From Noise to Narrative: Tracing the Origins of Hallucinations in
  Transformers
Authors: name: Praneet Suresh; name: Jack Stanley; name: Sonia Joseph; name: Luca Scimeca; name: Danilo Bzdok
Published: 2025-09-08 17:50:45

Abstract: As generative AI systems become competent and democratized in science,
business, and government, deeper insight into their failure modes now poses an
acute need. The occasional volatility in their behavior, such as the propensity
of transformer models to hallucinate, impedes trust and adoption of emerging AI
solutions in high-stakes areas. In the present work, we establish how and when
hallucinations arise in pre-trained transformer models through concept
representations captured by sparse autoencoders, under scenarios with
experimentally controlled uncertainty in the input space. Our systematic
experiments reveal that the number of semantic concepts used by the transformer
model grows as the input information becomes increasingly unstructured. In the
face of growing uncertainty in the input space, the transformer model becomes
prone to activate coherent yet input-insensitive semantic features, leading to
hallucinated output. At its extreme, for pure-noise inputs, we identify a wide
variety of robustly triggered and meaningful concepts in the intermediate
activations of pre-trained transformer models, whose functional integrity we
confirm through targeted steering. We also show that hallucinations in the
output of a transformer model can be reliably predicted from the concept
patterns embedded in transformer layer activations. This collection of insights
on transformer internal processing mechanics has immediate consequences for
aligning AI models with human values, AI safety, opening the attack surface for
potential adversarial attacks, and providing a basis for automatic
quantification of a model's hallucination risk.