Reduced susceptibility to experimentally-induced complex visual hallucinations with age

Visual hallucinations occur across many clinical conditions, but can also be induced experimentally in healthy individuals, using high-frequency flicker (Ganzflicker) and sensory deprivation (Ganzfeld). It is unclear how hallucinatory proneness changes across the lifespan, with prior questionnaire-based studies showing mixed results. As factors such as multi-sensory acuity loss and relatively increased reliance on prior knowledge may increase as we age, and these are considered risk factors for hallucination proneness, we hypothesised that reported decreases in hallucinations might reflect underreporting due to stigma. We therefore sought to measure hallucination proneness in 44 healthy adults spanning the adult lifespan (younger age group; n = 22; age 19-39 years, mean 27.2 ± 5.5; older age group n = 22; age 59-79 years, mean 68.0 ± 5.8), quantifying the tendency to experience complex and simple hallucinations in an experimental environment designed to elicit hallucinations. We find that reports of complex hallucinations (those representing objects, scenes, etc) are lower in older adults than in younger adults, both in real time and retrospectively. None of our measured cognitive or sensory measures (visual acuity, contrast sensitivity, perceptual reorganisation, imagery vividness, memory inhibition, and suggestibility) mediate this relationship. We conclude that reduced complex hallucination proneness is likely to be a feature of typical ageing, and that experiencing hallucinations in older individuals may signal underlying pathology.