Measuring the Content of Visual Hallucinations Induced by Psychedelics and Stroboscopic Light: The Six-Dimensional Visual Hallucination Questionnaire (6D-VHQ)

Hallucinations arise from disruptions in neural processes that construct perceptual experience from sensory input. Progress in consciousness science may therefore benefit from methods that precisely characterise their phenomenology. We introduce and validate the 6-Dimensional Visual Hallucination Questionnaire (6D-VHQ), a brief instrument designed to quantify visual hallucination content across contexts. The 6D-VHQ measures six dimensions: geometric and semantic content, detail level, vividness, entropy, and focality. It focuses on perceptual content rather than broader altered-state features.We report three studies: an image-based validation study, an application to stroboscopically induced visual hallucinations, and an application to psychedelic closed-eye visuals. Across 962 responses, the questionnaire showed high internal consistency, a factor structure aligned with the intended dimensions, and stable performance across settings. High inter-item redundancy indicated an initially over-complete but coherent implementation of the instrument. Compared with the 11-Dimensional Altered States of Consciousness questionnaire (11D-ASC), both instruments similarly captured simple and complex visual hallucinations despite differing psychometric properties. While the 11D-ASC showed low internal consistency, the 6D-VHQ has high internal consistency alongside high inter-dimensional redundancy. While the 11D-ASC correlates to participant traits, and 6D-VHQ is largely invariant to these, capturing the differences in visual phenomenology between different classes of hallucinatory experiences. The 6D-VHQ showed that stroboscopic stimulation predominantly elicited vivid simple geometric hallucinations, whereas psychedelic experiences showed combined geometric content alongside more vivid, detailed, and semantic visuals. The 6D-VHQ provides an operationalisation of a six-dimensional model of visual hallucination content that distinguishes broad classes of hallucinations across induction methods.