Ablation is Not Enough to Emulate DPO: How Neuron Dynamics Drive
  Toxicity Reduction
Authors: name: Yushi Yang, name: Filip Sondej, name: Harry Mayne, name: Adam Mahdi

Safety fine-tuning algorithms are commonly used to fine-tune language models
to reduce harmful outputs, but the exact internal mechanisms of how those
models achieve this remain unclear. In studying direct preference optimisation
(DPO) for toxicity reduction, current explanations claim that DPO works by
dampening the most toxic MLP neurons to learn an offset to avert toxic regions
in the residual stream. However, by ablating the most toxic neurons and
applying activation patching, we find this explanation incomplete. By
projecting neuron activation changes onto a toxicity probe, we find that only
31.8\% of toxicity reduction comes from dampened toxic neurons. Instead, DPO
reduces toxicity by accumulating effects across multiple neuron groups, both
reducing writing in the toxic direction and promoting anti-toxicity in the
residual stream. Moreover, DPO gives noisy adjustments to neuron activations,
with many neurons actually increasing toxicity. This indicates that DPO is a
balancing process between opposing neuron effects to achieve toxicity
reduction.