Deep convolutional feature transfer across mobile activity recognition domains, sensor modalities and locations

Kernels in the convolutional layers of deep convolutional networks are believed to act as feature extractors, progressively highlighting more domain-specific features in the upper network layers. Thus lower-level features might be suitable for transfer. We analyse this in wearable activity recognition by reusing kernels learned on a source domain on another target domain. We consider transfer between users, application domains, sensor modalities and sensor locations. We characterize the trade-offs of transferring various convolutional layers along model size, learning speed, recognition performance and training data. Through novel kernel visualisations and comparative evaluations we identify what kernels are predominantly sensitive to, amongst sensor characteristics, motion dynamics and on-body placement. Kernel transfer reduces training time by ~17% without additional complexity. We derive recommendations on when transfer is most suitable.