Plasticity Collapse in Machine Unlearning
On the Plasticity Collapse in Continual Machine Unlearning
The research addresses a critical challenge for AI compliance with privacy regulations in ASEAN, where models must repeatedly forget data without losing effectiveness.
This paper identifies plasticity collapse as a fundamental limitation in continual machine unlearning. The research shows that sequential unlearning operations accumulate geometric constraints in parameter space, leading to saturated subspaces. Experiments across multiple architectures and datasets confirm two failure modes: forward failure with diminishing forgetting quality and backward failure with spontaneous re-memorization. The findings reveal a critical barrier to long-term reliability of unlearning systems.
On the Plasticity Collapse in Continual Machine Unlearning
Machine unlearning enables deep neural networks to selectively remove the influence of specific data in response to privacy and regulatory requirements. While prior work largely studies single-shot unlearning, real-world systems must accommodate continual unlearning, where multiple unlearning requests occur sequentially over time. In this work, we identify a fundamental limitation of this setting: plasticity collapse, a progressive breakdown in a model's ability to effectively forget. Through theoretical analysis of continual unlearning dynamics, we show that continual unlearning operations accumulate geometric constraints in parameter space, leading to saturated subspaces that restrict future updates. This structural effect induces two distinct failure modes: (1) Forward failure -- diminishing forgetting quality for subsequent tasks, and (2) Backward failure -- spontaneous re-memorization of previously forgotten information. Extensive experiments across multiple architectures, datasets, and methods in image classification confirm that plasticity collapse is not an artifact of specific implementations, but a pervasive phenomenon inherent to continual unlearning. Our findings reveal a critical barrier to the long-term reliability of machine unlearning systems and motivate the development of plasticity-preserving unlearning algorithms. Our code is available at https://github.com/TIML-Group/Continual-Machine-Unlearning-Plasticity-Collapse