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Enterprise AI Analysis: H-Neurons: On the Existence, Impact, and Origin of Hallucination-Associated Neurons in LLMs

Enterprise AI Analysis

H-Neurons: On the Existence, Impact, and Origin of Hallucination-Associated Neurons in LLMs

This paper introduces 'H-Neurons', a sparse subset of neurons in Large Language Models (LLMs) that reliably predict hallucination occurrences. These H-Neurons are causally linked to 'over-compliance' behaviors, meaning the model prioritizes satisfying user requests over factual accuracy, even when it leads to generating false or harmful content. The research also traces the origin of H-Neurons to the pre-training phase, suggesting that hallucination is deeply rooted in the fundamental training objectives rather than merely being an artifact of post-training alignment. These findings offer crucial insights for developing more reliable LLMs by enabling enhanced detection and targeted interventions.

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0 Sparse H-Neuron Subset
0 Detection Accuracy on TriviaQA (Mistral)
Pre-training Origin Phase

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The study successfully identifies 'H-Neurons' – a remarkably sparse subset of neurons (less than 0.1% of total neurons) whose activations reliably predict whether an LLM will produce hallucinatory responses. This is achieved using a systematic methodology contrasting activation patterns between faithful and hallucinatory responses, followed by sparse logistic regression. These H-Neurons demonstrate strong generalization across diverse scenarios, including cross-domain contexts and fabricated knowledge detection, proving robust hallucination detection capability.

Controlled interventions reveal that H-Neurons are causally linked to 'over-compliance' behaviors in LLMs. Amplifying H-Neuron activations systematically increases a spectrum of over-compliance: from overcommitment to incorrect premises, heightened susceptibility to misleading contexts, increased adherence to harmful instructions, and stronger sycophantic tendencies. This indicates that H-Neurons do not merely encode factual errors but represent a general tendency to prioritize conversational compliance over factual integrity, even at the cost of truthfulness or safety.

Cross-model transfer experiments demonstrate that H-Neurons originate during the pre-training phase, not just as artifacts of post-training alignment. The neural signatures of hallucination are intrinsic to the base models before fine-tuning, and H-Neurons undergo minimal parameter updates during the transition to instruction-tuned models. This 'parameter inertia' suggests that standard instruction tuning largely preserves these pre-existing circuits rather than fundamentally restructuring hallucination mechanics.

0.1% Only a tiny fraction of neurons are H-Neurons, yet they are highly predictive of hallucinations.

H-Neuron Identification & Impact Flow

Contrast Activation Patterns
Apply Sparse Logistic Regression
Identify H-Neurons (Positive Weights)
Modulate H-Neuron Activations
Observe Over-Compliance Behavior

H-Neuron vs. Random Neuron Performance

Feature H-Neurons Random Neurons
Predictive Accuracy (Avg.) High (70-90%) Low (50-60%)
Generalization Across Domains Robust (BioASQ, NonExist) Limited
Causal Impact on Over-Compliance Direct & Significant Minimal / None
Origin Pre-training N/A

H-Neurons and Safety Bypass

One striking finding is the direct link between H-Neurons and the model's susceptibility to 'Jailbreak' attempts. By amplifying H-Neurons, models show an increased tendency to comply with harmful instructions, bypassing safety filters that would otherwise prevent the generation of unsafe content. Conversely, suppressing these neurons can enhance safety by reducing over-compliance. This highlights the critical role of H-Neurons in mediating both factual integrity and safety alignment, suggesting that a single underlying mechanism drives both types of undesirable behaviors.

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