Graphical illustration and functional neuroimaging of visual hallucinations during prolonged blindfolding: A comparison to visual imagery

Abstract

 

Purpose: To compare the subjective experience and the brain activity associated with visual hallucinations produced by prolonged blindfolding with the activity associated with mental imagery of the same patterns. Methods: The subject was a 37-year-old healthy female who developed visual hallucinations during three weeks of complete visual deprivation. We acquired fMRI data with a Siemens 3T Magnetom Allegra towards the end of the deprivation period, to assess hallucination-related activity, and again after recovery from blindfolding, to measure imagery-related activity. Subjective descriptions and graphical illustrations were provided by the subject after blindfolding was completed. Results: During blindfolding, the subject experienced vivid visual hallucinations, consisting of flashes of light and coloured, moving patterns. The hallucinated images become gradually less vivid and lost their colour intensity, but gained in structural complexity during the three weeks of blindfolding. Neural activity related to hallucinations was found in occipital visual, posterior parietal and several prefrontal regions (the left medial frontal gyrus, the bilateral inferior frontal gyri and the bilateral middle frontal gyri). In contrast, mental imagery of the same percepts led to activation in prefrontal, but not in posterior parietal and occipital regions. Conclusions: These results suggest that deprivation-induced hallucinations result from increased excitability of early visual areas, while mentally-induced imagery involves active read-out under the volitional control of prefrontal structures. This agrees with the subject's report that visual hallucinations were more vivid than mental imagery.