  Due to the advantages of fusing information from various modalities,
multimodal learning is gaining increasing attention. Being a fundamental task
of multimodal learning, Visual Grounding (VG), aims to locate objects in images
through natural language expressions. Ensuring the quality of VG models
presents significant challenges due to the complex nature of the task. In the
black box scenario, existing adversarial testing techniques often fail to fully
exploit the potential of both modalities of information. They typically apply
perturbations based solely on either the image or text information,
disregarding the crucial correlation between the two modalities, which would
lead to failures in test oracles or an inability to effectively challenge VG
models. To this end, we propose PEELING, a text perturbation approach via
image-aware property reduction for adversarial testing of the VG model. The
core idea is to reduce the property-related information in the original
expression meanwhile ensuring the reduced expression can still uniquely
describe the original object in the image. To achieve this, PEELING first
conducts the object and properties extraction and recombination to generate
candidate property reduction expressions. It then selects the satisfied
expressions that accurately describe the original object while ensuring no
other objects in the image fulfill the expression, through querying the image
with a visual understanding technique. We evaluate PEELING on the
state-of-the-art VG model, i.e. OFA-VG, involving three commonly used datasets.
Results show that the adversarial tests generated by PEELING achieves 21.4% in
MultiModal Impact score (MMI), and outperforms state-of-the-art baselines for
images and texts by 8.2%--15.1%.
