Not Every Subject Should Stay: Machine Unlearning for Noisy Engagement Recognition

TL;DR

This paper introduces a method for post-hoc subject-level machine unlearning in engagement recognition models, effectively removing problematic subjects with significantly less retraining cost.

Contribution

It proposes a lightweight approximate unlearning approach for removing entire subjects from trained models, improving efficiency over full retraining.

Findings

Unlearning recovers 89.3% and 92.5% of oracle gains on two datasets.

Effectiveness peaks at intermediate forget-set sizes.

Unlearning reduces retraining costs to roughly one quarter.

Abstract

Engagement recognition datasets are typically subject-indexed and often contain noisy, subjective supervision, making post-hoc dataset revision a practical problem. Existing noisy-label and data-cleaning methods largely operate at the sample level before or during training, but do not directly address a different question: once a model has already been trained, can the influence of an entire problematic subject be removed without full retraining? We study this setting through subject-level machine unlearning as a post-hoc sanitization mechanism for engagement recognition. Starting from a baseline trained on all subjects, we rank candidate harmful subjects using a model-dependent proxy, apply a lightweight approximate unlearning update, and compare the result against an oracle model retrained from scratch on the retained subjects only. We instantiate this protocol on DAiSEE and EngageNet using Tensor-Convolution and Convolution-Transformer Network (TCCT-Net) as a fixed platform and evaluate three matched model states under the same removal scenario: baseline, unlearned, and oracle. In representative K=3 forget-set settings, the unlearned model recovers 89.3% and 92.5% of the oracle gain on EngageNet and DAiSEE, respectively, at roughly one quarter of retraining cost. Across the tested small-audit regimes, effectiveness is strongest at an intermediate forget-set size, indicating that approximate subject-level unlearning is a useful low-cost correction mechanism, but one whose benefit depends on subject selection quality and removal regime.