Human sensing with radio signals has emerged as a non-intrusive and occlusion-robust alternative to vision-based approaches, and WiFi signals further support device-free sensing. However, current approaches deeply rely on neural networks, whose black-box nature hinders model transparency and explainability, limiting the use of WiFi-based human sensing in critical fields. For model explainability, recent works have studied saliency methods which attribute model outputs to important features, but they mostly bias in favor of common modalities (e.g., images, time series). This paper proposes a Matryoshka-like saliency method, MatryMask, an initial exploration of feature attribution for human sensing with radio signals. Compared to existing methods that require empirical knowledge about the sparsity of important features, MatryMask regularizes multiple masks to highlight salient areas at different scales, adapting to the uncertain and varying sparsity of important features in radio signals. To effectively perturb radio signals, we devise a novel frequency-removal perturbation beyond existing spatial/time-domain perturbations. Experimentally, MatryMask outperforms state-of-the-art saliency methods and significantly improves the attribution performance by up to 38.1~70.6% for three tasks.

Human activity recognition (HAR) is a fundamental sensing and analysis technique that supports diverse applications, such as smart homes and healthcare. In device-free and non-intrusive HAR, WiFi channel state information (CSI) captures wireless signal variations caused by human interference without the need for video cameras or on-body sensors. However, current CSI-based HAR performance is hampered by incomplete CSI recordings due to fixed window sizes in CSI collection and human/machine errors that incur missing values in CSI. To address these issues, we propose DiffAR, a temporal-augmented HAR approach that improves HAR performance by augmenting CSI. DiffAR devises a novel Adaptive Conditional Diffusion Model (ACDM) to synthesize augmented CSI, which tackles the issue of fixed windows by forecasting and handles missing values with imputation. Compared to existing diffusion models, ACDM improves the synthesis quality by guiding progressive synthesis with step-specific conditions. DiffAR further exploits an ensemble classifier for activity recognition using both raw and augmented CSI. Extensive experiments on four public datasets show that DiffAR achieves the best synthesis quality of augmented CSI and outperforms state-of-the-art CSI-based HAR methods in recognition performance. The source code of DiffAR is available at https://github.com/huangshk/DiffAR.

Medical Visual Question Answering (VQA) has emerged as a promising solution to enhance clinic-decision making and patient interactions. Given a medical image and a corresponding question, medical VQA aims to predict an informative answer by reasoning the visual and textual information. However, datasets with limited samples circumscribe the generalization of medical VQA, reducing its accuracy when applied to unseen medical samples. Existing works tried to solve this problem with meta-learning or self-supervised learning but still failed to achieve satisfactory performance on medical VQA with insufficient samples. To address this problem, we propose multimodal hierarchical knowledge distillation for medical VQA (MHKD-MVQA). In the primary novelty of MHKD-MVQA, we distill knowledge from not only the output but also the intermediate layers, which leverages the knowledge from limited samples to a greater extent. Meanwhile, medical images and questions are embedded in a shared latent space, enabling our model to tackle multimodal samples. We evaluate our model on two medical VQA datasets, VQA-MED 2019 and VQA-RAD, where MHKD-MVQA achieves state-of-the-art performance and outperforms baselines by 3.6% and 1.6%, respectively. The extensive experiments also highlight the generalization of knowledge distillation by analyzing the class activation maps on medical images concerning specific questions.