Visual hallucinations induced by Ganzflicker and Ganzfeld differ in frequency, complexity, and content

Abstract

Visual hallucinations can be phenomenologically divided into those of a simple or complex nature. Both simple and complex hallucinations can occur in pathological and non-pathological states, and can also be induced experimentally by visual stimulation or deprivation—for example using a high-frequency, eyes-open flicker (Ganzflicker) and perceptual deprivation (Ganzfeld). Here we leverage the differences in visual stimulation that these two techniques involve to investigate the role of bottom-up and top-down processes in shifting the complexity of visual hallucinations, and to assess whether these techniques involve a shared underlying hallucinatory mechanism despite their differences. For each technique, we measured the frequency and complexity of the hallucinations produced, utilising button presses, retrospective drawing, interviews, and questionnaires. For both experimental techniques, simple hallucinations were more common than complex hallucinations. Crucially, we found that Ganzflicker was more effective than Ganzfeld at eliciting simple hallucinations, while complex hallucinations remained equivalent across the two conditions. As a result, the likelihood that an experienced hallucination was complex was higher during Ganzfeld. Despite these differences, we found a correlation between the frequency and total time spent hallucinating in Ganzflicker and Ganzfeld conditions, suggesting some shared mechanisms between the two methodologies. We attribute the tendency to experience frequent simple hallucinations in both conditions to a shared low-level core hallucinatory mechanism, such as excitability of visual cortex, potentially amplified in Ganzflicker compared to Ganzfeld due to heightened bottom-up input. The tendency to experience complex hallucinations, in contrast, may be related to top-down processes less affected by visual stimulation.

Current and future directions for research on hallucinations and delusions

“…Hallucinations and delusions are pat of a normal distribution of human diversity; their mere presence does not indicate psychosis or psychiatric illness. Experimentally-induced hallucinations are safe, controlled, and have no negative consequences 3 ; confabulating memories sometimes is perfectly normal 4 ; and despite the insistence of some individuals, simply having conspiracy beliefs is not a sign of pathology 5 .…”

“…The study by Shenyan et al 3 demonstrates that hallucinations can be reliably induced in non-clinical samples using experimental techniques called Ganzfeld 6 and Ganzflicker 7 , 8 . Prolonged, unstructured (Ganzfeld) or repetitive (Ganzflicker) stimulation to the visual system can elicit both simple and complex hallucinatory experiences, such as geometric patterns, illusory colors, or even meaningful real-world objects and scenes.…”

“…The ubiquity of hallucinations and delusions across clinical and non-clinical populations suggests common cognitive and neural mechanisms. We are developing techniques to experimentally induce hallucinations, which could potentially elucidate how they can develop into psychotic experiences 3 , 7 , 8 . In the study by Shenyan et al 3 , the authors purport that a (cortically hierarchical) low-level hallucinatory mechanism may be responsible for simple hallucinations, whereas top-down processes (such as mental imagery or beliefs) may contribute to complex hallucinations.…”

“…We are developing techniques to experimentally induce hallucinations, which could potentially elucidate how they can develop into psychotic experiences 3 , 7 , 8 . In the study by Shenyan et al 3 , the authors purport that a (cortically hierarchical) low-level hallucinatory mechanism may be responsible for simple hallucinations, whereas top-down processes (such as mental imagery or beliefs) may contribute to complex hallucinations. These same mechanisms are proposed to be involved in different kinds of hallucinatory experiences in clinical populations, as well 10 .…”

“…Environmental and social factors importantly contribute to this impact: conspiracy beliefs under a ‘sacred canopy’ (i.e., a social support buffer) can benefit an individual to a similar extent as being part of a religious or social organization; but these beliefs can exacerbate psychopathological characteristics if social support is lacking 5 . As another example, individuals who purposefully seek hallucinatory experiences are able to choose the environment, onset, and duration of the event; and can attribute the experience to an explicable source, such as Ganzflicker 3 . Controlling an unusual experience in this way can neutralize its potential negative effects.…”