgetting/interference.
These experiments show how pretraining a network on an environment with the right 'latent' characteristics (a more naturalistic environment) can result in the network learning deep perceptual rules which would dramatically improve subsequent generalization.

One-sentence Summary: CNNs are not, as commonly assumed, 'architecturally' invariant to translation, but we investigated the conditions in which they can learn to be invariant to translation.

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