Measuring the contents of hallucinations from psychedelics and stroboscopic light: the Six-Dimensional Visual Hallucination Questionnaire

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 characterize their phenomenology. We provide an initial validation of the Six-Dimensional Visual Hallucination Questionnaire (6D-VHQ), a brief instrument designed to quantify visual hallucination content across contexts. The 6D-VHQ measures six dimensions of visual experience: geometric and semantic content, detail level, vividness, entropy and focality. We report three studies: an online image-based validation study, a laboratory-based study with stroboscopically induced hallucinations and an online study in which participants reported closed-eye psychedelic hallucinations which occurred before enrolment. Across 962 responses, the questionnaire showed high internal consistency and a factor structure aligned with the intended dimensions, while also exhibiting substantial inter-item redundancy consistent with an over-complete initial implementation. Compared with the 11-Dimensional Altered States of Consciousness Questionnaire, both instruments captured simple and complex hallucinations, despite differing psychometric profiles. Stroboscopic stimulation was associated with simple geometric phenomena, whereas psychedelic reports showed greater semantic content, vividness and detail. These findings provide an initial operationalization of multi-dimensional visual hallucination content. Future work may expand and improve upon this work, including refinements to the instrument and applications to other hallucinatory contexts.