  In this paper, we study a novel problem in egocentric action recognition,
which we term as "Multimodal Generalization" (MMG). MMG aims to study how
systems can generalize when data from certain modalities is limited or even
completely missing. We thoroughly investigate MMG in the context of standard
supervised action recognition and the more challenging few-shot setting for
learning new action categories. MMG consists of two novel scenarios, designed
to support security, and efficiency considerations in real-world applications:
(1) missing modality generalization where some modalities that were present
during the train time are missing during the inference time, and (2)
cross-modal zero-shot generalization, where the modalities present during the
inference time and the training time are disjoint. To enable this
investigation, we construct a new dataset MMG-Ego4D containing data points with
video, audio, and inertial motion sensor (IMU) modalities. Our dataset is
derived from Ego4D dataset, but processed and thoroughly re-annotated by human
experts to facilitate research in the MMG problem. We evaluate a diverse array
of models on MMG-Ego4D and propose new methods with improved generalization
ability. In particular, we introduce a new fusion module with modality dropout
training, contrastive-based alignment training, and a novel cross-modal
prototypical loss for better few-shot performance. We hope this study will
serve as a benchmark and guide future research in multimodal generalization
problems. The benchmark and code will be available at
https://github.com/facebookresearch/MMG_Ego4D.
